RAINER MARIA RILKE, “THE FIFTH ELEGY”
(1923)
We were wanderers from the beginning. We
knew every stand of tree for a hundred miles. When the fruits or nuts were
ripe, we were there. We followed the herds in their annual migrations. We
rejoiced in fresh meat. through stealth, feint, ambush, and main-force assault,
a few of us cooperating accomplished what many of us, each hunting alone, could
not. We depended on one another. Making it on our own was as ludicrous to
imagine as was settling down.
Working together, we protected our
children from the lions and the hyenas. We taught them the skills they would
need. And the tools. Then, as now, technology was the key to our survival.
When the drought was prolonged, or when
an unsettling chill lingered in the summer air, our group moved on-sometimes to
unknown lands. We sought a better place. And when we couldn't get on with the
others in our little nomadic band, we left to find a more friendly bunch
somewhere else. We could always begin again.
For 99. 9 percent of the time since our
species came to be, we were hunters and foragers, wanderers on the savannahs
and the steppes. There were no border guards then, no customs officials. The
frontier was everywhere. We were bounded only by the Earth and the ocean and
the sky-plus occasional grumpy neighbors.
When the climate was congenial, though,
when the food was plentiful, we were willing to stay put. Unadventurous.
Overweight. Careless. In the last ten thousand years-an instant in our long
history—we've abandoned the nomadic fife. We've domesticated the plants and
animals. Why chase the food when you can make it come to you?
For all its material advantages, the
sedentary life has left us edgy, unfulfilled. Even after 400 generations in
villages and cities, we haven't forgotten. The open road still softly calls,
like a nearly forgotten song of childhood. We invest far-off places with a
certain romance. This appeal, I suspect, has been meticulously crafted by
natural selection as an essential element in our survival. Long summers, mild
winters, rich harvests, plentiful game-none of them lasts forever. It is beyond
our powers to predict the future. Catastrophic events have a way of sneaking up
on us, of catching us unaware. Your own life, or your band's, or even your
species' might be owed to a restless few-drawn, by a craving they can hardly
articulate or understand, to undiscovered lands and new worlds.
Herman Melville, in Moby Dick, spoke for
wanderers in all epochs and meridians: “I am tormented with an everlasting itch
for things remote. I love to sail forbidden seas...”
To the ancient Greeks and Romans, the
known world comprised Europe and an attenuated Asia and Africa, all surrounded
by an impassable World Ocean. Travelers might encounter inferior beings called
barbarians or superior beings called gods. Every tree had its dryad, every
district its legendary hero. But there were not very many gods, at least at
first, perhaps only a few dozen. They lived on mountains, under the Earth, in
the sea, or up there in the sky. They sent messages to people, intervened in
human affairs, and interbred with us.
As time passed, as the human exploratory
capacity hit its stride, there were surprises: Barbarians could be fully as
clever as Greeks and Romans. Africa and Asia were larger than anyone had
guessed. The World Ocean was not impassable. There were Antipodes. Three new
continents existed, had been settled by Asians in ages past, and the news had
never reached Europe. Also the gods were disappointingly hard to find.
The first large-scale human migration
from the Old World to the New happened during the last ice age, around 11,500
years ago, when the growing polar ice caps shallowed the oceans and made it
possible to walk on dry land from Siberia to Alaska. A thousand years later, we
were in Tierra del Fuego, the southern tip of South America. Long before
Columbus, Indonesian argonauts in outrigger canoes explored the western
Pacific; people from Borneo settled Madagascar; Egyptians and Libyans
circumnavigated Africa; and a great fleet of ocean going junks from Ming
Dynasty China crisscrossed the Indian Ocean, established a base in Zanzibar,
rounded the Cape of Good Hope, and entered the Atlantic Ocean. In the fifteenth
through seventeenth centuries, European sailing ships discovered new continents
(new, at any rate, to Europeans) and circumnavigated the planet. In the
eighteenth and nineteenth centuries, American and Russian explorers, traders,
and settlers raced west and east across two vast continents to the Pacific.
This zest to explore and exploit, however thoughtless its agents may have been,
has clear survival value. It is not restricted to any one nation or ethnic
group. It is an endowment that all members of the human species hold in common.
Since we first emerged, a few million
years ago in East Africa, we have meandered our way around the planet. There
are now people on every continent and the remotest islands, from pole to pole,
from Mount Everest to the Dead Sea, on the ocean bottoms and even,
occasionally, in residence 200 miles up-humans, like the gods of old, living in
the sky.
These days there seems to be nowhere
left to explore, at least on the land area of the Earth. Victims of their very
success the explorers now pretty much stay home.
Vast migrations of people-some
voluntary, most not- have shaped the human condition. More of us flee from war,
oppression, and famine today than at any other time in human history. As the
Earth's climate changes in the coming decade. there are likely to be far
greater numbers of environmental refugees. Better places will always call to
us. Tides of people will continue to ebb and flow across the planet. But the
lands we run to now have already been settled. Other people, often
unsympathetic to our plight, are there before us.
LATE IN THE NINETEENTH CENTURY, Leib
Gruber was growing up in Central Europe, in an obscure town in the immense,
polyglot, ancient Austro-Hungarian Empire. His father sold fish when he could.
But times were often hard. As a young man, the only honest employment Leib
could find was carrying people across the nearby river Bug. The customer, male
or female, would mount Leib's back; in his prized boots, the tools of his
trade, he would wade out in a shallow stretch of the river and deliver his
passenger to the opposite bank. Sometimes the water reached his waist. There
were no bridges here, no ferryboats. Horses might have served the purpose, but
they had other uses. That left Leib and a few other young men like him. They
had no other uses. no other work was available. They would lounge about the
riverbank, calling out their prices, boasting to potential customers about the
superiority of their drayage. They hired themselves out like four-footed
animals. My grandfather was a beast of burden
I don't think that in all his young
manhood Leib had ventured more than a hundred kilometers from his little
hometown of Sassow. But then, in 1904, he suddenly ran away to the New World to
avoid a murder rap, according to one family legend. He left his young wife
behind. How different from his tiny back-water hamlet the great German port
cities must have seemed, how vast the ocean, how strange the lofty skyscrapers
and endless hub-bub of his new land. We know nothing of his crossing, but have
found the ship's manifest for the journey undertaken later by his wife, Chaiya
joining Leib after he had saved enough to bring her over. She traveled in the
cheapest class on the Batavia, a vessel of Hamburg registry. There's something
heartbreakingly terse about the document: Can she read or write? No. Can she
speak English? No. How much money does she have? I can imagine her
vulnerability and her shame as she replies, “One dollar.”
She disembarked in New York, was
reunited with Leib, lived just long enough to give birth to my mother and her
sister, and then died from “complications” of childbirth. In those few years in
America, her name had sometimes been anglicized to Clara. A quarter century
later, my mother named her own firstborn, a son, after the mother she never
knew.
OUR DISTANT ANCESTORS, watching the
stars, noted five that did more than rise and set in stolid procession, as the
so-called “fixed” stars did. These five had a curious and complex motion. Over
the months they seemed to wander slowly among the stars. Sometimes they did
loops. Today we call them planets, the Greek word for wanderers. It was, I
imagine, a peculiarity our ancestors could relate to.
We know now that the planets are not
stars, but other worlds, gravitationally lashed to the Sun. Just as the
exploration of the Earth was being completed, we began to recognize it as one
world among an uncounted multitude of others, circling the Sun or orbiting the
other stars that make up the Milky Way galaxy. Our planet and our solar system
are surrounded by a new world ocean the depths of space. It is no more
impassable than the last.
Maybe it's a little early. Maybe the
time is not quite yet. But those other worlds-promising untold
opportunities-beckon.
In the last few decades, the United
States and the former Soviet Union have accomplished something stunning and
historic-the close-up examination of all those points of light, from Mercury to
Saturn, that moved our ancestors to wonder and to science. Since the advent of
successful interplanetary flight in 1962, our machines have flown by, orbited,
or landed on more than seventy new worlds. We have wandered among the
wanderers. We have found vast volcanic eminences that dwarf the highest
mountain on Earth; ancient river valleys on two planets enigmatically one too
cold and the other too hot for running water; a giant planet with an interior
of liquid metallic hydrogen into which a thousand Earths would fit; whole moons
that have melted; a cloud-covered place with an atmosphere of corrosive raids,
where even the high plateaus are above the melting point of lead ancient
surfaces on which a faithful record of the violent formation of the Solar
System is engraved; refugee ice worlds from the transplutonian depths;
exquisitely patterned ring systems, marking the subtle harmonies of gravity;
and a world surrounded by clouds of complex organic molecules like those that m
the earliest history of our planet led to the origin of life. Silently, they
orbit the Sun, waiting.
We have uncovered wonders undreamt by
our ancestors who first speculated on the nature of those wandering lights in
the night sky. We have probed the origins of our planet and ourselves. By
discovering what else is possible, by coming face to face with alternative
fates of worlds more or less like our own, we have begun to better understand
the Earth. Every one of these worlds is lovely and instructive. But, so far as
we know, they are also, every one of them, desolate and barren. Out there,
there are no “better places.” So far, at least.
During the Viking robotic mission,
beginning in July 1976, in a certain sense I spent a year on Mars. I examined
the boulders and sand dunes, the sky red even at high noon, the ancient river
valleys, the soaring volcanic mountains, the fierce wind erosion, the laminated
polar terrain, the two dark potato-shaped moons. But there was no life-not a
cricket or a blade of grass, or even, so far as we can tell for sure, a
microbe. These worlds have not been graced, as ours has, by life. Life is a
comparative rarity. You can survey dozens of worlds and find that on only one
of them does life arise and evolve and persist.
Having in all their lives till then
crossed nothing wider than a layer, Leib and Chaiya graduated to crossing
oceans. They had one great advantage: On the other side of the waters there
would be-invested with outlandish customs, it is true-other human beings
speaking their language and sharing at least some of their values, even people
to whom they were closely related.
In our time we've crossed the Solar
System and sent four ships to the stars. Neptune lies a million times farther
from Earth than New York City is from the banks of the Bug. But there are no
distant relatives, no humans, and apparently no life waiting for us on those other
worlds. no letters conveyed by recent emigres help us to understand the new
land only digital data transmitted at the speed of light by unfeeling, precise
robot emissaries. They tell us that these new worlds are not much like home.
But we continue to search for inhabitants. We can't help it. Life looks for
life.
No one on Earth, not the richest among
us, can afford the passage; so we can't pick up and leave for Mars or Titan on
a whim, or because we're bored, or out of work, or drafted into the army, or
oppressed, or because, justly or unjustly, we've been accused of a crime. There
does not seem to be sufficient short-term profit to motivate private industry.
If we humans ever go to these worlds, then, it will be because a nation or a
consortium of them believes it to be to its advantage-or to the advantage of
the human species. Just now, there are a great many matters pressing in on us
that compete for the money it takes to send people to other worlds.
That's what this book is about: other
worlds, what awaits us on them, what they tell us about ourselves, and-given
the urgent problems our species now faces-whether it makes sense to go. Should
we solve those problems first? Or are they a reason for going?
This book is, in many ways, optimistic
about the human prospect. The earliest chapters may at first sight seem to
revel overmuch in our imperfections. But they lay an essential spiritual and
logical foundation for the development of my argument.
I have tried to present more than one
facet of an issue. There will be places where I seem to be arguing with myself.
I am. Seeing some merit to more than one side, I often argue with myself. I
hope by the last chapter it will be clear where I come out.
The plan of the book is roughly this: We
first examine the widespread claims made over all of human history that our
world and our species are unique, and even central to the workings and purpose
of the Cosmos. We venture through the Solar System in the footsteps of the
latest voyages of exploration and discovery, and then assess the reasons
commonly offered for sending humans into space. In the last and most
speculative part of the book, I trace how I imagine that our long-term future
in space will work itself out.
Pale Blue Dot is about a new
recognition, still slowly overtaking us, of our coordinates, our place in the
Universe and how, even if the call of the open road is muted in our time, a
central element of the human future lies far beyond the Earth.
CHAPTER 1
You are here
The entire Earth is but a point, and the
place of our own habitation but a minute corner of it.
—MARCUS AURELIUS, ROMAN EMPEROR,
MEDITATIONS, BOOK 4 (CA. 170)
As the astronomers unanimously teach,
the circuit of the whole earth, which to us seems endless, compared with the
greatness of the universe has the likeness of a mere tiny point.
—AMMIANUS MARCELLINUS ACA. 330-395, THE
LAST MAJOR ROMAN HISTORIAN, IN THE CHRONICLE OF EVENTS
The spacecraft was a long way from home,
beyond the orbit of the outermost planet and high above the ecliptic plane-which
is an imaginary flat surface that we can think of as something like a racetrack
in which the orbits of the planets are mainly confined. The ship was speeding
away from the Sun at 40,000 miles per hour. But in early February of 1990, it
was overtaken by an urgent message from Earth.
Obediently, it turned its cameras back
toward the now-distant planets. Slewing its scan platform from one spot in the
sky to another, it snapped 60 pictures and stored them in digital form on its
tape recorder. Then, slowly, in March, April, and May, it radioed the data back
to Earth. Each image was composed of 640,000 individual picture elements
(“pixels”), like the dots in a newspaper wire-photo or a pointillist painting.
The spacecraft was 3. 7 billion miles away from Earth, so far away that it took
etch pixel 51/2 hours, traveling at the speed of light, to reach us. The
pictures might have been returned earlier, but the big radio telescopes in
California, Spain, and Australia that receive these whispers from the edge of
the Solar System had responsibilities to other ships that ply the sea of space
among them, Magellan, bound for Venus, and Galileo on its tortuous passage to
Jupiter.
Voyager 1 was so high above the ecliptic
plane because, in 1981, it had made a close pass by Titan, the giant moon of
Saturn. Its sister ship, Voyager 2, was dispatched on a different trajectory,
within the ecliptic plane, and so she was able to perform her celebrated
explorations of Uranus and Neptune The two Voyager robots have explored four
planets and nearly sixty moons. They are triumphs of human engineering an. one
of the glories of the American space program. They will he in the history books
when much else about our time forgotten.
The Voyagers were guaranteed to work
only until the Saturn encounter. I thought it might be a good idea, just after
Saturn, to have them take one last glance homeward. From Saturn, I knew the
Earth would appear too small for Voyager to make out any detail. Our planet
would be just a point of light, a lonely pixel, hardly distinguishable from the
many other points of light Voyager could see, nearby planets and far-off suns.
But precise because of the obscurity of our world thus revealed, such picture
might be worth having.
Mariners had painstakingly mapped the
coastlines of the continents. Geographers had translated these findings into
charts and globes. Photographs of tiny patches of the Earth had been obtained
first by balloons and aircraft, then by rockets in brief ballistic flight, and
at last by orbiting spacecraft-giving a perspective like the one you achieve by
positioning your eyeball about an inch above a large globe. While almost
everyone is taught that the Earth is a sphere with all of us somehow glued to
it by gravity, the reality of our circumstance did not really begin to sink in
until the famous frame-filling Apollo photograph of the whole Earth-the one
taken by the Apollo 17 astronauts on the last journey of humans to the Moon.
It has become a kind of icon of our age.
There's Antarctica at what Americans and Europeans so readily regard as the
bottom, and then all of Africa stretching up above it: You can see Ethiopia,
Tanzania, and Kenya, where the earliest humans lived. At top right are Saudi
Arabia and what Europeans call the Near East. Just barely peeking out at the
top is the Mediterranean Sea, around which so much of our global civilization
emerged. You can make out the blue of the ocean, the yellow-red of the Sahara
and the Arabian desert, the brown-green of forest and grassland.
And yet there is no sign of humans in
this picture, not our reworking of the Earth's surface, not our machines, not
ourselves: We are too small and our statecraft is too feeble to be seen by a
spacecraft between the Earth and the Moon. From this vantage point, our obsession
with nationalism is nowhere in evidence. The Apollo pictures of the whole Earth
conveyed to multitudes something well known to astronomers: On the scale of
worlds-to say nothing of stars or galaxies-humans are inconsequential, a thin
film of life on an obscure and solitary lump of rock and metal.
It seemed to me that another picture of
the Earth, this one taken from a hundred thousand times farther away, might
help in the continuing process of revealing to ourselves our true circumstance
and condition. It had been well understood by the scientists and philosophers
of classical antiquity that the Earth was a mere point in a vast encompassing
Cosmos, but no one had ever seen it as such. Here was our first chance (and
perhaps also our last for decades to come).
Many in NASA's Voyager Project were
supportive. But from the outer Solar System the Earth lies very near the Sun,
like a moth enthralled around a flame. Did we want to aim the camera so close
to the Sun as to risk burning out the spacecraft's vidicon system? Wouldn't it
be better to delay until all the scientific images from Uranus and Neptune, if
the spacecraft lasted that long, were taken?
And so we waited- and a good thing
too-from 1981 at Saturn, to 1986 at Uranus, to 1989, when both spacecraft had
passed the orbits of Neptune and Pluto. At last the time came But there were a
few instrumental calibrations that needed to be done first, and we waited a
little longer. Although the spacecraft were in the right spots, the instruments
were still working beautifully, and there were no other pictures to take, a few
project personnel opposed it. It wasn't science, they said. Then we discovered
that the technicians who devise and transmit the radio commands to Voyager
were, in a cash-strapped NASA, to be laid off immediately or transferred to
other jobs. If the picture were to be taken, it had to be done right then. At
the last minute actually, in the midst of the Voyager 2 encounter with Neptune,
the then NASA Administrator, Rear Admiral Richard Truly, stepped in and made
sure that these images were obtained. The space scientists Candy Hansen of
NASA's Jet Propulsion Laboratory (JPL) and Carolyn Porco of University of
Arizona designed the command sequence and calculated the camera exposure times.
So here they are-a mosaic of squares
laid down on top of the planets and a background smattering of more distant
stars. We were able to photograph not only the Earth, but also five other of
the Sun's nine known planets. Mercury, the innermost, was lost in the glare of
the Sun, and Mars and Pluto were too small, too dimly lit, and/or too far away.
Uranus and Neptune are so dim that to record their presence required long
exposures; accordingly, their images were smeared because of spacecraft motion.
This is how the planets would look to an alien spaceship approaching the Solar
System after a long interstellar voyage.
From this distance the planets seem only
points of light, smeared or unsmeared-even through the high-resolution
telescope aboard Voyager. They are like the planets seen with the naked eye
from the surface of the Earth-luminous dots, brighter than most of the stars.
Over a period of months the Earth, like the other planets, would seem to move
among the stars. You cannot tell merely by looking at one of these dots what
it's like, what's on it, what its past has been, and whether, n this particular
epoch, anyone lives there.
Because of the reflection of sunlight
off the spacecraft, the Earth seems to be sitting in a beam of light, as if
there were some special significance to this small world. But it's just an
accident of geometry and optics. The Sun emits its radiation equitably in all
directions. Had the picture been taken a little earlier or a little later,
there would have been no sunbeam highlighting the Earth.
And why that cerulean color? The blue
comes partly from the sea, partly from the sky. While water in a glass is
transparent, It absorbs slightly more red light than blue. If you have tens of
meters of the stuff or more, the red light is absorbed out and what gets
reflected back to space is mainly blue. In the same way, a short line of sight
through air seems perfectly transparent. Nevertheless-something Leonardo da
Vinci excelled at portraying-the more distant the object, the bluer it seems.
Why? Because the air scatters blue light around much better than it does red.
So the bluish cast of this dot comes from its thick but transparent atmosphere
and its deep oceans of liquid water. And the white? The Earth on an average day
is about half covered with white water clouds.
We can explain the wan blueness of this
little world because we know it well. Whether an alien scientist newly arrived
at the outskirts of our solar system could reliably deduce oceans and clouds
and a thickish atmosphere is less certain. Neptune, for instance, is blue, but
chiefly for different reasons. From this distant vantage point, the Earth might
not seem of any particular interest.
But for us, it's different. Look again
at that dot. That's here. That's home. That's us. On it everyone you love,
everyone you know, everyone you ever heard of, every human being who ever was,
lived out their lives. The aggregate of our joy and suffering, thousands of
confident religions, ideologies, and economic doctrines, every hunter and
forager, every hero and coward, every creator and destroyer of civilization,
ever king and peasant, every young couple in love, every moth and father,
hopeful child, inventor and explorer, every teacher of morals, every corrupt
politician, every “superstar,” every “supreme leader,” every saint and sinner
in the history of our species lived there-on a mote of dust suspended in a
sunbeam.
The Earth is a very small stage in a
vast cosmic arena. Think of the rivers of blood spilled by all those generals
and emperors so that, in glory and triumph, they could become momentary masters
of a fraction of a dot. Think of the endless visited by the inhabitants of one
corner of this pixel the scarcely distinguishable inhabitants of some other
corner, how frequent their misunderstandings, how eager they are to kill one
another, how fervent their hatreds.
Our posturings, our imagined
self-importance, the delusion that we have some privileged position in the
Universe, are challenged by this point of pale light. Our planet is a lonely
speck in the great enveloping cosmic dark. In our obscurity, in all this
vastness, there is no hint that help will come from elsewhere to save us from
ourselves.
The Earth is the only world known so far
to harbor life. There is nowhere else, at least in the near future, to which
our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the
moment the Earth is where we make our stand.
It has been said that astronomy is
a humbling and character-building experience. There is perhaps no better demonstration
of the folly of human conceits than this distant image of our tiny world. To
me, it underscores our responsibility to deal more kindly with one another, and
to preserve and cherish the pale blue dot, the only home we've ever known.
CHAPTER 2
Aberrations of light
If man were taken away from the world,
the rest would seem to be all astray, without aim or purpose...and to be
leading to nothing.
—FRANCIS BACON, WISDOM OF THE ANCIENTS
(1619)
Ann Druyan suggests an experiment: Look
back again at the pale blue dot of the preceding chapter. Take a good long look
at it. Stare at the dot for any length of time and then try to convince
yourself that God created the whole Universe for one of the 10 million or so
species of life that inhabit that speck of dust. Now take it a step further:
Imagine that everything was made just for a single shade of that species, or
gender, or ethnic or religious subdivision. If this doesn't strike you as
unlikely, pick another dot. Imagine it to be inhabited by a different form of
intelligent life. They, too, cherish the notion of a God who has created
everything for their benefit. How seriously do you take their claim?
“SEE THAT STAR?”
“You mean the bright red one?” his
daughter asks in return.
“Yes. You know, it might not be there
anymore. It might be gone by now-exploded or something. Its light is still
crossing space, just reaching our eyes now. But we don't see it as it is. We
see it as it was.”
Many people experience a stirring sense
of wonder when they first confront this simple truth. Why? Why should it be so
compelling? On our little world light travels, for all practical purposes,
instantaneously. If a lightbulb is glowing, then of course it's physically
where we see it, shining away. We reach out our hand and touch it: It's there
all right, and unpleasantly hot. If the filament fails, then the light goes
out. We don't see it in the same place, glowing, illuminating the room years
after the bulb breaks and it's removed from its socket. The very notion seems
nonsensical. But if we're far enough away, an entire sun can go out and we'll
continue to see it shining brightly; we won't learn of its death, it may be,
for ages to come-in fact, for how long it takes light, which travels fast but
not infinitely fast, to cross the intervening vastness.
The immense distances to the stars and
the galaxies mean that we see everything in space in the past-some as they were
before the Earth came to be. Telescopes are time machines. Long ago, when an
early galaxy began to pour light out into the surrounding darkness, no witness
could have known that billions of years later some remote clumps of rock and
metal, ice and organic molecules would fall together to make a place called
Earth; or that life would arise and thinking beings evolve who would one day
capture a little of that galactic light, and try to puzzle out what had sent it
on its way.
And after the Earth dies, some 5 billion
years from now, after it is burned to a crisp or even swallowed by the Sun,
there will be other worlds and stars and galaxies coming into being-and they
will know nothing of a place once called Earth.
IT ALMOST NEVER FEELS like prejudice.
Instead, it seems fitting and just-the idea that, because of an accident of
birth, our group (whichever one it is) should have a central position in the
social universe. Among Pharaonic princelings and Plantagenet pretenders,
children of robber barons and Central Committee bureaucrats, street gangs and
conquerors of nations, members of confident majorities, obscure sects, and reviled
minorities, this self-serving attitude seems as natural as breathing. It draws
sustenance from the same psychic wellsprings as sexism, racism, nationalism,
and the other deadly chauvinisms that plague our species. Uncommon strength of
character is needed to resist the blandishments of those who assure us that we
have an obvious, even God-given, superiority over our fellows. The more
precarious our self-esteem, the greater our vulnerability to such appeals.
Since scientists are people, it is not
surprising that comparable pretensions have insinuated themselves into the
scientific worldview. Indeed, many of the central debates in the history of
science seem to be, in part at least, contests over whether humans are special.
Almost always, the going-in assumption is that we are special. After the
premise is closely examined, though, it turns out-in dishearteningly many
cases-that we are not.
Our ancestors lived out of doors. They
were as familiar with the night sky as most of us are with our favorite television
programs. The Sun, the Moon, the stars, and the planets all rose in the east
and set in the west, traversing the sky overhead in the interim. The motion of
the heavenly bodies was not merely a diversion, eliciting a reverential nod and
grunt; it was the only way to tell the time of day and the seasons. For hunters
and gatherers, as well as for agricultural peoples, knowing about the sky was a
matter of life and death.
How lucky for us that the Sun, the Moon,
the planets, and the stars are part of some elegantly configured cosmic
clockwork! It seemed to be no accident. They were put here for a purpose, for
our benefit. Who else makes use of them? What else are they good for?
And if the lights in the sky rise and
set around us, isn't it evident that we're at the center of the Universe? These
celestial bodies-so clearly suffused with unearthly powers, especially the Sun
on which we depend for light and heat-circle us like courtiers fawning on a
king. Even if we had not already guessed, the most elementary examination of
the heavens reveals that we are special. The Universe seems designed for human
beings. It's difficult to contemplate these circumstances without experiencing
stirrings of pride and reassurance. The entire Universe, made for us! We must
really be something.
This satisfying demonstration of our
importance, buttressed by daily observations of the heavens, made the
geocentrist conceit a transcultural truth-taught in the schools, built into the
language, part and parcel of great literature and sacred scripture. Dissenters
were discouraged, sometimes with torture and death. It is no wonder that for
the vast bulk of human history, no one questioned it.
It was doubtless the view of our
foraging and hunting ancestors. The great astronomer of antiquity, Claudius
Ptolemaeus (Ptolemy), in the second century knew that the Earth was a sphere,
knew that its size was “a point” compared to the distance of the stars, and
taught that it lay “right in the middle of the heavens.” Aristotle, Plato, St.
Augustine, St. Thomas Aquinas, and almost all the great philosophers and
scientists of all cultures over the 3,000 years ending in the seventeenth
century bought into this delusion. Some busied themselves figuring out how the
Sun, the Moon, the stars, and the planets could be cunningly attached to
perfectly transparent, crystalline spheres-the big spheres, of course, centered
on the Earth-that would explain the complex motions of the celestial bodies so
meticulously chronicled by generations of astronomers. And they succeeded: With
later modifications, the geocentric hypothesis adequately accounted for the
facts of planetary motion as known in the second century, and in the sixteenth.
From there it was only a slight
extrapolation to an even more grandiose claim-that the “perfection” of the
world would be incomplete without humans, as Plato asserted in the Timaeus.
“Man...is all,” the poet and cleric John Donne wrote in 1625. “He is not a
piece of the world, but the world itself; and next to the glory of God, the
reason why there is a world.”
And yet-never mind how many kings,
popes, philosophers, scientists, and poets insisted on the contrary-the Earth
through those millennia stubbornly persisted in orbiting the Sun. You might
imagine an uncharitable extraterrestrial observer looking down on our species
over all that time-with us excitedly chattering, “The Universe is created for
us! We're at the center! Everything pays homage to us!”-and concluding that our
pretensions are amusing, our aspirations pathetic, that this must be the planet
of the idiots.
But such a judgment is too harsh. We did
the best we could. There was an unlucky coincidence between everyday
appearances and our secret hopes. We tend not to be especially critical when
presented with evidence that seems to confirm our prejudices. And there was
little countervailing evidence.
In muted counterpoint, a few dissenting
voices, counseling humility and perspective, could be heard down through the
centuries. At the dawn of science, the atomist philosophers of ancient Greece
and Rome- those who first suggested that matter is made of atoms-Democritus,
Epicurus, and their followers (and Lucretius, the first popularizer of science)
scandalously proposed many worlds and many alien life forms, all made of the
same kinds of atoms as we. They offered for our consideration infinities in
space and time. But in the prevailing canons of the West, secular and
sacerdotal, pagan and Christian, atomist ideas were reviled. Instead, the
heavens were not at all like our world. They were unalterable and “perfect.”
The Earth was mutable and “corrupt.” The Roman statesman and philosopher Cicero
summarized the common view: “In the heavens...there is nothing of chance or
hazard, no error, no frustration, but absolute order, accuracy, calculation and
regularity.”
Philosophy and religion cautioned that
the gods (or God) were far more powerful than we, jealous of their prerogatives
and quick to mete out justice for insufferable arrogance. At the same time,
these disciplines had not a clue that their own teaching of how the Universe is
ordered was a conceit and a delusion.
Philosophy and religion presented mere
opinion-opinion that might be overturned by observation and experiment-as
certainty. This worried them not at all. That some of their deeply held beliefs
might turn out to be mistakes was a possibility hardly considered. Doctrinal
humility was to be practiced by others. Their own teachings were inerrant and
Infallible. In truth, they had better reason to be humble than they knew.
BEGINNING WITH COPERNICUS in the middle
sixteenth century, the issue was formally joined. The picture of the Sun rather
than the Earth at the center of the Universe was understood to be dangerous.
Obligingly, many scholars were quick to assure the religious hierarchy that
this newfangled hypothesis represented no serious challenge to conventional
wisdom. In a kind of split-brain compromise, the Sun-centered system was
treated as a mere computational convenience, not an astronomical reality that
is, the Earth was really at the center of the Universe, as everybody knew; but
if you wished to predict where Jupiter would be on the second Tuesday of
November the year after next, you were permitted to pretend that the Sun was at
the center. Then you could calculate away and not affront the Authorities.
“This has no danger in it,” wrote Robert
Cardinal Bellarmine, the foremost Vatican theologian in the early seventeenth
century, and suffices for the
mathematicians. But, to affirm that the Sun is really fixed in the center of
the heavens and that the Earth revolves very swiftly around the Sun is a
dangerous thing, not only irritating the theologians and philosophers, but
injuring our holy faith and making the sacred scripture false.”
“Freedom of belief is pernicious,”
Bellarmine wrote on another occasion. “It is nothing but the freedom to be
wrong.”
Besides, if the Earth was going around
the Sun, nearby stars should seem to move against the background of more
distant stars as, every six months, we shift our perspective from one side of
the Earth's orbit to the other. no such “annual parallax” had been found. The
Copernicans argued that this was because the stars were extremely far
away-maybe a million times more distant than the Earth is from the Sun. Perhaps
better telescopes, in future times, would find an annual parallax. The
geocentrists considered this a desperate attempt to save a flawed hypothesis,
and ludicrous on the face of it.
When Galileo turned the first
astronomical telescope to the sky, the tide began to turn. He discovered that
Jupiter had a little retinue of moons circling it, the inner ones orbiting
faster than the outer ones, just as Copernicus had deduced for the motion of
the planets about the Sun. He found that Mercury and Venus went through phases
like the Moon (showing they orbited the Sun). Moreover, the cratered Moon and
the spotted Sun challenged the perfection of the heavens. This may in part
constitute the sort of trouble Tertullian was worried about thirteen hundred
years earlier, when he pleaded, “If you have any sense or modesty, have done
with prying into the regions of the sky, into the destiny and secrets of the
universe.”
In contrast, Galileo taught that we can
interrogate Nature by observation and experiment. Then, “facts which at first
sight seem improbable will, even on scant explanation, drop the cloak which had
hidden them and stand forth in naked and simple beauty.” Are not these facts,
available even for skeptics to confirm, a surer insight into God's Universe
than all the speculations of the theologians? But what if these facts
contradict the beliefs of those who hold their religion incapable of making
mistakes? The princes of the Church threatened the aged astronomer with torture
if he persisted in teaching the abominable doctrine that the Earth moved. He
was sentenced to a kind of house arrest for the remainder of his life.
A generation or two later, by the time
Isaac Newton demonstrated that simple and elegant physics could quantitatively
explain-and predict-all the observed lunar and planetary motions (provided you
assumed the Sun at the center of the Solar System), the geocentrist conceit
eroded further.
In 1725, in an attempt to discover
stellar parallax, the painstaking English amateur astronomer James Bradley
stumbled on the aberration of light. The term “aberration,” I suppose, conveys
something of the unexpectedness of the discovery. When observed over the course
of a year, stars were found to trace little ellipses against the sky. But all
the stars were found to do so. This could not be stellar parallax, where we
would expect a big parallax for nearby stars and an indetectible one for
faraway stars. Instead, aberration is similar to how raindrop falling directly
down on a speeding auto seem to the passenger, to be falling at a slant; the faster
the car goes, the steeper the slant. If the Earth were stationary at the center
of the Universe. and not speeding in its orbit around the Sun, Bradley would
nor have found the aberration of light. It was a compelling demonstration that
the Earth revolved about the Sun. It convinced most astronomers and some others
but not, Bradley thought, the “Anti-Copernicans.”
But not until 1837 did direct
observations of the stars prove in the clearest way that the Earth is indeed
circling the Sun. The long-debated annual parallax was at last discovered-not
by better arguments, but by better instruments. Because explaining what it
means is much more straightforward than explaining the aberration of light, its
discovery was very important. It pounded the final nail into the coffin of
geocentrism. You need only look at your finger with your left eye and then with
your right and see it seem to move. Everyone can understand parallax.
By the nineteenth century, all
scientific geocentrists had been converted or rendered extinct. Once most
scientists had been convinced, informed public opinion had swiftly changed, in
some countries in a mere three or four generations. Of course, in the time of
Galileo and Newton and even much later, there were still some who objected, who
tried to prevent the new Sun-centered Universe from becoming accepted, or even
known. And there were many who at least harbored secret reservations.
By the late twentieth century, just in
case there were any holdouts, we have been able to settle the matter directly.
We've been able to test whether we live in an Earth-centered system with
planets affixed to transparent crystal spheres, or in a Sun-centered system
with planets controlled at a distance by the gravity of the Sun. We have, for
example, probed the planets with radar. When we bounce a signal off a moon of
Saturn, we receive no radio echo from a nearer crystal sphere attached to
Jupiter. Our spacecraft arrive at their appointed destinations with astonishing
precision, exactly as predicted by Newtonian gravitation. When our ships fly to
Mars, say, their instruments do not hear a tinkling sound or detect shards of
broken crystal as they crash through the “spheres” that-according to the
authoritative opinions that prevailed for millennia-propel Venus or the Sun ill
their dutiful motions about the central Earth.
When Voyager 1 scanned the Solar System
from beyond the outermost planet, it saw, just as Copernicus and Galileo had
said we would, the Sun in the middle and the planets in concentric orbits about
it. Far from being the center of the Universe, the Earth is just one of the
orbiting dots. no longer confined to a single world, we are now able to reach
out to others and determine decisively what kind of planetary system we
inhabit.
EVERY OTHER PROPOSAL, and their number
is legion, to displace us from cosmic center stage has also been resisted, in
part for similar reasons. We seem to crave privilege, merited not by our work,
but by our birth, by the mere fact that, say, we are humans and born on Earth.
We might call it the anthropocentric-the “human-centered”-conceit.
This conceit is brought close to culmination
in the notion that we are created in God's image: The Creator and Ruler of the
entire Universe looks just like me. My, what a coincidence How convenient and
satisfying! The sixth-century-B. C. Green philosopher Xenophanes understood the
arrogance of the perspective:
“The Ethiopians make their gods black
and snub-nosed; the Thracians say theirs have blue eyes and red hair...Yes, and
if oxen and horses or lions had hands, and could paint with their hands, and
produce works of art as men do, horses would paint the forms of the gods like
horses, and oxen like oxen...”
Such attitudes were once described as
“provincial”-the naive expectation that the political hierarchies and social
conventions of an obscure province extend to a vast empire composed of many
different traditions and cultures; that the familiar boondocks, our boondocks,
are the center of the world. The country bumpkins know almost nothing about
what else is possible. They fail to grasp the insignificance of their province
or the diversity of the Empire. With ease, they apply their own standards and
customs to the rest of the planet. But plopped down in Vienna, say, or Hamburg,
or New York, ruefully they recognize how limited their perspective has been.
They become “deprovincialized.”
Modern science has been a voyage into the
unknown, with a lesson in humility waiting at every stop. Many passengers would
rather have stayed home.
CHAPTER 3
The great demotions
[One philosopher] asserted that he knew
the whole secret...[H]e surveyed the two celestial strangers from top to toe,
and maintained to their faces that their persons, their worlds, their suns, and
their stars, were created solely for the use of man. At this assertion our two
travelers let themselves fall against each other, seized with a fit
of...inextinguishable laughter.
—VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL
HISTORY (1752)
In the seventeenth century there was
still some hope that, even if the Earth was not the center of the Universe, it
might be the only “world.” But Galileo's telescope revealed that “the Moon
certainly does not possess a smooth and polished surface” and that other worlds
might look “just like the face of the Earth itself.” The Moon and the planets
showed unmistakably that they had as much claim to being worlds as the Earth
does-with mountains, craters, atmospheres, polar ice caps, clouds, and, in the
case of Saturn, a dazzling, unheard-of set of circumferential rings. After
millennia of philosophical debate, the issue was settled decisively in favor of
“the plurality of worlds.” They might be profoundly different from our planet.
None of them might be as congenial for life. But the Earth was hardly the only
one.
This was the next in the series of Great
Demotions, downlifting experiences, demonstrations of our apparent
insignificance, wounds that science has, in its search for Galileo's facts,
delivered to human pride.
WELL, SOME HOPED, even if the Earth
isn't at the center of the Universe, the Sun is. The Sun is our Sun. So the
Earth is approximately at the center of the Universe. Perhaps some of our pride
could in this way be salvaged. But by the nineteenth century, observational
astronomy had made it clear that the Sun is but one lonely star in a great
self-gravitating assemblage of suns called the Milky Way Galaxy. Far from being
at the center of the Galaxy, our Sun with its entourage of dim and tiny planets
lies in an undistinguished sector of an obscure spiral arm. We are thirty
thousand light years from the Center.
Well, our Milky Way is the only galaxy.
The Milky Way Galaxy is one of billions, perhaps hundreds of billions of
galaxies notable neither in mass nor in brightness nor in how its stars are
configured and arrayed. Some modern deep sky photographs show more galaxies
beyond the Milky Way than stars within the Milky Way. Every one of them is an
island universe containing perhaps a hundred billion suns. Such an image is a
profound sermon on humility.
Well, then, at least our Galaxy is at
the center of the Universe. No, this is wrong too. When the expansion of the
Universe was first discovered, many people naturally gravitated to the notion
that the Milky Way was at the center of the expansion, and all the other
galaxies running away from us. We now recognize that astronomers on any galaxy
would see all the others running away . from them; unless they were very
careful, they would all conclude that they were at the center of the Universe.
There is, in fact, no center to the expansion, no point of origin of the Big
Bang, at least not in ordinary three-dimensional space.
Well, even if there are hundreds of
billions of galaxies, each with hundreds of billions of stars, no other star
has planets. If there are n other planets beyond our Solar System, perhaps
there's no other life in the Universe. Our uniqueness might then be saved. Sing
planets are small and feebly shine by reflected sunlight, they're hard to find.
Although applicable technology is improving wit breathtaking speed, even a
giant world like Jupiter, orbiting the nearest star, Alpha Centauri, would
still be difficult to detect. Iii our ignorance, the geocentrists find hope.
There was once a scientific
hypothesis-not just well received but prevailing-that supposed our solar system
to have formed through the near collision of the ancient Sun with another star;
the gravitational tidal interaction pulled out tendrils of sunstuff that
quickly condensed into planets. Since space is mainly empty and near stellar collisions
most rare, it was concluded that few other planetary systems exist-perhaps only
one, around that other star that long ago co-parented the worlds of our solar
system. Early in my studies, I was amazed and disappointed that such a view had
ever been taken seriously, that for planets of other stars, absence of evidence
had been considered evidence of absence.
Today we have firm evidence for at least
three planets orbiting an extremely dense star, the pulsar designated B1257+12,
about which I'll say more later. And we've found, for more than half the stars
with masses like the Sun's, that early in their careers they're surrounded by
great disks of gas aid dust out of which planets seem to form. Other planetary
systems now look to be a cosmic commonplace, maybe even worlds something like
the Earth. We should be able, in the next few decades, to inventory at least
the larger planets, if they exist, of hundreds of nearby stars.
Well, if our position in space doesn't
reveal our special role, our position in time does: We've been in the Universe
since The Beginning (give or take a few days). We've been given special
responsibilities by the Creator. It once seemed very reasonable to think of the
Universe as beginning just a little before our collective memory is obscured by
the passage of time and the illiteracy of our ancestors. Generally speaking,
that's hundreds or thousands of years ago. Religions that purport to describe
the origin of the Universe often specify-implicitly or explicitly-a date of
origin of roughly such vintage, a birthday for the world.
If you add up all the “begats” in
Genesis, for example, you get an age for the Earth: 6,000 years old, plus or
minus a little. The universe is said to be exactly as old as the Earth. This is
still the standard of Jewish, Christian, and Moslem fundamentalists and is
clearly reflected in the Jewish calendar.
But so young a Universe raises an
awkward question: How is it that there are astronomical objects more than 6,000
light-years away? It takes light a year to travel a light-year, 10,000 years to
travel 10,000 light-years, and so on. When we look at the center of the Milky
Way Galaxy, the light we see left its source 30,000 years ago. The nearest
spiral galaxy like our own, M31 in the constellation Andromeda, is 2 million
light-years away, so we are seeing it as it was when the light from it set out
on its long journey to Earth-2 million years ago. And when we observe distant
quasars 5 billion light-years away, we are seeing them as they were 5 billion
years ago, before the Earth was formed. (They are, almost certainly, very
different today.)
If, despite this, we were to accept the
literal truth of such religious books, how could we reconcile the data? The
only plausible conclusion, I think, is that God recently made all the photons
of light arriving on the Earth in such a coherent format as to mislead
generations of astronomers into the misapprehension that there are such things
as galaxies and quasars, and intentionally driving them to the spurious
conclusion that the Universe is vast and old. This is such a malevolent
theology I still have difficulty believing that anyone, no matter how devoted
to the divine inspiration of any religious book, could seriously entertain it.
Beyond this, the radioactive dating of rocks,
the abundance of impact craters on many worlds, the evolution of the stars, and
the expansion of the Universe each provides compelling and independent evidence
that our Universe is many billions of years old-despite the confident
assertions of revered theologians that a world so old directly contradicts the
word of God, and that at any rate information on the antiquity of the world is
inaccessible except to faith. These
lines of evidence, as well, would have to be manufactured by a deceptive and
malicious deity-unless the world is much older than the literalists in the
Judeo-Christian-Islamic religion suppose. Of course, no such problem arises for
those many religious people who treat the Bible and the Qur'an as historical
and moral guides and great literature, but who recognize that the perspective
of these scriptures on the natural world reflects the rudimentary science of
the time in which they were written.
Ages rolled by before the Earth began.
More ages will run their course before it is destroyed. A distinction needs to
be drawn between how old the Earth is (around 4. 5 billion years) and how old
the Universe is (about 15 billion years since the Big Bang). The immense
interval of time between the origin of the Universe and our epoch was two-thirds
over before the Earth came to be. Some stars and planetary systems are billions
of years younger, others billions of years older. But in Genesis, chapter 1,
verse 1, the Universe and the Earth are created on the same day. The
Hindu-Buddhist-Jain religion tends not to confound the two events.
As for humans, we're latecomers. We
appear in the last instant of cosmic time. The history of the Universe till now
was 99. 998 percent over before our species arrived on the scene. In that vast
sweep of aeons, we could not have assumed any special responsibilities for our
planet, or life, or anything else. We were not here.
Well, if we can't find anything special
about our position or our epoch, maybe there's something special about our
motion. Newton and all the other great classical physicists held that the
velocity of the Earth in space constituted a “privileged frame of reference.”
That's actually what it was called. Albert Einstein, a keen critic of prejudice
and privilege all his life, considered this “absolute” physics a remnant of an
increasingly discredited Earth chauvinism. It seemed to him that the laws of
Nature must be the same no matter what the velocity or frame of reference of
the observer. With this as his starting point, he developed the Special Theory
of Relativity. Its consequences are bizarre, counter intuitive, and grossly
contradict common sense-but only at very high speeds. Careful and repeated
observations show that his justly celebrated theory is an accurate description
of how the world is made. Our commonsense intuitions can be mistaken. Our
preferences don't count. We do not live in a privileged reference frame.
One consequence of special relativity is
time dilation-the slowing down of time as the observer approaches light speed.
You can still find claims that time dilation applies to watches and elementary
particles-and, presumably, to circadian and other rhythms in plants, animals,
and microbes-but not to human biological clocks. Our species has been granted,
it is suggested, special immunity from the laws of Nature, which must
accordingly be able to distinguish deserving from undeserving collections of
matter. (In fact, the proof Einstein gave for special, relativity admits no
such distinctions.) The idea of humans as exceptions to relativity seems
another incarnation of the notion of special creation:
Well, even if our position, our epoch,
our motion, and our world are not unique, maybe we are. We're different from
the other animals. We're specially created. The particular devotion of the Creator
of the Universe is evident in us. This position was passionately defended on
religious and other grounds. But in the middle nineteenth century Charles
Darwin showed convincingly how one species can evolve into another by entirely
natural processes, which come down to the heartless business of Nature saving
the heredities that work and rejecting those that don't. “Man in his arrogance
thinks himself a great work worthy [of] the interposition of a deity,” Darwin
wrote telegraphically in his notebook. “More humble and I think truer to
consider him created from animals.” The profound and intimate connections of
humans with the other life forms, on Earth have been compellingly demonstrated
in the late twentieth century by the new science of molecular biology.
IN EACH AGE the self-congratulatory
chauvinisms are challenged in yet another arena of scientific debate-in this
century, for example, in attempts to understand the nature of human sexuality,
the existence of the unconscious mind, and the fact that many psychiatric
illnesses and character “defects” have a molecular origin. But also:
Well, even if we're closely related to
some of the other animals, we're different-not just in degree, but in kind-on
what really matters: reasoning, selfconsciousness, tool making, ethics,
altruism, religion, language, nobility of character. While humans, like all
animals, have traits that set them apart-otherwise, how could we distinguish
one species from another? -human uniqueness has been exaggerated, sometimes
grossly so. Chimps reason, are self-conscious, make tools, show devotion, and
so on. Chimps and humans have 99. 6 percent of their active genes in common.
(Ann Druyan and I run through the evidence in our book Shadows of Forgotten
Ancestors.)
In popular culture, the very opposite
position is also embraced, although it too is driven by human chauvinism (plus
a failure of the imagination): Children's stories and cartoons make animals
dress in clothes, live in houses, use knives and forks, and speak. The three
bears sleep in beds. The owl and the pussycat go to sea in a beautiful
pea-green boat. Dinosaur mothers cuddle their young. Pelicans deliver the mail.
Dogs drive cars. A worm catches a thief. Pets have human names. Dolls,
nutcrackers, cups, and saucers dance and have opinions. The dish runs away with
the spoon. In the Thomas the Tank Engine series, we even have anthropomorphic
locomotives and railway cars, charmingly portrayed. no matter what we're
thinking about, animate or inanimate, we tend to invest it with human traits.
We can't help ourselves. The images come readily to mind. Children are clearly
fond of them.
When we talk about a “threatening” sky,
a “troubled” sea diamonds “resisting” being scratched, the Earth “attracting”
passing asteroid, or an atom being “excited,” we are again drawn to a kind of
animist worldview. We reify. Some ancient level of our thinking endows
inanimate Nature with life, passions, and forethought.
The notion that the Earth is self-aware
has lately been growing at the fringes of the “Gaia” hypothesis. But this was
commonplace belief of both the ancient Greeks and the earl Christians. Origen
wondered whether “the earth also, according to its own nature, is accountable
for some sin.” A host of ancient scholars thought the stars alive. This was
also the position of Origen, of St. Ambrose (the mentor of St. Augustine), and
even, in a more qualified form, of St. Thomas Aquinas. The Stoic philosophical
position on the Sun's nature was stated by Cicero, in the first century B. C. :
“Since the Sun resembles those fires which are contained in the bodies of
living creatures, the Sun must also be alive.”
Animist attitudes in general seem to
have been spreading recently. In a 1954 American survey, 75 percent of people
polled were willing to state that the Sun is not alive; in 1989, only 30
percent would support so rash a proposition. On whether an automobile tire can
feel anything, 90 percent of respondents denied it emotions in 1954, but only
73 percent in 1989.
We can recognize here a shortcoming-in some
circumstances serious-in our ability to understand the world.
Characteristically, willy-nilly, we seem compelled to project our own nature
onto Nature. Although this may result in a consistently distorted view of the
world, it does have one great virtue-projection is the essential precondition
for compassion.
Okay, maybe we're not much, maybe we're
humiliatingly related t0 apes, but at least we're the best there is. God and
angels aside, we're the only intelligent beings in the Universe. One
correspondent writes to me, “I am as sure of this as anything in my experience.
There is no conscious life anywhere else in the Universe. Mankind thus returns
to its rightful position as center of the universe.” However, partly through
the influence of science acid science fiction, most people today, in the United
States at least, reject this proposition-for reasons essentially stated by the
ancient Greek philosopher Chrysippus: “For any human being in existence to
think that there is nothing in the whole world superior to himself would be an
insane piece of arrogance.”
But the simple fact is that we have not
yet found extraterrestrial life. We are in the earliest stages of looking. The
question is wide open. If I had to guess-especially considering our long
sequence of failed I would guess that the Universe is filled with beings far
more intelligent, tar more advanced than we are. But of course I might be
wrong. Such a conclusion is at best based on a plausibility argument, derived
from the numbers of planets, the ubiquity of organic matter, the immense
timescales available for evolution, and so on. It is not a scientific
demonstration. The question is among the most fascinating in all of science. As
described in this book, we are just developing the tools to treat it seriously.
What about the related matter of whether
we are capable of creating intelligences smarter than ourselves? Computers
routinely do mathematics that no unaided human can manage, outperform world
champions in checkers and grand masters in chess, speak and understand English
and other languages, write presentable short stories and musical compositions,
learn from their mistakes, and competently pilot ships, airplanes, and
spacecraft. Their abilities steadily improve. They're getting smaller, faster, and
cheaper. Each year, the tide of scientific advance laps a little further ashore
on the island of human intellectual uniqueness with its embattled castaways.
If, at so early a stage in our technological evolution, we have been able to go
so far in creating intelligence out of silicon and metal, what will be possible
in the following decades and centuries? What happens when smart machines are
able to manufacture smarter machines?
PERHAPS THE CLEAREST INDICATION that the
search for an unmerited privileged position for humans will never be wholly
abandoned is what in physics and astronomy is called the Anthropic Principle.
It would be better named the Anthropocentric Principle. It comes in various
forms. The “Weak” Anthropic Principle merely notes that if the laws of Nature
and the physical constants-such as the speed of light, the electrical charge of
the electron, the Newtonian gravitational constant, or Planck's quantum
mechanical constant had been different, the course of events leading to the
origin of humans would never have transpired. Under other laws and constants,
atoms would not hold together, stars would evolve too quickly to leave
sufficient time for life to evolve on nearby planets, the chemical elements of
which life is made would never have been generated, and so on. Different laws,
no humans.
There is no controversy about the Weak
Anthropic Principle: Change the laws and constants of Nature, if you could, and
a very different universe may emerge-in many cases, a universe incompatible
with life. The mere fact that we exist
implies (but does not impose) constraints on the laws of Nature. In contrast,
the various “Strong” Anthropic Principles go much farther; some of their
advocates come close to deducing that the laws of Nature and the values of the
physical constants were established (don't ask how or by Whom) so that humans
would eventually come to be. Almost all of the other possible universes, they
say, are inhospitable. In this way, the ancient conceit that the Universe was
made for us is resuscitated.
To me it echoes Dr. Pangloss in
Voltaire's Candide, convinced that this world, with all its imperfections, is
the best possible. It sounds like playing my first hand of bridge, winning,
knowing that there are 54 billion billion billion (5. 4 X 1028) possible other
hands that I was equally likely to have been dealt...and then foolishly
concluding that a god of bridge exists and favors me, a god who arranged the
cards and the shuffle with my victory foreordained from The Beginning. We do
not know how many other winning hands there are in the cosmic deck, how many
other kinds of universes, laws of Nature, and physical constants: that could
also lead to life and Intelligence and perhaps even delusions of
self-importance. Since we know next to nothing about how the Universe was
made-or even if it was made-it's difficult to pursue these notions
productively.
Voltaire asked “Why is there anything?”
Einstein's formulation was to ask whether God had any choice in creating the
Universe. But if the Universe is infinitely old-if the Big Bang some 15 billion
years ago is only the most recent cusp in an infinite series of cosmic
contractions and expansions-then it was never created and the question of why
it is as it is is rendered meaningless.
If, on the other hand, the Universe has
a finite age, why is it the way it is? Why wasn't it given a very different
character? Which laws of Nature go with which others? Are there meta-laws
specifying the connections? Can we possibly discover them? Of all conceivable
laws of gravity, say, which ones can exist simultaneously with which
conceivable laws of quantum physics that determine the very existence of
macroscopic matter? Are all laws we can think of possible, or is there only a
restricted number that can somehow be brought into existence? Clearly we have
not a glimmering of how to determine which laws of Nature are “possible” and
which are not. Nor do we have more than the most rudimentary notion of what
correlations of natural laws are “permitted.”
For example, Newton's universal law of
gravitation specifies that the mutual gravitational force attracting two bodies
towards each other is inversely proportional to the square of how far they are
apart. You move twice as far from the center of the Earth and you weigh a quarter
as much; ten times farther and you weigh only a hundredth of your ordinary
weight; etc. It is this inverse square law that permits the exquisite circular
and elliptical orbits of planets around the Sun, and moons around the
planets-as well as the precision trajectories of our interplanetary spacecraft.
If r is the distance between the centers of two masses, we say that the
gravitational force varies as 1/r2.
But if this exponent were different-if
the gravitational law were 1/r4, say, rather than 1/r2 -then the orbits would
not close; over billions of revolutions, the planets would spiral in and be
consumed in the fiery depths of the Sun, or spiral out and be lost to
interstellar space. If the Universe were constructed with an inverse fourth
power law rather than an inverse square law, soon there would be no planets for
living beings to inhabit.
So of all the possible gravitational
force laws, why are we so lucky as to live in a universe sporting a law
consistent with life? First of course, we're so “lucky,” because if we weren't,
we wouldn't be here to ask the question. It is no mystery that inquisitive
beings who evolve on planets can be found only in universes that admit planets.
Second, the inverse square law is not is the only one consistent with stability
over billions of years. Any power law less steep than 1/r3 (1/r2. 99 or 1/r,
for example) will keep a planet in the vicinity of a circular orbit even if
it's given a shove. We have a tendency to overlook the possibility that other
conceivable laws of Nature might also be consistent with life.
But there's a further point: It's not
arbitrary that we have an inverse square law of gravitation. When Newton's
theory is understood in terms of the more encompassing general theory of
relativity, we recognize that the exponent of the gravity law is 2 because the
number of physical dimensions we live in is 3. All gravity laws aren't
available, free for a Creator's choosing. Even given an infinite number of
three-dimensional universes for some great god to tinker with, the gravity law
would always lave to be the law of the inverse square. Newtonian gravity, we
might say, is not a contingent facet of our universe, but a necessary one.
In general relativity, gravity is due to
the dimensionality and curvature of space. When we talk about gravity we are
talking about local dimples in space-time. This is by no means obvious and even
affronts commonsense notions. But when examined deeply, the ideas of gravity
and mass are not separate matters, but ramifications of the underlying geometry
of space-time.
I wonder if something like this doesn't
apply generally to all anthropic hypotheses. The laws or physical constants on
which our lives depend turn out to be members of a class, perhaps even a vast
class, of other laws and other physical constants-but some of these are also
compatible with a kind of life. Often we do not (or cannot) work through what
those other universes allow. Beyond that, not every arbitrary choice of a law
of Nature or a physical constant may be available, even to a maker of
universes. Our understanding of which laws of Nature and which physical
constants are up for grabs is fragmentary at best.
Moreover, we have no access to any of
those putative alternative universes. We have no experimental method by which anthropic
hypotheses may be tested. Even if the existence of such universes were to
follow firmly from well-established theories-of quantum mechanics or
gravitation, say-we could not be sure that there weren't better theories that
predict no alternative universes. Until that time comes, if it ever does, it
seems to me premature to put faith in the Anthropic Principle as an argument
for human centrality or uniqueness.
Finally, even if the Universe were
intentionally created to allow for the emergence of life or intelligence, other
beings may exist on countless worlds. If so, it would be cold comfort to
anthropocentrists that we inhabit one of the few universes that allow life and
intelligence.
There is something stunningly narrow
about how the Anthropic Principle is phrased. Yes, only certain laws and
constants of nature are consistent with our kind of life. But essentially the
same laws and constants are required to make a rock. So why not talk about a
Universe designed so rocks could one day come to be, and strong and weak Lithic
Principles? If stones could philosophize, I imagine Lithic Principles would be
at the intellectual frontiers.
There are cosmological models being
formulated today in which even the entire Universe is nothing special. Andrei
Linde, formerly of the Lebedev Physical Institute in Moscow and now at Stanford
University, has incorporated current understanding of the strong and weak
nuclear forces and quantum physics into a new cosmological model. Linde
envisions a vast Cosmos, much larger than our Universe-perhaps extending to
infinity both in space and time-not the paltry 15 billion light-years or so in
radius and 15 billion years in age which are the usual understanding. In this
Cosmos there is, as here, a kind of quantum fluff in which tiny structures-
much smaller than an electron-are everywhere forming, reshaping, and
dissipating; in which, as here, fluctuations in absolutely empty space create
pairs of elementary particles-an electron and a positron, for example. In the
froth of quantum bubbles, the vast majority remain submicroscopic. But a tiny
fraction inflate, grow, and achieve respectable universehood. They are so far
away from us, though-much farther than the 15 billion light-years that is the
conventional scale of our universe-that, if they exist, they appear to be
wholly inaccessible and undetectable.
Most of these other universes reach a
maximum size and then collapse, contract to a point, and disappear forever.
Others may oscillate. Still others may expand without limit. In different
universes there will be different laws of nature. We live, Linde argues, in one
such universe-one in which the physics is congenial for growth, inflation,
expansion, galaxies, stars, worlds, life. We imagine our universe to be unique,
but it is one of an immense number-perhaps an infinite number-of equally valid,
equally independent, equally isolated universes. There will be life in some,
and not in others. In this view the observable Universe is just a newly formed
backwater of a much vaster, infinitely old, and wholly unobservable Cosmos. If
something like this is right, even our residual pride, pallid as it must be, of
living in the only universe is denied to us.
Maybe someday, despite current evidence,
a means will be devised to peer into adjacent universes, sporting very
different laws of nature, and we will see what else is possible. Or perhaps
inhabitants of adjacent universes can peer into ours. Of course, in such
speculations we have far exceeded the bounds of knowledge. But if something
like Linde's Cosmos is true, there is-amazingly-still another devastating
deprovincialization awaiting us.
Our powers are far from adequate to be
creating universes anytime soon. Strong Anthropic Principle ideas are not
amenable to proof (although Linde's cosmology does have some testable
features). Extraterrestrial life aside, if self-congratulatory pretensions to
centrality have now retreated to such bastions impervious to experiment, then
the sequence of scientific battles with human chauvinism would seem to have
been, at least largely, won.
THE LONG-STANDING VIEW, as summarized by
the philosopher Immanuel Kant, that “without man...the whole of creation would
be a mere wilderness, a thing in vain, and have no final end” is revealed to be
self-indulgent folly. A Principle of Mediocrity seems to apply to all our
circumstances. We could not have known beforehand that the evidence would be,
so repeatedly and thoroughly, incompatible with the proposition that human
beings are at center stage in the Universe. But most of the debates have now
been settled decisively in favor of a position that, however painful, can be
encapsulated in a single sentence: We have not been given the lead in the
cosmic drama.
Perhaps someone else has. Perhaps no one
else has. In either case, we have good reason for humility.
CHAPTER 4
A universe not made for us
The Sea of Faith
Was once, too, at the full, and round
earth's shore
Lay like the folds of a bright girdle
furl'd.
But now I only hear
Its melancholy, long, withdrawing roar,
Retreating, to the breath
Of the night-wind, down the vast edges
drear
And naked shingles of the world.
—MATTHEW ARNOLD, “DOVER BEACH” (1867)
What a beautiful sunset,” we say, or
“I'm up before sunrise.” no matter what the scientists allege, in everyday
speech we often ignore their findings. We don't talk about the Earth turning,
but about the Sun rising and setting. Try formulating it in Copernican
language. Would you say, “Billy, be home by the time the Earth has rotated
enough so as to occult the Sun below the local horizon”? Billy would be long
gone before you're finished. We haven't been able even to find a graceful
locution that accurately the Heliocentric insight. We at the center and
everything else circling us is built into our languages; we teach it to our
children. We are unreconstructed geocentrists hiding behind a Copernican
veneer.
In 1633 the Roman Catholic Church
condemned Galileo for teaching that the Earth goes around the Sun. Let's take a
closer look at this famous controversy. In the preface to his book comparing
the two hypotheses-an Earth-centered and a Sun-centered universe-Galileo had
written,
The celestial phenomena will be
examined, strengthening the Copernican hypothesis until it might seem that this
must triumph absolutely.
And later in the book he confessed,
Nor can I ever sufficiently admire
[Copernicus and his followers]; they have through sheer force of intellect done
such violence to their own senses as to prefer what reason told them over what
sensible experience plainly showed them...
The Church declared, in its indictment
of Galileo,
The doctrine that the earth is neither
the center of the universe nor immovable, but moves even with a daily rotation,
is absurd, and both psychologically and theologically false, and at the least
an error of faith.
Galileo replied,
The doctrine of the movements of the
earth and the fixity of the sun is condemned on the ground that the Scriptures
speak in many places of the sun moving and the earth standing still...It is
piously spoken that the Scriptures cannot lie. But none will deny that they are
frequently abstruse and their true meaning difficult to discover, and more than
the bare words signify. I think that in the discussion of natural problems we
ought to begin not with the Scriptures, but with experiments and
demonstrations.
But in his recantation (June 22, 1633)
Galileo was made to say,
Having been admonished by the Holy
Office entirely to abandon the false opinion that the Sun was the center of the
universe and immovable, and that the Earth was not the center of the same and
that it moved...I have been...suspected of heresy, that is, of having held and
believed that the Sun is the center of the universe and immovable, and that the
Earth is not the center of the same, and that it does move...1 abjure with a
sincere heart and unfeigned faith, I curse and detest the same errors and
heresies, and generally all and every error and sect contrary to the Holy
Catholic Church.
It took the Church until 1832 to remove
Galileo's work from its list of books which Catholics were forbidden to read at
the risk of dire punishment of their immortal souls.
Pontifical disquiet with modern science
has ebbed and flowed since the time of Galileo. The high-water mark in recent
history is the 1864 Syllabus of Errors of Pius IX, the pope who also convened
the Vatican Council at which the doctrine of papal infallibility was, at his
insistence, first proclaimed. Here are a few excerpts:
Divine revelation is perfect and,
therefore, it is not subject to continual and indefinite progress in order to
correspond with the progress of human reason...no man is free to embrace and
profess that religion which he believes to be true, guided by the light of
reason...The Church has power to define dogmatically the religion of the Catholic
Church to be the only true religion...It is necessary even in the present day
that the Catholic religion shall be held as the only religion of the state, to
the exclusion of all other forms of worship...The civil liberty of every mode
of worship, and full power given to all of openly and publicly manifesting
their opinions and their ideas conduce more easily to corrupt the morals and
minds of the people...The Roman Pontiff cannot and ought not to reconcile
himself or agree with, progress, liberalism and modern civilization.
To its credit, although belatedly and
reluctantly, the Church in 1992 repudiated its denunciation of Galileo. It
still cannot quite bring itself, though, to see the significance of its
opposition. In a 1992 speech Pope John Paul II argued,
From the beginning of the Age of
Enlightenment down to our own day, the Galileo case has been a sort of “myth”
in which the image fabricated out of the events is quite far removed from
reality. In this perspective, the Galileo case was a symbol of the Catholic
Church's supposed rejection of scientific progress, or of “dogmatic”
obscurantism opposed to the free search for truth.
But surely the Holy Inquisition ushering
the elderly and infirm Galileo in to inspect the instruments of torture in the
dungeons of the Church not only admits but requires just such an
interpretation. This was not mere scientific caution and restraint, a
reluctance to shift a paradigm until compelling evidence, such as the annual
parallax, was available. This was fear of discussion and debate. Censoring
alternative views and threatening to torture their proponents betray a lack of
faith in the very doctrine and parishioners that are ostensibly being
protected. Why were threats and Galileo's house arrest needed? Cannot truth
defend itself in its confrontation with error?
The Pope does, though, go on to add:
The error of the theologians of the
time, when they maintained the centrality of the earth, was to think that our
understanding of the physical world's structure was in soiree way imposed by
the literal sense of Sacred Scriptures.
Here indeed considerable progress has
been made-although proponents of fundamentalist faiths will be distressed to
hear from the Pontiff that Sacred Scripture is not always literally true.
But if the Bible is not everywhere
literally true, which parts are divinely inspired and which are merely fallible
and human? As soon as we admit that there are scriptural mistakes (or
concessions to the ignorance of the times), then how can the Bible be an
inerrant guide to ethics and morals? Might sects and individuals now accept as
authentic the parts of the Bible they like, and reject those that are
inconvenient or burdensome? Prohibitions against murder, say, are essential for
a society to function, but if divine retribution for murder is considered
implausible, won't more people think they can get away with it?
Many felt that Copernicus and Galileo
were up to no good and erosive of the social order. Indeed any challenge from
any source, to the literal truth of the Bible might have such consequences. We
can readily see how science began to make people nervous. Instead of
criticizing those who perpetuated the myths, public rancor was directed at
those who discredited them.
OUR ANCESTORS UNDERSTOOD origins by
extrapolating from their own experience. How else could they have done it? So
the Universe was hatched from a cosmic egg, or conceived in the sexual congress
of a mother god and a father god, or was a kind of product of the Creator's
workshop-perhaps the latest of many flawed attempts. And the Universe was not
much bigger than we see, and not much older than our written or oral records,
and nowhere very different from places that we know.
We've tended in our cosmologies to make
things familiar. Despite all our best efforts, we've not been very inventive.
In the West, Heaven is placid and fluffy, and Hell is like the inside of a
volcano. In many stories, both realms are governed by dominance hierarchies
headed by gods or devils. Monotheists talked about the king of kings. In every
culture we imagined something like our own political system running the
Universe. Few found the similarity suspicious.
Then science came along and taught us
that we are not the measure of all things, that there are wonders unimagined,
that the Universe is not obliged to conform to what we consider comfortable or
plausible. We have learned something about the idiosyncratic nature of our
common sense. Science has carried human self-consciousness to a higher level.
This is surely a rite of passage, a step towards maturity. It contrasts starkly
with the childishness and narcissism of our pre-Copernican notions.
But why should we want to think that the
Universe was made for us? Why is the idea so appealing? Why do we nurture it?
Is our self-esteem so precarious that nothing short of a universe custom-made
for us will do?
Of course it appeals to our vanity.
“What a man desires, he also imagines to be true,” said Demosthenes. “The light
of faith makes us see what we believe,” cheerfully admitted St. Thomas Aquinas.
But I think there may be something else. There's a kind of ethnocentrism among
primates. To whichever little group we happen to be born, we owe passionate
love and loyalty. Members of other groups are beneath contempt, deserving of
rejection and hostility. That both groups are. of the same species, that to an
outside observer they are virtually indistinguishable, makes no difference.
This is certainly the pattern among the chimpanzees, our closest relatives in
the animal kingdom. Ann Druyan and I have described how this way of viewing the
world may have made enormous evolutionary sense a few million years ago,
however dangerous it has become today. Even members of hunter-gatherer
groups-as far from the technological feats of our present global civilization
as it is possible for humans to be-solemnly describe their little band,
whichever it is, as “the people.” Everyone else is something different,
something less than human.
If this is our natural way of viewing
the world, then it should occasion no surprise that every time we make a naive
judgment about our place in the Universe-one untempered by careful and
skeptical scientific examination-we almost always opt for the centrality of our
group and circumstance. We want to believe, moreover, that these are objective
facts, and not our prejudices finding a sanctioned vent.
So it's not much fun to have a gaggle of
scientists incessantly haranguing us with “You're ordinary, you're unimportant
your privileges are undeserved, there's nothing special about you.” Even
unexcitable people might, after a while, grow annoyed at this incantation and
those who insist on chanting it. It almost seems that the scientists are
getting some weird satisfaction out of putting humans down. Why can't they find
a way in which we're superior? Lift our spirits! Exalt us! In such debates
science, with its mantra of discouragement, feels cold and remote,
dispassionate, detached, unresponsive to human needs.
And, again, if we're not important, not
central, not the apple of God's eye, what is implied for our theologically
based moral codes? The discovery of our true bearings in the Cosmos was
resisted for so long and to such a degree that many traces of the debate
remain, sometimes with the motives of the geocentrists laid bare. Here, for
example, is a revealing unsigned commentary in the British review The Spectator
in 1892:
[I]t is certain enough that the
discovery of the heliocentric motion of the planets which reduced our earth to
its proper “insignificance” in the solar system, did a good deal to reduce to a
similar but far from proper “insignificance” the moral principles by which the
predominant races of the earth had hitherto been guided and restrained. Part of
this effect was no doubt due to the evidence afforded that the physical science
of various inspired writers was erroneous instead of being infallible, -a
conviction which unduly shook the confidence felt even in their moral and
religious teaching. But a good deal of it was due only to the mere sense of
“insignificance” with which man has contemplated himself, since he has
discovered that he inhabits nothing but a very obscure corner of the universe,
instead of the central world round which sun, moon, and stars alike revolved.
There can be no doubt that man may feel himself, and has often felt himself, a
great deal too insignificant to be the object of any particular divine training
or care. If the earth be regarded as a sort of ant-hill, and the life and death
of human beings as the life and death of so many ants which run into and out of
so many holes in search of food and sunlight, it is quite certain that no
adequate importance will be attached to the duties of human life, and that a
profound fatalism and hopelessness, instead of new hopefulness, will attach to
human effort...
[F]or the present at least, our
horizons are quite vast enough...; till we can get used to the infinite
horizons we already have, and not lose our balance so much as we usually do in
contemplating them, the yearning for still wider horizons is premature.
WHAT DO WE REALLY WANT from philosophy
and religion? Palliatives? Therapy? Comfort? Do we want reassuring fables or an
understanding of our actual circumstances? Dismay that the Universe does not
conform to our preferences seems childish. You might think that grown-ups would
be ashamed to put such disappointments into print. The fashionable way of doing
this is not to blame the Universe-which seems truly pointless-but rather to
blame the means by which we know the Universe, namely science.
George Bernard Shaw, in the preface to
his play St. Joan, described a sense of science preying on our credulity,
forcing on us an alien worldview, intimidating belief:
In the Middle Ages, people believed that
the Earth was flat, for which they had at least the evidence of their senses:
we believe it to be round, not because as many as one per cent of us could give
the physical reason for so quaint a belief, but because modern science has
convinced us that nothing that is obvious is true, and that everything that is
magical, improbable, extraordinary, gigantic, microscopic, heartless, or
outrageous is scientific.
A more recent and very instructive
example is Understanding the Present: Science and the Soul of Modern Man, by
Bryan Appleyard, a British journalist. This book makes explicit what many
people feel, all over the world, but are too embarrassed to say. Appleyard's
candor is refreshing. He is a true believer and will not let us slough over the
contradictions between modern science and traditional religion:
“Science has taken away our religion,”
he laments. And what sort of religion is it that he longs for? One in which
“the human race was the point, the heart, the final cause of the whole system.
It placed our selves definitively upon the universal map.”...“We were the end,
the purpose, the rational axle around which the great aetherian shells
rotated.” He longs for “the universe of Catholic orthodoxy” in which “the
cosmos is shown to be a machine constructed around the drama of salvation”-by
which Appleyard means that, despite explicit orders to the contrary, a woman
and a man once ate of an apple, and that this act of insubordination
transformed the Universe into a contrivance for operant-conditioning their
remote descendants.
By contrast, modern science “presents us
as accidents. We are caused by the cosmos, but we are not the cause of it.
Modem man is not finally anything, he has no role in creation.” Science is
“spiritually corrosive, burning away ancient authorities and traditions. It
cannot really co-exist with anything.”...“Science, quietly and inexplicitly, is
talking us into abandoning our selves, our true selves.” It reveals “the mute,
alien spectacle of nature.”...“Human beings cannot live with such a revelation.
The only morality left is that of the consoling lie.” Anything is better than
grappling with the unbearable burden of being tiny.
In a passage reminiscent of Plus IX,
Appleyard even decries the fact that “a modern democracy can be expected to
include a number of contradictory religious faiths which are obliged to agree
on a certain limited number of general injunctions, but no more. They must not
burn each other's places of worship, but they may deny, even abuse each other's
God. This is the effective, scientific way of proceeding.”
But what is the alternative? Obdurately
to pretend to certainty in an uncertain world? To adopt a comforting belief
system, no matter how out of kilter with the facts it is? If we don't know
what's real, how can we deal with reality? For practical reasons, we cannot
live too much in fantasyland. Shall we censor one another's religions and burn
down one another's places of worship? How can we be sure which of the thousands
of human belief systems should become unchallenged, ubiquitous, mandatory?
These quotations betray a failure of
nerve before the Universe its grandeur and magnificence, but especially its
indifference. Science has taught us that, because we have a talent for
deceiving ourselves, subjectivity may not freely reign. This is one reason
Appleyard so mistrusts science: It seems too reasoned, measured, and
impersonal. Its conclusions derive from the interrogation of Nature, and are
not in all cases predesigned to satisfy our wants. Appleyard deplores
moderation. He yearns for inerrant doctrine, release from the exercise of
judgment, and an obligation to believe but not to question. He has not grasped
human fallibility. He recognizes no need to institutionalize error-correcting
machinery either in our social institutions or in our view of the Universe.
This is the anguished cry of the infant
when the Parent does not come. But most people eventually come to grips with
reality, and with the painful absence of parents who will absolutely guarantee
that no harm befalls the little ones so long as they do what they are told.
Eventually most people find ways to accommodate to the Universe-especially when
given the tools to think straight.
“All that we pass on to our children” in
the scientific age, Appleyard complains, “is the conviction that nothing is
true, final or enduring, including the culture from which they sprang.” How
right he is about the inadequacy of our legacy. But would it be enriched by
adding baseless certainties? He scorns “the pious hope that science and
religion are independent realms which can easily be separated.” Instead,
“science, as it is now, is absolutely not compatible with religion.”
But isn't Appleyard really saying that
some religions now find it difficult to make unchallenged pronouncements on the
nature of the world that are straight-out false? We recognize that even revered
religious leaders, the products of their time as we are of ours, may have made
mistakes. Religions contradict one another on small matters, such as whether we
should put on a hat or take one off on entering a house of worship, or whether
we should eat beef and eschew pork or the other way around, all the way to the
most central issues, such as whether there are no gods, one God, or many gods.
Science has brought many of us to that
state in which Nathaniel Hawthorne found Herman Melville: “He can neither
believe, nor be comfortable in his unbelief.” Or Jean-Jacques Rousseau: “They
had not persuaded me, but they had troubled me. Their arguments had shaken me
without ever convincing me...It is hard to prevent oneself from believing what
one so keenly desires.” As the belief systems taught by the secular and
religious authorities are undennined, respect for authority in general probably
does erode. The lesson is clear: Even politics] leaders must be wary of
embracing false doctrine. This is not a failing of science, but one of its
graces.
Of course, worldview consensus is
comforting, while clashes of opinion may be unsettling, and demand more of us.
But unless we insist, against all evidence, that our ancestors were perfect,
the advance of knowledge requires us to unravel and then restitch the consensus
they established.
In some respects, science has far
surpassed religion in delivering awe. How is it that hardly any major religion
has looked at science and concluded, “This is better than we thought! The
Universe is much bigger than our prophets said, grander, more subtle, more
elegant. God must be even greater than we dreamed”? Instead they say, “No, no,
no! My god is a little god, and I want him to stay that way.” A religion, old or
new, that stressed the magnificence of the Universe as revealed by modern
science might be able to draw forth reserves of reverence and awe hardly tapped
by the conventional faiths. Sooner or later, such a religion will emerge.
IF YOU LIVED two or three millennia ago,
there was no shame in holding that the Universe was made for us. It was an
appealing thesis consistent with everything we knew; it was what the most
learned among us taught without qualification. But we have found out much since
then. Defending such a position today amounts to willful disregard of the
evidence, and a flight from self-knowledge.
Still, for many of us, these
deprovincializations rankle. Even if they do not fully cant' the day, they
erode confidence-unlike the happy anthropocentric certitudes, rippling with
social utility, of an earlier age. We long to be here for a purpose, even
though, despite much self-deception, none is evident. “The meaningless
absurdity of life,” wrote Leo Tolstoy, “is the only incontestable knowledge
accessible to man.” Our time is burdened under the cumulative weight of
successive debunkings of our conceits: We're Johnny-come-latelies. We live in
the cosmic boondocks. We emerged from microbes and muck. Apes are our cousins.
Our thoughts and feelings are not fully under our own control. There may be
much smarter and very different beings elsewhere. And on top of all this, we're
making a mess of our planet and becoming a danger to ourselves.
The trapdoor beneath our feet swings
open. We find ourselves in bottomless free fall. We are lost in a great
darkness, and there's no one to send out a search party. Given so harsh a
reality, of course we're tempted to shut our eyes and pretend that we're safe
and snug at home, that the fall is only a bad dream.
We lack consensus about our place in the
Universe. There is no generally agreed upon long-term vision of the goal of our
species-other than, perhaps, simple survival. Especially when times are hard,
we become desperate for encouragement, unreceptive to the litany of great
demotions and dashed hopes, and much more willing to hear that we're special,
never mind if the evidence is paper-thin. If it takes a little myth and ritual
to get us through a night that seems endless, who among us cannot sympathize
and understand?
But if our objective is deep knowledge
rather than shallow reassurance, the gains from this new perspective far
outweigh the losses. Once we overcome our fear of being tiny, we find ourselves
on the threshold of a vast and awesome Universe that utterly dwarfs-in time, in
space, and in potential-the tidy anthropocentric proscenium of our ancestors.
We gaze across billions of light-years of space to view the Universe shortly
after the Big Bang, and plumb the fine structure of matter. We peer down into
the core of our planet, and the blazing interior of our star. We read the
genetic language in which is written the diverse skills and propensities of
every being on Earth. We uncover hidden chapters in the record of our own
origins, and with some anguish better understand our nature and prospects. We
invent and refine agriculture, without which almost all of us would starve to
death. We create medicines and vaccines that save the lives of billions. We
communicate at the speed of light, and whip around the Earth in an hour and a
half. We have sent dozens of ships to more than seventy worlds, and four
spacecraft to the stars. We are right to rejoice in our accomplishments, to be
proud that our species has been able to see so far, and to judge our merit in
part by the very science that has so deflated our pretensions.
To our ancestors there was much in
Nature to be afraid of-lightning, storms, earthquakes, volcanos, plagues,
drought, long winters. Religions arose in part as attempts to propitiate and
control, if not much to understand, the disorderly aspect of Nature. The
scientific revolution permitted us to glimpse an underlying ordered Universe in
which there was a literal harmony of the worlds (Johannes Kepler's phrase). If
we understand Nature, there is a prospect of controlling it or at least
mitigating the harm it may bring. In this sense, science brought hope.
Most of the great deprovincializing
debates were entered into with no thought for their practical implications.
Passionate and curious humans wished to understand their actual circumstances,
how unique or pedestrian they and their world are, their ultimate origins and
destinies, how the Universe works. Surprisingly, some of these debates have
yielded the most profound practical benefits. The very method of mathematical
reasoning that Isaac Newton introduced to explain the motion of the planets
around the Sun has led to most of the technology Of our modern world. The
Industrial Revolution, for all its shortcomings, is still the global model of
how an agricultural nation can emerge from poverty. These debates have
bread-and-butter consequences.
It might have been otherwise. It might
have been that the balance lay elsewhere, that humans by and large did not want
to yaw about a disquieting Universe, that we were unwilling to hermit
challenges to the prevailing wisdom. Despite determined resistance in every
age, it is very much to our credit that we have allowed ourselves to follow the
evidence, to draw conclusions that at first seem daunting: a Universe so much larger
and older that our personal and historical experience is dwarfed and humbled, a
Universe in which, every day, suns are born and worlds obliterated, a Universe
in which humanity, newly arrived, clings to an obscure clod of matter.
How much more satisfying had we been
placed in a garden custom-made for us, its other occupants put there for us to
use as we saw fit. There is a celebrated story in the Western tradition like
this, except that not quite everything was there for us. There was one
particular tree of which we were not to partake, a tree of knowledge. Knowledge
and understanding and wisdom were forbidden to us in this story. We were to be
kept ignorant. But we couldn't help ourselves. We were starving for
knowledge-created hungry, you might say. This was the origin of all our
troubles. In particular, it is why we no longer live in a garden: We found out
too much. So long as we were incurious and obedient, I imagine, we could
console ourselves with our importance and centrality, and tell ourselves that
we were the reason the Universe was made. As we began to indulge our curiosity,
though, to explore, to learn how the Universe really is, we expelled ourselves
from Eden. Angels with a flaming sword were set as sentries at the gates of
Paradise to bar our return. The gardeners became exiles and wanderers.
Occasionally we mourn that lost world, but that, it seems to me, is maudlin and
sentimental. We could not happily have remained ignorant forever.
There is in this Universe much of what
seems to he design. Every time we come upon it, we breathe a sigh of relief. We
are forever hoping to find, or at least safely deduce, a Designer. But instead,
we repeatedly discover that natural processes-collisional selection of worlds,
say, or natural selection of gene pools, or even the convection pattern in a
pot of boiling water-can extract order out of chaos, and deceive us into
deducing purpose where there is none. In everyday life, we often sense-in the
bedrooms of teenagers, or in national politics-that chaos is natural, and order
imposed from above. While there are deeper regularities in the Universe than
the simple circumstances we generally describe as orderly, all that order,
simple and complex, seems to derive from laws of Nature established at the Big
Bang (or earlier), rather than as a consequence of belated intervention by an
imperfect deity. “God is to be found in the details” is the famous dictum of
the German scholar Abu Warburg. But, amid much elegance and precision, the
details of life and the Universe also exhibit haphazard, jury-rigged
arrangements and much poor planning. What shall we make of this: an edifice
abandoned early in construction by the architect?
The evidence, so far at least and laws
of Nature aside, does not require a Designer. Maybe there is one hiding,
maddeningly unwilling to be revealed. Sometimes it seems a very slender hope.
The significance of our lives and our
fragile planet is then determined only by our own wisdom and courage. We are
the custodians of life's meaning. We long for a Parent to care for us, to
forgive us our errors, to save us from our childish mistakes. But knowledge is
preferable to ignorance. Better by far to embrace the hard truth than a
reassuring fable.
If we crave some cosmic purpose, then
let us find ourselves a worthy goal.
CHAPTER 5
Is there intelligent life on Earth?
They journeyed a long time and found
nothing. At length they discerned a small light, which was the Earth...[But]
they could not find the smallest reason to suspect that we and our fellow-citizens
of this globe have the honor to exist.
—VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL
HISTORY (1752)
There are places, in and around our
great cities, where the natural world has all but disappeared. You can make out
streets and sidewalks, autos, parking garages, advertising billboards,
monuments of glass and steel, but not a tree or a blade of grass or any
animal-besides, of course, the humans. There are lots of humans. Only when you
look straight up through the skyscraper canyons can you make out a star or a
patch of blue-reminders of what was there long before humans came to be. But
the bright lights of the big cities bleach out the stars, and even that patch
of blue is sometimes gone, tinted brown by industrial technology.
It's not hard, going to work every
day in such a place, to be impressed with ourselves. How we've transformed the
Earth for our benefit and convenience! But a few hundred miles up or down there
are no humans. Apart from a thin film of life at the very surface of the Earth,
an occasional intrepid spacecraft, and some radio static, our impact on the
Universe is nil. It knows nothing of us.
YOU'RE AN ALIEN EXPLORER entering the
Solar System after a long journey through the blackness of interstellar space.
You examine the planets of this humdrum star from afar-a pretty handful, some
gray, some blue, some red, some yellow. You're interested in what kinds of
worlds these are, whether their environments are static or changing, and
especially whether there are life and intelligence. You have no prior knowledge
of the Earth. You've just discovered its existence.
There's a galactic ethic, let's imagine:
Look but don't touch. You can fly by these worlds; you can orbit them; but you
are strictly forbidden to land. Under such constraints, could you figure out
what the Earth's environment is like and whether anyone lives there?
As you approach, your first impression
of the whole Earth is white clouds, white polar caps, brown continents, and
some bluish substance that covers two thirds of the surface. When you take the
temperature of this world from the infrared radiation it emits, you find that
most latitudes are above the freezing point of water, while the polar caps are
below freezing. Water is a very abundant material in the Universe; polar caps
made of solid water would be a reasonable guess, as well as clouds of solid and
liquid water.
You might also he tempted by the idea
that the blue stuff is enormous quantities-kilometers deep-of liquid water. The
suggestion is bizarre, though, at least as far as this solar system is
concerned, because surface oceans of liquid water exist nowhere else. When you
look in the visible and near-infrared spectrum for telltale signatures of
chemical composition, sure enough you discover water ice in the polar caps, and
enough water vapor in the air to account for the clouds; this is also just the
right amount that must exist because of evaporation if the oceans are in fact
made of liquid water. The bizarre hypothesis is confirmed.
The spectrometers further reveal that
the air on this world is one fifth oxygen, O2. no other planet in the Solar
System has anything close to so much oxygen. Where does it all come from? The
intense ultraviolet light from the Sun breaks water, H20, down into oxygen and
hydrogen, and hydrogen, the lightest gas, quickly escapes to space. This is a
source of O2, certainly, but it doesn't easily account for so much oxygen.
Another possibility is that ordinary
visible light, which the Sun pours out in vast amounts, is used on Earth to
break water apart-except that there's no known way to do this without life.
There would have to be plants, life-forms colored by a pigment that strongly
absorbs visible light, that knows how to split a water molecule by saving up
the energy of two photons of light, that retains the H and excretes the O, and
that uses the hydrogen thus liberated to synthesize organic molecules. The
plants would have to be spread over much of the planet. All this is asking a
lot. If you're a good skeptical scientist, so much Oz would not be proof of
life. But it certainly might be cause for suspicion.
With all that oxygen you're not
surprised to discover ozone (O3) in the atmosphere, because ultraviolet light
makes ozone out of molecular oxygen (O2). The ozone then absorbs dangerous
ultraviolet radiation. So if the oxygen is due to life, there's a curious sense
in which the life is protecting itself. But this life “night be mere
photosynthetic plants. A high level of intelligence is not implied.
When you examine the continents more
closely, you find there are, crudely speaking, two kinds of regions. One shows
the spectrum of ordinary rocks and minerals as found on many worlds. The other
reveals something unusual: a material, covering vast areas, that strongly
absorbs red light. (The Sun, of course, shines in light of all colors, with a
peak in the yellow.) This pigment might be just the agent needed if ordinary
visible light is being used to break water apart and account for the oxygen in
the air. It's another hint, this time a little stronger, of life, not a bug
here and there, but a planetary surface overflowing with life. The pigment is
in fact chlorophyll: It absorbs blue light as well as red, and is responsible
for the fact that plants are green. What you're seeing is a densely vegetated
planet.
So the Earth is revealed to possess
three properties unique at least in this solar system-oceans, oxygen, life.
It's hard not to think they're related, the oceans being the sites of origin,
and the oxygen the product, of abundant life.
When you look carefully at the infrared
spectrum of the Earth, you discover the minor constituents of the air. In
addition to water vapor, there's carbon dioxide (CO2), methane (CH4), and other
gases that absorb the heat that the Earth tries to radiate away to space at night.
These gases warm the planet. Without them, the Earth would everywhere be below
the freezing point of water. You've discovered this world's greenhouse effect.
Methane and oxygen together in the same
atmosphere is peculiar. The laws of chemistry are very clear: In an excess of
O2, CH4 should be entirely converted into H2O and CO2, The process is so
efficient that not a single molecule in all the Earth's atmosphere should be
methane. Instead, you find that one out of every million molecules is methane, ail
immense discrepancy. What could it mean?
The only possible explanation is that
methane is being injected into the Earth's atmosphere so quickly that its
chemical reaction with Oz can't keep pace. Where does all this methane come
from? Maybe it seeps out of the deep interior of the Earth-but quantitatively
this doesn't seem to work, and Mars and Venus don't have anything like this
much methane. The only alternatives are biological, a conclusion that makes no
assumptions about the chemistry of life, or what it looks like, but follows
merely from how unstable methane is in an oxygen atmosphere. In fact, the
methane arises from such sources as bacteria in bogs, the cultivation of rice,
the burning of vegetation, natural gas from oil wells, and bovine flatulence.
In an oxygen atmosphere, methane is a sign of life.
That the intimate intestinal activities
of cows should be detectable from interplanetary space is a little
disconcerting, especially when so much of what we hold dear is not. But an
alien scientist flying by the Earth would, at this point, be unable to deduce
bogs, rice, fire, oil, or cows. Just life.
All the signs of life that we've
discussed so far are due to comparatively simple forms (the methane in the
rumens of cows is generated by bacteria that homestead there). Had your
spacecraft flown by the Earth a hundred million years ago, in the age of the
dinosaurs when there were no humans and no technology, you would still have
seen oxygen and ozone, they chlorophyll pigment, and far too much methane. At
present, though, your instruments are finding signs not just of life, but of
high technology-something that couldn't possibly have been detected even a
hundred years ago:
You are detecting a particular kind of
radio wave emanating from Earth. Radio waves don't necessarily signify life and
intelligence Many natural processes generate them. You've already found radio
emissions from other, apparently uninhabited worlds-generated by electrons
trapped in the strong magnetic fields of planets, by chaotic motions at the
shock front that separates these magnetic fields from the interplanetary
magnetic field, and by lightning. (Radio “whistlers” usually sweep from high
notes to low, and then begin again.) Some of these radio emissions are
continuous; some come in repetitive bursts; some last a few minutes and then
disappear.
But this is different: A portion of the
radio transmission from Earth is at just the frequencies where radio waves
begin to leak out of the planet's ionosphere, the electrically charged region
above the stratosphere that reflects and absorbs radio waves. There is a
constant central frequency for each transmission, added to which is a modulated
signal (a complex sequence of ons and offs). no electrons in magnetic fields,
no shock waves, no lightning discharges can generate something like this.
Intelligent life seems to be the only possible explanation. Your conclusion
that the radio transmission is due to technology on Earth holds no matter what
the ons and offs mean: You don't have to decode the message to be sure it is a
message. (This signal is really, let us suppose, communications from the U. S.
Navy to its distant nuclear-armed submarines.)
So, as an alien explorer, you would know
that at least one species on Earth has achieved radio technology. Which one is
it? The beings that make methane? Those that generate oxygen? The ones whose
pigment colors the landscape green? Or somebody else, somebody more subtle,
someone not otherwise detectable to a spacecraft plummeting by? To search for
this technological species, you might want to examine the Earth at finer and
finer resolution-seeking, if not the beings themselves, at least their
artifacts.
You look first with a modest telescope,
so the finest detail you can resolve is one or two kilometers across. You can
make out no monumental architecture, no strange formations, no unnatural
reworking of the landscape, no signs of life. You see a dense atmosphere in
motion. The abundant water must evaporate and then rain back down. Ancient
impact craters, apparent on the Earth's nearby Moon, are almost wholly absent.
There must, then, be a set of processes whereby new land is created and then
eroded away in much less time than the age of this world. Running water is
implicated. As you look with finer and finer definition you find mountain
ranges, river valleys, and many other indications that the planet is
geologically active. There are also odd places surrounded by vegetation, but
which are themselves denuded of plants. They look like discolored smudges on
the landscape.
When you examine the Earth at about
100-meter resolution, everything changes. The planet is revealed to be covered
with straight lines, squares, rectangles, circles-sometimes huddling along
river banks or nestling on the lower slopes of mountains, sometimes stretching
over plains, but rarely in deserts or high mountains, and absolutely never in
the oceans. Their regularity, complexity, and distribution would be hard to
explain except by life and intelligence, although a deeper understanding of function
and purpose might be elusive. Perhaps you would conclude only that the dominant
life-forms have a simultaneous passion for territoriality and Euclidean
geometry. At this resolution you could not see them, much less know them.
Many of the devegetated smudges are
revealed to have an underlying checkerboard geometry. These are the planets
cities. Over much of the landscape, and not just in the cities, there is a
profusion of straight lines, squares, rectangles, circles. The dark smudges of
the cities are revealed to be highly geometrized, with only a few patches of
vegetation-themselves with highly regular boundaries-left intact. There are
occasional triangles, and in one city there is even a pentagon.
When you take pictures at a meter
resolution or better, you find that the crisscrossing straight lines within the
cities and the long straight lines that join them with other cities are filled
with streamlined, multicolored beings a few meters in length, politely running
one behind the other, in long, slow orderly procession. They are very patient.
One stream of beings stops so another stream can continue at right angles.
Periodically, the favor is returned. At night, they turn on two bright lights
in front so they can see where they're going. Some, a privileged few, go into
little houses when their workday is done and retire for the night. Most are
homeless and sleep in the streets.
At last! You've detected the source of
all the technology. the dominant life-forms on the planet. The streets of the
cities and the roadways of the countryside are evidently built for their
benefit. You might believe that you were really beginning to understand life on
Earth. And perhaps you'd be right.
If the resolution improved just a little
further, you'd discover tiny parasites that occasionally enter and exit the
dominant organisms. They play some deeper role, though, because a stationary
dominant organism will often start up again just after it's reinfected by a
parasite, and stop again just before the parasite is expelled. This is
puzzling. But no one said life on Earth would be easy to understand.
All the images you've taken so far are
in reflected sunlight-that is, on the day side of the planet. Something most
interesting is revealed when you photograph the Earth at light: The planet is
lit up. The brightest region, near the Arctic Circle, is illuminated by the
aurora borealis-generated not by life , but by electrons and protons from the
Sun, beamed down by the Earth's magnetic field. Everything else you see is due
to life. The lights recognizably outline the same continents you can make out
in daytime; and many correspond to cities you've already mapped. The cities are
concentrated near coastlines. They tend to be sparser in continental interiors.
Perhaps the dominant organisms are desperate for seawater (or maybe oceangoing
ships were once essential for commerce and emigration).
Some of the lights, though, are not due
to cities. In North Africa, the Middle East, and Siberia, for example, there
are very bright lights in a comparatively barren landscape due, it turns out,
to burnoff in oil and natural gas wells. In the Sea of Japan on the day you
first look, there is a strange, triangular-shaped area of light. In daytime it
corresponds to open ocean. This is no city. What could it be? It is in fact the
Japanese squid fishing fleet, using brilliant illumination to attract schools
of squid up to their deaths. On other days, this pattern of light wanders all
over the Pacific Ocean, seeking prey. In effect, what you have discovered here
is sushi.
It seems sobering to me that from space
you can so readily detect some of the odds and ends of life on Earth-the
gastrointestinal habits of ruminants, Japanese cuisine, the means of
communicating with nomadic submarines that carry death for 200 cities-while so
much of our monumental architecture, our greatest engineering works, our
efforts to care for one another, are almost wholly invisible. It's a kind of
parable.
BY THIS POINT your expedition to the
Earth must be considered highly successful. You've characterized the
environment; you've detected life; you've found manifestations of intelligent
beings; you may even have identified the dominant species, the one transfixed
with geometry and rectilinearity. Surely this planet is worth a longer and more
detailed study. That's why you've now inserted your spacecraft into orbit
around the Earth.
Looking down on the planet, you uncover
new puzzles. All over the Earth, smokestacks are pouring carbon dioxide and
toxic chemicals into the air. So are the dominant beings who run on the
roadways. But carbon dioxide is a greenhouse gas. As you watch, the amount of
it in the atmosphere increases steadily, year after year. The same is true of
methane and other greenhouse gases. If this keeps up, the temperature of the
planet is going to increase. Spectroscopically, you discover another class of
molecules being injected into the air, the chlorofluorocarbons. Not only are
they greenhouse gases, but they are also devastatingly effective in destroying
the protective ozone layer.
You look more closely at the center of
the South American continent, which-as you know by now-is a vast rain forest.
Every night you see thousands of fires. In the daytime, you find the region
covered with smoke. Over the years, all over the planet, you find less and less
forest and more and more scrub desert.
You look down on the large island of
Madagascar. The rivers are colored brown, generating a vast stain in the
surrounding ocean. This is topsoil being washed out to sea at a rate so high
that in another few decades there will be none left. The same thing is
happening, you note, at the mouths of rivers all over the planet.
But no topsoil means no agriculture. In
another century, what will they eat? What will they breathe? How will they cope
with a changing and more dangerous environment?
From your orbital perspective, you can
see that something has unmistakably gone wrong. The dominant organisms, whoever
they are-who have gone to so much trouble to rework the surface-are
simultaneously destroying their ozone layer and their forests, eroding their
topsoil, and performing massive, uncontrolled experiments on their planet's
climate. Haven't they noticed what's happening? Are they oblivious to their
fate? Are they unable to work together on behalf of the environment that
sustains them all?
Perhaps, you think, it's time to
reassess the conjecture that there's intelligent life on Earth.
LOOKING FOR LIFE ELSEWHERE:A CALIBRATION
Spacecraft from the Earth have now flown
by dozens of planets, moons, comets, and asteroids-equipped with cameras,
instruments for measuring heat and radio waves, spectrometers to determine
composition, and a host of other devices. We have found not a hint of life
anywhere else in the Solar System. But you might be skeptical about our ability
to detect life elsewhere, especially life different from the kind we know.
Until recently we had never performed the obvious calibration test: to fly a
modern interplanetary spacecraft by the Earth and see whether we could detect
ourselves. This all changed on December 8, 1990.
Galileo is a NASA spacecraft designed to
explore the giant planet Jupiter, its moons, and its rings. It's named after
the heroic Italian scientist who played so central a role in toppling the
geocentric pretension. It is he who first saw Jupiter as a world, and who
discovered its four big moons. To get to Jupiter, the spacecraft had to fly
close by Venus (once) and the Earth (twice) and be accelerated by the gravities
of these planets-otherwise there wasn't enough oomph to get it where it was
going. This necessity of trajectory design permitted us, for the first time, to
look systematically at the Earth from an alien perspective.
Galileo passed only 960 kilometers
(about 600 miles) above the Earth's surface. With some exceptions-including
pictures showing features finer than 1 kilometer across, and the images of the
Earth at night-much of the spacecraft data described in this chapter were
actually obtained by Galileo. With Galileo we were able to deduce an oxygen atmosphere,
water, clouds, oceans, polar ice, life, and intelligence. The use of
instruments and protocols developed to explore the planets to monitor the
environmental health of our own-something NASA is now doing in earnest-was
described by the astronaut Sally Ride as “Mission to Planet Earth.”
Other members of the NASA scientific
team who worked with me on Galileo's detection of life on Earth were Drs. W.
Reid Thompson, Cornell University; Robert Carlson, JPL; Donald Gurnett,
University of Iowa; and Charles Hord, University of Colorado.
Our success in detecting life on Earth
with Galileo, without making any assumptions beforehand about what kind of life
it must be, increases our confidence that when we fail to find life on other
planets, that negative result is meaningful. Is this judgment anthropocentric,
geocentric, provincial? I don't think so. We're not looking only for our kind
of biology. Any widespread photosynthetic pigment, any gas grossly out of
equilibrium with the rest of the atmosphere, any rendering of the surface into
highly geometrized patterns, any steady constellation of lights on the night
hemisphere, any non-astrophysical sources of radio emission would betoken the
presence of life. On Earth we have found of course only our type, but many other
types would have been detectable elsewhere. We have not found them. This
examination of the third planet strengthens our tentative conclusion that, of
all the worlds in the Solar System, only ours is graced by life.
We have just begun to search. Maybe life
is hiding on Mars or Jupiter, Europa or Titan. Maybe the Galaxy is filled with
worlds as rich in life as ours. Maybe we are on the verge of making such
discoveries. But in terms of actual knowledge, at this moment the Earth is
unique. no other world is yet known to harbor even a microbe, much less a
technical civilization.
CHAPTER 6
The triumph ov Voyger
They that go down to the sea in ships,
that do business in great waters; these see the works of the Lord, and his
wonders in the deep.
—PSALMS, 107 (CA. 150 B. C)
The visions we offer our children shape
the future. It matters what those visions are. Often they become
self-fulfilling prophecies. Dreams are maps.
I do not think it irresponsible to
portray even the direst futures; if we are to avoid them, we must understand
that they are possible. But where are the alternatives? Where are the dreams
that motivate and inspire? We long for realistic maps of a world we can be
proud to give to our children. Where are the cartographers of human purpose? Where
are the visions of hopeful futures, of technology as a tool for human
betterment and not a gun on hair trigger pointed at our heads?
NASA, in its ordinary course of doing
business, offers such a vision. But in the late 1980s and early '90s, many
people saw the U. S. space program as, instead, a succession of
catastrophes-seven brave Americans killed on a mission whose main function was
to put up a communications satellite that could have been launched at less cost
without risking anybody; a billion-dollar telescope sent up with a bad case of
myopia; a spacecraft to Jupiter whose main antenna-essential for returning data
to Earth-did not unfurl; a probe lost just as it was about to orbit Mars. Some
people cringe every time NASA describes as exploration sending a few astronauts
200 miles up in a small capsule that endlessly circles the Earth and goes
nowhere. Compared to the brilliant achievements of robotic missions, it is
striking how rarely fundamental scientific findings emerge from manned
missions. Except for repairing ineptly manufactured or malfunctioning
satellites, or launching a satellite that could just as well have been sent up
in an unmanned booster, the manned program has, since the 1970s, seemed unable
to generate accomplishments commensurate with the cost. Others looked at NASA
as a stalking horse for grandiose schemes to put weapons into space, despite
the fact that an orbiting weapon is in many circumstances a sitting duck. And
NASA showed many symptoms of an aging, arteriosclerotic, overcautious,
unadventurous bureaucracy. The trend is perhaps beginning to be reversed.
But these criticisms-many of them surely
valid-should not blind us to NASA triumphs in the same period: the first
exploration of the Uranus and Neptune systems, the in-orbit repair of the
Hubble space telescope, the proof that the existence of galaxies is compatible
with the Big Bang, the first close-up observations of asteroids, mapping Venus
pole to pole, monitoring ozone depletion, demonstrating the existence of a
black hole with the mass of a billion suns at the center of a nearby galaxy,
and a historic commitment to joint space endeavors by the U. S. and Russia.
There are far-reaching, visionary, and
even revolutionary implications to the space program. Communications satellites
link up the planet, are central to the global economy, and, through television,
routinely convey the essential fact that We live in a global community.
Meteorological satellites predict the weather, save lives in hurricanes and
tornados, and avoid many billions of dollars in crop losses every year.
Military-reconnaissance and treaty-verification satellites make nations and the
global civilization more secure; in a world with tens of thousands of nuclear
weapons, they calm the hotheads and paranoids on all sides; they are essential
tools for survival on a troubled and unpredictable planet.
Earth-observing satellites, especially a
new generation soon to be deployed, monitor the health of the global
environment: greenhouse warming, topsoil erosion, ozone layer depletion, ocean
currents, acid rain, the effects of floods and droughts, and new dangers we
haven't yet discovered. This is straightforward planetary hygiene.
Global positioning systems are now in
place so that your locale is radio-triangulated by several satellites. Holding
a small instrument the size of a modern shortwave radio, you can read out to
high precision your latitude and longitude. no crashed airplane, no ship in fog
and shoals, no driver in an unfamiliar city need ever be lost again.
Astronomical satellites peering outward
from Earth's orbit observe with unsurpassed clarity-studying questions ranging
from the possible existence of planets around nearby stars to the origin and
fate of the Universe. Planetary probes from close range explore the gorgeous
array of other worlds in our solar system comparing their fates with ours.
All of these activities are
forward-looking, hopeful, stirring and cost-effective. None of them requires
“manned" spaceflight. A key issue facing the future of NASA and addressed
in this book is whether the purported justifications for human spaceflight are
coherent and sustainable. Is it worth the cost?
But first, let's consider the visions of
a hopeful future vouchsafed by robot spacecraft out among the planets.
VOYAGER 1 AND VOYAGER 2 are the ships
that opened the Solar System for the human species, trailblazing a path for
future generations. Before their launch, in August and September 1977, we were
almost wholly ignorant about most of the planetary part of the Solar System. In
the next dozen years, they provided our first detailed, close-up information on
many new worlds-some of them previously known only as fuzzy disks in the
eyepieces of ground-based telescopes, some merely as points of light, and some
whose very existence was unsuspected. They are still returning reams of data.
These spacecraft have taught us about
the wonders of other worlds, about the uniqueness and fragility of our own,
about beginnings and ends. They have given us access to most of the Solar System-both
in extent and in mass. They are the ships that first explored what may be
homelands of our remote descendants.
U. S. launch vehicles are these days too
feeble to get such a spacecraft to Jupiter and beyond in only a few years by
rocket propulsion alone. But if we're clever (and lucky), there's something
else we can do: We can (as Galileo also did, years later) fly close to one
world, and have its gravity fling us on to the next. A gravity assist, it's
called. It costs us almost nothing but ingenuity. lt's something like grabbing
hold of a post on a moving merry-go-round as it passes-to speed you up and
fling you in some new direction. The spacecraft's acceleration is compensated
for by a deceleration in the planet's orbital motion around the Sun. But
because the planet is so massive compared to the spacecraft, it slows down
hardly at all. Each Voyager spacecraft picked up a velocity boost of nearly
40,000 miles per hour from Jupiter's gravity. Jupiter in turn was slowed down
in its motion around the Sun. By how much? Five billion years from now, when
our Sun becomes a swollen red giant, Jupiter will be one millimeter short of
where it would have been had Voyager not flown by it in the late twentieth
century.
Voyager 2 took advantage of a rare
lining-up of the planets: A close flyby of Jupiter accelerated it on to Saturn,
Saturn to Uranus, Uranus to Neptune, and Neptune to the stars. But you can't do
this anytime you like: The previous opportunity for such a game of celestial
billiards presented itself during the presidency of Thomas Jefferson. We were
then only at the horseback, canoe, and sailing ship stage of exploration.
(Steamboats were the transforming new technology just around the corner.)
Since adequate funds were unavailable,
NASA's Jet Propulsion Laboratory (JPL) could afford to build spacecraft that
would work reliably only as far as Saturn. Beyond that, all bets were off.
However, because of the brilliance of the engineering design and the fact that
the JPL engineers who radioed instructions up to the spacecraft got smarter
faster than the spacecraft got stupid-both spacecraft went on to explore Uranus
and Neptune. These days they are broadcasting back discoveries from beyond the
most distant known planet of the Sun.
We tend to hear much more about the
splendors returned than the ships that brought them, or the shipwrights. It has
al, ways been that way. Even those history books enamored of the voyages of
Christopher Columbus do not tell us much about the builders of the Nina, the
Pinta, and the Santa Maria, or about the principle of the caravel. These
spacecraft, their designers, builders, navigators, and controllers are examples
of what science and engineering, set free for well-defined peaceful purposes,
can accomplish. Those scientists and engineers should be role models for an
America seeking excellence and international competitiveness. They should be on
our stamps.
At each of the four giant
planets-Jupiter, Saturn, Uranus, and Neptune-one or both spacecraft studied the
planet itself, its rings, and its moons. At Jupiter, in 1979, they braved a
dose of trapped charged particles a thousand times more intense than what it
takes to kill a human; enveloped in all that radiation, they discovered the
rings of the largest planet, the first active volcanos outside Earth, and a
possible underground ocean on an airless world-among a host of surprising
discoveries. At Saturn, in 1980 and 1981, they survived a blizzard of ice and
found not a few new rings, but thousands. They examined frozen moons mysteriously
melted in the comparatively recent past, and a large world with a putative
ocean of liquid hydrocarbons surmounted by clouds of organic matter.
On January 25, 1986, Voyager 2 entered
the Uranus system and reported a procession of wonders. The encounter lasted
only a few hours, but the data faithfully relayed back to Earth have
revolutionized our knowledge of the aquamarine planet, its 15 moons. its
pitch-black rings, and its belt of trapped high-energy charged particles. On
August 25, 1989, Voyager 2 swept through the Neptune system and observed, dimly
illuminated by the distant Sun, kaleidoscopic cloud patterns and a bizarre moon
on which plumes of fine organic particles were being blown about by the
astonishingly thin air. And in 1992, having flown beyond the outermost known
planet, both Voyagers picked up radio emission thought to emanate from the
still remote heliopause-the place where the wind from the Sun gives way to the
wind from the stars.
Because we're stuck on Earth, we're
forced to peer at distant worlds through an ocean of distorting air. Much of
the ultraviolet, infrared, and radio waves they emit do not penetrate our
atmosphere. It's easy to see why our spacecraft have revolutionized the study
of the Solar System: We ascend to stark clarity in the vacuum of space, and
there approach our objectives, flying past them, as did Voyager, or orbiting
them, or landing on their surfaces.
These spacecraft have returned four
trillion bits of information to Earth, the equivalent of about 100,000 encyclopedia
volumes. I described the Voyagers 1 and 2 encounters with the Jupiter system in
Cosmos. In the following pages, I'll say something about the Saturn, Uranus,
and Neptune encounters.
JUST BEFORE VOYAGER 2 was to encounter
the Uranus system, the mission design had specified a final maneuver, a brief
firing of the on-board propulsion system to position the spacecraft correctly
so it could thread its way on a preset path among the hurtling moons. But the
course correction proved unnecessary. The spacecraft was already within 200
kilometers of its designed trajectory-after a journey along an arcing path 5
billion kilometers long. This is roughly the equivalent of throwing a pin
through the eye of a needle 50 kilometers away, or firing your rifle in Washington
and hitting the bull's-eye in Dallas.
Mother lodes of planetary treasure were
radioed back to Earth. But Earth is so far away that by the time the signal
frog Neptune was gathered in by radio telescopes on our planet, the received
power was only 10-16 watts (fifteen zeros between the decimal point and the
one). This weak signal bears the same pro, portion to the power emitted by an
ordinary reading lamp as the diameter of an atom bears to the distance from the
Earth to the Moon. It's like hearing an amoeba's footstep.
The mission was conceived during the
late 1960s. It was first funded in 1972. But it was not approved in its final
form (including the encounters with Uranus and Neptune) until after the ships
had completed their reconnaissance of Jupiter. The two spacecraft were lifted
off the Earth by a nonreusable Titan/Centaur booster configuration. Weighing
about a ton, a Voyager would fill a small house. Each draws about 400 watts of
power-considerably less than an average American home-from a generator that
converts radioactive plutonium into electricity. (If it had to rely on solar
energy, the available power would diminish quickly as the ship ventured farther
and farther from the Sun Were it not for nuclear power, Voyager would have
returned no data at all from the outer Solar System, except perhaps a little
from Jupiter.)
The flow of electricity through the
innards of the spacecraft would generate enough magnetism to overwhelm the
sensitive instrument that measures interplanetary magnetic fields. So the
magnetometer is placed at the end of along boom, far from the offending
electrical currents. With other projections, it gives Voyager alter a slightly
porcupine appearance. Cameras, infrared and ultraviolet spectrometers, and an
instrument called a photopolarimeter are on a scan platform that swivels on
command so these device can be aimed at a target world. The spacecraft must
know where Earth is if the antenna is to be pointed properly and the data
rereceived back home. It also needs to know where the Sun is and at least one
bright star, so it can orient itself in three dimensions and point properly
toward any passing world. If you can't point the cameras, it does no good to be
able to return pictures over billions of miles.
Each spacecraft cost about as much as a
single modern strategic bomber. But unlike bombers, Voyager cannot, once
launched, be returned to the hangar for repairs. The ship's computers and
electronics are therefore designed redundantly. Much key machinery, including
the essential radio receiver, had at least one backup-waiting to be called upon
should the hour of need ever arrive. When either Voyager finds itself in
trouble, the computers use branched contingency tree logic to work out the
appropriate course of action. If that doesn't work, the ship radios home for
help.
As the spacecraft journeys increasingly
far from Earth, the roundtrip radio travel time also increases, approaching
eleven hours by the time Voyager is at the distance of Neptune. Thus, in case
of emergency, the spacecraft needs to know how to put itself into a safe
standby mode while awaiting instructions from Earth. As it ages, more and more
failures are expected, both in its mechanical parts and in its computer system,
although there is no sign, even now, of a serious memory deterioration, some
robotic Alzheimer's disease.
This is not to say that Voyager is
perfect. Serious mission-threatening, white-knuckle mishaps did occur. Each
time, special teams of engineers-some of whom had been with the Voyager program
since its inception-were assigned to “work” the problem. They would study the
underlying science and draw upon their previous experience with the failed
subsystems. They would experiment with identical Voyager spacecraft equipment
that had never been launched, or even manufacture a large number of components
of the sort that failed in order to gain some statistical understanding of the
failure mode.
In April 1978, almost eight months after
launch, and while the ship was approaching the asteroid belt, an omitted ground
command-a human error-caused Voyager 2's on-board computer to switch from the
prime radio receiver to its backup, During the next ground transmission to the
spacecraft, the backup receiver refused to lock onto the signal from Earth. A
component called a tracking loop capacitor had failed. After seven days in
which Voyager 2 was entirely out of contact, its fault protection software
suddenly commanded the backup receiver to be switched off and the prime
receiver to be switched back on. Mysteriously-to this day, no one knows why-the
prime receiver failed moments later. It was never heard from again. To top it
off, the on-board computer now foolishly insisted on using the failed primary
receiver. Through an unlucky concatenation of human and robotic error, the
spacecraft was now in real jeopardy. no one could think of a way to get Voyager
2 to revert to the backup receiver. Even if it did, the backup receiver
couldn't receive the commands from Earth, because of that failed capacitor.
There were many project personnel who feared that all was lost.
But after a week of obdurate
unresponsiveness to all commands, instructions to switch automatically between
receivers were accepted and programmed into the skittish onboard computer.
During that same week the JPL engineers designed an innovative command
frequency control procedure to make sure that essential orders would be
understood by the damaged backup receiver.
The engineers were now able to
recommunicate, at least in a rudimentary way, with the spacecraft. Unfortunately
the backup receiver now turned giddy, becoming extremely sensitive to stray
heat dumped when various components of the spacecraft powered up or down. In
the following months the JPL engineers devised and conducted tests that let
them thoroughly understand the thermal implications of most spacecraft
operational modes: What would prevent and what would permit receipt of commands
from Earth?
With this information, the backup
receiver problem was entirely circumvented. It subsequently acquired all the
commands sent from Earth on how to gather data in the Jupiter, Saturn, Uranus,
and Neptune systems. The engineers had saved the mission. (To be on the safe
side, during most of Voyager 2's subsequent flight a nominal data-taking
sequence for the next planet to be encountered was always sitting in the
on-board computers-should the spacecraft again become deaf to entreaties from
home.)
Another heart-wrenching failure occurred
just after Voyager 2 emerged from behind Saturn (as seen from the Earth) in
August 1981. The scan platform had been moving feverishly-pointing here and
there among the rings, moons, and the planet itself during the all-too-brief
moments of close approach. Suddenly, the platform jammed. A stuck scan platform
is a maddening predicament: knowing that the spacecraft is flying past wonders
that have never been witnessed, that we will not see again for years or
decades, and the incurious spacecraft staring fixedly off into space, ignoring
everything.
The scan platform is driven by actuators
containing gear trains. So first the JCL engineers ran an identical copy of a
flight actuator in a simulated mission. This actuator failed after 348 turns;
the actuator on the spacecraft had failed after 352 turns. The problem turned
out to be a lubrication failure. Good to know, but what to do about it?
Plainly, it would be impossible to overtake Voyager with ail oilcan.
The engineers wondered whether they
could restart the tailed actuator by alternate heating and cooling; maybe the
resulting thermal stresses would induce the components of the actuator to
expand and contract at different rates and unjam the system. They tested this
notion with specially manufactured actuators in the laboratory, and then
jubilantly found that in this way they could start the scan platform up again
in space. Project personnel also devised ways to diagnose any additional trend
toward actuator failure early enough to work around the problem. Thereafter,
Voyager 2's scan platform worked perfectly. All the pictures taken in the Uranus
and Neptune systems owe their existence to this work. The engineers had saved
the day again.
Voyagers 1 and 2 were designed to
explore the Jupiter and Saturn systems only. It is true that their trajectories
would carry them on past Uranus and Neptune, but officially these planets were
never contemplated as targets for Voyager exploration: The spacecraft were not
supposed to last that long. Because of our wish to fly close to the mystery
world Titan, Voyager 9 was flung by Saturn on a path that could never encounter
any other known world; it is Voyager 2 that flew on to Uranus and Neptune with
brilliant success. At these immense distances, sunlight is getting
progressively dimmer, and the radio signals transmitted to Earth are getting
progressively fainter. These were predictable but still very serious problems
that the JPL engineers and scientists also had to solve.
Because of the low light levels at
Uranus and Neptune, the Voyager television cameras were obliged to take long
time exposures. But the spacecraft was hurtling so fast through, say, the
Uranus system (at about 35,000 miles per hour) that the image would have been
smeared or blurred. To compensate, the entire spacecraft had to be moved during
the time exposures to cancel out the motion, like panning in the direction
opposite yours while taking a photograph of a street scene from a moving car.
This may sound easy, but it's not: You have to neutralize the most innocent of
motions. At zero gravity, the mere start and stop of the on-board tape recorder
can jiggle the spacecraft enough to smear the picture.
This problem was solved by sending up
commands to the spacecraft's little rocket engines (called thrusters), machines
of exquisite sensitivity. With a little puff of gas at the start and stop of
each data-taking sequence, the thrusters compensated for the tape-recorder
jiggle by turning the entire spacecraft just a little. To deal with the low
radio power received at Earth, the engineers devised a new and more efficient
way to record and transmit the data, and the radio telescopes on Earth were
electronically linked together with others to increase their sensitivity.
Overall, the imaging system worked, by many criteria, better at Uranus and
Neptune than it did at Saturn or even at Jupiter.
Voyager may not yet be done exploring.
There is, of course, a chance that some vital subsystem will fail tomorrow, but
as far as the radioactive decay of the plutonium power source is concerned, the
two Voyager spacecraft should be able to return data to Earth roughly through
the year 2015.
Voyager is an intelligent being-part
robot, part human. It extends the human senses to far-off worlds. For simple
tasks and short-term problems, it relies on its own intelligence; but for more
complex tasks and longer-term problems, it turns to the collective intelligence
and experience of the JPL engineers. This trend is sure to grow. The Voyagers
embody the technology of the early 1970s; if spacecraft were designed for such
a mission today, they would incorporate stunning advances in artificial
intelligence, in miniaturization, in data-processing speed, in the ability to
self-diagnose and repair, and in the propensity to learn from experience They
would also be much cheaper.
In the many environments too dangerous
for people, on Earth as well as in space, the future belongs to robot-human
partnerships that will recognize the two Voyagers as antecedents and pioneers.
For nuclear accidents, mine disasters, undersea exploration and archaeology,
manufacturing, prowling the interiors of volcanos, and household help, to name
only a few potential applications, it could make an enormous difference to have
a ready corps of smart, mobile, compact, commandable robots that can diagnose
and repair their own malfunctions. There are likely to be many more of this
tribe in the near future.
It is conventional wisdom now that
anything built by the government will be a disaster. But the two Voyager
spacecraft were built by the government (in partnership with that other
bugaboo, academia). They came in at cost, on time, and vastly exceeded their
design specifications-as well as the fondest dreams of their makers. Seeking
not to control, threaten, wound, or destroy, these elegant machines represent
the exploratory part of our nature set free to roam the Solar System and
beyond. This kind of technology, the treasures it uncovers freely available to
all humans everywhere, has been, over the last few decades, one of the few
activities of the United States admired as much by those who abhor many of its
policies as by those who agree with it on every issue. Voyager cost each
American less than a penny a year from launch to Neptune encounter. Missions to
the planets are one of those things-and I mean this not just for the United
States, but for the human species-that we do best.
CHAPTER 7
Among the moons of Saturn
Seat thyself sultanically among the
moons of Saturn.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 107
(1851)
There is a world, midway in size between
the Moon and Mars, where the upper air is rippling with electricity-streaming
in from the archetypical ringed planet next door, where the perpetual brown
overcast is tinged with an odd burnt orange, and where the very stuff of life
falls out of the skies onto the unknown surface below. It is so far away that
light takes more than an hour to get there from the Sun. Spacecraft take years.
Much about it is still a mystery-including whether it holds great oceans. We
know just enough, though, to recognize that within reach may be a place where
certain processes ate today working themselves out that aeons ago on Earth led
to the origin of life.
On our own world a long-standing-and in
some respects quite promising-experiment has been under way on the evolution of
matter. The oldest known fossils are about 3. 6 billion years old. Of course,
the origin of life had to have happened well before that. But 4. 2 or 4. 3
billion years ago the Earth was being so ravaged by the final stages of its
formation that life could not yet have come into being: Massive collisions were
melting the surface, turning the oceans into steam and driving any atmosphere
that had accumulated since the last impact off into space. So around 4 billion
years ago, there was a fairly narrow window-perhaps only a hundred million
years wide in which our most distant ancestors came to be. Once conditions
permitted, life arose fast. Somehow. The first living things very likely were
inept, far less capable than the most humble microbe alive today-perhaps just
barely able to make crude copies of themselves. But natural selection, the key
process first coherently described by Charles Darwin, is an instrument of such
enormous power that from the most modest beginnings there can emerge all the
richness and beauty of the biological world.
Those first living things were made of
pieces, parts, building blocks which had to come into being on their own-that
is, driven by the laws of physics and chemistry on a lifeless Earth. The
building blocks of all terrestrial life are called organic molecules, molecules
based on carbon. Of the stupendous ]lumber of possible organic molecules, very
few are used at the heart of life. The two most important classes are the amino
acids, the building blocks of proteins, and the nucleotide bases, the building
blocks of the nucleic acids. Mist before the origin of life, where did these
molecules come from? There are only two possibilities: from the outside or from
the inside. We know that vastly more comets and asteroids were hitting the
Earth than do so today, that these small worlds are rich storehouses of complex
organic molecules, and that some of these molecules escaped being fried on
impact. Here I'm describing homemade, not imported, goods: the organic
molecules generated in the air and waters of the primitive Earth.
Unfortunately, we don't know very much
about the composition of the early air, and organic molecules are far easier to
make in some atmospheres than in others. There couldn't have been much oxygen,
because oxygen is generated by green plants and there weren't any green plants
yet. There was probably more hydrogen, because hydrogen is very abundant in the
Universe and escapes from the upper atmosphere of the Earth into space better
than any other atom (because it's so light). If we can imagine various possible
early atmospheres, we can duplicate them in the laboratory, supply some energy,
and see which organic molecules are made and in what amounts. Such experiments
have over the years proved provocative and promising. But our ignorance of
initial conditions limits their relevance.
What we need is a real world whose
atmosphere still retains some of those hydrogen-rich gases, a world in other
respects something like the Earth, a world in which the organic building blocks
of life are being massively generated in our own time, a world we can go to to
seek our own beginnings. There is only one such world in the Solar System. That
world is Titan, the big moon of Saturn. It's about 5,150 kilometers (3,200
miles) in diameter, a little less than half the size of the Earth. It takes 16
of our days to complete one orbit of Saturn.
No world is a perfect replica of any
other, and in at least one important respect Titan is very different from the
primitive Earth: Being so far from the Sun, its surface is extremely cold, far
below the freezing point of water, around 180° below zero Celsius. So while the
Earth at the time of the origin of life was, as now, mainly ocean-covered,
plainly there can be no oceans of liquid water on Titan. (Oceans made of other
stuff are a different story, as we shall see.) The low temperatures provide an
advantage, though, because once molecules are synthesized on Titan, they tend
to stick around: The higher the temperature, the faster molecules fall to
pieces. On Titan the molecules that have been raining down like manna from
heaven for the last 4 billion years might still be there, largely unaltered,
deep-frozen, awaiting the chemists from Earth.
THE INVENTION OF THE TELESCOPE In the
seventeenth century led to the discovery of many new worlds. In 1610 Galileo
first spied the four large satellites of Jupiter. It looked like a miniature
solar system, the little moons racing around Jupiter as the planets were
thought by Copernicus to orbit the Sun. It was another blow to the
geocentrists. Forty-five years later, the celebrated Dutch physicist
Christianus Huygens discovered a moon moving about the planet Saturn and named
it Titan. † It was a dot of light a billion miles away, gleaming in reflected
sunlight. From the time of its discovery, when European men wore long curly
wigs, to world War II, when American men cut their hair down to stubble, almost
nothing more was discovered about Titan except the fact it had a curious, tawny
color. Ground-based telescopes could, even in principle, barely make out some
enigmatic detail. The Spanish astronomer J. Comas Sola reported at the turn of
the twentieth century some faint and indirect evidence of an atmosphere.
In a way, I grew up with Titan. I did my
doctoral dissertation at the University of Chicago under the guidance of Gerard
P. Kuiper, the astronomer who made the definitive discovery that Titan has an
atmosphere. Kuiper was Dutch and in a direct line of intellectual descent from
Christianus Huygens. In 1914, while making a spectroscopic examination of
Titan, Kuiper was astonished to find the characteristic spectral features of
the gas methane. When he pointed the telescope at Titan, there was the
signature of methane. When he pointed it away, not a hint of methane. But moons
were not supposed to hold onto sizable atmospheres, and the Earth's Moon
certainly doesn't. Titan could retain an atmosphere, Kuiper realized, even
though its gravity was less than Earth's, because its upper atmosphere is very
cold. The molecules simply aren't moving fast enough for significant numbers to
achieve escape velocity and trickle away to space.
Daniel Harris, a student of Kuiper's,
showed definitively that Titan is red. Maybe we were looking at a rusty
surface, like that of Mars. If you wanted to learn more about Titan, you could
also measure the polarization of sunlight reflected off it. Ordinary sunlight
is unpolarized. Joseph Veverka, now a fellow faculty member at Cornell
University, was my graduate student at Harvard University, and therefore, so to
speak, a grandstudent of Kuiper's. In his doctoral work, around 1970, he
measured the polarization of Titan and found that it changed as the relative
positions of Titan, the Sun, and the Earth changed. But the change was very
different from that exhibited by, say, the Moon. Veverka concluded that the
character of this variation was consistent with extensive clouds or haze on
Titan. When we looked at it through the telescope, we weren't seeing its
surface. We knew nothing about what the surface was like. We had no idea how
fat below the clouds the surface was.
So, by the early 1970s, as a kind of
legacy from Huygens and his line of intellectual descent, we knew at least that
Titan has a dense methane-rich atmosphere, and that it's probable enveloped by
a reddish cloud veil or aerosol haze. But what kind of cloud is red? By the
early 1970s my colleague Bishun Khare and I had been doing experiments at
Cornell in which we irradiated various methane-rich atmospheres with
Ultraviolet light or electrons and were generating reddish or brownish solids;
the stuff would coat the interiors of our reaction vessels. It seemed to me
that, if methane-rich Titan had red-brown clouds, those clouds might very well
be similar to what we were making in the laboratory. We called this material
tholin, after a Greek word for “muddy.” At the beginning we had yen little idea
what it was made of. It was some organic stew made by breaking apart our
starting molecules, and allowing the atoms-carbon, hydrogen, nitrogen-and
molecular fragments to recombine.
The word “organic” carries no imputation
of biological origin; following long-standing chemical usage dating back mots
than a century, it merely describes molecules built out of car bon atoms
(excluding a few very simple ones such as carbon monoxide, CO, and carbon
dioxide, CO2). Since life on Earth is based oil organic molecules, and since
there was a time before there was life on Earth, some process must have made
organic molecules on our planet before the time of the first organism.
Something sitar, I proposed, might be happening on Titan today.
The epochal event in our understanding
of Titan was the arrival in 1980 and 1981 of the Voyager 1 and 2 spacecraft in
the Saturn system. The ultraviolet, infrared, and radio instruments revealed
the pressure and temperature through the atmosphere-from the hidden surface to
the edge of space. We learned how high the cloud tops are. We found that the
air on Titan is composed mainly of nitrogen, N2, as on the Earth today. The
other principal constituent is, as Kuiper found, methane. CH4 the starting material
from which carbon-based organic molecules are generated there.
A variety of simple organic molecules
was found, present as gases, mainly hydrocarbons and nitriles. The most complex
of them have four “heavy” (carbon and/or nitrogen) atoms. Hydrocarbons are
molecules composed of carbon and hydrogen atoms only, and are familiar to us as
natural gas, petroleum, and waxes. (They're quite different from carbohydrates,
such as sugars and starch, which also have oxygen atoms.) Nitriles are
molecules with a carbon and nitrogen atom attached in a particular way. The
best known nitrile is HCN, hydrogen cyanide, a deadly gas for humans. But
hydrogen cyanide is implicated in the steps that on Earth led to the origin of
life.
Finding these simple organic molecules in
Titan's upper atmosphere-even if present only in a part per million or a part
per billion-is tantalizing. Could the atmosphere of the primeval Earth have
been similar? There's about ten times more air oil Titan than there is on Earth
today, but the early Earth may well have had a denser atmosphere.
Moreover, Voyager discovered an
extensive region of energetic electrons and protons surrounding Saturn, trapped
by the planet's magnetic field. During the course of its orbital motion around
Saturn, Titan bobs in and out of this magnetosphere. Beams of electrons (plus
solar ultraviolet light) fall on the upper air of Titan, just as charged
particles (plus solar ultraviolet light) were intercepted by the atmosphere of
the primitive Earth.
So it's a natural thought to irradiate
the appropriate mixture of nitrogen and methane with ultraviolet light or
electrons at very low pressures, and find out what more complex molecules can
be made. Can we simulate what's going on in Titan's high atmosphere? In our
laboratory at Cornell-with my colleague W. Reid Thompson playing a key
role-we've replicated some of Titan's manufacture of organic gases. The
simplest hydrocarbons on Titan are manufactured by ultraviolet light from the
Sun. But for all the other gas products, those made most readily by electrons
in the laboratory correspond to those discovered by Voyager on Titan, and in
the same proportions. The correspondence is one to one. The next most abundant
gases that we've found in the laboratory will be looked for in future studies
of Titan. The most complex organic gases we make have six or seven carbon
and/or nitrogen atoms. These product molecules are on their way to forming
tholins.
WE HAD HOPED FOR A BREAK In the weather
as Voyager 1 approached Titan. A long distance away, it appeared as a tiny
disk; at closest approach, our camera's field of view was filled by a small
province of Titan. If there had been a break in the haze and clouds, even only
a few miles across, as we scanned the disk we would have seen something of its
hidden surface. But there was no hint of a break. This world is socked in. no
one on Earth knows what's on Titan's surface. And an observer there, looking up
ill ordinary visible light, would have no idea of the glories that await upon
ascending through the haze and beholding Saturn and its magnificent rings.
From measurements by Voyager, by the
International Ultraviolet Explorer observatory in Earth orbit, and by
ground-based telescopes, we know a fair amount about the orange-brown haze
particles that obscure the surface: which colors of light they like to absorb,
which colors they pretty much let pass through them, how much they bend, the
light that does pass through them, and how big they are. (They're mostly the
size of the particles in cigarette smoke.) The “optical properties” will
depend, of course, on the composition of the haze particles.
In collaboration with Edward Arakawa of
Oak Ridge National Laboratory in Tennessee, Khare and I have measured the
optical properties of Titan tholin. It turns out to be a dead ringer for the
real Titan haze. no other candidate material, mineral or organic, matches the
optical constants of Titan. So we can fairly claim to have bottled the haze of
Titan-formed high in its atmosphere, slowly falling out, and accumulating in
copious amounts on its surface. What is this stuff made of?
It's very hard to know the exact
composition of a complex organic solid. For example, the chemistry of coal is
still not fully understood, despite a long-standing economic incentive. But
we've found out some things about Titan tholin. It contains many of the
essential building blocks of life on Earth. Indeed, if you drop Titan tholin
into water you make a large number of amino acids, the fundamental constituents
of proteins, and nucleotide bases also, the building blocks of DNA and RNA.
Some of the amino acids so formed are widespread in living things on Earth.
Others are of a completely different sort. A rich array of other organic
molecules is present also, some relevant to life, some not. During the past
four billion years, immense quantities of organic molecules sedimented out of
the atmosphere onto the surface of Titan. If it's all deep-frozen and unchanged
in the intervening aeons, the amount accumulated should be at least tens of meters
(a hundred feet) thick; outside estimates put it at a kilometer deep.
But at 180°C below the freezing point of
water, you might very well think that amino acids will never be made. Dropping
tholins into water may be relevant to the early Earth, but not, it would seem,
to Titan. However, comets and asteroids must on occasion come crashing into the
surface of Titan. (The other nearby moons of Saturn show abundant impact
craters, and the atmosphere of Titan isn't thick enough to prevent large,
high-speed objects from reaching the surface.) Although we've never seen the
surface of Titan, planetary scientists nevertheless know something about its
composition. The average density of Titan lies between the density of ice and
the density of rock. Plausibly it contains both. Ice and rock are abundant on
nearby worlds, some of which are made of nearly pure rice. If the surface of
Titan is icy, a high-speed cometary impact will temporarily melt the ice.
Thompson and I estimate that any given spot on Titan's surface has a better
than 50-50 chance of having once been melted, with an average lifetime of the
impact melt and slurry of almost a thousand years.
This makes for a very different story.
The origin of life on Earth seems to have occurred in oceans and shallow tidepools.
Life on Earth is made mainly of water, which plays an essential physical and
chemical role. Indeed, it's hard, for us water-besotted creatures to imagine
life without water. If on our planet the origin of life took less than a
hundred million years, is there any chance that on Titan it took a thousand?
With tholins mixed into liquid water-even for only a thousand years the surface
of Titan may be much further along toward the origin of life than we thought.
DESPITE ALL THIS we understand pitifully
little about Titan. This was brought home forcefully to me at a scientific
symposium on Titan held in Toulouse, France, and sponsored by the European
Space Agency (ESA). While oceans of liquid water are impossible on Titan,
oceans of liquid hydrocarbons are not. Clouds of methane (CH4), the most
abundant hydrocarbon, are expected not far above the surface. Ethane (C2H6),
the next most abundant hydrocarbon, must condense out at the surface in the
same way that water vapor becomes a liquid near the surface of the Earth, where
the temperature is generally between the freezing and melting points. Vast
oceans of liquid hydrocarbons should have accumulated over the lifetime of
Titan. They would lie far beneath the haze and clouds. But that doesn't mean
they would be wholly inaccessible to us-because radio waves readily penetrate
the atmosphere of Titan and its suspended, slowly falling fine particles.
In Toulouse, Duane O. Muhleman of the
California Institute of Technology described to us the very difficult technical
feat of transmitting a set of radio pulses from a radio telescope in
California's Mojave Desert, so they reach Titan, penetrate through the haze and
clouds to its surface, are reflected back into space, and then returned to
Earth. Here, the greatly enfeebled signal is picked up by an array of radio
telescopes near Socorro, New Mexico. Great. If Titan has a rocky or icy
surface, a radar pulse reflected off its surface should be detectable on Earth.
But if Titan were covered with hydrocarbon oceans, Muhleman shouldn't see a
thing: Liquid hydrocarbons are black to these radio waves, and no echo would
have been returned to Earth. In fact, Muhleman's giant radar system sees a
reflection when some longitudes of Titan are turned toward Earth, and not at
other longitudes. All right, you might say, so Titan has oceans and continents,
and it was a continent that reflected the signals back to Earth. But if Titanic
in this respect like the Earth-for some meridians (through Europe and Africa,
say) mainly continent, and for others (through the central Pacific, say) mainly
ocean-then we must confront another problem:
The orbit of Titan around Saturn is not
a perfect circle. it's noticeably squashed out, or elliptical. If Titan has
extensive oceans, though, the giant planet Saturn around which it orbits will
raise substantial tides on Titan, and the resulting tidal friction will
circularize Titan's orbit in much less than the age of the Solar System. In a
1982 scientific paper called “The Tide in the Seas of Titan,” Stanley Dermott,
now at the University of Florida, and I argued that for this reason Titan must
be either an all-ocean or an all-land world. Otherwise the tidal friction in
places where the ocean is shallow would have taken its toll. Lakes and islands
might be permitted, but anything more and Titan would have a very different
orbit than the one we see.
We have, then, three scientific
arguments-one concluding that this world is almost entirely covered with
hydrocarbon oceans, another that it's a mix of continents and oceans, and a
third requiring us to choose, counseling that Titan can't have extensive oceans
and extensive continents at the same time. It will be interesting to see what
the answer turns out to be.
What I've just told you is a kind of
scientific progress report. Tomorrow there might be a new finding that clears
up these mysteries and contradictions. Maybe there's something wrong with
Muhleman's radar results, although it's hard to see what it might be: His
system tells him he's seeing Titan when it's nearest, when he ought to be
seeing Titan. Maybe there's something wrong with Dermott's and my calculation
about the tidal evolution of the orbit of Titan, but no one has been able to
find any errors so far. And it's Bard to see how ethane can avoid condensing
out at the surface of Titan. Maybe, despite the low temperatures, over billions
of years there's been a change in the chemistry; maybe some combination of
comets impacting from the sky and volcanoes and other tectonic events, helped
along by cosmic rays, can congeal liquid hydrocarbons, turning them into some
complex organic solid that reflects radio waves back to space. Or maybe
something reflective to radio waves is floating on the ocean surface. But
liquid hydrocarbons are very underdense: Every known organic solid, unless
extremely frothy, would sink like a stone in the seas of Titan.
Dermott and I now wonder whether, when
we imagined continents and oceans on Titan, we were too transfixed by our
experience on our own world, too Earth-chauvinist in our thinking. Battered,
cratered terrain and abundant impact basins cover other moons in the Saturn
system. If we pictured liquid hydrocarbons slowly accumulating on one of those
worlds, we would wind up not with global oceans, but with isolated large craters
filled, although not to the brim, with liquid hydrocarbons. Many circular seas
of petroleum, some over a hundred miles across, would be splattered across the
surface-but no perceptible waves would be stimulated by distant Saturn and, it
is conventional to think, no ships, no swimmers, no surfers, and no fishing.
Tidal friction should, we calculate, be negligible in such a case, and Titan's
stretched-out, elliptical orbit would not have become so circular. We can't
know for sure until we start getting radar or near-infrared images of the
surface. But perhaps this is the resolution of our dilemma: Titan as a world of
large circular hydrocarbon lakes, more of them in some longitudes than in
others.
Should we expect an icy surface covered
with deep tholin sediments, a hydrocarbon ocean with at most a few organic
encrusted islands poking up here and there, a world of crater lakes, or
something more subtle that we haven't yet figured out? This isn't just an
academic question, because there's a real spacecraft being designed to go to
Titan. In a joint NASA/ESA program, a spacecraft called Cassini will be
launched in October 1997-if all goes well. With two flybys of Venus, one of
Earth, and one of Jupiter for gravitational assists, the ship will, after a
seven-year voyage, be injected into orbit around Saturn. Each time the
spacecraft comes close to Titan, the moon will be examined by an array of
instruments, including radar. Because Cassini will be so much closer to Titan,
it will be able to resolve many details on Titan's surface indetectable to
Muhleman's pioneering Earth-based system. It's also likely that the surface can
be viewed in the near infrared. Maps of the hidden surface of Titan may be in
our hands sometime in the summer of 2004.
Cassini is also carrying an entry probe,
fittingly called Huygens, which will detach itself from the main spacecraft and
plummet into Titan's atmosphere. A great parachute will be deployed. The
instrument package will slowly settle through the organic haze down into the
lower atmosphere, through the methane clouds. It will examine organic chemistry
as it descends, and-if it survives the landing-on the surface of this world as
well.
Nothing is guaranteed. But the mission
is technically feasible, hardware is being built, an impressive coterie of
specialists, including many young European scientists, are hard at work on it,
and all the nations responsible seem committed to the project. Perhaps it will
actually come about. Perhaps winging across the billion miles of intervening
interplanetary space will be, in the not too distant future. news about how far
along the path to life Titan has come.
CHAPTER 8
The first new planet
I implore you, you do not hope to be
able to give the reasons for the number of planets, do you? This worry has been
resolved...
— JOHANNES KEPLER, EPITOME OF COPERNICAN
ASTRONOMY, BOOK 4 / 1621
Before we invented civilization, our
ancestors lived mainly in the open, out under the sky. Before we devised
artificial lights and atmospheric pollution and modern forms of nocturnal
entertainment, we watched the stars. There were practical calendrical reasons,
of course, but there was more to it than that. Even today, the most jaded city
dweller can be unexpectedly moved upon encountering a clear night sky studded
with thousands of twinkling stars. When it happens to me after all these years,
it still takes my breath away.
In every culture, the sky and the
religious impulse are intertwined. I lie back in an open field and the sky
surround me. I'm overpowered by its scale. It's so vast and so far away that my
own insignificance becomes palpable. But I don't feel rejected by the sky. I'm
a part of it, tiny, to be sure, but everything is tiny compared to that
overwhelming immensity, And when I concentrate on the stars, the planets, and
their motions, I have an irresistible sense of machinery, clockwork, elegant
precision working on a scale that, however lofty our aspirations, dwarfs and
humbles us.
Most of the great inventions in human
history-from stone tools and the domestication of fire to written language-were
made by unknown benefactors. Our institutional memory of long-gone events is
feeble. We do not know the name of that ancestor who first noted that planets
were different from stars. She or he must have lived tens, perhaps even
hundreds of thousands of years ago. But eventually people all over the world
understood that five, no more, of the bright points of light that grace the
night sky break lockstep with the others over a period of months, moving
strangely-almost as if they had minds of their own.
Sharing the odd apparent motion of these
planets were the Sun and Moon, making seven wandering bodies in all. These
seven were important to the ancients, and they named them after gods not any
old gods, but the main gods, the chief gods, the ones who tell other gods (and
mortals) what to do. One of the planets, bright and slow-moving, was named by
the Babylonians after Marduk, by the Norse after Odin, by the Greeks after
Zeus, and by the Romans after Jupiter, in each case the king of the gods. The
faint, fast-moving one that was never far from the Sun the Romans named
Mercury, after the messenger of the gods; the most brilliant of them was named
Venus, after the goddess of love and beauty; the blood red one Mars, after the god
of war; and the most sluggish of the bunch Saturn, after the god of time. These
metaphors and allusions were the best our ancestors could do: They possessed no
scientific instruments beyond the naked eye, they were confined to the Earth,
and they had no idea that it, too, is a planet.
When it got to be time to design the
week-a period of time, unlike the day, month, and year, with no intrinsic
astronomical significance-it was assigned seven days, each named after one of
the seven anomalous lights in the night sky. We can readily make out the
remnants of this convention. In English, Saturday is Saturn's day. Sunday and
Mo[o]nday are clear enough. Tuesday through Friday are named after the gods of
the Saxon and kindred Teutonic invaders of Celtic/Roman Britain: Wednesday, for
example, is Odin's (or Wodin's) day, which would be more apparent if we
pronounced it as it's spelled, “Wedn's Day”; Thursday is Thor's day; Friday is
the day of Freya, goddess of love. The last day of the week stayed Roman, the
rest of it became German.
In all Romance languages, such as
French, Spanish, and Italian, the connection is still more obvious, because
they 4 derive from ancient Latin, in which the days of the week were named (in
order, beginning with Sunday) after the Sun, the Moon, Mars, Mercury, Jupiter,
Venus, and Saturn. (The Sun's day became the Lord's day.) They could have named
the days in order of the brightness of the corresponding astronomical
bodies-the Sun, the Moon, Venus, Jupiter, Mars, Saturn, Mercury (and thus Sunday,
Monday, Friday, Thursday, Tuesday, Saturday, Wednesday)-but they did not. If
the days of the week in Romance languages had been ordered by distance from the
Sun, the sequence would be Sunday, Wednesday, Friday, Monday, Tuesday,
Thursday, Saturday. no one knew the order of the planets, though, back when we
were naming planets, gods, and days of the week. The ordering of the days of
the week seems arbitrary, although perhaps it does acknowledge the primacy of
the Sun.
This collection of seven gods, seven
days, and seven worlds the Sun, the Moon, and the five wandering planets
entered the perceptions of people everywhere. The number seven began to acquire
supernatural connotations. There were seven “heavens,” the transparent
spherical shells, centered on the Earth, that were imagined to make these
worlds move. The outermost-the seventh heaven-is where the “fixed” stars were
imagined to reside. There are Seven Days of Creation (if we include God's day
of rest), seven orifices to the head, seven virtues, seven deadly sins, seven
evil demons in Sumerian myth. seven vowels in the Greek alphabet (each
affiliated with a planetary god), Seven Governors of Destiny according to the
Hermetists, Seven Great Books of Manichaeism, Seven Sacraments, Seven Sages of
Ancient Greece, and seven alchemical “bodies” (gold, silver, iron, mercury,
lead, tin, and copper-gold still associated with the Sun, silver with the Moon,
iron with Mars, etc.). The seventh son of a seventh son is endowed with
supernatural powers. Seven is a “lucky” number. In the New Testament's Book of
Revelations, seven seals on a scroll are opened, seven trumpets are sounded,
seven bowls are filled. St. Augustine obscurely argued for the mystic
importance of seven on the grounds that three “is the first whole number that
is odd” (what about one?), “four the first that is even” (what about two?), and
“of these...seven is composed.” And so on. Even in our time these associations
linger.
The existence even of the four
satellites of Jupiter that Galileo discovered-hardly planets-was disbelieved on
the grounds that it challenged the precedence of the number seven. As
acceptance of the Copernican system grew, the Earth was added to the list of
planets, and the Sun and Moon were removed. Thus, there seemed to be only six
planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). So learned academic
arguments were invented showing why there had to be six. For example, six is
the first “perfect” number, equal to the sum of its divisors (1 + 2 + 3). Q. E.
D. And anyway, there were only six days of creation, not seven. People found
ways to accommodate from seven planets to six.
As those adept at numerological
mysticism adjusted to the Copernican system, this self-indulgent mode of
thinking spilled over from planets to moons. The Earth had one moon; Jupiter
had the four Galilean moons. That made five. Clearly one was missing. (Don't
forget: Six is the first perfect number.) When Huygens discovered Titan in
1655, he and many others convinced themselves that it was the last: Six
planets, six moons, and God's in His Heaven.
The historian of science I. Bernard
Cohen of Harvard University has pointed out that Huygens actually gave up
searching for other moons because it was apparent, from such arguments, that no
more were to be found. Sixteen years later, ironically with Huygens in
attendance, G. D. Cassim of the Paris Observatory discovered a seventh
moon-Iapetus, a bizarre world with one hemisphere black and the other white, in
an orbit exterior to Titan's. Shortly after, Cassim discovered Rhea, the next
Saturnian moon interior to Titan.
Here was another opportunity for
numerology, this time harnessed to the practical task of flattering patrons.
Cassim added up the number of planets (six) and the number of satellites
(eight) and got fourteen. Now it so happened that the man who built Cassim's
observatory for him and paid his salary was Louis XIV of France, the Sun King.
The astronomer promptly “presented” these two new moons to his sovereign and
proclaimed that Louis's “conquests” reached to the ends of the Solar System.
Discreetly, Cassim then backed off from looking for more moons; Cohen suggests
he was afraid one more might now offend Louis-a monarch not to be trifled with,
who would shortly be throwing his subjects into dungeons for the crime of being
Protestants. Twelve years later, though, Cassim returned to the search and
found-doubtless with a measure of trepidation-another two moons. (It is
probably a good thing that we have not continued in this vein; otherwise France
would have been burdened by seventy-some-odd Bourbon kings named Louis.)
WHEN CLAIMS OF NEW WORLDS WERE MADE In
the late eighteenth century, the force of such numerological arguments had much
dissipated. Still, it was with a real sense of surprise that people heard in
1781 about a new planet, discovered through the telescope. New moons were
comparatively unimpressive, especially after the first six or eight. But that
there were new planets to be found and that humans had devised the means to do
so were both considered astonishing, and properly so. If there is one
previously unknown planet, there may be many more-in this solar system and in
others. Who can tell what might be found if a multitude of new worlds are
hiding in the dark?
The discovery was made not even by a
professional astronomer but by William Herschel, a musician whose relatives had
come to Britain with the family of another anglified German, the reigning
monarch and future oppressor of the American colonists, George III. It became
Herschel's wish to call the planet George (“George's Star,” actually), after
his patron. but, providentially, the name didn't stick. (Astronomers seem to
have been very busy buttering up kings.) Instead, the planet that Herschel
found is called Uranus (an inexhaustible source of hilarity renewed in each
generation of English-speaking nine-year-olds). It is named after the ancient
sky god who, according to Greek myth, was Saturn's father and thus the
grandfather of the Olympian gods.
We no longer consider the Sun and Moon to
be planets, and ignoring the comparatively insignificant asteroids and comets,
count Uranus as the seventh planet in order from the Sun (Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto). It is the first planet
unknown to the ancients. The four outer, Jovian, planets turn out to be very
different from the four inner, terrestrial, planets. Pluto is a separate case.
As the years passed and the quality of
astronomical instruments unproved, we began to learn more about distant Uranus.
What reflects the dim sunlight back to us is no solid surface, but atmosphere
and clouds just as for Titan, Venus, Jupiter Saturn, and Neptune. The air on
Uranus is made of hydrogen and helium, the two simplest gases. Methane and
other hydrocarbons are also present. Just below the clouds visible to
Earthbound observers is a massive atmosphere with enormous quantities of
ammonia, hydrogen sulfide, and, especially, water.
At depth on Jupiter and Saturn, the
pressures are so great that atoms sweat electrons, and the air becomes a metal.
That does not seem to happen on less massive Uranus, because the pressures at
depth are less. Still deeper, discovered only by its subtle tugs on Uranus'
moons, wholly inaccessible to view, under the crushing weight of the overlying
atmosphere, is a rocky surface. A big Earthlike planet is hiding down there,
swathed in an immense blanket of air.
The Earth's surface temperature is due
to the sunlight it intercepts. Turn off the Sun and the planet soon chills-not
to trifling Antarctic cold, not just so cold that the oceans freeze, but to a
cold so intense that the very air precipitates out, forming a ten-meter-thick
layer of oxygen and nitrogen snows covering the whole planet. The little bit of
energy that trickles up from the Earth's hot interior would be insufficient to
melt these snows. For Jupiter, Saturn, and Neptune it's different. There's
about as much heat pouring out from their interiors as they acquire from the
warmth of the distant Sun. Turn off the Sun, and they would be only a little
affected.
But Uranus is another story. Uranus is
an anomaly among the Jovian planets. Uranus is like the Earth: There's very
little intrinsic heat pouring out. We have no good understanding of why this
should be, why Uranus-which in many respects is so similar to Neptune-should
lack a potent source of internal heat. For this reason, among others, we cannot
say we understand what is going on in the deep interiors of these mighty
worlds.
Uranus is lying on its side as it goes
around the Sun. In the 1990s. the south pole is heated by the Sun, and it is
this pole that Earthbound observers at the end of the twentieth century see
when they look at Uranus. It takes Uranus 84 Earth years to make one circuit of
the Sun. So in the 2030s, the north pole will be sunward (and Earthward). In
the 2070s the south pole will be pointing to the Sun once again. In between,
Earthbound astronomers will be looking mainly at equatorial latitudes.
All the other planets spin much more
upright in their orbits. no one is sure of the reason for Uranus' anomalous
spin; the most promising suggestion is that sometime in its early history,
billions of years ago, it was struck by a rogue planet, about the size of the
Earth, in a highly eccentric orbit. Such a collision, if it ever happened, must
have worked much tumult in the Uranus system; for all we know, there may be
other vestiges of ancient havoc still left for us to find. But Uranus'
remoteness tends to guard its mysteries.
In 1977 a team of scientists led by
James Elliot, then of Cornell University, accidentally discovered that, like
Saturn, Uranus has rings. The scientists were flying over the Indian Ocean in a
special NASA airplane-the Kuiper Airborne Observatory-to witness the passage of
Uranus in front of a star. (Such passages, or occultations as they're called,
happen from time to time, precisely because Uranus slowly moves with respect to
the distant stars.) The observers were surprised to find that the star winked
on and off several times just before it passed behind Uranus and its
atmosphere, then several times more just after it emerged. Since the patterns
of winking on and off were the same before and after occultation, this finding
(and much subsequent work) has led to the discovery of nine very thin, very
dark circumplanetary rings, giving Uranus the appearance of a bull's-eye in the
sky.
Surrounding the rings, Earthbound
observers understood were the concentric orbits of the five moons then known:
Miranda, Ariel, Umbriel, Titania, and Oberon. They're named after characters in
Shakespeare's A Midsummer Night's Dream and The Tempest, and in Alexander
Pope's The Rape of the Lock. Two of them were found by Herschel himself. The
innermost of the five, Miranda, was discovered as recently as 1948, by my
teacher G. P. Kuiper. †† I remember how great an achievement the discovery of a
new moon of Uranus was considered back then. The near-infrared light reflected
by all five moons subsequently revealed the spectral signature of ordinary
water ice on their Surfaces. And no wonder-Uranus is so far from the Sun that
it is no brighter there at noontime than it is after sunset on Earth. The
temperatures are frigid. Any water must be frozen.
A REVOLUTION IN OUR UNDERSTANDING of the
Uranus system-the planet, its rings, and its moons-began on January 24, 1986.
On that day, after a journey of 81/2 years, the Voyager 2 spacecraft sailed
very near to Miranda, and hit the bull's-eye in the sky. Uranus' gravity then
flung it on to Neptune. The spacecraft returned 4,300 close-up pictures of the Uranus
system and a wealth of other data.
Uranus was found to be surrounded by an
intense radiation belt, electrons and protons trapped by the planet's magnetic
field. Voyager flew through this radiation belt, measuring the magnetic field
and the trapped charged particles as it went. It also detected-in changing
timbres, harmonies, and nuance, but mainly in fortissimo-a cacophony of radio
waves generated by the speeding, trapped particles. Something similar was
discovered on Jupiter and Saturn and would be later found at Neptune-but always
with a theme and counterpoint characteristic of each world.
On Earth the magnetic and geographical
poles are quite close together. On Uranus the magnetic axis and the axis of
rotation are tilted away from each other by some 60 degrees. no one yet
understands why: Some have suggested that we are catching Uranus in a reversal
of its north and south magnetic poles, as periodically happens on Earth. Others
propose that this too is the consequence of that mighty, ancient collision that
knocked the planet over. But we do not know.
Uranus is emitting much more ultraviolet
light than it's receiving from the Sun, probably generated by charged particles
leaking out of the magnetosphere and striking its upper atmosphere. From a
vantage point in the Uranus system, the spacecraft examined a bright star
winking on and off as the rings of Uranus passed by. New faint dust bands were
found. From the perspective of Earth, the spacecraft passed behind Uranus; so
the radio signals it was transmitting back home passed tangentially through the
Uranian atmosphere, probing it-to below its methane clouds. A vast and deep
ocean, perhaps 8,000 kilometers thick, of super-heated liquid water floating in
the air is inferred by some.
Among the principal glories of the
Uranus encounter were the pictures. With Voyager's two television cameras, we
discovered ten new moons, determined the length of the day in the clouds of
Uranus (about 17 hours), and studied about a dozen rings. The most spectacular
pictures were those returned from the five larger, previously known moons of
Uranus, especially the smallest of them, Kuiper's Miranda. Its surface is a
tumult of fault valleys, parallel ridges, sheer cliffs, low mountains, impact
craters, and frozen floods of once-molten surface material. This turmoiled
landscape is unexpected for a small, cold, icy world so distant from the Sun.
Perhaps the surface was melted and reworked in some long-gone epoch when a
gravitational resonance between Uranus, Miranda, and Ariel pumped energy from
the nearby planet into Miranda's interior. Or perhaps we are seeing the results
of the primordial collision that is thought to have knocked Uranus over. Or,
just conceivably, maybe Miranda was once utterly destroyed, dismembered,
blasted into smithereens by a wild careening world, with many collision
fragments still left in Miranda's orbit. The shards and remnants, slowly
colliding, gravitationally attracting one another, may have re-aggregated into
just such a jumbled, patchy, unfinished world as Miranda is today.
For me, there's something eerie about
the pictures of dusky Miranda, because I can remember so well when it `vas only
a faint point of light almost lost in the glare of Uranus, discovered through
great difficulty by dint of the astronomer's skills and patience. In only half
a lifetime it has gone from an undiscovered world to a destination whose
ancient and idiosyncratic secrets have been at least partially revealed.
CHAPTER 9
An American ship at the frontiers of the solar
system
... by the shore Of Triton's Lake... I
will clear my breast of secrets.
—EURIPIDES, ION (CA. 413 B. C.)
Neptune was the final port of call in
Voyager 2's grand tour of the Solar System. Usually, it is thought of as the
penultimate planet, with Pluto the outermost. But because of Pluto's
stretched-out, elliptical orbit, Neptune has lately been the outermost planet,
and will remain so until 1999. Typical temperatures in its upper clouds are
about -240°C, because it is so far from the warming rays of the Sun. It would
be colder still except for the heat welling up from its interior. Neptune
glides along the hem of interstellar night. It is so far away that, in its sky,
the Sun appears as little more than an extremely bright star.
How far? So far away that it has
yet to complete a single trip around the Sun, a Neptunian year, since its
discovery in 1846. It's so far away that it cannot be seen with the naked eye.
It's so far away that it takes light-faster than which nothing can go-more than
five hours to get from Neptune to Earth.
When Voyager 2 raced through the Neptune
system in 1989, its cameras, spectrometers, particle and field detectors, and
other instruments were feverishly examining the planet, its moons, and its
rings. The planet itself, like its cousins Jupiter, Saturn, and Uranus, is a
giant. Every planet is an Earthlike world at heart-but the four gas giants wear
elaborate, cumbersome disguises. Jupiter and Saturn are great gas worlds with
relatively small rocky and icy cores. But Uranus and Neptune are fundamentally
rock and ice worlds swaddled in dense atmospheres that hide them from view.
Neptune is four times bigger than the
Earth. When we look down on its cool, austere blueness, again we are seeing
only atmosphere and clouds-no solid surface. Again, the atmosphere is made
mainly of hydrogen and helium, with a little methane and traces of other
hydrocarbons. There may also be some nitrogen. The bright clouds, which seem to
be methane crystals, float above thick, deeper clouds of unknown composition. From
the motion of the clouds we discovered fierce winds, approaching the local
speed of sound. A Great Dark Spot was found, curiously at almost the same
latitude as the Great Red Spot on Jupiter. The azure color seems appropriate
for a planet named after the god of the sea.
Surrounding this dimly lit, chilly,
stormy, remote world is-here also-a system of rings, each composed of
innumerable orbiting objects ranging in size from the fine particles in
cigarette smoke to small trucks. Like the rings of the other Jovian planets,
those of Neptune seem to be evanescent-it is calculated that gravity and solar
radiation will disrupt them in much less than the age of the Solar System. If
they are destroyed quickly, we must see them only because they were made recently.
But how can rings be made?
The biggest moon in the Neptune system
is called Triton. Nearly six of our days are required for it to orbit Neptune,
which-alone among big moons in the Solar System-it does in the opposite
direction to which its planet spins (clockwise if we say Neptune rotates
counterclockwise). Triton has a nitrogen-rich atmosphere, somewhat similar to
Titan's; but, because the air and haze are much thinner, we can see its
surface. The landscapes are varied and splendid. This is a world of ices-nitrogen
ice, methane ice, probably underlain by more familiar water ice and rock. There
are impact basins, which seem to have been flooded with liquid before
refreezing (so there once were lakes on Triton); impact craters; long
crisscrossing valleys; vast plains covered by freshly fallen nitrogen snow;
puckered terrain that resembles the skin of a cantaloupe; and more or less
parallel, long, dark streaks that seem to have been blown by the wind and then
deposited on the icy surface despite how sparse Triton's atmosphere is (about
1/10,000 the thickness of the Earth's).
All the craters on Triton are
pristine-as if stamped out by some vast milling device. There are no slumped
walls or muted relief. Even with the periodic falling and evaporation of snow,
it seems that nothing has eroded the surface of Triton in billions of years. So
the craters that were gouged out during the formation of Triton must have all
been filled in and covered over by some early global resurfacing event. Triton
orbits Neptune in the opposite direction to Neptune's rotation-unlike the
situation with the Earth and its moon, and with most of the large moons in the
Solar System. If Triton had formed out of the same spinning disk that made
Neptune, it ought to be going around Neptune in the same direction that Neptune
rotates. So Triton was not made from the original local nebula around Neptune,
but arose somewhere else-perhaps far beyond Pluto-and was by chance
gravitationally captured when it passed too close to Neptune. This event should
have raised enormous solid-body tides in Triton, melting the surface and
sweeping away all the past topography.
In some places the surface is as bright
and white as freshly fallen Antarctic snows (and may offer a skiing experience
unrivaled in all the Solar System). Elsewhere there's a tint, ranging from pink
to brown. One possible explanation: Freshly fallen snows of nitrogen, methane,
and other hydrocarbons are irradiated by solar ultraviolet light and by
electrons trapped in the magnetic field of Neptune, through which Triton plows.
We know that such irradiation will convert the snows (like the corresponding
gases) to complex, dark, reddish organic sediments, ice tholins-nothing alive,
but here too composed of some of the molecules implicated in the origin of life
on Earth four billion years ago.
In local winter, layers of ice and snow
build up on the surface. (Our winters, mercifully, are only 4 percent as long.)
Through the spring, they are slowly transformed, more and more reddish organic
molecules accumulating. By summertime, the ice and snow have evaporated; the
gases so released migrate halfway across the planet to the winter hemisphere
and there cover the surface with ice and snow again. But the reddish organic
molecules do not vaporize and are not transported-a lag deposit, they are next
winter covered over by new snows, which are in turn irradiated, and by the
following summer the accumulation is thicker. As time goes on, substantial
amounts of organic matter are built up on the surface of Triton, which may
account for its delicate color markings.
The streaks begin in small, dark source
regions, perhaps when the warmth of spring and summer heats subsurface volatile
snows. As they vaporize, gas comes gushing out as in a geyser, blowing off
less-volatile surface snows and dark organics. Prevailing low-speed winds carry
away the dark organics, which slowly sediment out of the thin air, are
deposited on the ground, and generate the appearance of the streaks. This, at
least, is one reconstruction of recent Tritonian history.
Triton may have large, seasonal polar
caps of smooth nitrogen ice underlying layers of dark organic materials.
Nitrogen snows seem recently to have fallen at the equator. Snowfalls, geysers,
windblown organic dust, and high-altitude hazes were entirely unexpected on a
world with so thin an atmosphere.
Why is the air so thin? Because Triton
is so far from the Sun. Were you somehow to pick this world up and move it into
orbit around Saturn, the nitrogen and methane ices would quickly evaporate, a
much denser atmosphere of gaseous nitrogen and methane would form, and
radiation would generate an opaque tholin haze. It would become a world very
like Titan. Conversely, if you moved Titan into orbit about Neptune, almost all
its atmosphere would freeze out as snows and ices, the tholin would fall out
and not be replaced, the air would clear, and the surface would become visible
in ordinary light. It would become a world very like Triton.
These two worlds are not identical. The
interior of Titan seems to contain much more ice than that of Triton, and much
less rock. Titan's diameter is almost twice that of Triton. Still, if placed at
the same distance from the Sun they would look like sisters. Alan Stern of the
Southwest Research Institute suggests that they are two members of a vast
collection of small worlds rich in nitrogen and methane that formed in the
early Solar System. Pluto, yet to be visited by a spacecraft, appears to be
another member of this group. Many more may await discovery beyond Pluto. The
thin atmospheres and icy surfaces of all these worlds are being irradiated-by
cosmic rays, if nothing else and nitrogen-rich organic compounds are being
formed. It looks as if the stuff of life is sitting not just on Titan, but
throughout the cold, dimly lit outer reaches of our planetary system.
Another class of small objects has
recently been discovered, whose orbits take them-at least part of the
time-beyond Neptune and Pluto. Sometimes called minor planets or asteroids,
they are more likely to be inactive comets (with no tails, of course; so far
from the Sun, their ices cannot readily vaporize). But they are much bigger
than the run-of-the-mill comets we know. They may be the vanguard of a vast
array of small worlds that extends from the orbit of Pluto halfway to the
nearest star. The innermost province of the Oort Comet Cloud, of which these
new objects may be members, is called the Kuiper Belt, after my mentor Gerard
Kuiper, who first suggested that it should exist. Short-period comets-like
Halley's-arise in the Kuiper Belt, respond to gravitational tugs, sweep into
the inner part of the Solar System, grow their tails, and grace our skies.
Back in the late nineteenth century,
these building blocks of worlds-then mere hypotheses-were called “planetesimals.”
The flavor of the word is, I suppose, something like that of “infinitesimals”:
You need an infinite number of them to make anything. It's not quite that
extreme with planetesimals, although a very large number of them would be
required to make a planet. For example, trillions of bodies each a kilometer in
size would be needed to coalesce to make a planet with the mass of the Earth.
Once there were much larger numbers of worldlets in the planetary part of the
Solar System. Most of them are now gone-ejected into interstellar space, fallen
into the Sun, or sacrificed in the great enterprise of building moons and
planets. But out beyond Neptune and Pluto the discards, the leftovers that were
never aggregated into worlds, may be waiting-a few largish ones in the
100-kilometer range, and stupefying numbers of kilometer-sized and smaller
bodies peppering the outer Solar System all the way out to the Oort Cloud.
In this sense there are planets beyond
Neptune and Pluto-but they are not nearly as big as the Jovian planets, or even
Pluto. Larger worlds may, for all we know, also be hiding in the dark beyond
Pluto, worlds that can properly be called planets. The farther away they are,
the less likely it is that we would have detected them. They cannot lie just
beyond Neptune, though; their gravitational tugs would have perceptibly altered
the orbits of Neptune and Pluto, and the Pioneer 10 and 11 and Voyager 1 and 2
spacecraft.
The newly discovered cometary bodies
(with names like 1992QB and 1993FW) are not planets in this sense. If our
detection threshold has just encompassed them, many more of them probably
remain to be discovered in the outer Solar System-so far away that they're hard
to see from Earth, so distant that it's a long journey to get to them. But
small, quick ships to Pluto and beyond are within our ability. It would make
good sense to dispatch one by Pluto and its moon Charon, and then, if we can,
to make a close pass by one of the denizens of the Kuiper Comet Belt.
The rocky Earthlike cores of Uranus and
Neptune seem to have accreted first, and then gravitationally attracted massive
amounts of hydrogen and helium gas from the ancient nebula out of which the
planets formed. Originally, they lived in a hailstorm. Their gravities were
just sufficient to eject icy worldlets, when they came too close, far out
beyond the realm of the planets, to populate the Oort Comet Cloud. Jupiter and
Saturn became gas giants by the same process. But their gravities were too
strong to populate the Oort Cloud: Ice worlds that came close to them were
gravitationally pitched out of the Solar System entirely-destined to wander
forever in the great dark between the stars.
So the lovely comets that on occasion
rouse us humans to wonder and to awe, that crater the surfaces of inner planets
and outer moons, and that now and then endanger life on Earth would be unknown
and unthreatening had Uranus and Neptune not grown to be giant worlds four and
a half billion years ago.
THIS IS THE PLACE for a brief interlude
on planets far beyond Neptune and Pluto, planets of other stars.
Many nearby stars are surrounded by thin
disks of orbiting gas and dust, often extending to hundreds of astronomical
units (AU) from the local star (the outermost planets, Neptune and Pluto, are
about 40 AU from our Sun). Younger Sun-like stars are much more likely to have
disks than older ones. In some cases, there's a hole in the center of the disk
as in a phonograph record. The hole extends out from the star to perhaps 30 or
40 AU. This is true, for example, for the disks surrounding the stars Vega and
Epsilon Eridam. The hole in the disk surrounding Beta Pictoris extends to only
15 AU from the star. There is a real possibility that these inner, dust-free
zones have been cleaned up by planets that recently formed there. Indeed, this
sweeping-out process is predicted for the early history of our planetary
system. As observations improve, perhaps we will see telltale details in the
configuration of dust and dust-free zones that will indicate the presence of
planets too small and dark to be seen directly. Spectroscopic data suggest that
these disks are churning and that matter is falling in on the central
stars-perhaps from comets formed in the disk, deflected by the unseen planets,
and evaporating as they approach too close to the local sun.
Because planets are small and shine by
reflected light, they tend to be washed out in the glare of the local sun.
Nevertheless, many efforts are now under way to find fully formed planets
around nearby stars-by detecting a faint brief dimming of starlight as a dark
planet interposes itself between the star and the observer on Earth; or by
sensing a faint wobble in the motion of the star as it's tugged first one way
and then another by an otherwise invisible orbiting companion. Spaceborne
techniques will be much more sensitive. A Jovian planet going around a nearby
star is about a billion times fainter than its sun; nevertheless, a new
generation of ground-based telescopes that can compensate for the twinkling in
the Earth's atmosphere may soon be able to detect such planets in only a few
hours' observing time. A terrestrial planet of a neighboring star is a hundred
times fainter still; but it now seems that comparatively inexpensive
spacecraft, above the Earth's atmosphere, might be able to detect other Earths.
None of these searches has succeeded yet, but we are clearly on the verge of
being able to detect at least Jupiter-sized planets around the nearest stars-if
there are any to be found.
A most important and serendipitous
recent discovery is of a bona fide planetary system around an unlikely star,
some 1,300 light-years away, found by a most unexpected technique: The pulsar
designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense
sun, the remnant of a massive star that suffered a supernova explosion. It
spins, at a rate measured to impressive precision, once every 0.
0062185319388187 seconds. This pulsar is pushing 10,000 rpm.
Charged particles trapped in its intense
magnetic field generate radio waves that are cast across the Earth, about 160
flickers a second. Small but discernible changes in the flash rate were
tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State
University, in 1991-as a tiny reflex motion of the pulsar in response to the
presence of planets. In 1994 the predicted mutual gravitational interactions of
these planets were confirmed by Wolszczan from a study of timing residuals at
the microsecond level over the intervening years. The evidence that these are
truly new planets and not starquakes on the neutron star surface (or something)
is now overwhelming-or, as Wolszczan put it, “irrefutable”; a new solar system
is “unambiguously identified.” Unlike all the other techniques, the pulsar
timing method makes close-in terrestrial planets comparatively easy and more
distant Jovian planets comparatively difficult to detect.
Planet C, some 2. 8 times more massive
than the Earth, orbits the pulsar every 98 days at a distance of 0. 47
astronomical units (AU); Planet B, with about 3. 4 Earth masses, has a
67-Earth-day year at 0. 36 AU. A smaller world, Planet A, still closer to the
star, with about 0. 015 Earth masses, is at 0. 19 AU. Crudely speaking, Planet
B is roughly at the distance of Mercury from our Sun; Planet C is midway
between the distances of Mercury and Venus; and interior to both of them is
Planet A, roughly the mass of the Moon at about half Mercury's distance from
our Sun. Whether these planets are the remnants of an earlier planetary system
that somehow survived the supernova explosion that produced the pulsar, or
whether they formed from the resulting circumstellar accretion disk subsequent
to the supernova explosion, we do not know. But in either case, we have now
learned that there are other Earths.
The energy put out by B1257+12 is about
4. 7 times that of gun. But, unlike the Sun, most of this is not in visible
light, but in a fierce hurricane of electrically charged particles. Suppose
that these particles impinge on the planets and heat them. Then, even a planet
at 1 AU would have a surface around 280 Celsius degrees above the normal
boiling point of water, greater than the temperature of Venus.
These dark and broiling planets do not
seem hospitable for life. But there may be others, farther from B1257+12, that
are. (Hints of at least one cooler, outer world in the B1257+12 system exist.)
Of course, we don't even know that such worlds would retain their atmospheres;
perhaps any atmospheres were stripped away in the supernova explosion, if they
date back that far. But we do seem to be detecting a recognizable planetary
system. Many more are likely to become known in coming decades, around ordinary
Sun-like stars as well as white dwarfs, pulsars, and other end states of
stellar evolution.
Eventually, we will have a list of
planetary systems-each perhaps with terrestrials and Jovians and maybe new
classes of planets. We will examine these worlds, spectroscopically and in
other ways. We will be searching for new Earths and other life.
ON NONE OF THE WORLDS In the outer Solar
System did Voyager find signs of life, much less intelligence. There was
organic Matter galore-the stuff of life, the premonitions of life, perhaps but
as far as we could see, no life. There was no oxygen in their atmospheres, and
no gases profoundly out of chemical equilibrium, as methane is in the Earth's
oxygen. Many of the worlds were painted with subtle colors, but none with such
distinctive, sharp absorption features as chlorophyll provides over much of the
Earth's surface. On very few worlds was Voyager able to resolve details as
small as a kilometer across. By this standard, it would not have detected even
our own technical civilization had it been transplanted to the outer Solar
System. But for what it's worth, we found no regular patterning, no
geometrization, no passion for small circles, triangles, squares, or
rectangles. There were no constellations of steady points of light on the night
hemispheres. There were no signs of a technical civilization reworking the
surface of any of these worlds.
The Jovian planets are prolific
broadcasters of radio waves-generated in part by the abundant trapped and
beamed charged particles in their magnetic fields, in part by lightning, and in
part by their hot interiors. But none of this emission has the character of
intelligent life-or so it seems to the experts in the field.
Of course our thinking may be too
narrow. We may be missing something. For example, there is a little carbon
dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere
out of chemical equilibrium. I think the CO2 is provided by the steady
pitter-patter of comets falling into Titan's atmosphere-but maybe not. Maybe
there's something on the surface unaccountably generating CO2 in the face of
all that methane.
The surfaces of Miranda and Triton are
unlike anything else we know. There are vast chevron-shaped landforms and
crisscrossing straight lines that even sober planetary geologists once
mischievously described as “highways.” We think we (barely) understand these landforms
in terms of faults and collisions, but of course we might be wrong.
The surface stains of organic
matter-sometimes, as on Triton, delicately hued-are attributed to charged
particles producing chemical reactions in simple hydrocarbon ices, generating
more complex organic materials, and all this having nothing to do with the
intermediation of life. But of course we might be wrong.
The complex pattern of radio static,
bursts, and whistles that we receive from all four Jovian planets seems, in a
general way, explicable by plasma physics and thermal emission. (Much of the
detail is not yet well understood.) But of course we might be wrong.
We have found nothing on dozens of
worlds so clear and striking as the signs of life found by the Galileo
spacecraft in its passages by the Earth. Life is a hypothesis of last resort.
You invoke it only when there's no other way to explain what you see. If I had
to judge, I would say that there's no life on any of the worlds we've studied,
except of course our own. But I might be wrong, and, right or wrong, my
judgment is necessarily confined to this Solar System. Perhaps on some new
mission we'll find something different, something striking, something wholly
inexplicable with the ordinary tools of planetary science-and tremulously,
cautiously, we will inch toward a biological explanation. However, for now
nothing requires that we go down such a path. So far, the only life in the
Solar System is that which comes from Earth. In the Uranus and Neptune systems,
the only sign of life has been Voyager itself.
As we identify the planets of other
stars, as we find other worlds of roughly the size and mass of the Earth, we
will scrutinize them for life. A dense oxygen atmosphere may be detectable even
on a world we've never imaged. As for the Earth, that may by itself be a sign
of life. An oxygen atmosphere with appreciable quantities of methane would
almost certainly be a sign of life, as would modulated radio emission. Someday,
from observations of our planetary system or another, the news of life
elsewhere may be announced over the morning coffee.
THE VOYAGER SPACECRAFT are bound for the
stars. They are on escape trajectories from the Solar System, barreling along
at almost a million miles a day. The gravitational fields of Jupiter, Saturn,
Uranus, and Neptune have flung them at such high speeds that they have broken
the bonds that once tied them to the Sun.
Have they left the Solar System yet? The
answer depends very much on how you define the boundary of the Sun's realm. If
it's the orbit of the outermost good-sized planet, then the Voyager spacecraft
are already long gone; there are probably no undiscovered Neptunes. If you mean
the outermost planet, it may be that there are other-perhaps
Triton-like-planets far beyond Neptune and Pluto; if so, Voyager 1 and Voyager
2 are still within the Solar System. If you define the outer limits of the
Solar System as the heliopause-where the interplanetary particles and magnetic
fields are replaced by their interstellar counterparts-then neither Voyager has
yet left the Solar System, although they may do so in the next few decades. †††
But if your definition of the edge of the Solar System is the distance at which
our star can no longer hold worlds in orbit about it, then the Voyagers will not
leave the Solar System for hundreds of centuries.
Weakly grasped by the Sun's gravity, in
every direction in the sky, is that immense horde of a trillion comets or more,
the port Cloud. The two spacecraft will finish their passage through the Oort
cloud in another 20,000 years or so. Then, at last, completing their long
good-bye to the Solar System, broken free of the gravitational shackles that
once bound them to the Sun, the Voyagers will make for the open sea of
interstellar space. only then will Phase Two of their mission begin.
Their radio transmitters long dead, the
spacecraft will wander for ages in the calm, cold interstellar blackness-where
there is almost nothing to erode them. Once out of the Solar System, they will
remain intact for a billion years or more, as they circumnavigate the center of
the Milky Way galaxy.
We do not know whether there are other
space-faring civilizations in the Milky Way. If they do exist, we do not know
how abundant they are, much less where they are. But there is at least a chance
that sometime in the remote future one of the Voyagers will be intercepted and
examined by an alien craft.
Accordingly, as each Voyager left Earth
for the planets and the stars, it carried with it a golden phonograph record
encased in a golden, mirrored jacket containing, among other things; greetings
in 59 human languages and one whale language; a 12-minute sound essay including
a kiss, a baby's cry, and an EEG record of the meditations of a young woman in
love; 116 encoded pictures, on our science, our civilization, and ourselves;
and 90 minutes of the Earth's greatest hits-Eastern and Western, classical and
folk, including a Navajo night chant, a Japanese shakuhachi piece, a Pygmy
girl's initiation song, a Peruvian wedding song, a 3,000-year-old composition
for the ch'in called “Flowing Streams,” Bach, Beethoven, Mozart, Stravinsky,
Louis Armstrong, Blind Willie Johnson, and Chuck Berry's “Johnny B. Goode.”
Space is nearly empty. There is
virtually no chance that one of the Voyagers will ever enter another solar
system-and this is true even if every star in the sky is accompanied by
planets. The instructions on the record jackets, written in what we believe to
be readily comprehensible scientific hieroglyphics, can be read, and the
contents of the records understood, only if alien beings, somewhere in the
remote future, find Voyager in the depths of interstellar space. Since both
Voyagers will circle the center of the Milky Way Galaxy essentially forever,
there is plenty of time for the records to be found-if there's anyone out there
to do the finding.
We cannot know how much of the records
they would understand. Surely the greetings will be incomprehensible, but their
intent may not be. (We thought it would be impolite not to say hello.) The hypothetical
aliens are bound to be very different from us-independently evolved on another
world. Are we really sure they could understand anything at all of our message?
Every time I feel these concerns stirring, though, I reassure myself. Whatever
the incomprehensibilities of the Voyager record, any alien ship that finds it
will have another standard by which to judge us. Each Voyager is itself a
message. In their exploratory intent, in the lofty ambition of their
objectives, in their utter lack of intent to do harm, and in the brilliance of
their design and performance, these robots speak eloquently for us.
But being much more advanced scientists
and engineers than we-otherwise they would never be able to find and retrieve
the small, silent spacecraft in interstellar space-perhaps the aliens would
have no difficulty understanding what is encoded on these golden records.
Perhaps they would recognize the tentativeness of our society, the mismatch
between our technology and our wisdom. Have we destroyed ourselves since
launching Voyager, they might wonder, or have we gone on to greater things?
Or perhaps the records will never be
intercepted. Perhaps no one in five billion years will ever come upon them.
Five billion years is a long time. In five billion years, all humans will have
become extinct or evolved into other beings, none of our artifacts will have
survived on Earth, the continents will have become unrecognizably altered or
destroyed, and the evolution of the Sun will have burned the Earth to a crisp or
reduced it to a whirl of atoms.
Far from home, untouched by these remote
events, the Voyagers, bearing the memories of a world that is no more, will fly
on.
CHAPTER 10
Sacred black
Deep sky is, of all visual impressions,
the nearest akin to a feeling.
—SAMUEL TAYLOR COLERIDGE, NOTEBOOKS
(1805)
The blue of a cloudless May morning, or
the reds and oranges of a sunset at sea, have roused humans to wonder, to
poetry, and to science. no matter where on Earth we live, no matter what our
language, customs, or politics, we share a sky in common. Most of us expect
that azure blue and would, for good reason, be stunned to wake up one sunrise
to find a cloudless sky that was black or yellow or green. (Inhabitants of Los
Angeles and Mexico City have grown accustomed to brown skies, and those of
London and Seattle to gray ones-but even they still consider blue the planetary
norm.)
And yet there are worlds with black or
yellow skies, and maybe even green. The color of the sky characterizes the
world. Plop me down on any planet in the Solar System; without sensing the
gravity, without glimpsing the ground, let me take a quick look at the Sun and
sky, and I can, I think, pretty well tell you where I am. That familiar shade
of blue, interrupted here and there by fleecy white clouds, is a signature of
our world. The French have an expression, sacre-bleu!, which translates roughly
as “Good heavens!" Literally, it means “sacred blue!” Indeed. If there
ever is a true flag of Earth, this should be its color.
Birds fly through it, clouds are
suspended in it, humans admire and routinely traverse it, light from the Sun
and stars flutters through it. But what is it? What is it made of? Where does
it end? How much of it is there? Where does all that blue come from? If it's a
commonplace for all humans, if it typifies our world, surely we should know
something about it. What is the sky?
In August 1957, for the first time, a
human being rose above the blue and looked around-when David Simons, a retired
Air Force officer and a physician, became the highest human in history. Alone,
he piloted a balloon to an altitude of over 100,000 feet (30 kilometers) and
through his thick windows glimpsed a different sky. Now a professor at the
University of California Medical School in Irvine, Dr. Simons recalls it was a
dark, deep purple overhead. He had reached the transition region where the blue
of ground level is being overtaken by the perfect black of space.
Since Simons' almost forgotten flight,
people of many nations have flown above the atmosphere. It is now clear from
repeated and direct human (and robotic) experience that in space the daytime
sky is black. The Sun shines brightly on your ship. The Earth below you is
brilliantly illuminated. But the sky above is black as night.
Here is the memorable description by
Yuri Gagarin of what he saw on the first spaceflight of the human species,
aboard Vostok 1, on April 12, 1961:
The sky is completely black; and against
the background of this black sky the stars appear somewhat brighter and more
distinct. The Earth has a very characteristic, very beautiful blue halo, which
is seen well when you observe the horizon. There is a smooth color transition
from tender blue, to blue, to dark blue and purple, and then to the completely
black color of the sky. It is a very beautiful transition.
Clearly, the daylit sky-all that blue-
is somehow connected with the air. But as you look across the breakfast table,
your companion is not (usually) blue; the color of the sky must be a property
not of a little air, but of a great deal. If you look closely at the Earth from
space, you see it surrounded by a thin band of blue, as thick as the lower
atmosphere; indeed, it is the lower atmosphere. At the top of that band you can
make out the blue sky fading into the blackness of space. This is the
transition zone that Simons was the first to enter and Gagarin the first to
observe from above. In routine spaceflight, you start at the bottom of the
blue, penetrate entirely through it a few minutes after liftoff; and then enter
that boundless realm where a simple breath of air is impossible without
elaborate life-support systems. Human life depends for its very existence on
that blue sky. We are right to consider it tender and sacred.
We see the blue in daytime because
sunlight is bouncing off the air around and above us. On a cloudless night, the
sky is black because there is no sufficiently intense source of light to be
reflected off the air. Somehow, the air preferentially bounces blue light down
to us. How?
The visible light from the Sun comes in
many colors-violet, blue, green, yellow, orange, red-corresponding to light of
different wavelengths. (A wavelength is the distance front crest to crest as
the wave travels through air or space.) Violet and blue light waves have the
shortest wavelengths; orange and red the longest. What we perceive as color is
how our eyes and brains read the wavelengths of light. (We Might just as
reasonably translate wavelengths of light into, say, heard tones rather than
seen colors-but that's not how our senses evolved.)
When all those rainbow colors of the
spectrum are mixed together, as in sunlight, they seem almost white. These
waves travel together in eight minutes across the intervening 93 million miles
(150 million kilometers) of space from the Sun to the Earth. They strike the
atmosphere, which is made mostly of nitrogen and oxygen molecules. Some waves
are reflected by the air back into space. Some are bounced around before the
light reaches the ground and they can be detected by a passing eyeball. (Also,
some bounce off clouds or the ground back into space.) This bouncing around of
light waves in the atmosphere is called “scattering.”
But not all waves are equally well
scattered by the molecules of air. Wavelengths that are much longer than the size
of the molecules are scattered less; they spill over the molecules, hardly
influenced by their presence. Wavelengths that are closer to the size of the
molecules are scattered snore. And waves have trouble ignoring obstacles as big
as they are. (You can see this in water waves scattered by the pilings of
piers, or bathtub waves from a dripping faucet encountering a rubber duck.) The
shorter wavelengths, those that we sense as violet and blue light, are more
efficiently scattered than the longer wavelengths, those that we sense as
orange and red light. When we look up on a cloudless day and admire the blue
sky, we are witnessing the preferential scattering of the short waves in
sunlight. This is called Rayleigh scattering, after the English physicist who offered
the first coherent explanation for it. Cigarette smoke is blue for just the
same reason: The particles that make it up are about as small as the wavelength
of blue light.
So why is the sunset red? The red of the
sunset is what's left of sunlight after the air scatters the blue away. Since
the atmosphere is a thin shell of gravitationally bound gas surrounding the
solid Earth, sunlight must pass through a longer slant path of air at sunset
(or sunrise) than at noon. Since the violet and blue waves are scattered even
more during their now-longer path through the air than when the Sun is
overhead, what we see when we look toward the Sun is the residue-the waves of
sunlight that are hardly scattered away at all, especially the oranges and
reds. A blue sky makes a red sunset. (The noontime Sun seems yellowish partly
because it emits slightly more yellow light than other colors, and partly
because, even with the Sun overhead, some blue light is scattered out of the
sunbeams by the Earth's atmosphere.)
It is sometimes said that scientists are
unromantic, that their passion to figure out robs the world of beauty and
mystery. But is it not stirring to understand how the world actually works-that
white light is made of colors, that color is the way we perceive the
wavelengths of light, that transparent air reflects light, that in so doing it
discriminates among the waves, and that the sky is blue for the same reason
that the sunset is red? It does no harm to the romance of the sunset to know a
little bit about it.
Since most simple molecules are about
the same size (roughly a hundred millionth of a centimeter), the blue of the
Earth's sky doesn't much depend on what the air is made of-as long as the air
doesn't absorb the light. Oxygen and nitrogen molecules don't absorb visible
light; they only bounce it away in some other direction. Other molecules,
though, can gobble up the light. Oxides of nitrogen-produced in automotive
engines and in the fires of industry-are a source of the murky brown coloration
of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light.
Absorption, as well as scattering, can color a sky.
OTHER WORLDS, OTHER SKIES: Mercury, the
Earth's Moon, and most satellites of the other planets are small worlds;
because of their feeble gravities, they are unable to retain their
atmospheres-which instead trickle of into space. The near-vacuum of space then
reaches the ground. Sunlight strikes their surfaces unimpeded, neither
scattered nor absorbed along the way. The skies of these worlds are black, even
at noon. This has been witnessed firsthand so far by only 12 humans, the lunar
landing crews of Apollos 11, 12, and 14-17.
A full list of the satellites in the
Solar System, known as of this writing, is given in the accompanying table.
(Nearly half of them were discovered by Voyager.) All have black skies-except
Titan of Saturn and perhaps Triton of Neptune, which are big enough to have
atmospheres. And all asteroids as well.
Venus has about 90 times more air than
Earth. It isn't mainly oxygen and nitrogen as here-it's carbon dioxide. But
carbon dioxide doesn't absorb visible light either. What would the sky look
like from the surface of Venus if Venus had no clouds. With so much atmosphere
in the way, not only are violet and blue waves scattered, but all the other
colors as well-green yellow, orange, red. The air is so thick, though, that
hardly any blue light makes it to the ground; it's scattered back to space by
successive bounces higher up. Thus, the light that does reach the ground should
be strongly reddened-like an Earth sunset all over the sky. Further, sulfur in
the high clouds will stain the sky yellow. Pictures taken by the Soviet Venera
landers confirm that the skies of Venus are a kind of yellow-orange.
SIXTY-TWO WORLDS FOR THE THIRD
MILLENNIUM: KNOWN MOONS OF THE PLANETS (AND ONE ASTEROID)—LISTED IN ORDER OF
DISTANCE FROM THEIR PLANET
EARTH, 1
MARS, 2
IDA, 1
JUPITER, 16
SATURN, 18
URANUS, 15
NEPTUNE, 8
PLUT0, 1
Moon
Phobos
Dactyl
Metis
Pan
Cordelia
Naiad
Charon
Deimos
Adrastea
Atlas
Ophelia
Thalassa
Amalthea
Prometheus
Bianca
Despina
Thebe
Pandora
Cressida
Galatea
Epimetheus
Desdemona
Larissa
Europa
Janus
Juliet
Proteus
Ganymede
Mimas
Portia
Triton
Callisto
Enceladus
Rosalind
Nereid
Leda
Tethys
Belinda
Himalia
Telesto
Puck
Lysithea
Calypso
Miranda
Elara
Diane
Ariel
Ananke
Helene
Umbriel
Carme
Rhea
Titania
Pasiphae
Titan
Oberon
Sinope
Hyperion
Iapetus
Phoebe
Mars is a different story. It is a
smaller world than Earth, with a much thinner atmosphere. The pressure at the
surface of Mars is, in fact, about the same as the altitude in the Earth's
stratosphere to which Simons rose. So we might expect the Martian sky to be
black or purple-black. The first color picture from the surface of Mars was
obtained in July 1976 by the American Viking 1 lander-the first spacecraft to
touch down successfully on the surface of the Red Planet. The digital data were
dutifully radioed from Mars back to Earth, and the color picture assembled by
computer. To the surprise of all the scientists and nobody else, that first
image, released to the press, showed the Martian sky to be a comfortable, homey
blue-impossible for a planet with so insubstantial an atmosphere. Something had
gone wrong.
The picture on your color television set
is a mixture of three monochrome images, each in a different color of
light-red, green, and blue. You can see this method of color compositing in
video projection systems, which project separate beams of red, green, and blue
light to generate a full-color picture (including yellows). To get the right
color, your set needs to mix or balance these three monochrome images
correctly. If you turn up the intensity of, say, blue, the picture will appear
too blue. Any picture returned from space requires a similar color balance.
Considerable discretion is sometimes left to the computer analysts in deciding
this balance. The hiking analysts were not planetary astronomers, and with this
first color picture from Mars they simply mixed the colors until it looked
“right.” We are so conditioned by our experience on Earth that “right,” of
course, means a blue sky. The color of the picture was soon corrected-using
color calibration standards placed for this very purpose on board the
spacecraft-and the resulting composite showed no blue sky at all; rather it was
something between ochre and pink. Not blue, but hardly purple-black either.
This is the right color of the Martian
sky. Much of the surface of Mars is desert-and red because the sands are rusk.
There are occasional violent sandstorms that lift fine particles from the
surface high into the atmosphere. It takes a long time for them to fall out,
and before the sky has fully cleaned itself, there's always another sandstorm.
Global or near-global sandstorms occur almost every Martian year. Since rusty
particles are always suspended in this sky, future generations of humans, born
and living out their lives on Mars, will consider that salmon color to be as
natural and familiar as we consider our homey blue. From a single glance at the
daytime sky, they'll probably be able to tell how long it's been since the last
big sandstorm.
The planets in the outer Solar System-Jupiter,
Saturn, Uranus, and Neptune- are of a different sort. These are huge worlds
with giant atmospheres made mainly of hydrogen and helium. Their solid surfaces
are so deep inside that no sunlight penetrates there at all. Down there, the
sky is black, with no prospect of a sunrise-not ever. The perpetual starless
night is perhaps illuminated on occasion by a bolt of lightning. But higher in
the atmosphere, where the sunlight reaches, a much more beautiful vista awaits.
On Jupiter, above a high-altitude haze
layer composed of ammonia (rather than water) ice particles, the sky is almost
black. Farther down, in the blue sky region, are multicolored clouds-in various
shades of yellow-brown, and of unknown composition. (The candidate materials
include sulfur, phosphorus, and complex organic molecules.) Even farther down,
the sky will appear red-brown, except that the clouds there are of varying thicknesses,
and where they are thin, you might see a patch of blue. Still deeper, we
gradually return to perpetual night. Something similar is true on Saturn, but
the colors there are more muted.
Uranus and especially Neptune have an
uncanny, austere blue color through which clouds-some of them a little
whiter-are carried by high-speed winds. Sunlight reaches a comparatively clean
atmosphere composed mainly of hydrogen and helium but also rich in methane.
Long paths of methane absorb yellow and especially red light and let the green
and blue filter through. A thin hydrocarbon haze removes a little blue. There
may be a depth where the sky is greenish.
Conventional wisdom holds that the
absorption by methane and the Rayleigh scattering of sunlight by the deep atmosphere
together account for the blue colors on Uranus and Neptune. But analysis of
Voyager data by Kevin Baines of JPL seems to show that these causes are
insufficient. Apparently very deep-maybe in the vicinity of hypothesized clouds
of hydrogen sulfide-there is an abundant blue substance. So far no one has been
able to figure out what it might be. Blue materials are very rare in Nature. As
always happens in science, the old mysteries are dispelled only to be replaced
by new ones. Sooner or later we'll find out the answer to this one, too.
ALL WORLDS WITH NONBLACK SKIES have
atmospheres. If you're standing on the surface and there's an atmosphere thick
enough to see, there's probably a way to fly through it. We're now sending our
instruments to fly in the variously colored skies of other worlds. Someday we
will go ourselves.
Parachutes have already been used in the
atmospheres of Venus and Mars, and are planned for Jupiter and Titan. In 1985
two French-Soviet balloons sailed through the yellow skies of Venus. The Vega 9
balloon, about 4 meters across, dangled an instrument package 13 meters below.
The balloon inflated in the night hemisphere, floated about 54 kilometers above
the surface, and transmitted data for almost two Earth days before its batteries
failed. In that time it traveled 11,600 kilometers (nearly 7,000 miles) over
the surface of Venus, far below. The Vega 2 balloon had an almost identical
profile. The atmosphere of Venus has also been used for aerobraking-changing
the Magellan spacecraft's orbit by friction with the dense air; this is a key
future technology for converting flyby spacecraft to Mars into orbiters and
landers.
A Mars mission, scheduled to be launched
in 1998, and led by Russia, includes an enormous French hot air balloon-looking
something like a vast jellyfish, a Portuguese man-of-war. It's designed to sink
to the Martian surface every chilly twilight and rise high when heated by
sunlight the next day. The winds are so fast that, if all goes well, it will be
carried hundreds of kilometers each day, hopping and skipping over the north
pole. In the early morning, when close to the ground, it will obtain very high
resolution pictures and other data. The balloon has an instrumental guide-rope,
essential for its stability, conceived and designed by a private membership
organization based in Pasadena, California, The Planetary Society.
Since the surface pressure on Mars is
approximately that at an altitude of 100,000 feet on Earth, we know we can fly
airplanes there. The U-2, for example, or the SR-71 Blackbird routinely
approaches such low pressures. Aircraft with even larger wingspans have been
designed for Mars.
The dream of flight and the dream of
space travel are twins, conceived by similar visionaries, dependent on allied
technologies, and evolving more or less in tandem. As certain practical and
economic limits to flight on Earth are reached, the possibility arises of
flying through the multihued skies of other worlds.
IT IS NOW ALMOST POSSIBLE to assign
color combinations, based on the colors of clouds and sky, to every planet in
the Solar System-from the sulfur-stained skies of Venus and the rusty skies of
Mars to the aquamarine of Uranus and the hypnotic and unearthly blue of
Neptune. Sacre-jaunt, sacre-rouge, sacre-vert. Perhaps they will one day adorn
the flags of distant human outposts in the Solar System, in that time when the
new frontiers are sweeping out from the Sun to the stars, and the explorers are
surrounded by the endless black of space. Sacre-noir.
CHAPTER 11
Evening and morning star
This is another world
Which is not of men.
—LI BAI, “QUESTION AND ANSWER IN THE
MOUNTAINS”
(CHINA, TANG DYNASTY, CA. 730)
You can see it shining brilliantly in
the twilight, chasing the Sun down below the western horizon. Upon first
glimpsing it each night, people were accustomed to make a wish (“upon a star”).
Sometimes the wish came true.
Or you can spy it in the east before
dawn, fleeing the rising Sun. In these two incarnations, brighter than anything
else in the sky except only the Sun and the Moon it was known as the evening
and the morning star. Our ancestors did not recognize it was a world, the same
world, never too far from the Sun because it is in an orbit about it interior
to the Earth's. Just before sunset or just after sunrise, we can sometimes see
it near some fluffy white cloud, and then discover by the comparison that Venus
has a color, a pale lemon-yellow.
You peer through the eyepiece of a
telescope-even a big telescope, even the largest optical telescope on Earth-and
you can make out no detail at all. Over the months, you see a featureless disk
methodically going through phases, like the Moon: crescent Venus, full Venus,
gibbous Venus, new Venus. There is not a hint of continents or oceans.
Some of the first astronomers to see
Venus through the telescope immediately recognized that they were examining a
world enshrouded by clouds. The clouds, we now know, are droplets of
concentrated sulfuric acid, stained yellow by a little elemental sulfur. They
lie high above the ground. In ordinary visible light there's no hint of what
this planet's surface, some 50 kilometers below the cloud tops, is like, and
for centuries the best we had were wild guesses.
You might conjecture that if we could
take a much finer look there might be breaks in the clouds, revealing day by
day, in bits and pieces, the mysterious surface ordinarily hidden from our
view. Then the time of guesses would be over. The Earth is on average half
cloud-covered. In the early days of Venus exploration, we saw no reason that
Venus should be 100 percent overcast. If instead it was only 90 percent, or
even 99 percent, cloud-covered, the transient patches of clearing might tell us
much.
In 1960 and 1961, Mariners 1 and 2, the
first American spacecraft designed to visit Venus, were being prepared. There
were those, like me, who thought the ships should carry video cameras so they
could radio pictures back to Earth. The same technology would be used a few
years later when Rangers 7, 8, and 9 would photograph the Moon on the way to
their crash landings-the last making a bull's-eye in the crater Alphonsus. But
time was short for the Venus mission, and cameras were heavy. There were those
who maintained that cameras weren't really scientific instruments, but rather
catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to
answer a single straightforward, well-posed scientific question. I thought
myself that whether there are breaks in the clouds was one such question. I
argued that cameras could also answer questions that we were too dumb even to
pose. I argued that pictures were the only way to show the public-who were,
after all, footing the bill-the excitement of robotic missions. At any rate, no
camera was flown, and subsequent missions have, for this particular world, at
least partly vindicated that judgment: Even at high resolution from close
flybys, in visible light it turns out there are no breaks in the clouds of
Venus, any more than in the clouds of Titan. These worlds are permanently
overcast.
In the ultraviolet there is detail, but
due to transient patches of high-altitude overcast, far above the main cloud
deck. The high clouds race around the planet much faster than the planet itself
turns: super-rotation. We have an even smaller chance of seeing the surface in
the ultraviolet.
When it became clear that the atmosphere
of Venus was much thicker than the air on Earth-as we now know, the pressure at
the surface is ninety times what it is here-it immediately followed that in
ordinary visible light we could not possibly see the surface, even if there
were breaks in the clouds. What little sunlight is able to make its tortuous
way through the dense atmosphere to the surface would be reflected back, all
right; but the photons would be so jumbled by repeated scattering off molecules
in the lower air that no image of surface features could be retained. It would
be like a “whiteout” in polar snowstorm. However, this effect, intense Rayleigh
scattering, declines rapidly with increasing wavelength; in the near-infrared,
it was easy to calculate, you could see the surface if there were breaks in the
clouds or if the clouds were transparent there.
So in 1970 Jim Pollack, Dave Morrison,
and I went to the McDonald Observatory of the University of Texas to try to
observe Venus in the near-infrared. We “hypersensitized” our emulsions; the
good old-fashioned glass photographic plates were treated with ammonia, and
sometimes heated or briefly illuminated, before being exposed at the telescope
to light from Venus. For a time the cellars of McDonald Observatory reeked of
ammonia. We took many pictures. None showed any detail. We concluded that
either we hadn't gone far enough into the infrared or the clouds of Venus were
opaque and unbroken in the near infrared.
More than 20 years later, the Galileo
spacecraft, making a close flyby of Venus, examined it with higher resolution
and sensitivity, and at wavelengths a little further into the infrared than we
were able to reach with our crude glass emulsions. Galileo photographed great
mountain ranges. We already knew of their existence, though; a much more
powerful technique had earlier been employed: radar. Radio waves effortlessly
penetrate the clouds and thick atmosphere of Venus, bounce off the surface, and
return to Earth, where they are gathered in and used to make a picture. The
first work had been done, chiefly, by. American ground-based radar at JPL's
Goldstone tracking station in the Mojave Desert and at the Arecibo Observatory
in Puerto Rico, operated by Cornell University.
Then the U. S. Pioneer 12, the Soviet
Venera 15 and ?6 and the U. S. Magellan missions inserted radar telescopes into
orbit around Venus and mapped the place pole to pole. Each spacecraft would
transmit a radar signal to the surface and then catch it as it bounced back.
From how reflective each patch of surface was and how long it took the signal
to return (shorter from mountains, longer from valleys), a detailed map of the
entire surface was slowly and painstakingly constructed.
The world so revealed turns out to be
uniquely sculpted by lava flows (and, to a much lesser degree, by wind), as
described in the next chapter. The clouds and atmosphere of Venus have now
become transparent to us, and another world has been visited by the doughty
robot explorers from Earth. Our experience with Venus is now being applied
elsewhere-especially to Titan, where once again impenetrable clouds hide an
enigmatic surface, and radar is beginning to give us hints of what might lie
below.
VENUS HAD LONG BEEN THOUGHT of as our
sister world. It is the nearest planet to the Earth. It has almost the same
mass, size, density, and gravitational pull as the Earth does. It's a little
closer to the Sun than the Earth, but its bright clouds reflect more sunlight
back to space than our clouds do. As a first guess you might very well imagine
that, under those unbroken clouds, Venus was rather like Earth. Early
scientific speculation included fetid swamps crawling with monster amphibians,
like the Earth in the Carboniferous Period; a world desert; a global petroleum
sea; and a seltzer ocean dotted here and there with limestone-encrusted
islands. While based on some scientific data, these “models” of Venus-the first
dating from the beginnings of the century, the second from the 1930s, and the
last two from the raid-1950s-were little more than scientific romances, hardly
constrained by the sparse data available.
Then, in 1956, a report was published in
The Astrophysical Journal by Cornell H. Mayer and his colleagues. They had
pointed a newly completed radio telescope, built in part for classified
research, on the roof of the Naval Research Laboratory in Washington, D. C., at
Venus and measured the flux of radio waves arriving at Earth. This was not
radar: no radio waves were bounced off Venus. This was listening to radio waves
that Venus on its own emits to space. Venus turned out to be much brighter than
the background of distant stars and galaxies. This in itself was not very
surprising. Every object warmer than absolute zero (-273°C) gives off radiation
throughout the electromagnetic spectrum, including the radio region. You, for
example, emit radio waves at an effective or “brightness” temperature of about
35°C, and if you were in surroundings colder than you are, a sensitive radio
telescope could detect the faint radio waves you are transmitting in all
directions. Each of us is a source of cold static.
What was surprising about Mayer's
discovery was that the brightness temperature of Venus is more than 300°C, far
higher than the surface temperature of the Earth or the measured infrared
temperature of the clouds of Venus. Some places on Venus seemed at least 200°
hotter than the normal boiling point of water. What could this mean?
Soon there was a deluge of explanations.
I argued that the high radio brightness temperature was a direct indication of
a hot surface, and that the high temperatures were due to a massive carbon
dioxide/water vapor greenhouse effect-in which some sunlight is transmitted
through the clouds and heats the Surface, but the surface experiences enormous
difficulty in radiating back to space because of the high infrared opacity of
carbon dioxide and water vapor. Carbon dioxide absorbs at a range of
wavelengths through the infrared, but there seemed to be “windows” between the CO2
absorption bands through which the surface could readily cool off to space.
Water vapor, though, absorbs at infrared frequencies that correspond in part to
the windows in the carbon dioxide opacity. The two gases together, it seemed to
me, could pretty well absorb almost all the infrared emission, even if there
was very little water vapor-something like two picket fences, the slats of one
being fortuitously positioned to cover the gaps of the other.
There was another very different
category of explanation, in which the high brightness temperature of Venus had
nothing to do with the ground. The surface could still be temperate, clement,
congenial. It was proposed that some region in the atmosphere of Venus or in
its surrounding magnetosphere emitted these radio waves to space. Electrical
discharges between water droplets in the Venus clouds were suggested. A glow
discharge in which ions and electrons recombined at twilight and dawn in the
upper atmosphere was offered. A very dense ionosphere had its advocates, in
which the mutual acceleration of unbound electrons (“free-free emission”) gave
off radio waves. (One proponent of this idea even suggested that the high
ionization required was due to an average of 10,000 times greater radioactivity
on Venus than on Earth-perhaps from a recent nuclear war there.) And, in the
light of the discovery of radiation from Jupiter's magnetosphere, it was
natural to suggest that the radio emission came from an immense cloud of
charged particles trapped by some hypothetical very intense Venusian magnetic
field.
In a series of papers I published in the
middle 1960s, many in collaboration with Jim Pollack, these conflicting models
of a high hot emitting region and a cold surface were subjected to a critical
analysis. By then we had two important new clues: the radio spectrum of Venus,
and the Mariner 2 evidence that the radio emission was more intense at the
center of the disk of Venus than toward its edge. By 1967 we were able to
exclude the alternative models with some confidence, and conclude that the
surface of Venus was at a scorching and un-Earthlike temperature, in excess of
400°C. But the argument was inferential, and there were many intermediate
steps. We longed for a more direct measurement.
In October 1967-commemorating the tenth
anniversary of Sputnik 1-the Soviet Venera 4 spacecraft dropped an entry
capsule into the clouds of Venus. It returned data from the hot lover
atmosphere, but did not survive to the surface. One day later, the United
States spacecraft Mariner 5 flew by Venus, its radio transmission to Earth
skimming the atmosphere at progressively greater depths. The rate of fading of
the signal gave information about atmospheric temperatures. Although there
seemed to be some discrepancies (later resolved) between the two sets of
spacecraft data, both clearly indicated that the surface of Venus is very hot.
Since then a progression of Soviet
Venera spacecraft and one cluster of American spacecraft from the Pioneer 12
mission have entered the deep atmosphere or landed on the surface and measured
directly-essentially by sticking out a thermometer-the surface and near-surface
temperatures. They turn out to be about 470°C, almost 900°F. When such factors
as calibration errors of terrestrial radio telescopes and surface emissivity
are taken into account, the old radio observations and the new direct
spacecraft measurements turn out to be in good accord.
Early Soviet landers were designed for
an atmosphere somewhat like our own. They were crushed by the high pressures
like a tin can in the grasp of a champion arm wrestler, or a World War II
submarine in the Tonga Trench. Thereafter, Soviet Venus entry vehicles were
heavily reinforced, like modern submarines, and successfully landed on the
searing surface. When it became clear how deep the atmosphere is and how thick
the clouds, Soviet designers became concerned that the surface might be
pitch-black. Veneras 9 and 10 were equipped with floodlights. They proved
unnecessary. A few percent of the sunlight that falls on the top of the clouds
makes it through to the surface, and Venus is about as bright as on a cloudy
day on Earth.
The resistance to the idea of a hot
surface on Venus can, I suppose, be attributed to our reluctance to abandon the
notion that the nearest planet is hospitable for life, for future exploration,
and perhaps even, in the longer term, for human settlement. As it turns out
there are no Carboniferous swamps no global oil or seltzer oceans. Instead,
Venus is a stifling, brooding inferno. There are some deserts, but it's mainly
a world of frozen lava seas. Our hopes are unfulfilled. The call of this world
is now more muted than in the early days of spacecraft exploration, when almost
anything was possible and our most romantic notions about Venus might, for all
we then knew, be realized.
MANY SPACECRAFT CONTRIBUTED to our
present understanding of Venus. But the pioneering mission was Mariner 2.
Mariner 1 failed at launch and-as they say of a racehorse with a broken leg-had
to be destroyed. Mariner 2 worked beautifully and provided the key early radio
data on the climate of Venus. It made infrared observations of the properties
of the clouds. On its way from Earth to Venus, it discovered and measured the
solar wind-the stream of charged particles that flows outward from the Sun,
filling the magnetospheres of any planets in its way, blowing back the tails of
comets, and establishing the distant heliopause. Mariner 2 was the first
successful planetary probe, the ship that ushered in the age of planetary
exploration.
It's still in orbit around the Sun,
every few hundred days still approaching, more or less tangentially, the orbit
of Venus. Each time that happens, Venus isn't there. But if we wait long
enough, Venus will one day be nearby and Mariner 2 will be accelerated by the
planet's gravity into some quite different orbit. Ultimately, Mariner 2, like
some planetesimal from ages past, will be swept up by another planet, fall into
the Sun, or be ejected from the Solar System.
Until then, this harbinger of the age of
planetary exploration, this minuscule artificial planet, will continue silently
orbiting the Sun. It's a little as if Columbus's flagship, the Santa Maria,
were still making regular runs with a ghostly crew across the Atlantic between
Cadiz and Hispaniola. In the vacuum of interplanetary space, Mariner 2 should
be in mint condition for many generations.
My wish on the evening and morning star
is this: that late in the twenty-first century some great ship, on its regular
gravity-assisted transit to the outer Solar System, intercepts this ancient
derelict and heaves it aboard, so it can be displayed in a museum of early
space technology-on Mars, perhaps, or Europa, or Iapetus.
CHAPTER 12
The ground melts
Midway between Thera and Therasia, fires broke forth from the sea
and continued for four days, so that the
whole sea boiled and blazed, and the fires cast up an island which was
gradually elevated as though by levers... After the cessation of the eruption,
the Rhodians, at the time of their
maritime supremacy, were first to venture upon
the scene and to erect on the island a temple.
—STRABO, GEOGRAPHY (CA. 7 B. C)
All over the Earth, you can find a kind
of mountain with one striking and unusual feature. Any child can recognize it:
The top seems sheared or squared off: If you climb to the summit or fly over
it, you discover that the mountain has a hole or crater at its peak. In some
mountains of this sort, the craters are small; in others, they are almost as
big as the mountain itself. Occasionally, the craters are filled with water.
Sometimes they're filled with a more amazing liquid: You tiptoe 10 the edge,
and see vast, glowing lakes of yellow-red liquid and fountains of fire. These
holes in the tops of mountains are called calderas, after the word “caldron,”
and the mountains on which they sit are known, of course, as volcanos-after
Vulcan, the Roman god of fire. There are perhaps 600 active volcanos discovered
on Earth. Some, beneath the oceans, are yet to be found.
A typical volcanic mountain looks safe
enough. Natural vegetation runs up its sides. Terraced fields decorate its
flanks. Hamlets and shrines nestle at its base. And yet, without warning, after
centuries of lassitude, the mountain may explode. Barrages of boulders,
torrents of ash drop out of the sky. Rivers of molten rock come pouring down
its sides. All over the Earth people imagined that an active volcano was an
imprisoned giant or demon struggling to get out.
The eruptions of Mt. St. Helens and Mt.
Pinatubo are recent reminders, but examples can be found throughout history. In
1902 a hot, glowing volcanic cloud swept down the slopes of Mt. Pelee and
killed 35,000 people in the city of St. Pierre on the Caribbean island of
Martinique. Massive mudflows from the eruption of the Nevado del Ruiz volcano
in 1985 killed more than 25,000 Colombians. The eruption of Mt. Vesuvius in the
first century buried in ash the hapless inhabitants of Pompeii and Herculaneum
and killed the intrepid naturalist Pliny the Elder as he made his way up the
side of the volcano, intent on arriving at a better understanding of its
workings. (Pliny was hardly the last: Fifteen volcanologists have been killed
in sundry volcanic eruptions between 1979 and 1993.) The Mediterranean island
of Santorin (also called Thera) is in reality the only part above water of the
rim of a volcano now inundated by the sea. The explosion of the Santorin
volcano in 1623 B. C. may, some historians think, have helped destroy the great
Minoan civilization on the nearby island of Crete and changed the balance of
power in early classical civilization. This disaster may be the origin of the
Atlantis legend as related by Plato, in which a civilization was destroyed “in
a single day and night of misfortune.” It must have been easy back then to
think that a god was angry.
Volcanos have naturally been regarded
with fear and awe. When medieval Christians viewed the eruption of Mt. Hekla in
Iceland and saw churning fragments of soft lava suspended over the summit, they
imagined they were seeing the souls of the damned awaiting entrance to Hell.
“Fearful howlings, weeping and gnashing of teeth,” “melancholy cries and loud
wailings” were dutifully reported. The glowing red lakes and sulfurous gases
within the Hekla caldera were thought to be a real glimpse into the underworld
and a confirmation of folk beliefs in Hell sand, by symmetry, in its partner,
Heaven).
A volcano is, in fact, an aperture to an
underground realm much vaster than the thin surface layer that humans inhabit,
and far more hostile. The lava that erupts from a volcano is liquid rock-rock
raised to its melting point, generally around 1000°C. The lava emerges from a
hole in the Earth; as it cools and solidifies, it generates and later remakes
the flanks of a volcanic mountain.
The most volcanically active locales on
Earth tend to be along ridges on the ocean floor and island arcs-at the
junction of two great plates of oceanic crust-either separating from each
other, or one slipping under the other. On the seafloor there are long zones of
volcanic eruptions-accompanied by swarms of earthquakes and plumes of abyssal
smoke and hot water-that we a are just beginning to observe with robot and
manned submersible vehicles.
Eruptions of lava must mean that the
Earth's interior is extremely hot. Indeed, seismic evidence shows that, only a
few hundred kilometers beneath the surface, nearly the entire body of the Earth
is at least slightly molten. The interior of the Earth is hot, in part, because
radioactive elements there, such as uranium, give off heat as they decay; and
in part because the Earth retains some of the original heat released in its
formation, when many small worlds fell together by their mutual gravity to make
the Earth, and when iron drifted down to form our planet's core.
The molten rock, or magma, rises through
fissures in the surrounding heavier solid rocks. We can imagine vast
subterranean caverns filled with glowing, red, bubbling, viscous liquids that
shoot up toward the surface if a suitable channel is by chance provided. The
magma, called lava as it pours out of the summit caldera, does indeed arise
from the underworld. The souls of the damned have so far eluded detection.
Once the volcano is fully built from
successive outpourings, and the lava is no longer spewing up into the caldera,
then it becomes just like any other mountain-slowly eroding because of rainfall
and windblown debris and, eventually, the movement of continental plates across
the Earth's surface. “How many years can a mountain exist before it is washed
to the sea?” asked Bob Dylan in the ballad “Blowing in the Wind.” The answer
depends on which planet we're talking about. For the Earth, it's typically
about ten million years. So mountains, volcanic and otherwise, must be built on
the same timescale; otherwise the Earth would be everywhere smooth as Kansas.
§§§§ Volcanic explosions can punch vast quantities of matter-mainly fine
droplets of sulfuric acid-into the stratosphere. There, for a year or two, they
reflect sunlight back to space and cool the Earth. This happened recently with
the Philippine volcano, Mt. Pinatubo, and disastrously in 1815-16 after the
eruption of the Indonesian volcano Mt. Tambora, which resulted in the
famine-ridden “year without a summer.” A volcanic eruption in Taupo, New
Zealand, in the year 177 cooled the climate of the Mediterranean, half a world
away, and dropped fine particles onto the Greenland ice cap. The explosion of
Mt. Mazama in Oregon (which left the caldera now called Crater Lake) in 4803 B.
C. had climatic consequences throughout the northern hemisphere. Studies of
volcanic effects on the climate were on the investigative path that eventually
led to the discovery of nuclear winter. They provide important tests of our use
of computer models to predict future climate change. Volcanic particles
injected into the upper air are also an additional cause of thinning of the
ozone layer.
So a large volcanic explosion in some
unfrequented and obscure part of the world can alter the environment on a
global scale. Both in their origins and in their effects, volcanos remind us of
how vulnerable we are to minor burps and sneezes in the Earth's internal
metabolism, and how important it is for us to understand how this subterranean
heat engine works.
IN THE FINAL STAGES of formation of the
Earth-as well as the Moon, Mars, and Venus- impacts by small worlds are thought
to have generated global magma oceans. Molten rock flooded the pre-existing
topography. Great floods, tidal waves kilometers high, of flowing, red-hot
liquid magma welled up from the interior and poured over the surface of the
planet, burying everything in their path: mountains, channels, craters, perhaps
even the last evidence of much earlier, more clement times. The geological
odometer was reset. All accessible records of surface geology begin with the
last global magma flood. Before the, cool and solidify, oceans of lava may be
hundreds or even thousands of kilometers thick. In our time, billions of years
later, the surface of such a world may be quiet, inactive, with no hint of
current vulcanism. Or there may be-as on Earth-a few small-scale but active
reminders of an epoch when the entire surface was flooded with liquid rock.
In the early days of planetary geology,
ground-based telescopic observations were all the data we had. A fervent debate
had been running for half a century on whether the craters of the Moon were due
to impacts or volcanos. A few low mounds with summit calderas were found-almost
certainly lunar volcanos. But the big craters-bowl or pan-shaped and sitting on
the flat ground and not the tops of mountains-were a different story. Some
geologists saw in them similarities with certain highly eroded volcanos on
Earth. Others did not. The best counter-argument was that we know there are
asteroids and comets that fly past the Moon; they must hit it sometimes; and
the collisions must make craters. Over the history of the Moon a large number
of such craters should have been punched out. So if the craters we see are not
due to impacts, where then are the impact craters? We now know from direct
laboratory examination of lunar craters that they are almost entirely of impact
origin. But 4 billion years ago this little world, nearly dead today, was
bubbling and churning away, driven by primeval vulcanism from sources of
internal heat now long gone.
In November 1971, NASA's Mariner 9
spacecraft arrived at Mars to find the planet completely obscured by a global
dust storm. Almost the only features to be seen were four circular spots rising
out of the reddish murk. But there was something peculiar about them: They had
holes in their tops. As the storm cleared, we were able to see unmistakably
that we had been viewing four huge volcanic mountains penetrating through the
dust cloud, each capped by a great summit caldera.
After the storm dissipated, the true
scale of these volcanos became clear. The largest-appropriately named Olympus
Mons, or Mt. Olympus, after the home of the Greek gods-is more than 25
kilometers (roughly 15 miles) high, dwarfing not only the largest volcano on
Earth but also the largest mountain of any sort, Mt. Everest, which stands 9
kilometers above the Tibetan plateau. There are some 20 large volcanos on Mars,
but none so massive as Olympus Mons, which has a volume about 100 times that of
the largest volcano on Earth, Mauna Loa in Hawaii.
By counting the accumulated impact
craters (made by small impacting asteroids, and readily distinguished from
summit calderas) on the flanks of the volcanos, estimates of their ages can be
derived. Some Martian volcanos turn out to be a few billion years old, although
none dates back to the very origin of Mars, about 4. 5 billion years ago. Some,
including Olympus Mons, are comparatively new-perhaps only a few hundred
million years old. It is clear that enormous volcanic explosions occurred early
in Martian history, perhaps providing an atmosphere much denser than the one
Mars holds today. What would the place have looked like if we had visited it
then?
Some volcanic flows on Mars (for
example, in Cerberus) formed as recently as 200 million years ago. It is, I
suppose, even possible-although there is no evidence either way-that Olympus
Mons, the largest volcano we know about for certain in the Solar System, will
be active again. Volcanologists, a patient sort, would doubtless welcome the
event.
In 1990-93 the Magellan spacecraft
returned surprising radar data about the landforms of Venus. Cartographers
prepared maps of almost the entire planet, with fine detail down to about 100
meters, the goal-line-to-goal-line distance in an American football stadium.
More data were radioed home by Magellan than by all other planetary missions
combined. Since much of the ocean floor remains unexplored (except perhaps for
still-classified data acquired by the U. S. and Soviet navies), we may know
more about the surface topography of Venus than about any other planet, Earth
included. Much of the geology of Venus is unlike anything seen on Earth or
anywhere else. Planetary geologists have given these landforms names, but that
doesn't mean we fully understand how they're formed.
Because the surface temperature of Venus
is almost 470°C (900°F), the rocks there are much closer to their melting
points than are those at the surface of the Earth. Rocks begin to soften and
flow at much shallower depths on Venus than on Earth. This is very likely the
reason that many geological features on Venus seem to be plastic and deformed.
The planet is covered by volcanic plains
and highland plateaus. The geological constructs include volcanic cones,
probable shield volcanos, and calderas. There are many places where we can see
that lava has erupted in vast floods. Some plains features ranging to over 200
kilometers in size are playfully called “ticks” and “arachnoids” (which
translates roughly as “spiderlike things”)-because they are circular
depressions surrounded by concentric rings, while long, spindly surface cracks
extend radially out from the center. Odd, flat “pancake domes”-a geological
feature unknown on Earth, but probably a kind of volcano-are perhaps formed by
thick, viscous lava slowly flowing uniformly in all directions. There are many
examples of more irregular lava flows. Curious ring structures called “coronae”
range up to some 2,000 kilometers across. The distinctive lava flows on
stifling hot Venus offer up a rich menu of geological mysteries.
The most unexpected and peculiar
features are the sinuous channels-with meanders and oxbows, looking just like
river valleys on Earth. The longest are longer than the greatest rivers on
Earth. But it is much too hot for liquid water on Venus. And we can tell from
the absence of small impact craters that the atmosphere has been this thick,
driving as great a greenhouse effect, for as long as the present surface has
been in existence. (If it had been much thinner, intermediate-sized asteroids
would not have burned up on entry into the atmosphere, but would have survived
to excavate craters as they impact this planet's surface.) Lava flowing
downhill does make sinuous channels (sometimes under the ground, followed by
collapse of the roof of the channel). But even at the temperatures of Venus,
the lavas radiate heat, cool, slow, congeal, and stop. The magma freezes solid.
Lava channels cannot go even 10 percent of the length of the long Venus
channels before they solidify. Some planetary geologists think there must be a
special thin, watery, inviscid lava generated on Venus. But this is a
speculation supported by no other data, and a confession of our ignorance.
The thick atmosphere moves sluggishly;
because it's so dense, though, it's very good at lifting and moving fine
particles. There are wind streaks on Venus, largely emanating from impact
craters, in which the prevailing winds have scoured piles of sand and dust and
provided a sort of weather vane imprinted on the surface. Here and there we
seem to see fields of sand dunes, and provinces where wind erosion has sculpted
volcanic landforms. These aeolian processes take place in slow motion, as if at
the bottom of the sea. The winds are feeble at the surface of Venus. It may
take only a soft gust to raise a cloud of fine particles, but in that stifling
inferno a gust is hard to come by.
There are many impact craters on Venus,
but nothing like the number on the Moon or Mars. Craters smaller than a few
across are oddly missing. The reason is understood: Small asteroids and comets
are broken up on entry into the dense Venus atmosphere before they can hit the
surface. The observed cutoff in crater size corresponds very well to the
present density of the atmosphere of Venus. Certain irregular splotches seen on
the Magellan images are thought to be the remains of impactors that broke up in
the thick air before they could gouge out a crater.
Most of the impact craters are
remarkably pristine and well preserved; only a few percent of them have been
engulfed by subsequent lava flows. The surface of Venus as revealed by Magellan
is very young. There are so few impact craters that everything older than about
500 million years must have been eradicated-on a planet almost certainly 4. 5
billion years old. There is only one plausible erosive agent adequate for what
we see: vulcanism. All over the planet craters, mountains, and other geological
features have been inundated by seas of lava that once welled up from the
inside, flowed far, and froze.
After examining so young a surface
covered with congealed magma, you might wonder if there are any active volcanos
left. None has been found for certain, but there are a few-for example, one
called Maat Mons-that appear to be surrounded by fresh lava and which may
indeed still be churning and belching. There is some evidence that the
abundance of sulfur compounds in the high atmosphere varies with time, as if
volcanos at the surface were episodically injecting these materials into the
atmosphere. When the volcanos are quiescent, the sulfur compounds simply fall
out of the air. There's also disputed evidence of lightning playing around the
mountaintops of Venus, as sometimes happens on active volcanos on Earth. But we
do not know for certain whether there is ongoing vulcanism on Venus. That's a
matter for future missions.
Some scientists believe that until about
500 million years ago the Venus surface was almost entirely devoid of
landforms. Streams and oceans of molten rock were relentlessly pouring out of
the interior, filling in and covering over any relief that had managed to form.
Had you plummeted down through the clouds in that long-ago time, the surface
would have been nearly uniform and featureless. At night the landscape would
have been hellishly glowing from the red heat of molten lava. In this view, the
great internal heat engine of Venus, which supplied copious amounts of magma to
the surface until about 500 million years ago, has now turned off. The
planetary heat engine has finally run down.
In another provocative theoretical
model, this one by the geophysicist Donald Turcotte, Venus has plate tectonics
like the Earth's but it turns off and on. Right now, he proposes, the plate
tectonics are off; “continents” do not move along the surface, do not crash into
one another, do not thereby raise mountain ranges, and are not later subducted
into the deep interior. After hundreds of millions of years of quiescence,
though, plate tectonics always breaks out and surface features are flooded by
lava, destroyed by mountain building, subducted, and otherwise obliterated. The
last such breakout ended about 500 million years ago, Turcotte suggests, and
everything has been quiet since. However, the presence of coronae may
signify-on timescales that are geologically in the near future-that massive
changes on the surface of Venus are about to break out again.
EVEN MORE UNEXPECTED than the great
Martian volcanos or the magma-flooded surface of Venus is what awaited us when
the Voyager 1 spacecraft encountered lo, the innermost of the four large
Galilean moons of Jupiter, in March 1979. There we found a strange, small,
multihued world positively awash in volcanos. As we watched in astonishment,
eight active plumes poured gas and fine particles up into the sky. The largest,
now called Pele-after the Hawaiian volcano goddess-projected a fountain of
material 250 kilometers into space, higher above the surface of Io than some
astronauts have ventured above the Earth. By the time Voyager 2 arrived at Io,
four months later, Pele had turned itself off, although six of the other plumes
were still active, at least one new plume had been discovered, and another
caldera, named Surt, had changed its color dramatically.
The colors of Io, even though
exaggerated in NASA's color-enhanced images, are like none elsewhere in the
Solar System. The currently favored explanation is that the Ionian volcanos are
driven not by upwelling molten rock, as on the Earth, the Moon, Venus, and
Mars, but by upwelling sulfur dioxide and molten sulfur. The surface is covered
with volcanic mountains, volcanic calderas, vents, and lakes of molten sulfur.
Various forms and compounds of sulfur have been detected on the surface of Io
and in nearby space-the volcanos blow some of the sulfur off Io altogether.
These findings have suggested to some an underground sea of liquid sulfur that
issues to the surface at points of weakness, generates a shallow volcanic
mound, trickles downhill, and freezes, its final color determined by its
temperature on eruption.
On the Moon or Mars, you can find many
places that have changed little in a billion years. On Io, in a century, much
of the surface should be reflooded, filled in or washed away by new volcanic
flows. Maps of Io will then quickly become obsolete, and cartography of Io will
have become a growth industry.
All this seems to follow readily enough
from the Voyager observations. The rate at which the surface is covered over by
current volcanic flows implies massive changes in 50 or 100 years, a prediction
that luckily can be tested. The Voyager images of to can be compared with much
poorer images taken by ground-based telescopes 50 years earlier, and by the
Hubble Space Telescope 13 years later. The surprising conclusion seems to be
that the big surface markings on Io have hardly changed at all. Clearly, we're
missing something.
A VOLCANO in one sense represents the
insides of a planet gushing out, a wound that eventually heals itself by
cooling, only to be replaced by new stigmata. Different worlds have different
insides. The discovery of liquid-sulfur vulcanism on Io was a little like
finding that an old acquaintance, when cut, bleeds green. You had no idea such
differences were possible. He seemed so ordinary.
We are naturally eager to find
additional signs of vulcanism on other worlds. On Europa, the second of the
Galilean moons of Jupiter and Io's neighbor, there are no volcanic mountains at
all; but molten ice-liquid water-seems to have gushed to the surface through an
enormous number of crisscrossing dark markings before freezing. And further
out, among the moons of Saturn, there are signs that liquid water has gushed up
from the interior and wiped away impact craters. Still, we have never seen
anything that might plausibly be an ice volcano in either the Jupiter or Saturn
systems. On Triton, we may have observed nitrogen or methane vulcanism.
The volcanos of other worlds provide a
stirring spectacle. They enhance our sense of wonder, our joy in the beauty and
diversity of the Cosmos. But these exotic volcanos perform another service as
well: They help us to know the volcanos of our own world-and perhaps will help
one day even to predict their eruptions. If we cannot understand what's
happening in other circumstances, where the physical parameters are different,
how deep can our understanding be of the circumstance of most concern to us? A
general theory of vulcanism must cover all cases. When we stumble upon vast
volcanic eminences on a geologically quiet Mars; when we discover the surface
of Venus wiped clean only yesterday by floods of magma; when we find a world
melted not by the heat of radioactive decay, as on Earth, but by gravitational
tides exerted by nearby worlds; when we observe sulfur rather than silicate
vulcanism; and when we begin to wonder, in the moons of the outer planets,
whether we might be viewing water, ammonia, nitrogen, or methane vulcanism-then
we are learning what else is possible.
CHAPTER 13
The gift of Apollo
The gates of Heaven are open wide;
Off I ride...
—CH'U TZ'U /ATTRIBUTED TO CH'U YUAN),
“THE NINE SONGS,” SONG V, “THE GREAT
LORD OF LIVES”
(CHINA, CA. THIRD CENTURY B. C.)
It's a sultry night in July. You've
fallen asleep in the armchair. Abruptly, you startle awake, disoriented. The
television set is on, but not the sound. You strain to understand what you're
seeing. Two ghostly white figures in coveralls and helmets are softly dancing
under a pitch-black sky. They make strange little skipping motions, which
propel them upward amid barely perceptible clouds of dust. But something is
wrong. They take too long to come down. Encumbered as they are, they seem to be
flying a little. You rub your eyes, but the dreamlike tableau persists.
Of all the events surrounding Apollo
11's landing on the Moon on July 20, 1969, my most vivid recollection is its unreal
quality. Neil Armstrong and Buzz Aldrin shuffled along the gray, dusty lunar
surface, the Earth looming large in their sky, while Michael Collins, now the
Moon's own moon, orbited above them in lonely vigil. Yes, it was an astonishing
technological achievement and a triumph for the United States. Yes, the
astronauts displayed death-defying courage. Yes, as Armstrong said as he first
alighted, this was a historic step for the human species. But if you turned off
the by-play between Mission Control and the Sea of Tranquility, with its
deliberately mundane and routine chatter, and stared into that black-and-white
television monitor, you could glimpse that we humans had entered the realm of
myth and legend.
We knew the Moon from our earliest days.
It was there when our ancestors descended from the trees into the savannahs,
when we learned to walk upright, when we first devised stone tools, when we
domesticated fire, when we invented agriculture and built cities and set out to
subdue the Earth. Folklore and popular songs celebrate a mysterious connection
between the Moon and love. The word “month” and the second day of the week are
both named after the Moon. Its waxing and waning-from crescent to full to
crescent to new-was widely understood as a celestial metaphor of death and
rebirth. It was connected with the ovulation cycle of women, which has nearly
the same period-as the word “menstruation” (Latin mensis = month, from the word
“to measure”) reminds us. Those who sleep in moonlight go mad; the connection
is preserved in the English word “lunatic.” In the old Persian story, a vizier
renowned for his wisdom is asked which is more useful, the Sun or the Moon.
“The Moon,” he answers, “because the Sun shines in daytime when it's light out
anyway.” Especially when we lived out-of-doors, it was a major-if oddly
intangible-presence in our lives.
The Moon was a metaphor for the
unattainable: “You plight as well ask for the Moon,” they used to say. Or “You
can no more do that than fly to the Moon.” For most of our history, eve had no
idea what it was. A spirit? A god? A thing? It didn't look like something big
far away, but more like something small nearby-something the size of a plate,
maybe, hanging in the sky a little above our heads. Ancient Greek philosophers debated
the proposition “that the Moon is exactly as large as it looks” (betraying a
hopeless confusion between linear and angular size). Walking on the Moon would
have seemed a screwball idea; it made more sense to imagine somehow climbing up
into the sky on a ladder or on the back of a giant bird, grabbing the Moon, and
bringing it down to Earth. Nobody ever succeeded, although there were myths
aplenty about heroes who had tried.
Not until a few centuries ago did the
idea of the Moon as a place, a quarter-million miles away, gain wide currency.
And in that brief flicker of time, we've gone from the earliest steps in
understanding the Moon's nature to walking and joy-riding on its surface. We
calculated how objects move in space; liquefied oxygen from the air; invented
big rockets, telemetry, reliable electronics, inertial guidance, and much else.
Then we sailed out into the sky.
I was lucky enough to be involved in the
Apollo program, but I don't blame people who think the whole thing was faked in
a Hollywood movie studio. In the late Roman Empire, pagan philosophers had
attacked Christian doctrine on the ascension to Heaven of the body of Christ
and on the promised bodily resurrection of the dead-because the force of
gravity pulls down all “earthly bodies.” St. Augustine rejoined: “If human
skill can by some contrivance fabricate vessels that float, out of metals which
sink... how much more credible is it that God, by some hidden mode of
operation, should even more certainly effect that these earthly masses be
emancipated” from the chains that bind them to Earth? That humans should one
day discover such a “mode of operation” was beyond imagining. Fifteen hundred
years later, we emancipated ourselves.
The achievement elicited an amalgam of
awe and concern. Some remembered the story of the Tower of Babel. Some,
orthodox Moslems among them, felt setting foot on the Moon's surface to be
impudence and sacrilege. Many greeted it as a turning point in history.
The Moon is no longer unattainable. A
dozen humans, all Americans, have made those odd bounding motions they called
“moonwalks” on the crunchy, cratered, ancient gray lava-beginning on that July
day in 1969. But since 1972, no one from any nation has ventured back. Indeed,
none of us has gone anywhere since the glory days of Apollo except into low
Earth orbit-like a toddler who takes a few tentative steps outward and then,
breathless, retreats to the safety of his mother's skirts.
Once upon a time, we soared into the
Solar System. For a few years. Then we hurried back. Why? What happened? What
was Apollo really about?
The scope and audacity of John Kennedy's
May 25, 1961 message to a joint session of Congress on “Urgent National
Needs”-the speech that launched the Apollo program-dazzled me. We would use
rockets not yet designed and alloys not yet conceived, navigation and docking
schemes not yet devised, in order to send a man to an unknown world-a world not
yet explored, not even in a preliminary way, not even by robots-and we would
bring him safely back, and we would do it before the decade was over. This
confident pronouncement was made before any American had even achieved Earth
orbit.
As a newly minted Ph. D., I actually
thought all this had something centrally to do with science. But the President
did not talk about discovering the origin of the Moon, or even about bringing
samples of it back for study. All he seemed to be interested in was sending
someone there and bringing him home. It was a kind of gesture. Kennedy's
science advisor, Jerome Wiesner, later told me he had made a deal with the
President: If Kennedy would not claim that Apollo was about science, then he.
Wiesner, would support it. So if not science, what?
The Apollo program is really about
politics, others told me. This sounded more promising. Nonaligned nations would
be tempted to drift toward the Soviet Union if it was ahead in space
exploration, if the United States showed insufficient “national vigor.” I
didn't follow. Here was the United States, ahead of the Soviet Union in
virtually every area of technology-the world's economic, military, and, on
occasion, even moral leader-and Indonesia would go Communist because Yuri
Gagarin beat John Glenn to Earth orbit? What's so special about space
technology? Suddenly I understood.
Sending people to orbit the Earth or
robots to orbit the Sun requires rockets-big, reliable, powerful rockets. Those
same rockets can be used for nuclear war. The same technology that transports a
man to the Moon can carry nuclear warheads halfway around the world. The same technology
that puts an astronomer and a telescope in Earth orbit can also put up a laser
“battle station.” Even back then, there was fanciful talk in military circles,
East and West, about space as the new “high ground,” about the nation that
“controlled” space “controlling” the Earth. Of course strategic rockets were
already being tested on Earth. But heaving a ballistic missile with a dummy
warhead into a target zone in the middle of the Pacific Ocean doesn't buy much
glory. Sending people into space captures the attention and Imagination of the
world.
You wouldn't spend the money to launch
astronauts for this reason alone, but of all the ways of demonstrating rocket
potency, this one works best. It was a rite of national manhood; the shape of
the boosters made this point readily understood without anyone actually having
to explain it. The communication seemed to be transmitted from unconscious mind
to unconscious mind without the higher mental faculties catching a whiff of
what was going on.
My colleagues today-struggling for every
space science dollar-may have forgotten how easy it was to get money for
“space” in the glory days of Apollo and just before. Of many examples, consider
this exchange before the Defense Appropriations Subcommittee of the House of Representatives
in 1958, only a few months after Sputnik 1. Air Force Assistant Secretary
Richard E. Horner is testifying; his interlocutor is Rep. Daniel J. Flood
(Democrat of Pennsylvania):
HORNER: [W]hy is it desirable from the
military point of view to have a man on the moon? Partly, from the classic
point of view, because it is there. Partly because we might be afraid that the
U. S. S. R. might get one there first and realize advantages which we had not
anticipated existed there...
FLOOD: [I]f we gave you all the money
you said was necessary, regardless of how much it was, can you in the Air Force
hit the moon with something, anything, before Christmas?
HORNER: I feel sure we can. There is
always a certain amount of risk in this kind of undertaking, but we feel that
we can do that; yes, sir.
FLOOD: Have you asked anybody in the Air
Force or the Department of Defense to give you enough money, hardware, and
people, starting at midnight tonight, to chip a. piece out of that ball of
green cheese for a Christmas present to Uncle Sam? Have you asked for that?
HORNER: We have submitted such a program
to the Office of the Secretary of Defense. It is currently under consideration.
FLOOD: I am for giving it to them as of
this minute, Mr. Chairman, with our supplemental, without waiting for somebody
downtown to make up his mind to ask for it. If this man means what he says and
if he knows what he is talking about-and I think he does-then this committee
should not wait five minutes more today. We should give him all the money and
all the hardware and all the people he wants, regardless of what anybody else
says or wants, and tell him to go up on top of some hill and do it without any
question.
When President Kennedy formulated the
Apollo program, the Defense Department had a slew of space projects under
development-ways of carrying military personnel up into space, means of
conveying them around the Earth, robot weapons on orbiting platforms intended
to shoot down satellites and ballistic missiles of other nations. Apollo
supplanted these programs. They never reached operational status. A case can be
made then that Apollo served another purpose-to move the U. S. -Soviet space
competition from a military to a civilian arena. There are some who believe
that Kennedy intended Apollo as a substitute for an anus race in space. Maybe.
For me, the most ironic token of that
moment in history is the plaque signed by President Richard M. Nixon that
Apollo 11 took to the Moon. It reads: “We came in peace for all mankind.” As
the United States was dropping 71/2 megatons of conventional explosives on
small nations in Southeast Asia, we congratulated ourselves on our humanity: We
would harm no one on a lifeless rock. That plaque is there still, attached to
the base of the Apollo 11 Lunar Module, on the airless desolation of the Sea of
Tranquility. If no one disturbs it, it will still be readable a million years
from now.
Six more missions followed Apollo 11,
all but one of which successfully landed on the lunar surface. Apollo 17 was
the first to carry a scientist. As soon as he got there, the program was
canceled. The first scientist and the last human to land on the Moon were the
same person. The program had already served its purpose that July night in
1969. The half-dozen subsequent missions were just momentum.
Apollo was not mainly about science. It
was not even mainly about space. Apollo was about ideological confrontation and
nuclear war-often described by such euphemisms as world “leadership” and
national “prestige.” Nevertheless, good space science was done. We now know
much more about the composition, age, and history of the Moon and the origin of
the lunar landforms. We have made progress in understanding where the Moon came
from. Some of us have used lunar cratering statistics to better understand the
Earth at the time of the origin of life. But more important than any of this,
Apollo provided an aegis, an umbrella under which brilliantly engineered robot
spacecraft were dispatched throughout the Solar System, making that preliminary
reconnaissance of dozens of worlds. The offspring of Apollo have now reached
the planetary frontiers.
If not for Apollo-and, therefore, if not
for the political purpose it served-I doubt whether the historic American
expeditions of exploration and discovery throughout the Solar System would have
occurred, The Mariners, Vikings, Pioneers, Voyagers ,and Galileo are among the
gifts of Apollo. Magellan and Cassini are more distant descendants. Something
similar is true for the pioneering Soviet efforts in Solar System exploration,
including the first soft landings of robot spacecraft-Luna 9, Mars 3, Venera
8-on other worlds.
Apollo conveyed a confidence, energy,
and breadth of vision that did capture the imagination of the world. That too
was part of its purpose. It inspired an optimism about technology, an
enthusiasm for the future. If we could fly to the Moon, as so many have asked,
what else were we capable of? Even those who opposed the policies and actions
of the United States-even those who thought the worst of us-acknowledged the
genius and heroism of the Apollo program. With Apollo, the United States
touched greatness.
When you pack your bags for a big trip,
you never know what's in store for you. The Apollo astronauts on their way to
and from the Moon photographed their home planet. It was a natural thing to do,
but it had consequences that few foresaw. For the first time, the inhabitants
of Earth could see their world from above-the whole Earth, the Earth in color,
the Earth as an exquisite spinning white and blue ball set against the vast
darkness of space. Those images helped awaken our slumbering planetary
consciousness. They provide incontestable evidence that we all share the same
vulnerable planet. They remind us of what is important and what is not. They
were the harbingers of Voyager's pale blue dot.
We may have found that perspective just
in time, just as our technology threatens the habitability of our world.
Whatever the reason we first mustered the Apollo program, however mired it was
in Cold War nationalism and the instruments of death, the inescapable
recognition of the unity and fragility of the Earth is its clear and luminous
dividend, the unexpected final gift of Apollo. What began in deadly competition
has helped us to see that global cooperation is the essential precondition for
of survival.
Travel is broadening.
It's time to hit the road again.
CHAPTER 14
Exploring other worlds and protecting this one
The planets, in their various stages of
development, are subjected to the same formative forces that operate on our
earth, and have, therefore, the same geologic formation, and probably life, of
our own past, and perhaps future; but. further than this, these forces are
acting, in some cases, under totally different conditions from those under
which they operate on the earth, and hence must evolve forms different from
those ever known to man. The value of such material as this to the comparative
sciences is too obvious to need discussion.
—ROBERT H. GODDARD, NOTEBOOK (1907)
For the first time in my life, I saw the
horizon as a curved line. It was accentuated by a thin seam of dark blue
light-our atmosphere. Obviously, this was not the “ocean” of air I had been
told it was so many times in my life. I was terrified by its fragile appearance.
—ULF MERBOLD, GERMAN SPACE SHUTTLE
ASTRONAUT (1988)
When you look down at the Earth from
orbital altitudes, you see a lovely, fragile world embedded in black vacuum.
But peering at a piece of the Earth through a spacecraft porthole is nothing
like the joy of seeing it entire against the backdrop of black,
or-better-sweeping across your field of view as you float in space unencumbered
by a spacecraft. The first human to have this experience was Alexei Leonov, who
on March 18, 1965, left Voskhod 2 in the original space “walk”: “I looked down
at the Earth,” he recalls, “and the first thought that crossed my mind was `The
world is round, after all. ' In one glance I could see from Gibraltar to the
Caspian Sea...I felt like a bird-with wings, and able to fly.”
When you view the Earth from farther
away, as the Apollo astronauts did, it shrinks in apparent size, until nothing
but a little geography remains. You're struck by how self-contained it is. An
occasional hydrogen atom leaves; a pitter-patter of cometary dust arrives.
Sunlight, generated in the immense, silent thermonuclear engine deep in the
solar interior, pours out of the Sun in all directions, and the Earth
intercepts enough of it to provide a little illumination and enough heat for
our modest purposes. Apart from that, this small world is on its own.
From the surface of the Moon you can see
it, perhaps as a crescent, even its continents now indistinct. And from the
vantage point of the outermost planet it is a mere point of pale light.
From Earth orbit, you are struck by the
tender blue arc of the horizon-the Earth's thin atmosphere seen tangentially.
You can understand why there is no longer such a thing as a local environmental
problem. Molecules are stupid. Industrial poisons, greenhouse gases, and substances
that attack the protective ozone layer, because of their abysmal ignorance, do
not respect borders. They are oblivious of the notion of national sovereignty.
And so, due to the almost mythic powers of our technology (and the prevalence
of short-term thinking), we are beginning-on Continental and on planetary
scales-to pose a danger to ourselves. Plainly, if these problems are to be
solved, it will require many nations acting in concert over many years.
I'm struck again by the irony that
spaceflight-conceived in tile cauldron of nationalist rivalries and
hatreds-brings with it a stunning transnational vision. You spend even a little
time contemplating the Earth from orbit and the most deeply engrained
nationalisms begin to erode. They seem the squabbles of mites on a plum.
If we're stuck on one world, we're
limited to a single case; we don't know what else is possible. Then-like an art
fancier familiar only with Fayoum tomb paintings, a dentist who knows only
molars, a philosopher trained merely in NeoPlatonism, a linguist who has
studied only Chinese, or a physicist whose knowledge of gravity is restricted
to falling bodies on Earth-our perspective is foreshortened, our insights
narrow, our predictive abilities circumscribed. By contrast, when we explore
other worlds, what once seemed the only way a planet could be turns out to be
somewhere in the middle range of a vast spectrum of possibilities. When we look
at those other worlds, we begin to understand what happens when we have too
much of one thing or too little of another. We learn how a planet can go wrong.
We gain a new understanding, foreseen by the spaceflight pioneer Robert
Goddard, called comparative planetology.
The exploration of other worlds has
opened our eyes in the study of volcanos, earthquakes, and weather. It may one
day have profound implications for biology, because all life on Earth is built
on a common biochemical master plan. The discovery of a single extraterrestrial
organism-even something as humble as a bacterium-would revolutionize our
understanding of living things. But the connection between exploring other
worlds and protecting this one is most evident in the study of Earth's climate
and the burgeoning threat to that climate that our technology poses. Other
worlds provide vital insights about what dumb things not to do on Earth.
Three potential environmental
catastrophes-all operating on a global scale-have recently been uncovered:
ozone layer depletion, greenhouse warming, and nuclear winter. All three
discoveries, it turns out, have strong ties to the exploration of the planets.
(1) It was disturbing to find that an
inert material with all sorts of practical applications-it serves as the
working fluid in refrigerators and air conditioners, as aerosol propellant for
deodorants and other products, as lightweight foamy packaging for fast foods,
and as a cleaning agent in microelectronics, to name only a few-can pose a
danger to life on Earth. Who would have figured?
The molecules in question are called
chlorofluorocarbons (CFCs). Chemically, they're extremely inert, which means
they're invulnerable-until they find themselves up in the ozone layer, where
they're broken apart by ultraviolet light from the Sun. The chlorine atoms thus
liberated attack and break down the protective ozone, letting more ultraviolet
light reach the ground. This increased ultraviolet intensity ushers in a
ghastly procession of potential consequences involving not just skin cancer and
cataracts, but weakening of the human immune system and, most dangerous of all,
possible harm to agriculture and to photosynthetic organisms at the base of the
food chain on which most life on Earth depends.
Who discovered that CFCs posed a threat
to the ozone layer? Was it the principal manufacturer, the DuPont Corporation,
exercising corporate responsibility? Was it the Environmental Protection Agency
protecting us? Was it the Department of Defense defending us? No, it was two
Ivory-tower, white-coated university scientists working on something
else-Sherwood Rowland and Mario Molina of the University of California, Irvine.
Not even an Ivy League university. no one instructed them to look for dangers
to the environment. They were pursuing fundamental research. They were
scientists following their own interests. Their names should be known to every
schoolchild.
In their original calculations, Rowland
and Molina used rate constants of chemical reactions involving chlorine and
other halogens that had been measured in part with NASA support. Why NASA?
Because Venus has chlorine and fluorine molecules in its atmosphere, and
planetary aeronomers had wanted to understand what's happening there.
Confirming theoretical work on the role
of CFCs in ozone depletion was soon done by a group led by Michael McElroy at
Harvard How is it they had all these branching networks of halogen chemical
kinetics in their computer ready to go? Because they were working on the
chlorine and fluorine chemistry of the atmosphere of Venus. Venus helped make
and helped confirm the discovery that the Earth's ozone layer is in danger. An
entirely unexpected connection was found between the atmospheric
photochemistries of the two planets. A result of importance to everyone on
Earth emerged from what might well have seemed the most blue-sky, abstract,
impractical kind of work, understanding the chemistry of minor constituents in
the upper atmosphere of another world.
There's also a Mars connection. With
Viking we found the surface of Mars to be apparently lifeless and remarkably
deficient even in simple organic molecules. But simple organic molecules ought
to be there, because of the impact of organic-rich meteorites from the nearby
asteroid belt. This deficiency is widely attributed to the lack of ozone on
Mars. The Viking microbiology experiments found that organic matter carried
from Earth to Mars and sprinkled on Martian surface dust is quickly oxidized
arid destroyed. The materials in the dust that do the destruction are molecules
something like hydrogen peroxide-which we use as an antiseptic because it kills
microbes by oxidizing there, Ultraviolet light from the Sun strikes the surface
of Mars unimpeded by an ozone layer; if any organic matter were there, it would
be quickly destroyed by the ultraviolet light itself and its oxidation
products. Thus part of the reason the topmost layers of Martian soil are
antiseptic is that Mars has an ozone hole of planetary dimensions-by itself a
useful cautionary tale for us, who are busily thinning and puncturing our ozone
layer.
(2) Global warming is predicted to
follow from the increasing greenhouse effect caused largely by carbon dioxide
generated in the burning of fossil fuels-but also from the buildup of other
infrared-absorbing gases (oxides of nitrogen, methane, those same CFCs, and
other molecules).
Suppose that we have a three-dimensional
general circulation computer model of the Earth's climate. Its programmers
claim it's able to predict what the Earth will be like if there's more of one
atmospheric constituent or less of another. The model does very well at
“predicting” the present climate. But there is a nagging worry: The model has
been “tuned” so it will come out right-that is, certain adjustable parameters
are chosen, not from first principles of physics, but to get the right answer.
This is not exactly cheating, but if we apply the same computer model to rather
different climatic regimes-deep global warming, for instance-the tuning might
then be inappropriate. The model might be valid for today's climate, but not
extrapolatable to others.
One way to test this program is to apply
it to the very different climates of other planets. Can it predict the
structure of the atmosphere on Mars and the climate there? The weather? What
about Venus? If it were to fail these test cases, we would be right in
mistrusting it when it makes predictions for our own planet. In fact, climate
models now in use do very well in predicting from first principles of physics
the climates on Venus and Mars.
On Earth, huge upwellings of molten lava
are known and attributed to superplumes convecting up from the deep mantle and
generating vast plateaus of frozen basalt. A spectacular example occurred about
a hundred million years ago, and added perhaps ten times the present carbon
dioxide content to the atmosphere, inducing substantial global warming. These
plumes, it is thought, occur episodically throughout Earth's history. Similar
mantle upwelling seem to have occurred on Mars and Venus. There are sound
practical reasons for us to want to understand how a major change to the
Earth's surface and climate could suddenly arrive unannounced from hundreds of
kilometers beneath our feet.
Some of the most important recent work
on global warming has been done by James Hansen and his colleagues at the
Goddard Institute for Space Sciences, a NASA facility in New York City. Hansen
developed one of the major computer climate models and employed it to predict
what will happen to our climate as the greenhouse gases continue to build up.
He has been in the forefront of testing these models against ancient climates of
the Earth. (During the last ice ages, it is of interest to note, more carbon
dioxide and methane are strikingly correlated with higher temperatures.) Hansen
collected a wide range of weather data from this century and last, to see what
actually happened to the global temperature, and then compared it to the
computer model's predictions of what should have happened. The two agree to
within the errors of measurement and calculation, respectively. He courageously
testified before Congress in the face of a politically generated order from the
White House Office of Management and Budget (this was in the Reagan years) to
exaggerate the uncertainties and minimize the dangers. His calculation on the
explosion of the Philippine volcano Mt. Pinatubo and his prediction of the
resulting temporary decline in the Earth's temperature (about half a degree
Celsius) were right on the money. He has been a force in convincing governments
worldwide that global warming is something to be taken seriously.
How did Hansen get interested in the
greenhouse effect in the first place? His doctoral thesis (at the University of
Iowa in 1967) was about Venus. He agreed that the high radio brightness of
Venus is due to a very hot surface, agreed that greenhouse gases keep the heat
in, but proposed that heat from the interior rather than sunlight was the
principal energy source. The Pioneer 12 mission to Venus in 1978 dropped entry
probes into the atmosphere; they showed directly that the ordinary greenhouse
effect-the surface heated by the Sun and the heat retained by the blanket of
air-was the operative cause. But it's Venus that got Hansen thinking about the
greenhouse effect.
Radio astronomers, you note, find Venus
to be an intense source of radio waves. Other explanations of the radio emission
fail. You conclude that the surface must be ridiculously hot. You try to
understand where the high temperatures come from and are led inexorably to one
or another kind of greenhouse effect. Decades later you find that this training
has prepared you to understand and help predict an unexpected threat to our
global civilization. I know many other instances where scientists who first
tried to puzzle out the atmospheres of other worlds are making important and
highly practical discoveries about this one. The other planets are a superb
training ground for students of the Earth. They require both breadth and depth
of knowledge, and they challenge the imagination.
Those who are skeptical about carbon
dioxide greenhouse warming might profitably note the massive greenhouse effect
on Venus. no one proposes that Venus's greenhouse effect derives froth
imprudent Venusians who burned too much coal, drove fuel-inefficient autos, and
cut down their forests. My point is different. The climatological history of
our planetary neighbor, an otherwise Earthlike planet on which the surface
became hot enough to melt tin or lead, is worth considering-especially by those
who say that the increasing greenhouse effect on Earth will be self-correcting,
that we don't really have to worry about it, or (you can see this in the
publications of some groups that call themselves conservative) that the
greenhouse effect itself is a “hoax.”
(3) Nuclear winter is the predicted
darkening and cooling of the Earth-mainly from fine smoke particles injected
into the atmosphere from the burning of cities and petroleum facilities-that is
predicted to follow a global thermonuclear war. A vigorous scientific debate
ensued on just how serious nuclear winter might be. The various opinions have
now converged. All three-dimensional general circulation computer models
predict that the global temperatures resulting from a worldwide thermonuclear
war would be colder than those in the Pleistocene ice ages. The implications
for our planetary civilization-especially through the collapse of
agriculture-are very dire. It is a consequence of nuclear war that was somehow
overlooked by the civil and military authorities of the United States, the
Soviet Union, Britain, France, and China when they decided to accumulate well
over 60,000 nuclear weapons. Although it's hard to be certain about such
things, a case can be made that nuclear Winter played a constructive role
(there were other causes, of course) in convincing the nuclear-armed nations,
especially the Soviet Union, of the futility of nuclear war.
Nuclear winter was first calculated and
named in 1982/83 by a group of five scientists, to which I'm proud to belong.
This team was given the acronym TTAPS (for Richard P. Turco, (even B. Toon,
Thomas Ackerman, James Pollack, and myself). Of the five TTAPS scientists, two
were planetary scientists, and the other three had published many papers in
planetary science, The earliest intimation of nuclear winter came during that
same Mariner 9 mission to Mars, when there was a global dust storm and we were
unable to see the surface of the planet; the infrared spectrometer on the
spacecraft found the high atmosphere to be warmer and the surface colder than
they ought to have been. Jim Pollack and I sat down and tried to calculate how
that could come about. Over the subsequent twelve years, this line of inquiry
led from dust storms on Mars to volcanic aerosols on Earth to the possible
extinction of the dinosaurs by impact dust to nuclear winter. You never know
where science will take you.
PLANETARY SCIENCE fosters a broad
interdisciplinary point of view that proves enormously helpful in discovering
and attempting to defuse these looming environmental catastrophes. When you cut
your teeth on other worlds, you gain a perspective about the fragility of
planetary environments and about what other, quite different, environments are
possible. There may well be potential global catastrophes still to be
uncovered. If there are, I bet planetary scientists will play a central role in
understanding them.
Of all the fields of mathematics,
technology, and science, the one with the greatest international cooperation
(as determined by how often the co-authors of research papers hail from two or
more countries) is the field called “Earth and space sciences.” Studying this
world and others, by its very nature, tends to be non-local, non-nationalist,
non-chauvinist. Very rarely do people go into these fields because they are
internationalists. Almost always, they enter for other reasons, and then
discover that splendid work, work that complements their own, is being done by
researchers in other nations; or that to solve a problem, you need data or a
perspective (access to the southern sky, for example) that is unavailable in
your country. And once you experience such cooperation-humans from different
parts of the planet working in a mutually intelligible scientific language as
partners on matters of common concern-it's hard not to imagine it happening on
other, nonscientific matters. I myself consider this aspect of Earth and space
sciences as a healing and unifying force in world politics; but, beneficial or
not, it is inescapable.
When I look at the evidence, it seems to
me that planetary exploration is of the most practical and urgent utility for
us here on Earth. Even if we were not roused by the prospect of exploring other
worlds, even if we didn't have a nanogram of adventuresome spirit in us, even
if we were only concerned for ourselves and in the narrowest sense, planetary
exploration would still constitute a superb investment.
CHAPTER 15
The gates of the wonder world open
The great floodgates of the wonder-world
swung open.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 1
(1851)
Sometime coming up, perhaps just around
the corner, there will be a nation-more likely, a consortium of nations-that
will work the next major step in the human venture into space. Perhaps it will
be brought about by circumventing bureaucracies and making efficient use of
present technologies. Perhaps it will require new technologies, transcending
the great blunderbuss chemical rockets. The crews of these ships will set foot
on new worlds. The first baby will be born somewhere up there. Early steps
toward living off the land will be made. We will be on our way. And the future
will remember.
TANTALIZING AND MAJESTIC, Mars is the
world next door, the nearest planet on which an astronaut or cosmonaut could
safely land. Although it is sometimes as warm as a New England October, Mars is
a chilly place, so cold that some of its thin carbon dioxide atmosphere freezes
out as dry ice at the winter pole.
It is the nearest planet whose surface
we can see with a small telescope. In all the Solar System, it is the planet
most like Earth. Apart from flybys, there have been only two fully successful
missions to Mars: Mariner 9 in 1971, and Vikings 1 and 2 in 1976. They revealed
a deep rift valley that would stretch from New York to San Francisco; immense
volcanic mountains, the largest of which towers 80,000 feet above the average
altitude of the Martian surface, almost three times the height of Mount
Everest; an intricate layered structure in and among the polar ices, resembling
a pile of discarded poker chips, and probably a record of past climatic change;
bright and dark streaks painted down on the surface by windblown dust,
providing high-speed wind maps of Mars over the past decades and centuries;
vast globe-girdling dust storms; and enigmatic surface features.
Hundreds of sinuous channels and valley
networks dating back several billion years can be found, mainly in the cratered
southern highlands. They suggest a previous epoch of more benign and Earthlike
conditions-very different from what we find beneath the tenuous and frigid
atmosphere of our time. Some ancient channels seem to have been carved by
rainfall, some by underground sapping and collapse, and some by great floods
that gushed up out of the ground. Rivers were pouring into and filling great
thousand-kilometer-diameter impact basins that today are dry as dust.
Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars.
Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently
lapped shorelines barely discernible today. That would have been a world to
explore. We are four billion years late.
On Earth in just the same period, the
first microorganisms arose and evolved. Life on Earth is intimately connected,
for the most basic chemical reasons, with liquid water. We humans are ourselves
made of some three-quarters water. The same sorts of organic molecules that
fell out of the sky and were generated in the air and seas of ancient Earth,
should also have accumulated on ancient Mars. Is it plausible that life quickly
came to be in the waters of early Earth, but was somehow restrained and
inhibited in the waters of early Mars? Or might the Martian seas have been
filled with life-floating, spawning, evolving? What strange beasts once swum
there?
Whatever the dramas of those distant
times, it all started to go wrong around 3. 8 billion years ago. We can see
that the erosion of ancient craters dramatically began to slow about then. As
the atmosphere thinned, as the rivers flowed no more, as the oceans began to
dry, as the temperatures plummeted, life would have retreated to the few
remaining congenial habitats, perhaps huddling at the bottom of ice-covered
lakes, until it too vanished and the dead bodies and fossil remains of exotic
organisms-built, it might be, on principles very different from life on
Earth-were deep-frozen, awaiting the explorers who might in some distant future
arrive on Mars.
METEORITES ARE FRAGMENTS OF OTHER WORLDS
recovered on Earth. Most originate in collisions among the numerous asteroids
that orbit the Sun between the orbits of Mars and Jupiter. But a few are
generated when a large meteorite impacts a planet or asteroid at high speed,
gouges out a crater, and propels the excavated surface material into space. A
very small fraction of the ejected rocks, millions of years later, may
intercept another world.
In the wastelands of Antarctica, the ice
is here and there dotted with meteorites, preserved by the low temperatures and
until recently undisturbed by humans. A few of them, called SNC (pronounced
“snick”) meteorites§§§§§ have an aspect about them that at first seemed almost
unbelievable: Deep inside their mineral and glassy structures, locked away from
the contaminating influence of the Earth's atmosphere, a little gas is trapped.
When the gas is analyzed, it turns out to have exactly the same chemical
composition and isotopic ratios as the air on Mars. We know about Martian air
not just from spectroscopic inference but from direct measurement on the
Martian surface by the Viking landers. To the surprise of nearly everyone, the
SNC meteorites come from Mars.
Originally, they were rocks that had
melted and refrozen. Radioactive dating of all the SNC meteorites shows their
parent rocks condensed out of lava between 180 million and 1. 3 billion years
ago. Then they were driven off the planet by collisions from space. From how
long they've been exposed to cosmic rays on their interplanetary journeys
between Mars and Earth, we can tell how old they are-how long ago they were
ejected from Mars. In this sense, they are between 10 million and 700,000 years
old. They sample the most recent 0. l percent of Martian history.
Some of the minerals they contain show
clear evidence of having once been in water, warm liquid water. These
hydro-thermal minerals reveal that somehow, probably all over Mars, there was
recent liquid water. Perhaps it came about when the interior heat melted
underground ice. But however it happened, it's natural to wonder if life is not
entirely extinct, if somehow it's managed to hang on into our time in transient
underground lakes, or even in thin films of water wetting subsurface grains.
The geochemists Everett Gibson and Hal
Karlsson of NASA's Johnson Space Flight Center have extracted a single drop of
water from one of the SNC meteorites. The isotopic ratios of the oxygen and
hydrogen atoms that it contains are literally unearthly. I look on this water
from another world as an encouragement for future explorers and settlers.
Imagine what we might find if a large
number of samples, including never melted soil and rocks, were returned to
Earth from Martian locales selected for their scientific interest. We are very
close to being able to accomplish this with small roving robot vehicles.
The transportation of subsurface
material from world to world raises a tantalizing question: Four billion years
ago there were two neighboring planets, both warm, both wet. Impacts from
space, in the final stages of the accretion of these planets, were occurring at
a much higher rate than today. Samples from each world were being flung out
into space. We are sure there was life on at least one of them in this period.
We know that a fraction of the ejected debris stays cool throughout the
processes of impact, ejection, and interception by another world. So could some
of the early organisms on Earth have been safely transplanted to Mars four
billion years ago, initiating life on that planet' Or, even more speculative,
could life on Earth have arisen by such a transfer from Mars? Might the two
planets have regularly exchanged life-forms for hundreds of millions of years?
The notion might be testable. If we were to discover life on Mars and found it
very similar to life on Earth-and if, as well, e were sure it wasn't microbial
contamination that we ourselves had introduced in the course of our
explorations-the proposition that life was long ago transferred across
interplanetary space would have to be taken seriously.
IT WAS ONCE THOUGHT that life is
abundant on Mars. Even the dour and skeptical astronomer Simon Newcomb (in his
Astronomy for Everybody, which went through many editions in the early decades
of this century and was the astronomy text of my childhood) concluded, “There
appears to be life on the planet Mars. A few years ago this statement was
commonly regarded as fantastic. Now it is commonly accepted.” Not “intelligent
human life,” he was quick to add, but green plants. However, we have now been
to Mars and looked for plants-as well as animals, microbes, and intelligent
beings. Even if the other forms were absent, we might have imagined, as in
Earth's deserts today, and as on Earth for almost all its history, abundant
microbial life.
The “life detection” experiments on
Viking were designed to detect only a certain subset of conceivable biologies;
they were biased to find the kind of life about which we know. It would have
been foolish to send instruments that could not even detect life on Earth. They
were exquisitely sensitive, able to find microbes in the most unpromising, arid
deserts and wastelands on Earth.
One experiment measured the gases
exchanged between Martian soil and the Martian atmosphere in the presence of
organic matter from Earth. A second brought a wide variety organic foodstuffs
marked by a radioactive tracer to see if there were bugs in the Martian soil
who ate the food and oxidized it to radioactive carbon dioxide. A third
experiment introduced radioactive carbon dioxide (and carbon monoxide) to the
Martian soil to see if any of it was taken up by Martian microbes. To the
initial astonishment of, I think, all the scientists involved, each of the
three experiments gave what at first seemed to be positive results. Gases were
exchanged; organic matter was oxidized; carbon dioxide was incorporated into
the soil.
But there are reasons for caution. These
provocative results are not generally thought to be good evidence for life on
Mars: The putative metabolic processes of Martian microbes occurred under a
very wide range of conditions inside the Viking landers-wet (with liquid water
brought from Earth) and dry, light and dark, cold (only a little above
freezing) to hot (almost the normal boiling point of water). Many
microbiologists deem it unlikely that Martian microbes would be so capable
under such varied conditions. Another strong inducement to skepticism is that a
fourth experiment, to look for organic chemicals in the Martian soil, gave uniformly
negative results despite its sensitivity. We expect life on Mars, like life on
Earth, to be organized around carbon-based molecules. To find no such molecules
at all was daunting for optimists among the exobiologists.
The apparently positive results of the
life detection experiments is now generally attributed to chemicals that
oxidize the soil, deriving ultimately from ultraviolet sunlight (as discussed
in the previous chapter). There is still a handful of Viking scientists who
wonder if there might be extremely tough and competent organisms very thinly
spread over the Martian soil-so their organic chemistry could not be detected,
but their metabolic processes could. Such scientists do not deny that
ultraviolet-generated oxidants are present in the Martian soil, but stress that
no thorough explanation of the liking life detection results from oxidants
alone has been forthcoming. Tentative claims have been made of organic matter
in SNC meteorites, but they seem instead to be contaminants that have entered
the meteorite after its arrival on our world. So far, there are no claims of
Martian microbes in these rocks from the sky.
Perhaps because it seems to pander to
public interest, NASA and most Viking scientists have been very chary about
pursuing the biological hypothesis. Even now, much more could be done in going
over the old data, in looking with Viking-type instruments at Antarctic and
other soils that have few microbes in them, in laboratory simulation of the
role of oxidants in the Martian soil, and in designing experiments to elucidate
these matters-not excluding further searches for life-with future Mars landers.
If indeed no unambiguous signatures of
life were determined by a variety of sensitive experiments at two sites 5,000
kilometers apart on a planet marked by global wind transport of fine particles,
this is at least suggestive that Mars may be, today at least, a lifeless
planet. But if Mars is lifeless, we have two planets, of virtually identical
age and early conditions, evolving next door to one another in the same polar
system: Life evolves and proliferates on one, but not the other. Why?
Perhaps the chemical or fossil remains
of early Martian life can still be found-subsurface, safely protected from the
ultraviolet radiation and its oxidation products that today fry the surface.
Perhaps in a rock face exposed by a landslide, or in the banks of an ancient
river valley or dry lake bed, or in the polar, laminated terrain, key evidence
for life on another planet is waiting.
Despite its absence on the surface of
Mars, the planet's two moons, Phobos and Deimos, seem to be rich in complex
organic matter dating back to the early history of the Solar System. The Soviet
Phobos 2 spacecraft found evidence of water vapor being out-gassed from Phobos,
as if it has an icy interior heated by radioactivity. The moons of Mars may
have long ago been captured from somewhere in the outer Solar System;
conceivably, they are among the nearest available examples of unaltered stuff
from the earliest days of the Solar System. Phobos and Deimos are very small,
each roughly 10 kilometers across; the gravity they exert is nearly negligible.
So it's comparatively easy to rendezvous with them, land on them, examine them,
use them as a base of operations to study Mars, and then go home.
Mars calls, a storehouse of scientific
information-important in its own right but also for the light it casts on the
environment of our own planet. There are mysteries waiting to be resolved about
the interior of Mars and its mode of origin, the nature of volcanos on a world
without plate tectonics, the sculpting of landforms on a planet with sandstorms
undreamt of on Earth, glaciers and polar landforms, the escape of planetary
atmospheres, and the capture of moons-to mention a more or less random sampling
of scientific puzzles. If Mars once had abundant liquid water and a clement
climate, what went wrong? How did an Earthlike world become so parched, frigid,
and comparatively airless? Is there something here we should know about our own
planet?
We humans have been this way before. The
ancient explorers would have understood the call of Mars. But mere scientific
exploration does not require a human presence. We tan always send smart robots.
They are far cheaper, they don't talk back, you can send them to much more
dangerous locales, arid, with some chance of mission failure always before us,
no lives are put at risk.
“HAVE YOU SEEN ME?” the back of the milk
carton read. “Mars Observer, 6' x 4. 5' x 3', 2500 kg. Last heard from on
8/21/93, 627,000 km from Mars.”
“M. O. call home” was the plaintive
message on a banner hung outside the jet Propulsion Laboratory's Mission
Operations Facility in late August 1993. The failure of the United States' Mars
Observer spacecraft just before it was to insert itself into orbit around Mars
was a great disappointment. It was the first post-launch mission failure of an
American lunar or planetary spacecraft in 26 years. Many scientists and
engineers had devoted a decade of their professional lives to M. O. It was the
first U. S. mission to Mars in 17 years-since Viking's two orbiters and two
landers in 1976. It was also the first real post-Cold War spacecraft: Russian
scientists were on several of the investigator teams, and Mars Observer was to
act as an essential radio relay link for larders from what was then scheduled
to be the Russian Mars '94 mission, as well as for a daring rover and balloon
mission slated for Mars '96.
The scientific instruments aboard Mars
Observer would have napped the geochemistry of the planet and prepared the way
for future missions, guiding landing site decisions. It might have cast a new
light on the massive climate change that seems to have occurred in early
Martian history. It would have photographed some of the surface of Mars with
detail better than two meters across. Of course, we do not know what wonders
Mars Observer would have uncovered. But every time we examine a world with new
instruments and in vastly improved detail, a dazzling array of discoveries
emerges just as it did when Galileo turned the first telescope toward the
heavens and opened the era of modern astronomy.
According to the Commission of Inquiry,
the cause of the failure was probably a rupture of the fuel tank during
pressurization, gases and liquids sputtering out, and the wounded spacecraft
spinning wildly out of control. Perhaps it was avoidable. Perhaps it was an
unlucky accident. But to keep this matter in perspective, let's consider the
full range of missions to the Moon and the planets attempted by the United
States and the former Soviet Union:
In the beginning, our track records were
poor. Space vehicles blew up at launch, missed their targets, or failed to
function when they got there. As time went on, we humans got ;)otter at
interplanetary flight. There was a learning curve. The ,adjacent figures show
these curves (based on NASA data with NASA definitions of mission success). We
learned very well. Our present ability to fix spacecraft in flight is best
illustrated by the Voyager missions described earlier.
We see that it wasn't until about its
thirty-fifth launch to the Moon or the planets that the cumulative U. S.
mission success rate got as high as 50 percent. The Russians took about 50
launches to get there. Averaging the shaky start and the better recent
performance, we find that both the United States and Russia have a cumulative
launch success rate of about 80 percent. But the cumulative mission success
rate is still under 70 percent for the U. S. and under 60 percent for the U. S.
S. R. /Russia. Equivalently, lunar and planetary missions have failed on
average 30 or 40 percent of the time.
Missions to other worlds were from the
beginning at the cutting edge of technology. They continue to be so today. They
. ire designed with redundant subsystems, and operated by dedicated and
experienced engineers, but they are not perfect. The amazing thing is not that
we have done so poorly, but that we leave done so well.
We don't know whether the Mars Observer
failure was due to incompetence or just statistics. But we must expect a steady
background of mission failures when we explore other worlds. no human lives are
risked when a robot spacecraft is lost. Even if we were able to improve this
success rate significantly, it would be far too costly. It is much better to
take more risks and fly more spacecraft:.
Knowing about irreducible risks, why do
we these days fly only one spacecraft per mission? In 1962 Mariner 1, intended
for Venus, fell into the Atlantic; the nearly identical Mariner 2 became the
human species' first successful planetary mission. Mariner 3 failed, arid its
twin Mariner 4 became, in 1964, the first spacecraft to take close-up pictures
of Mars. Or consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars.
Mariner 8 Was to map the planet. Mariner 9 was to study the enigmatic seasonal
and secular changes of surface markings. The spacecraft were otherwise
identical. Mariner 8 fell into the ocean. Mariner 9 flew on to Mars arid became
the first spacecraft in human history to orbit another planet. It discovered
the volcanos, the laminated terrain in the polar caps, the ancient river
valleys, and the aeolian nature of the surface changes. It disproved the
“canals.” It mapped the planet pole to pole and revealed all the major
geological features of Mars known to us today. It provided the first close-up
observations of members of a whole class of small worlds (by targeting the
Martian moons, Phobos and Deimos). If we had launched only Mariner 8, the
endeavor would have been an unmitigated failure. With a dual launch it became a
brilliant and historic success.
There were also two Vikings, two
Voyagers, two Vegas, many pairs of Veneras. Why was only one Mars Observer
flown? The standard answer is cost. Part of the reason it was so costly,
though, is that it was planned to be launched by shuttle, which is an almost
absurdly expensive booster for planetary missions-in this case too expensive
for two M. O. launches. After many shuttle-connected delays and cost increases,
NASA changed its mind and decided to launch Mars Observer on a Titan booster.
This required an additional two-year delay and an adapter to mate the
spacecraft to the new launch vehicle. If NASA had not been so intent on
providing business for the increasingly uneconomic shuttle, we could have
launched a couple of years earlier and maybe with two spacecraft instead of
one.
But whether in single launches or in
pairs, the space-faring nations have clearly decided that the time is ripe to
return robot explorers to Mars. Mission designs change; new nations enter the
field; old nations find they no longer have the resources. Even already funded
programs cannot always be relied upon. But current plans do reveal something of
the intensity of effort and the depth of dedication.
As I write this book, there are
tentative plans by the United States, Russia, France, Germany, Japan, Austria,
Finland, Italy, Canada, the European Space Agency, and other entities for a
coordinated robotic exploration of Mars. In the seven years between 1996 and
2003, a flotilla of some twenty-five spacecraft-most of them comparatively
small and cheap-are to be sent from Earth to Mars. There will be no quick
flybys among them; these are all long-duration orbiter and lander missions. The
United States will re-fly all of the scientific instruments that were lost on
Mars Observer. The Russian spacecraft will contain particularly ambitious
experiments involving some twenty nations. Communications satellites will
permit experimental stations anywhere on Mars to relay their data back to
Earth. Penetrators screeching down from orbit will punch into the Martian soil,
transmitting data from underground. Instrumented balloons and roving
laboratories will wander over the sands of Mars. Some microrobots will weigh no
more than a few pounds. Landing sites are being planned and coordinated.
Instruments will be cross-calibrated. Data will be freely exchanged. There is
every reason to think that in the coming years Mars and its mysteries will
become increasingly familiar to the inhabitants of the planet Earth.
IN THE COMMAND CENTER on Earth, in a
special room, you are helmeted and gloved. You turn your head to the left, and
the cameras on the Mars robot rover turn to the left. You see, in very high
definition and in color, what the cameras see. You take a step forward, and the
rover walks forward. You reach out your arm to pick up something shiny in the
soil, and the robot arm does likewise. The sands of Mars trickle through your
fingers. The only difficulty with this remote reality technology is that all
this must occur in tedious slow motion: The round-trip travel time of the
up-link commands from Earth to Mars and the down-link data returned from Mars
to Earth might take half an hour or more. But this is something we can learn to
do. We can learn to contain our exploratory impatience if that's the price of
exploring Mars. The rover can be made smart enough to deal with routine
contingencies. Anything more challenging, and it makes a dead stop, puts itself
into a safeguard mode, and radios for a very patient human controller to take
over.
Conjure up roving, smart robots, each of
them a small scientific laboratory, landing in the safe but dull places and
wandering to view close-up some of that profusion of Martian Wonders. Perhaps
every day a robot would rove to its own horizon; each morning we would see
close-up what had yesterday been only a distant eminence. The lengthening
progress of a traverse route over the Martian landscape would appear oil news
programs and in schoolrooms. People would speculate on what will be found.
Nightly newscasts from another planet, with their revelations of new terrains
and new scientific findings would make everyone on Earth a party to the
adventure.
Then there's Martian virtual reality:
The data sent back from Mars, stored in a modern computer, are fed into your
helmet and gloves and boots. You are walking in an empty room on Earth, but to
you you are on Mars: pink skies, fields of boulders, sand dunes stretching to
the horizon where an immense volcano looms; you hear the sand crunching under
your boots, you turn rocks over, dig a hole, sample the thin air, turn a
corner, and come face to face with...whatever new discoveries we will make on
Mars-all exact copies of what's on Mars, and all experienced from the safety of
a virtual reality salon in your hometown. This is not why we explore Mars, but
clearly we will need robot explorers to return the real reality before it can
be reconfigured into virtual reality.
Especially with continuing investment in
robotics and machine intelligence, sending humans to Mars can't be justified by
science alone. And many more people can experience the virtual Mars than could
possibly be sent to the real one. We can do very well with robots. If we're
going to send people, we'll need a better reason than science and exploration.
In the 1980s, I thought I saw a coherent
justification for human missions to Mars. I imagined the United States and the
Soviet Union, the two Cold War rivals that had put our global civilization at
risk, joining together in a far-seeing, high-technology endeavor that would
give hope to people everywhere. I pictured a kind of Apollo program in reverse,
in which cooperation, not competition, was the driving force, in which tire two
leading space-faring nations would together lay the groundwork for a major
advance in human history-the eventual settlement of another planet.
The symbolism seemed so apt. The same
technology that can propel apocalyptic weapons from continent to continent
would enable the first human voyage to another planet. It was a choice of
fitting mythic power: to embrace the planet named after, rather the madness
ascribed to, the god of war.
We succeeded in interesting Soviet
scientists and engineers in such a joint endeavor. Roald Sagdeev, then director
of the Institute for Space Research of the Soviet Academy of Sciences in
Moscow, was already deeply engaged in international cooperation on Soviet
robotic missions to Venus, Mars, and Halley's Comet, long before the idea
became fashionable. Projected joint use of the Soviet Mir space station and the
Saturn V-class launch vehicle Energiya made cooperation attractive to the
Soviet organizations that manufactured these items of hardware; they were
otherwise having difficulty justifying their wares. Through a sequence of
arguments (helping to bring the Cold War to an end being chief among them),
then-Soviet leader Mikhail S. Gorbachev was convinced. During the December 1987
Washington summit, Mr. Gorbachev-asked what was the most important joint
activity through which the two countries might symbolize the change in their
relationship-unhesitatingly replied, “Let's go to Mars together.”
But the Reagan Administration was not
interested. Cooperating with the Soviets, acknowledging that certain Soviet
technologies were more advanced than their American counterparts, making some
American technology available to the Soviets, sharing credit, providing an
alternative for the arms manufacturers-these were not to the Administration's
liking. The offer was turned down. Mars would have to wait.
In only a few years, times have changed.
The Cold War is over. The Soviet Union is no more. The benefit deriving from
the two nations working together has lost some of its force. Other
nations-especially Japan and the constituent members of the European Space
Agency-have become interplanetary travelers. Many just and urgent demands are
levied on the discretionary budgets of the nations.
But the Energiya heavy-lift booster
still awaits a mission. The workhorse Proton rocket is available. The Mir space
station-with a crew on board almost continuously-still orbits the Earth every
hour and a half. Despite internal turmoil, the Russian space program continues
vigorously. Cooperation between Russia and America in space is accelerating. A
Russian cosmonaut, Sergei Krikalev, in 1994 flew on the shuttle Discovery (for
the usual one-week shuttle mission duration; Krikalev had already logged 464
days aboard the Mir space station). U. S. astronauts will visit Mir. American
instruments-including one to examine the oxidants thought to destroy organic
molecules in the Martian soil-are to be carried by Russian space vehicles to
Mars. Mars Observer was designed to serve as a relay station for landers in
Russian Mars missions. The Russians have offered to include a U. S. orbiter in
a forthcoming Proton-launched multipayload mission to Mars.
The American and Russian capabilities in
space science and technology mesh; they interdigitate. Each is strong where the
other is weak. This is a marriage made in heaven-but one that has been
surprisingly difficult to consummate.
On September 2, 1993, an agreement to
cooperate in depth was signed in Washington by Vice President Al Gore and Prime
Minister Viktor Chernomyrdin. The Clinton Administration has ordered NASA to
redesign the U. S. space station (called Freedom in the Reagan years) so it is
in the same orbit as Mir and can be mated to it: Japanese and European modules
will be attached, as will a Canadian robot arm. The designs have now evolved
into what is called Space Station Alpha, involving almost all the spacefaring
nations. (China is the most notable exception.)
In return for U. S. space cooperation
and an infusion of hard currency, Russia in effect agreed to halt its sale of
ballistic missile components to other nations, and generally to exercise tight
controls on its export of strategic weapons technology. In this Way, space
becomes once again, as it was at the height of the Cold War, an instrument of
national strategic policy.
This new trend has, though, made some of
the American aerospace industry and some key members of Congress profoundly
uneasy. Without international competition, can we motivate such ambitious
efforts? Does every Russian launch vehicle used cooperatively mean less support
for the American aerospace industry? Can Americans rely on stable support and
continuity of effort in joint projects with the Russians? (The Russians, of
course, ask similar questions about the Americans.) But cooperative programs in
the long term save money, draw upon the extraordinary scientific and
engineering talent distributed over our planet, and provide inspiration about
the global future. There may be fluctuations in national commitments. We are
likely to take backward as well as forward steps. But the overall trend seems
clear.
Despite growing pains, the space
programs of the two former adversaries are beginning to join. It is now
possible to foresee a world space station-not of any one nation but of the
planet Earth-being assembled at 51° inclination to the equator and a few
hundred miles up. A dramatic joint mission, called “Fire and Ice,” is being
discussed in which a fast flyby would be sent to Pluto, the last unexplored
planet; but to get there, a gravity assist from the Sun would be employed, in
the course of Which small probes would actually enter the Sun's atmosphere. And
we seem to be on the threshold of a World Consortium for the scientific
exploration of Mars. It very much looks as though such projects will be done
cooperatively or not at all.
WHETHER THERE ARE VALID, cost-effective,
broadly supportable reasons for people to venture to Mars is an open question.
Certainly there is no consensus. The matter is treated in the next chapter.
I would argue that if we are not
eventually going to send people to worlds as far away as Mars, we have lost the
chief reason for a space station-a permanently (or intermittently) occupied
human outpost in Earth orbit. A space station is far from an optimum platform
for doing science either looking down at the Earth, or looking out into space,
or for utilizing microgravity (the very presence of astronauts messes things
up). For military reconnaissance it is much inferior to robotic spacecraft.
There are no compelling economic or manufacturing applications. It is expensive
compared to robotic spacecraft. And of course it runs some risk of losing human
lives. Every shuttle launch to help build or supply a space station has an
estimated 1 or 2 percent chance of catastrophic failure. Previous civilian and
military space activities have littered low Earth orbit with fast-moving
debris-that sooner or later will collide with a space station (although, so
far, Mir has had no failures from this hazard). A space station is also
unnecessary for human exploration of the Moon. Apollo got there very well with
no space station at all. With Saturn V or Energiya class launchers, it also may
be possible to get to near-earth asteroids or even Mars without having to
assemble the interplanetary vehicle on an orbiting space station.
A space station could serve
inspirational and educational purposes, and it certainly can help to solidify
relations among the spacefaring nations-particularly the United States and
Russia. But the only substantive function of a space station, as far as I can
see, is for long-duration spaceflight. How do humans behave in micro gravity?
How can we counter progressive changes in blood chemistry and an estimated 6
percent bone loss per year in zero gravity? (For a three- or four-year mission
to Mars this adds up, if the travelers have to go at zero g.)
These are hardly questions in
fundamental biology such as DNA or the evolutionary process; instead they
address issues of applied human biology. It's important to know the answers,
but only if we intend to go somewhere in space that's far away and takes a long
time to get there. The only tangible and coherent goal of a space station is
eventual human missions to near-Earth asteroids, Mars, and beyond. Historically
NASA has been cautious about stating this fact clearly, probably for fear that
members of Congress will throw up their hands in disgust, denounce the space
station as the thin edge of an extremely expensive wedge, and declare the
country unready to commit to launching people to Mars. In effect, then, NASA
has kept quiet about what the space station is really for. And yet if we had
such a space station, nothing would require us to go straight to Mars. We could
use a space station to accumulate and refine the relevant knowledge, and take
as long as we like to do so-so that when the time does come, when we are ready
to go to the planets, we will have the background and experience to do so
safely.
The Mars Observer failure, and the
catastrophic loss of the space shuttle Challenger in 1986, remind us that there
will be a certain irreducible chance of disaster in future human flights to
Mars and elsewhere. The Apollo 13 mission, which was unable to land on the Moon
and barely returned safely to Earth, underscores how lucky we've been. We
cannot make perfectly safe autos or trains even though we've been at it for
more than a century. Hundreds of thousands of years after we first domesticated
fire, every city in the world has a service of firefighters biding their time
until there's a blaze that needs putting out. In Columbus' four voyages to the
New World, he lost ships left and right, including one third of the little
fleet that set out in 1492.
If we are to send people, it must be for
a very good reason-and with a realistic understanding that almost certainly we
will lose lives. Astronauts and cosmonauts have always understood this.
Nevertheless, there has been and will be no shortage of volunteers.
But why Mars? Why not return to the
Moon? It's nearby, and we've proved we know how to send people there. I'm
concerned that the Moon, close as it is, is a long detour, if not a dead end.
We've been there. We've even brought some of it back. People have seen the Moon
rocks, and, for reasons that I believe are fundamentally sound, they are bored
by the Moon. It's a static, airless, waterless, black-sky, dead world. Its most
interesting aspect perhaps is its cratered surface, a record of ancient
,catastrophic impacts, on the Earth as well as on the Moon.
Mars, by contrast, has weather, dust
storms, its own moons, volcanos, polar ice caps, peculiar landforms, ancient
river valleys, and evidence of massive climatic change on a once-Earthlike
world. It holds some prospect of past or maybe even present life, and is the
most congenial planet for future life-humans transplanted from Earth, living
off the land. None of this is true for the Moon. Mars also has its own legible
cratering history. It Mars, rather than the Moon, had been within easy reach,
we would not have backed off from manned space flight.
Nor is the Moon an especially desirable
test bed or way station for Mars. The Martian and lunar environments are very
different, and the Moon is as distant from Mars as is the Earth. The machinery
for Martian exploration can at least equally well be tested in Earth orbit, or
on near-Earth asteroids, or on the Earth itself-in Antarctica, for instance.
Japan has tended to be skeptical of the
commitment of the United States and other nations to plan and execute major
cooperative projects in space. This is at least one reason that Japan, more
than any other spacefaring nation, has tended to go it alone. The Lunar and
Planetary Society of Japan is an organization representing space enthusiasts in
the government, universities, and major industries. As I write, the Society is
proposing to construct and stock a lunar base entirely with robot labor. It is
said to take about 30 years and to cost about a billion U. S. dollars a year
(which would represent 7 percent of the present U. S. civilian space budget).
Humans would arrive only when the base is fully ready. The use of robot
construction crews under radio command from Earth is said to reduce the cost
tenfold. The only trouble with the scheme, according to reports, is that other
scientists in Japan keep asking, “What's it for?” That's a good question in
every nation.
The first human mission to Mars is now
probably too expensive for any one nation to pull off by itself. Nor is it
fitting that such a historic step be taken by representatives of only a small
fraction of the human species. But a cooperative venture among the United
States, Russia, Japan, the European Space Agency-and perhaps other nations,
such as China-might be feasible in the not too distant future. The
international space station will have tested our ability to work together on
great engineering projects in space.
The cost of sending a kilogram of
something no farther away than low Earth orbit is today about the same as the
cost of kilogram of gold. This is surely a major reason we have yet to stride
the ancient shorelines of Mars. Multistage chemical rockets are the means that
first took us into space, and that's what we've been using ever since. We've
tried to refine them, to make there safer, more reliable, simpler, cheaper. But
that hasn't happened, or at least not nearly as quickly as many had hoped.
So maybe there's a better way: maybe
single-stage rockets that can launch their payloads directly to orbit; maybe
many small payloads shot from guns or rocket-launched from airplanes; maybe
supersonic ramjets. Maybe there's something much better that we haven't thought
of yet. If we can manufacture propellants for the return trip from the air and
soil of our destination world, the difficulty of the voyage would be greatly
eased.
Once we're up there in space, venturing
to the planets, rocketry is not necessarily the best means to move large
payloads around, even with gravity assists. Today, we make a few early rocket
burns and later midcourse corrections, and coast the rest of the way. But there
are promising ion and nuclear/ electric propulsion systems by which a small and
steady acceleration is exerted. Or, as the Russian space pioneer Konstantin
Tsiolkovsky first envisioned, we could employ solar sails-vast but very thin
films that catch sunlight and the solar wind, a caravel kilometers wide plying
the void between the worlds. Especially for trips to Mars and beyond, such
methods are far better than rockets.
As with most technologies, when
something barely works, when it's the first of its kind, there's a natural
tendency to improve it, develop it, exploit it. Soon there's such an
institutional investment in the original technology, no matter how flawed, that
it's very hard to move on to something better. NASA has almost no resources to
pursue alternative propulsion technologies. That money would have to come out
of near-term missions, missions which could provide concrete results and
improve NASA's success record. Spending money on alternative technologies pays
off a decade or two in the future. We tend to be very little interested in a
decade or two in the future. This is one of the ways by which initial success
can sow the seeds of ultimate failure; and is very similar to what sometimes
happens in biological evolution. But sooner or later some nation-perhaps one
without a huge investment in marginally effective technology-will develop
effective alternatives.
Even before then, if we take a
cooperative path, there will come a time-perhaps in the first decades of the
new century and the new millennium-when an interplanetary spacecraft is
assembled in Earth orbit, the progress in full view on the evening news.
Astronauts and cosmonauts, hovering like gnats, guide and mate the
prefabricated parts. Eventually the ship, tested and ready, is boarded by its
international crew, and boosted to escape velocity. For the whole of the voyage
to Mars and back, the lives of the crew members depend on one another, a
microcosm of our actual circumstances down here on Earth. Perhaps the first
joint interplanetary mission with human crews will be only a flyby or orbit of
Mars. Earlier, robot vehicles, with aerobraking, parachutes, and retrorockets,
will have set gently down on the Martian surface to collect samples and return
them to Earth, and to emplace supplies for future explorers. But whether or not
we have compelling, coherent reasons, I am sure-unless we destroy ourselves
first-that the day will come when we humans set foot on Mars. It is only a
matter of when.
According to solemn treaty, signed in
Washington and Moscow on January 27, 1967, no nation may lay claim to part or all
of another planet. Nevertheless-for historical reasons that Columbus would have
understood well-some people are concerned about who first sets foot on Mars. If
this really worries us, we can arrange for the ankles of the crew members to be
tied together as they alight in the gentle Martian gravity.
The crews would acquire new and
previously sequestered samples, in part to search for life, in part to
understand the past and future of Mars and Earth. They would experiment, for
later expeditions, on extracting water, oxygen, and hydrogen from the rocks and
the air and from the underground permafrost-to drink, to breathe, to power
their machines and, as rocket fuel and oxidizer, to propel the return voyage.
They would test Martian materials for eventual fabrication of bases and
settlements on Mars.
And they would go exploring. When I
imagine the early human exploration of Mars, it's always a roving vehicle, a
little like a jeep, wandering down one of the valley networks, the crew with
geological hammers, cameras, and analytic instruments at the ready. They're
looking for rocks from ages past, signs of ancient cataclysms, clues to climate
change, strange chemistries, fossils, or-most exciting and most
unlikely-something alive. Their discoveries are televised back to Earth at the
speed of light. Snuggled up in bed with the kids, you explore the ancient
riverbeds of Mars.
CHAPTER 16
Scaling heaven
Who, my friend, can scale heaven?
—THE EPIC OF GILGAMESH (SUMER, THIRD
MILLENNIUM B. C.)
What?, I sometimes ask myself in
amazement: Our ancestors walked from East Africa to Novaya Zemlya and Ayers
Rock and Patagonia, hunted elephants with stone spearpoints, traversed the
polar seas in open boats 7,000 years ago, circumnavigated the Earth propelled
by nothing but wind, walked the Moon a decade after entering space-and we're
daunted by a voyage to Mars? But then I remind myself of the avoidable human
suffering on Earth, how a few dollars can save the life of a child dying of
dehydration, how many children we could save for the cost of a trip to Mars-and
for the moment I change my mind. Is it unworthy to stay home or unworthy to go?
Or have I posed a false dichotomy? Isn't it possible to make a better life for
everyone on Earth and to reach for the planets and the stars?
We had an expansive run in the '60s and
'70s. You might have thought, as I did then, that our species would be on Mars
before the century was over. But instead, we've pulled inward. Robots aside,
we've backed off from the planets and the stars. 1 keep asking myself Is it a
failure of nerve or a sign of maturity?
Maybe it's the most we could reasonably
have expected. In a way it's amazing that it was possible at all: We sent a
dozen humans on week-long excursions to the Moon. And we were given the
resources to make a preliminary reconnaissance of the whole Solar System, out
to Neptune anyway-missions that returned a wealth of data, but nothing of
short-term, everyday, bread-on-the-table practical value. They lifted the human
spirit, though. They enlightened us about our place in the Universe. It's easy
to imagine skeins of historical causality in which there were no race to the
Moon and no planetary program.
But it's also possible to imagine a much
more serious devotion to exploration, because of which we would today have
robot vehicles probing the atmospheres of all the Jovian planets and dozens of
moons, comets, and asteroids; a network of automatic scientific stations
emplaced on Mars would daily be reporting their findings; and samples from many
worlds would be under examination in the laboratories of Earth-revealing their
geology, chemistry, and perhaps even their biology. Human outposts might be
already established on the near-Earth asteroids, the Moon, and Mars.
There were many possible historical
paths. Our particular causality skein has brought us to a modest and
rudimentary, although in many respects heroic, series of explorations. But it
is far interior to what might have been-and what may one day be.
TO CARRY THE GREEN Promethean spark of
Life with us into the sterile void and ignite there a firestorm of animate
matter is the very destiny of our race,” reads the brochure of something called
the First Millennial Foundation. It promises, for $120 a year, “citizenship” in
“space colonies-when the time comes.” “Benefactors” who contribute more also
receive “the undying gratitude of a star-flung civilization, and their name
carved on the monolith to be erected on the Moon.” This represents one extreme
in the continuum of enthusiasm for a human presence in space. The other
extreme-better represented in Congress-questions why we should be in space at
all, especially people rather than robots. The Apollo program was a
“moondoggle,” the social critic Amitai Etzioni once called it; with the Cold
War over, there is no justification whatever, proponents of this orientation
hold, for a manned space program. Where in this spectrum of policy options
should we be?
Ever since the United States beat the
Soviet Union to the Moon, a coherent, widely understood justification for
humans in space seems to have vanished. Presidents and Congressional committees
puzzle over what to do with the manned space program. What is it for? Why do we
need it? But the exploits of the astronauts and the moon landings had
elicited-and for good reason-the admiration of the world. It would be a
rejection of that stunning American achievement, the political leaders tell
themselves, to back off from manned spaceflight. Which President, which
Congress wishes to be responsible for the end of the American space program?
And in the former Soviet Union a similar argument is heard: Shall we abandon,
they ask themselves, the one remaining high technology in which we are still
world leaders? Shall we be faithless heirs of Konstantin Tsiolkovsky, Sergei Korolev,
and Yuri Gagarin?
The first law of bureaucracy is to
guarantee its own continuance. Left to its own devices, without clear
instructions from above, NASA gradually devolved into a program that would
maintain profits, jobs, and perquisites. Pork-barrel politics, with Congress
playing a leading role, became an increasingly powerful force in the design and
execution of missions and long-term goals. The bureaucracy ossified. NASA lost
its way.
On July 20, 1989, the twentieth
anniversary of the Apollo 11 landing on the Moon, President George Bush
announced a long-term direction for the U. S. space program. Called the Space
Exploration Initiative (SEI), it proposed a sequence of goals including a U. S.
space station, a return of humans to the Moon, and the first landing of humans
on Mars. In a later statement, Mr. Bush set 2019 as the target date for the
first footfall on that planet.
And yet the Space Exploration
Initiative, despite clear direction from the top, foundered. Four years after
it was mandated, it did not even have a NASA office dedicated to it. Small and
inexpensive lunar robotic missions-that otherwise might well have been
approved-were canceled by Congress because of guilt by association with SEI.
What went wrong?
One problem was the timescale. SEI
extended five or so presidential terms of office into the future (taking the
average presidency as one and a half terms). That makes it easy for a president
to attempt to commit his successors, but leaves in considerable doubt how
reliable such a commitment might be. SEI contrasted dramatically with the
Apollo program-which, it might have been conjectured at the time it began,
could have triumphed when President Kennedy or his immediate political heir was
still in office.
Second, there was concern about whether
NASA, which had recently experienced great difficulty in safely lifting a few
astronauts 200 miles above the Earth, could send astronauts on an arcing
year-long trajectory to a destination 100 million miles away and bring them
back alive.
Third, the program was conceived
exclusively in nationalist terms. Cooperation with other nations was not
fundamental to either design or execution. Vice President Dan Quayle, who had
nominal responsibility for space, justified the space station as a demonstration
that the United States was “the world's only superpower.” But since the Soviet
Union had an operational space station that was a decade ahead of the United
States, Mr. Quayle's argument proved difficult to follow.
Finally, there was the question of
where, in terms of practical politics, the money was supposed to come from. The
costs of getting the first humans to Mars had been variously estimated, ranging
as high as $500 billion.
Of course, it's impossible to predict
costs before you have a mission design. And the mission design depends on such
matters as the size of the crew; the extent to which you take mitigating steps
against solar and cosmic radiation hazards, or zero gravity; and what other
risks you are willing to accept with the lives of the men and women on board.
If every crew member has one essential specialty, what happens if one of them
falls ill? The larger the crew, the more reliable the backups. You would almost
certainly not send a full-time oral surgeon, but what happens if you need root
canal work and you're a hundred million miles from the nearest dentist? Or
could it be done by an endodontist on Earth, using telepresence?
Wernher von Braun was the Nazi-American
engineer who, more than anyone else, actually took us into space. His 1952 book
Das Marsprojekt envisioned a first mission with 10 interplanetary spacecraft,
70 crew members, and 3 “landing boats.” Redundancy was uppermost in his mind.
The logistical requirements, he wrote, “are no greater than those for a minor
military operation extending over a limited theater of war.” He meant to
“explode once and for all the theory of the solitary space rocket and its
little band of bold interplanetary adventurers,” and appealed to Columbus'
three ships without which “history tends to prove that he might never have
returned to Spanish shores.” Modern Mars mission designs have ignored this
advice. They are much less ambitious than yon Braun's, typically calling for
one or two spacecraft crewed by three to eight astronauts, with another robotic
cargo ship or two. The solitary rocket and the little band of adventurers are
still with us.
Other uncertainties affecting mission
design and cost include whether you pre-emplace supplies from Earth and launch
humans to Mars only after the supplies are safely landed; whether you can use
Martian materials to generate oxygen to breathe, water to drink, and rocket
propellants to get home; whether you land using the thin Martian atmosphere for
aerobraking; the degree of redundancy in equipment thought prudent; the extent
to which you use closed ecological systems or just depend on the food, water,
and waste disposal facilities you've brought from Earth; the design of roving
vehicles for the crew to explore the Martian landscape; and how much equipment
you're willing to carry to test our ability to live off the land in later
voyages.
Until such questions are decided, it's
absurd to accept any figure for the cost of the program. On the other hand, it
was equally clear that SEI would be extremely expensive. For all these reasons,
the program was a nonstarter. It was stillborn.
There was no effective attempt by the
Bush Administration to spend political capital to get SEI going.
The lesson to me seems clear: There may
be no way to Send humans to Mars in the comparatively near future-despite the
fact that it is entirely within our technological capability. Governments do
not spend these vast sums just for science, or merely to explore. They need
another purpose, and it must make real political sense.
It may be impossible to go just yet, but
when it is possible, the mission, I think, must be international from the
start, with costs and responsibilities equitably shared and the expertise of
many nations tapped; the price must be reasonable; the time from approval to
launch must fit within practical political timescales; and the space agencies
concerned must demonstrate their ability to muster pioneering exploratory
missions with human crews safely, on time, and on budget. If it were possible
to imagine such a mission for less than $100 billion, and for a time from
approval to launch less than 15 years, maybe it would be feasible. (In terms of
cost, this would represent only a fraction of the annual civilian space budgets
of the present spacefaring nations.) With aerobraking and manufacturing fuel
and oxygen for the return trip out of Martian air, it's now beginning to look
as if such a budget and such a timescale might actually be realistic.
The cheaper and quicker the mission is,
necessarily the more risk we must be willing to take with the lives of the
astronauts and cosmonauts aboard. But as is illustrated, among countless
examples, by the samurai of medieval Japan, there are always competent
volunteers for highly dangerous missions in what is perceived as a great cause.
no budget, no timeline can be really reliable when we attempt to do something
on so grand a scale, something that has never been done before. The more leeway
we ask, the greater is the cost and the longer it takes to get there. Finding
the right compromise between political feasibility and mission success may be
tricky.
IT'S NOT ENOUGH to go to Mars because
some of us have dreamt of doing so since childhood, or because it seems to us
the obvious long-term exploratory goal for the human species. If we're talking about
spending this much money, we must justify the expense.
There are now other matters-clear,
crying national needs-that cannot be addressed without major expenditures; at
the same time, the discretionary federal budget has become painfully
constrained. Disposal of chemical and radioactive poisons, energy efficiency,
alternatives to fossil fuels, declining rates of technological innovation, the
collapsing urban infrastructure, the AIDS epidemic, a witches' brew of cancers,
homelessness, malnutrition, infant mortality, education, jobs, health
care-there is a painfully long list. Ignoring them will endanger the well-being
of the nation. A similar dilemma faces all the spacefaring nations.
Nearly every one of these matters could
cost hundreds of billions of dollars or more to address. Fixing infrastructure
will cost several trillion dollars. Alternatives to the fossil-fuel economy
clearly represent a multitrillion-dollar investment worldwide, if we can do it.
These projects, we are sometimes told, are beyond our ability to pay. How then
can we afford to go to Mars?
If there were 20 percent more
discretionary funds in the U. S. federal budget (or the budgets of the other
spacefaring nations), I probably would not feel so conflicted about advocating
sending humans to Mars. If there were 20 percent less, I don't think the most
diehard space enthusiast would be urging such a mission. Surely there is some
point at which the national economy is in such dire straits that sending people
to Mats is unconscionable. The question is where we draw the line. plainly such
a line exists, and every participant in these debates should stipulate where
that line should be drawn, what fraction of the gross national product for
space is too much. I'd like the same thing done for “defense.”
Public opinion polls show that many
Americans think the NASA budget is about equal to the defense budget. In fact,
the entire NASA budget, including human and robotic missions and aeronautics,
is about 5 percent of the U. S. defense budget. How much spending for defense
actually weakens the country? And even if NASA were cancelled altogether, would
we free up what is needed to solve our national problems?
HUMAN SPACEFLIGHT In general-to say
nothing of expeditions to Mars-would be much more readily supportable if, as in
the fifteenth-century arguments of Columbus and Henry the Navigator, there were
a profit lure. Some arguments have been
advanced. The high vacuum or low gravity or intense radiation environment of
near-Earth space might be utilized, it is said, for commercial benefit. All
such proposals must be challenged by this question: Could comparable or better
products be manufactured down here on Earth if the development money made
available were comparable to what is being poured into the space program?
Judging by how little money corporations have been willing to invest in such
technology-apart from the entities building the rockets and spacecraft
themselves-the prospects, at least at present, seem to be not very high.
The notion that rare materials might be
available elsewhere is tempered by the fact that freightage is high. There may,
for all we know, be oceans of petroleum on Titan, but transporting it to Earth
will be expensive. Platinum-group metals may be abundant in certain asteroids.
If we could move these asteroids into orbit around the Earth, perhaps we could
conveniently mine them. But at least for the foreseeable future this seems
dangerously imprudent, as I describe later in this book.
In his classic science fiction novel The
Man Who Sold the Moon, Robert Heinlein imagined the profit motive as the key to
space travel. He hadn't foreseen that the Cold War would sell the Moon. But he
did recognize that an honest profit argument would be difficult to come by.
Heinlein envisioned, therefore, a scam in which the lunar surface was salted
with diamonds so later explorers could breathlessly discover them and initiate
a diamond rush. We've since returned samples from the Moon, though, and there
is not a hint of commercially interesting diamonds there.
However, Kiyoshi Kuramoto and Takafumi
Matsui of the University of Tokyo have studied how the central iron cores of
Earth, Venus, and Mars formed, and find that the Martian mantle (between crust
and core) should be rich in carbon-richer than that of the Moon or Venus or
Earth. Deeper than 300 kilometers, the pressures should transform carbon into
diamond. We know that Mars has been geologically active over its history.
Material from great depth will occasionally be extruded up to the surface, and
not just in the great volcanos. So there does seem to be a case for diamonds on
other worlds-on Mars, lied not the Moon. In what quantities, of what quality
and size, and in which locales we do not yet know.
The return to Earth of a spacecraft
stuffed with gorgeous multicarat diamonds would doubtless depress prices (as
well as the shareholders of the de Beers and General Electric corporations).
But because of the ornamental and industrial applications of diamonds, perhaps
there is a lower limit below which prices will not go. Conceivably, the
affected industries might find cause to promote the early exploration of Mars.
The idea that Martian diamonds will pay
for exploring Mars is at best a very long shot, but it's an example of how rare
and valuable substances may be discoverable on other worlds. It would be
foolish, though, to count on such contingencies. If we seek to justify missions
to other worlds, we'll have to find other reasons.
BEYOND DISCUSSIONS OF PROFITS and costs,
even reduced costs, we must also describe benefits, if they exist. Advocates of
human missions to Mars must address whether, in the long term, missions up
there are likely to mitigate any of the problems down here. Consider now the
standard set of justifications and see if you find them valid, invalid, or
indeterminate:
Human missions to Mars would
spectacularly improve our knowledge of the planet, including the search for
present and past life. The program is likely to clarify our understanding of
the environment of our own planet, as robotic missions have already begun to
do. The history of our civilization shows that the pursuit of basic knowledge
is the way the most significant practical advances come about. Opinion polls
suggest that the most popular reason for “exploring space” is “increased
knowledge.” But are humans in space essential to achieve this goal? Robotic
missions, given high national priority and equipped with improved machine
intelligence, seem to me entirely capable of answering, as well as astronauts
can, all the questions we need to ask-and at Maybe 10 percent the cost.
It is alleged that “spinoff” will
transpire-huge technological benefits that would otherwise fail to come
about-thereby improving our international competitiveness and the domestic
economy. But this is an old argument: Spend $80 billion (in contemporary money)
to send Apollo astronauts to the Moon, and we'll throw in a free stickless
frying pan. Plainly, if we're after frying pans, we can invest the money
directly and save almost all of that $80 billion.
The argument is specious for other
reasons as well, one of which is that DuPont's Teflon technology long antedated
Apollo. The same is true of cardiac pacemakers, ballpoint pens, Velcro, and
other purported spinoffs of the Apollo program. (I once had the opportunity to
talk with the inventor of the cardiac pacemaker, who himself nearly had a
coronary accident describing the injustice of what he perceived as NASA taking
credit for his device.) If there are technologies we urgently need, then spend
the money and develop them. Why go to Mars to do it?
Of course it would be impossible for so
much new technology as NASA requires to be developed and not have some
spillover into the general economy, some inventions useful down here. For
example, the powdered orange juice substitute Tang was a product of the manned
space program, and spinoffs have occurred in cordless tools, implanted cardiac
defibrillators, liquid-cooled garments, and digital imaging-to name a few. But
they hardly justify human voyages to Mars or the existence of NASA.
We could see the old spinoff engine
wheezing and puffing in the waning days of the Reagan-era Star Wars office.
Hydrogen bomb-driven X-ray lasers on orbiting battle stations will help perfect
laser surgery, they told us. But if we need laser surgery, if it's a high
national priority, by all means let's allocate the funds to develop it. just
leave Star Wars out of it. Spinoff justifications constitute an admission that
the program can't stand on its own two feet, cannot be justified by the purpose
for which it was originally sold.
Once upon a time it was thought, on the
basis of econometric models, that for every dollar invested in NASA many
dollars were pumped into the U. S. economy. If this multiplier effect applied
more to NASA than to most government agencies, it would provide a potent fiscal
and social justification for the space program. NASA supporters were not shy
about appealing to this argument. But a 1994 Congressional Budget Office study
found it to be a delusion. While NASA spending benefits some production
segments of the U. S. economy-especially the aerospace industry-there is no
preferential multiplier effect. Likewise, while NASA spending certainly creates
or maintains jobs and profits, it does so no more efficiently than many other
government agencies.
Then there's education, an argument that
has proved from time to time very attractive in the White House. Doctorates in
science peaked somewhere around the time of Apollo 11, maybe even with the
proper phase lag after the start of the Apollo program. The cause-and-effect
relationship is perhaps undemonstrated, although not implausible. But so what?
If we're interested in improving education, is going to Mars the best route?
Think of what we could do with $100 billion for teacher training and salaries,
school laboratories and libraries, scholarships for disadvantaged students,
research facilities, and graduate fellowships. Is it really true that the best
way to promote science education is to go to Mars?
Another argument is that human missions
to Mars will occupy the military-industrial complex, diffusing the temptation
to use its considerable political muscle to exaggerate external threats and
pump up defense funding. The other side of this particular coin is that by
going to Mars we maintain a standby technological capacity that might be
important for future military contingencies. Of course, we might simply ask
those guys to do something directly useful for the civilian economy. But as we
saw in the 1970s with Grumman buses and Boeing/Vertol commuter trains, the
aerospace industry experiences real difficulty in producing competitively for
the civilian economy. Certainly a tank may travel 1,000 miles a year and a bus
1,000 miles a week, so the basic designs must be different. But on matters of
reliability at least, the Defense Department seems to be much less demanding.
Cooperation in space, as I've already
mentioned, is becoming an instrument of international cooperation-for example,
in slowing the proliferation of strategic weapons to new nations. Rockets
decommissioned because of the end of the Cold War might be gainfully employed
in missions to Earth orbit, the Moon, the planets, asteroids, and comets. But
all this can be accomplished without human missions to Mars.
Other justifications are offered. It is
argued that the ultimate solution to world energy problems is to strip-mine the
Moon, return the solar-wind-implanted helium-3 back to Earth, and use it in
fusion reactors. What fusion reactors? Even if this were possible, even if it
were cost-effective, it is a technology 50 or 100 years away. Our energy
problems need to be solved at a less leisurely pace.
Even stranger is the argument that we
have to send human beings into space in order to solve the world population
crisis. But some 250,000 more people are born than die every day— which means
cans that we would have to launch 250,000 people per day into space to maintain
world population at its present levels. This appears to be beyond our present
capability.
I RUN THROUGH such a list and try to add
up the pros and cons, bearing in mind the other urgent claims on the federal
budget. To me, the argument so far comes down to this question: Can the sum of
a large number of individually inadequate Justifications add up to an adequate
justification?
I don't think any of the items on my
list of purported justifications is demonstrably worth $500 billion or even
$100 billion, certainly not in the short term. On the other hand, most of them
are worth something, and if I have five items each worth $20 billion, maybe it
adds up to $100 billion. If we can be clever about reducing costs and making
true international partnerships, the justifications become more compelling.
Until a national debate on this topic
has transpired, until we have a better idea of the rationale and the
cost/benefit ratio of human missions to Mars, what should we do? My suggestion
is that we pursue research and development projects that can be justified on
their own merits or by their relevance to other goals, but that can also
contribute to human missions to Mars should we later decide to go. Such an
agenda would include:
•U. S. astronauts on the Russian space
station Mir for joint flights of gradually increasing duration, aiming at one
to two years, the Mars flight time.
•Configuration of the international
space station so its principal function is to study the long-term effects of
the space environment on humans.
• Early implementation of a rotating or
tethered “artificial gravity” module on the international space station, for
other animals and then for humans.
• Enhanced studies of the Sun, including
a distributed set of robot probes in orbit about the Sun, to monitor solar
activity and give the earliest possible warning to astronauts of hazardous
“solar flares”-mass ejections of electrons and protons from the Sun's corona.
• U. S. /Russian and multilateral
development of Energiya and Proton rocket technology for the U. S. and
international space programs. Although the United States is unlikely to depend
primarily on a Soviet booster, Energiya has roughly the lift of the Saturn V
that sent the Apollo astronauts to the Moon. The United States let the Saturn V
assembly line die, and it cannot readily be resuscitated. Proton is the most
reliable large booster now in service. Russia is eager to sell this technology
for hard currency.
• Joint projects with NASDA (the
Japanese space agency) and Tokyo University, the
European Space Agency, and the Russian
Space Agency, along with Canada and other nations. In most cases these should
be equal partnerships, not the United States insisting on calling the shots.
For the robotic exploration of Mars, such programs are already under way. For
human flight, the chief such activity is clearly the international space
station. Eventually, we might muster joint simulated planetary missions in low
Earth orbit. One of the principal objectives of these programs should be to
build a tradition of cooperative technical excellence.
• Technological development-using
state-of-the-art robotics and artificial intelligence— of rovers, balloons, and
aircraft for the exploration of Mars, and implementation of the first
international return sample mission. Robotic spacecraft that can return samples
from Mars can be tested on near-Earth asteroids and the Moon. Samples returned
from carefully selected regions of the Moon can have their ages determined and
contribute in a fundamental way to our understanding of the early history of
the Earth.
•Further development of technologies to
manufacture fuel and oxidizer out of Martian materials. In one estimate, based
on a prototype instrument designed by Robert Zubrin and colleagues at the
Martin Marietta Corporation, several kilograms of Martian soil can be
automatically returned to Earth using a modest and reliable Delta launch
vehicle, all for no more than a song (comparatively speaking).
• Simulations on Earth of long-duration
trips to Mars, concentrating on potential social and psychological problems.
•Vigorous pursuit of new technologies
such as constant-thrust propulsion to get us to
Mars quickly; this may be essential if
the radiation or microgravity hazards make one—year (or longer) flight times
too risky.
•Intensive study of near-Earth
asteroids, which may provide superior intermediate— timescale objectives for human
exploration than does the Moon.
•A greater emphasis on science-including
the fundamental sciences behind space exploration, and the thorough analysis of
data already obtained-by NASA and other space agencies.
These recommendations add up to a
fraction of the full cost of a human mission to Mars and-spread out over a
decade or so and done jointly with other nations-a fraction of current space
budgets. But, if implemented, they would help us to make accurate cost
estimates and better assessment of the dangers and benefits. They would permit
us to maintain vigorous progress toward human expeditions to Mars without
premature commitment to any specific mission hardware. Most, perhaps all, of
these recommendations have other justifications, even if We were sure wed be
unable to send humans to any other world in the next few decades. And a steady
drumbeat of accomplishments increasing the feasibility of human voyages to Mars
would-in the minds of many at least-combat widespread pessimism about the
future.
THERE'S SOMETHING MORE. There's a set of
less tangible arguments, many of which, I freely admit, I find attractive and
resonant. Spaceflight speaks to something deep inside us-many of us, if not
all. An emerging cosmic perspective, an improved understanding of our place in
the Universe, a highly visible program affecting our view of ourselves might
clarify the fragility of our planetary environment and the common peril and
responsibility of all the nations and peoples of Earth. And human missions to
Mars would provide hopeful prospects, rich in adventure, for the wanderers
among us, especially the young. Even vicarious exploration has social utility.
I repeatedly find that when I give talks
on the future of the space program-to universities, business and military
groups, professional organizations-the audiences are much less patient with
practical, real-world political and economic obstacles than 1. They long to
sweep away the impediments, to recapture the glory days of Vostok and Apollo,
to get on with it and once more tread other worlds. We did it before; we can do
it again, they say. But, I caution myself, those who attend such talks are
self-selected space enthusiasts.
In 1969, less than half the American
people thought the Apollo program was worth the cost. But on the twenty-fifth
anniversary of the Moon landing, the number had risen to two thirds. Despite
its problems, NASA was rated as doing a good-to-excellent job by 63 percent of
Americans. With no reference to cost, 75 percent of Americans (according to a CBS
News poll) favored “the United States sending astronauts to explore Mars. ' For
young adults, the figure was 68 percent. I think “explore” is the operative
word.
It is no accident that, whatever their
human flaws, and how ever moribund the human space program has become (a trend
that the Hubble Space Telescope repair mission may have helped to reverse),
astronauts and cosmonauts are still widely regarded as heroes of our species. A
scientific colleague tells me about a recent trip to the New Guinea highlands
where she visited a stone age culture hardly contacted by Western civilization.
They were ignorant of wristwatches, soft drinks, and frozen food. But they knew
about Apollo 11. They knew that humans had walked on the Moon. They knew the
names of Armstrong and Aldrin and Collins. They wanted to know who was visiting
the Moon these days.
Projects that are future-oriented, that,
despite their political difficulties, can be completed only in some distant
decade are continuing reminders that there will be a future. Winning a foothold
on other worlds whispers in our ears that we're more than Picts or Serbs or
Tongans: We're humans.
Exploratory spaceflight puts scientific
ideas, scientific thinking, and scientific vocabulary in the public eye. It
elevates the general level of intellectual inquiry. The idea that we've now
understood something never grasped by anyone who ever lived before-that
exhilaration, especially intense for the scientists involved, but perceptible
to nearly everyone-propagates through the society, bounces off walls, and comes
back at us. It encourages us to address problems in other fields that have also
never before been solved. It increases the general sense of optimism in the
society. It gives currency to critical thinking of the sort urgently needed if
we are to solve hitherto intractable social issues. It helps stimulate a new
generation of scientists. The more science in the media-especially if methods
are described, as well as conclusions and implications-the healthier, I
believe, the society is. People everywhere hunger to understand.
WHEN I WAS A CHILD, my most exultant
dreams were about flying-not in some machine, but all by myself. I would be
skipping or hopping, and slowly I could pull my trajectory higher. It would
take longer to fall back to the ground. Soon I would be on such a high arc that
I wouldn't come down at all. I would alight like a gargoyle in a niche near the
pinnacle of a skyscraper, or gently settle down on a cloud. In the dream-which
I must have had in its many variations at least a hundred times-achieving
flight required a certain cast of mind. It's impossible to describe it in
words, but I can remember what it was like to this day. You did something
inside your head and at the pit of your stomach, and then you could lift
yourself up by an effort of will alone, your limbs hanging limply. Off you'd
soar.
I know many people have had similar
dreams. Maybe most people. Maybe everyone. Perhaps it goes back 10 million
years or more, when our ancestors were gracefully flinging themselves from
branch to branch in the primeval forest. A wish to soar like the birds
motivated many of the pioneers of flight, including Leonardo da Vinci and the
Wright brothers. Maybe that's part of the appeal of spaceflight, too.
In orbit about any world, or in
interplanetary flight, you are literally weightless. You can propel yourself to
the spacecraft ceiling with a slight push off the floor. You can go tumbling
through the air down the long axis of the spacecraft. Humans experience
weightlessness as joy; this has been reported by almost every astronaut and
cosmonaut. But because spacecraft are still so small, and because space “walks”
have been done with extreme caution, no human has yet enjoyed this wonder and
glory: propelling yourself by an almost imperceptible push, with no machinery
driving you, untethered, high up into the sky, into the blackness of
interplanetary space. You become a living satellite of the Earth, or a human
planet of the Sun.
Planetary exploration satisfies our
inclination for great enterprises and wanderings and quests that has been with
us since our days as hunters and gatherers on the East African savannahs a
million years ago. By chance-it is possible, I say, to imagine many skeins of
historical causality in which this would not have transpired-in our age we are
able to begin again.
Exploring other worlds employs precisely
the same qualities of daring, planning, cooperative enterprise, and valor that
mark the finest in the military tradition. Never mind the night launch of an
Apollo spacecraft bound for another world. That makes the conclusion foregone.
Witness mere F-14s taking off from adjacent flight decks, gracefully canting
left and right, afterburners flaming, and there's something that sweeps you
away-or at least it does me. And no amount of knowledge of the potential abuses
of carrier task forces can affect the depth of that feeling. It simply speaks
to another part of me. It doesn't want recriminations or politics. It just
wants to fly.
“I...had ambition not only to go farther
than anyone had done before,” wrote Captain James Cook, the eighteenth-century
explorer of the Pacific, “but as far as it was possible for man to go.” Two
centuries later, Yuri Romanenko, on returning to Earth after what was then the
longest space flight in history, said “The Cosmos is a magnet...Once you've
been there, all you can think of is how to get back.”
Even Jean-Jacques Rousseau, no
enthusiast of technology, felt it:
The stars are far above us; we need
preliminary instruction, instruments and machines, which are like so many
immense ladders enabling us to approach them and bring them within our grasp.
“The future possibilities of
space-travel,” wrote the philosopher Bertrand Russell in 1959which are now left
mainly to unfounded fantasy, could be more soberly treated without ceasing to
be interesting and could show to even the most adventurous of the young that a
world without war need not be a world without adventurous and hazardous glory.
To this kind of contest there is no limit. Each victory is only a prelude to
another, and no boundaries can be set to rational hope.
In the long run, these-more than any of
the “practical” justifications considered earlier-may be the reasons we will go
to Mars and other worlds. In the meantime, the most important step we can take
toward Mars is to make significant progress on Earth. Even modest improvements
in the social, economic, and political problems that our global civilization
now faces could release enormous resources, both material and human, for other
goals.
There's plenty of housework to be done
here on Earth, and our commitment to it must be steadfast. But we're the kind
of species that needs a frontier-for fundamental biological reasons.
Every time humanity stretches itself and
turns a new corner, it receives a jolt of productive vitality that can carry it
for centuries.
There's a new world next door. And we
know how to get there.
CHAPTER 17
Routine interplanetary violence
It is a law of nature that Earth and all
other bodies should remain in their proper places and be moved from them only
by violence.
—ARISTOTLE (384-322 B. C.), PHYSICS
There was something funny about Saturn.
When, in 1610, Galileo used the world's first astronomical telescope to view
the planet-then the most distant world known-he found two appendages, one on
either side. He likened them to “handles.” Other astronomers called them
“ears.” The Cosmos holds many wonders, but a planet with jug ears is dismaying.
Galileo went to his grave with this bizarre matter unresolved.
As the years passed, observers found the
ears...Well, waxing and waning. Eventually, it became clear that what Galileo
had discovered was an extremely thin ring that surrounds Saturn at its equator
but touches it nowhere. In some years, because of the changing orbital
positions of Earth and Saturn, the ring had been seen edge-on and, because of
its thinness, it seemed to disappear. In other years, it had been viewed more
face-oil, and the “ears” grew bigger. But what does it mean that there's a ring
around Saturn? A thin, flat, solid plate with a hole cut out for the planet to
fit into? Where does that come from?
This line of inquiry will shortly take
us to world-shattering collisions, to two quite different perils for our
species, and to a reason-beyond those already described-that we must, for our
very survival, be out there among the planets.
We now know that the rings (emphatically
plural) of Saturn are a vast horde of tiny ice worlds, each on its separate
orbit, each bound to Saturn by the giant planet's gravity. In size, these
worldlets range from particles of fine dust to houses. None is big enough to
photograph even from close flybys. Spaced out in an exquisite set of fine
concentric circles, something like the grooves on a phonograph record (which in
reality make, of course, a spiral), the rings were first revealed in their true
majesty by the two Voyager spacecraft in their 1980/81 flybys. In our century,
the Art Deco rings of Saturn have become an icon of the future.
At a scientific meeting in the late
1960s, I was asked to summarize the outstanding problems in planetary science.
One, I suggested, was the question of why, of all the planets, only Saturn had
rings. This, Voyager discovered, is a nonquestion. All four giant planets in
our Solar System- Jupiter, Saturn, Uranus, and Neptune-in fact have rings. But
no one knew it then.
Each ring system has distinctive
features. Jupiter's is tenuous and made mainly of dark, very small particles.
The bright rings of Saturn are composed mainly of frozen water; there are
thousands of separate rings here, some twisted, with strange, dusky, spoke-like
markings forming and dissipating. The dark rings of Uranus seem to be composed
of elemental carbon and organic molecules-something like charcoal or chimney
soot; Uranus has nine main rings, a few of which sometime seem to “breathe,”
expanding and contracting. Neptune's rings are the most tenuous of all, varying
so much in thickness that, when detected from Earth, they appear only as arcs
and incomplete circles. A number of rings seem to be maintained by the
gravitational tugs of two shepherd moons, one a little nearer and the other a
little farther from the planet than the ring. Each ring system displays its
own, appropriately unearthly, beauty.
How do rings form? One possibility is
tides: If an errant world passes close to a planet, the interloper's near side
is gravitationally pulled toward the planet more than its far side; if it comes
close enough, if its internal cohesion is low enough, it can be literally torn to
pieces. Occasionally we see this happening to comets as they pass too close to
Jupiter, or the Sun. Another possibility, emerging from the Voyager
reconnaissance of the outer Solar System, is this: Rings are made when worlds
collide and moons are smashed to smithereens. Both mechanisms may have played a
role.
The space between the planets is
traversed by an odd collection of rogue worldlets, each in orbit about the Sun.
A few are as big as a county or even a state; many more have surface areas like
those of a village or a town. More little ones are found than big ones, and
they range in size down to particles of dust. Some of them travel on long,
stretched-out elliptical paths, which make them periodically cross the orbit of
one or more planets.
Occasionally, unluckily, there's a world
in the way. The collision can shatter and pulverize both the interloper and the
moon that's hit (or at least the region around ground zero). The resulting
debris-ejected from the moon but not so fast-moving as to escape from the
planet's gravity-may form, for a time, a new ring. It's made of whatever the
colliding bodies were made of, but usually more of the target moon than the
rogue impactor. If the colliding worlds are icy, the net result will be rings
of ice particles; if they're made of organic molecules, the result will be
rings of organic particles (which will slowly be processed by radiation into
carbon). All the mass in the rings of Saturn is no more than 'would result from
the complete impact pulverization of a single icy moon. The disintegration of
small moons can likewise account for the ring systems of the three other giant
planets.
Unless it's very close to its planet, a
shattered moon gradually reaccumulates (or at least a fair fraction of it
does). The pieces, big and small, still in approximately the same orbit as the
moon was before the impact, fall together helter-skelter. What used to be a
piece of the core is now at the surface, and vice versa. The resulting
hodgepodge surfaces might seem very odd. Miranda, one of the moons of Uranus,
looks disconcertingly jumbled and may have had such an origin.
The American planetary geologist Eugene
Shoemaker proposes that many moons in the outer Solar System have been
annihilated and reformed-not just once but several times each over the 4. 5
billion years since the Sun and the planets condensed out of interstellar gas
and dust. The picture emerging from the Voyager reconnaissance of the outer
Solar System is of worlds whose placid and lonely vigils are spasmodically
troubled by interlopers from space; of world-shattering collisions; and of
moons re-forming from debris, reconstituting themselves like phoenixes from
their own ashes.
But a moon that lives very close to a
planet cannot re-form if it is pulverized-the gravitational tides of the nearby
planet prevent it. The resulting debris, once spread out into a ring system,
might be very long-lived-at least by the standard of a human lifetime. Perhaps
many of the small, inconspicuous moons now orbiting the giant planets will one
day blossom forth into vast and lovely rings.
These ideas are supported by the
appearance of a number of satellites in the Solar System. Phobos, the inner
moon of Mars, has a large crater named Stickney; Mimas, an inner moon of
Saturn, has a big one named Herschel. These craters-like those on our own Moon
and, indeed, throughout the Solar System-are produced by collisions. An
interloper smashes into a bigger world and makes an immense explosion at the
point of impact, A bowl-shaped crater is excavated, and the smaller impacting
object is destroyed. If the interlopers that dug out the Stickney and Herschel
craters had been only a little larger, they would have had enough energy to
blow Phobos and Mimas to bits. These moons barely escaped the cosmic wrecking
ball. Many others did not.
Every time a world is smashed into,
there's one less interloper-something like a demolition derby on the scale of
the Solar System, a war of attrition. The very fact that many such collisions
have occurred means that the rogue worldlets have been largely used up. Those
on circular trajectories around the Sun, those that don't intersect the orbits
of other worlds, will be unlikely to smash into a planet. Those on highly
elliptical trajectories, those that cross the orbits of other planets, Will
sooner or later collide or, by a near miss, be gravitationally ejected from the
Solar System.
The planets almost certainly accumulated
from worldlets which in turn had condensed out of a great flat cloud of gas and
dust Surrounding the Sun-the sort of cloud that can now be seen around young
nearby stars. So, in the early history of the solar System before collisions
cleaned things up, there should have been many more worldlets than we see
today.
Indeed, there is clear evidence for this
in our own backyard: If we count up the interloper worldlets in our
neighborhood in space, we can estimate how often they'll hit the Moon. Let us
make the very modest assumption that the population of interlopers has never
been smaller than it is today. We can then calculate how many craters there
should be on the Moon. The number we figure turns out to be much less than the
number we see on the Moon's ravaged highlands. The unexpected profusion of
craters on the Moon speaks to us of an earlier epoch when the Solar System was
in wild turmoil, churning with worlds on collision trajectories. This makes
good sense, because they formed from the aggregation of much smaller
worldlets-which themselves had grown out of interstellar dust. Four billion
years ago, the lunar impacts were hundreds of times more frequent than they are
today; and 4. 5 billion years ago, when the planets were still incomplete,
collisions happened perhaps a billion times more often than in our becalmed
epoch.
The chaos may have been relieved by much
more flamboyant ring systems than grace the planets today. If they had small
moons in that time, the Earth, Mars, and the other small planets may also have
been adorned with rings.
The most satisfactory explanation of the
origin of our own Moon, based on its chemistry (as revealed by samples returned
from the Apollo missions), is that it was formed almost 4. 5 billion years ago,
when a world the size of Mars struck the Earth. Much of our planet's rocky
mantle was reduced to dust and hot gas and blasted into space. Some of the
debris, in o: -bit around the Earth, then gradually reaccumulated-atom by atom,
boulder by boulder. If that unknown impacting world had been only a little
larger, the result would have been the obliteration of the Earth. Perhaps there
once were other worlds in our Solar System-perhaps even worlds on which life
was stirring-hit by some demon worldlet, utterly demolished, and of which today
we have not even an intimation.
The emerging picture of the early Solar
System does not resemble a stately progression of events designed to form the
Earth. Instead, it looks as if our planet was made, and survived, by mere lucky
chance, amid unbelievable violence. Our world does not seem to have been
sculpted by a master craftsman. Here too, there is no hint of a Universe made
for us.
THE DWINDLING SUPPLY of worldlets is
today variously labeled: asteroids, comets, small moons. But these are
arbitrary categories-real worldlets are able to breach these human-made
partitions. Some asteroids (the word means “starlike,” which they certainly are
not) are rocky, others metallic, still others rich in organic matter. None is
bigger than 1,000 kilometers across. They are found mainly in a belt between
the orbits of Mars and Jupiter. Astronomers once thought the “main-belt”
asteroids were the remains of a demolished world, but, as I've been describing,
another idea is now more fashionable: The Solar System was once filled with
asteroid-like worlds, some of which went into building the planets. Only in the
asteroid belt, near Jupiter, did the gravitational tides of this most massive
planet prevent the nearby debris from coalescing into a new world. The
asteroids, instead of representing a world that once was, seem to be the
building blocks of a world destined never to be.
Down to kilometer size, there may be
several million asteroids, but, in the enormous volume of interplanetary space,
even that's still far too few to cause any serious hazard to spacecraft on
their way to the outer Solar System. The first main-belt asteroids, Gaspra and
Ida, were photographed, in 1991 and 1993 respectively, by the Galileo
spacecraft on its tortuous journey to Jupiter.
Main-belt asteroids mostly stay at home.
To investigate them. we must go and visit them, as Galileo did. Comets, on the
other hand, sometimes come and visit us, as Halley's Comet did most recently in
1910 and 1986. Comets are made mainly of ice, plus smaller amounts of rocky and
organic material. When heated, the ice vaporizes, forming the long and lovely
tails blown outward by the solar wind and the pressure of sunlight. After many
passages by the Sun, the ice is all evaporated, sometimes leaving a dead rocky
and organic world. Sometimes the remaining particles, the ice that held them
together now gone, spread out in the comet's orbit, generating a debris trail
around the Sun.
Every time a bit of cometary fluff the
size of a grain of sand enters the Earth's atmosphere at high speed, it burns
up, producing a momentary trail of light that Earthbound observers call a
sporadic meteor or “shooting star.” Some disintegrating comets have orbits that
cross the Earth's. So every year, the Earth, on its steady circumnavigation of
the Sun, also plunges through belts of orbiting cometary debris. We may then
witness a meteor shower, or even a meteor storm-the skies ablaze with the body
parts of a comet. For example, the Perseid meteors, seen on or about August 12
of each year, originate in a dying comet called Swift-Tuttle. But the beauty of
a meteor shower should not deceive us: There is a continuum that connects these
shimmering visitors to our night skies with the destruction of worlds.
A few asteroids now and then give off
little puffs of gas or even form a temporary tail, suggesting that they are in
transition between cometdom and asteroidhood. Some small moons going around the
planets are probably captured asteroids or comets; the moons of Mars and the
outer satellites of Jupiter may be in this category.
Gravity smooths down everything that
sticks out too far. But only in large bodies is the gravity enough to make
mountains and other projections collapse of their own weight, rounding the
world. And, indeed, when we observe their shapes, almost always we find that
small worldlets are lumpy, irregular, potato-shaped.
THERE ARE ASTRONOMERS whose idea of a
good time is to stay up till dawn on a cold, moonless night taking pictures of
the sky-the same sky they photographed the year before...and the year before
that. If they got it right last time, you might well ask, why are they doing it
again? The answer is: The sky changes. In any given year there might be
worldlets wholly unknown, never seen before, that approach the Earth and are
spied by these dedicated observers.
On March 25, 1993, a group of asteroid
and comet hunters, looking at the photographic harvest from an intermittently
cloudy night at Mount Palomar in California, discovered a faint elongated
smudge on their films. It was near a very bright object in the sky, the planet
Jupiter. Carolyn and Eugene Shoemaker and David Levy then asked other observers
to take a look. The smudge turned out to be something astonishing: some twenty
small, bright objects orbiting Jupiter, one behind the other, like pearls on a
string. Collectively they are called Comet Shoemaker-Levy 9 (this is the ninth
time that these collaborators have together discovered a periodic comet).
But calling these objects a comet is
confusing. There was a horde of them, probably the fragmented remains of a
single, hitherto undiscovered, comet. It silently orbited the Sun for 4 billion
years before passing too close to Jupiter and being captured, a few decades
ago, by the gravity of the Solar System's largest planet. On July 7, 1992, it
was torn apart by Jupiter's gravitational tides.
You can recognize that the inner part of
such a comet would be pulled toward Jupiter a little more strongly than the
outer part, because the inner part is closer to Jupiter than the outer part.
The difference in pull is certainly small. Our feet are a little closer to the
center of the Earth than our heads, but we are not in consequence torn to
pieces by the Earth's gravity. For such tidal disruption to have occurred, the
original comet must have been held together very weakly. Before fragmentation,
it was, we think, a loosely consolidated mass of ice, rock, and organic matter,
maybe 10 kilometers (about 6 miles) across.
The orbit of this disrupted comet was
then determined to high precision. Between July 16 and 22, 1994, all the
cometary fragments, one after another, collided with Jupiter. The biggest
pieces seem to have been a few kilometers across. Their impacts with Jupiter
were spectacular.
No one knew beforehand what these
multiple impacts into the atmosphere and clouds of Jupiter would do. Perhaps
the cometary fragments, surrounded by halos of dust, were much smaller than
they seemed. Or perhaps they were not coherent bodies at all, but loosely
consolidated-something like a heap of gravel with all the particles traveling
through space together, in nearly identical orbits. If either of these
possibilities were true Jupiter might swallow the comets without a trace. Other
astronomers thought there would at least be bright fireballs and giant plumes
as the cometary fragments plunged into the atmosphere. Still others suggested
that the dense cloud of fine particles accompanying the fragments of Comet
Shoemaker-Levy 9 into Jupiter would disrupt the magnetosphere of Jupiter or
form a new ring.
A comet this size should impact Jupiter,
it is calculated, only once every thousand years. It's the astronomical event
not of one lifetime, but of a dozen. Nothing on this scale has occurred since
the invention of the telescope. So in mid July 1994, in a beautifully
coordinated international scientific effort, telescopes all over the Earth and
in space turned towards Jupiter.
Astronomers had over a year to prepare.
The trajectories of the fragments in their orbits around Jupiter were
estimated. It was discovered that they would all hit Jupiter. Predictions of
the timing were refined. Disappointingly, the calculations revealed that all
impacts would occur on the night side of Jupiter, the side invisible from the
Earth (although accessible to the Galileo and Voyager spacecraft in the outer
Solar System). But, happily, all impacts would occur only a few minutes before
the Jovian dawn, before the impact site would be carried by Jupiter's rotation
into the line of sight from Earth.
The appointed moment for the impact of
the first piece, Fragment A, came and went. There were no reports from
ground-based telescopes. Planetary scientists stared with increasing gloom at a
television monitor displaying the data transmitted to the Space Telescope
Science Institute in Baltimore from the Hubble Space Telescope. There was
nothing anomalous Shuttle astronauts took time off from the reproduction of
fruit flies, fish, and newts to look at Jupiter through binoculars. They
reported seeing nothing. The impact of the millennium was beginning to look
very much like a fizzle.
Then there was a report from a
ground-based optical telescope in La Palma in the Canary Islands, followed by
announcements from a radiotelescope in Japan; from the European Southern
Observatory in Chile; and from a University of Chicago instrument in the frigid
wastelands of the South Pole. In Baltimore the young scientists crowding around
the TV monitor-themselves monitored by the cameras of CNN-began to see
something, and in exactly the right place on Jupiter. You could witness
consternation turn into puzzlement, and then exultation. They cheered; they
screamed; they jumped up and down. Smiles filled the room. They broke out the
champagne. Here was a group of young American scientists-about a third of them,
including the team leader, Heidi Hammel, women-and you could imagine youngsters
all over the world thinking that it might be fun to be a scientist, that this
might be a good daytime job, or even a means to spiritual fulfillment.
For many of the fragments, observers
somewhere on Earth noticed the fireball rise so quickly and so high that it
could be seen even though the impact site below it was still in Jovian
darkness. Plumes ascended and then flattened into pancake-like forms. Spreading
out from the point of impact we could see sound and gravity waves, and a patch
of discoloration that for the largest fragments became as big as the Earth.
Slamming into Jupiter at 60 kilometers a
second (130,000 miles an hour), the large fragments converted their kinetic
energy partly into shock waves, partly into heat. The temperature in the
fireball was estimated at thousands of degrees. Some of the fireballs and
plumes were far brighter than all the rest of Jupiter put together.
What is the cause of the dark stains
left after the impact? It might be stuff from the deep clouds of Jupiter-from
the region to which ground-based observers cannot ordinarily see-that welled up
and spread out. However, the fragments do not seem to have penetrated to such
depths. Or the molecules responsible for the stains might have been in the
cometary fragments in the first place. We know from the Vega 1 and 2 Soviet
missions and the Giotto mission of the European Space Agency-both to Halley's
Comet-that comets may be as much as a quarter composed of complex organic
molecules. They are the reason that the nucleus of Halley's Comet is pitch
black. If some of the cometary organics survived the impact events, they may
have been responsible for the stain. Or, finally, the stain may be due to
organic matter not delivered by the impacting cometary fragments, but
synthesized by their shock waves from the atmosphere of Jupiter.
Impact of the fragments of Comet
Shoemaker-Levy 9 with Jupiter was witnessed on seven continents. Even amateur
astronomers with small telescopes could see the plumes and the subsequent
discoloration of the Jovian clouds. Just as sporting events are covered at all
angles by television cameras on the field and from a dirigible high overhead,
six NASA spacecraft deployed throughout the Solar System, with different
observational specialties, recorded this new wonder-the Hubble Space Telescope,
the International Ultraviolet Explorer, and the Extreme Ultraviolet Explorer
all in Earth orbit; Ulysses, taking time out from it investigation of the South
Pole of the Sun; Galileo, on the way to its own rendezvous with Jupiter; and
Voyager 2, far beyond Neptune on its way to the stars. As the data are
accumulated and analyzed, our knowledge of comets, of Jupiter, and of the
violent collisions of worlds should all be substantially improved.
For many scientists-but especially for
Carolyn and Eugene Shoemaker and David Levy-there was something poignant about
the cometary fragments, one after the other, making their death plunges into
Jupiter. They had lived with this comet, in a manner of speaking, for 16
months, watched it split, the pieces, enshrouded by clouds of dust, playing
hide-and-seek and spreading out in their orbits. In a limited way, each
fragment had its own personality. Now they're all gone, ablated into molecules
and atoms in the upper atmosphere of the Solar System's largest planet. In a
way, we almost mourn them. But we're learning from their fiery deaths. It is
perhaps some reassurance to know that there are a hundred trillion more of them
in the Sun's vast treasure-house of worlds.
THERE ARE ABOUT 200 known asteroids
whose paths take them near the Earth. They are called, appropriately enough,
“near-Earth” asteroids. Their detailed appearance (like that of their main-belt
cousins) immediately implies that they are the products of a violent
collisional history. Many of them may be the shards and remnants of once-larger
worldlets.
With a few exceptions, the near-Earth
asteroids are only a few kilometers across or smaller, and take one to a few
years to make a circuit around the Sun. About 20 percent of them, sooner or
later, are bound to hit the Earth-with devastating consequences. (But in
astronomy, “sooner or later” can encompass billions of years.) Cicero's
assurance that “nothing of chance or hazard” is to be found in an absolutely
ordered and regular heaven is a profound misperception. Even today, as Comet
Shoemaker-Levy 9's encounter with Jupiter reminds us, there is routine
interplanetary violence, although not on the scale that marked the early
history of the Solar System.
Like main-belt asteroids, many
near-Earth asteroids are rocky. A few are mainly metal, and it has been
suggested that enormous rewards might attend moving such an asteroid into orbit
around the Earth, and then systematically mining it-a mountain of high-grade
ore a few hundred miles overhead. The value of platinum-group metals alone in a
single such world has been estimated as many trillions of dollars-although the
unit price would plummet spectacularly if such materials became widely
available. Methods of extracting metals and minerals from appropriate asteroids
are being studied, for example by John Lewis, a planetary scientist at the
University of Arizona.
Some near-Earth asteroids are rich in
organic matter, apparently preserved from the very earliest Solar System. Some
have been found, by Steven Ostro of the Jet Propulsion Laboratory, to be
double, two bodies in contact. Perhaps a larger world has broken in two as it
passed through the strong gravitational tides of a planet like Jupiter; more
interesting is the possibility that two worlds on similar orbits made a gentle
overtaking collision and stuck. This process may have been key to the building
of planets and the Earth. At least one asteroid (Ida, as viewed by Galileo) has
its own small moon. We might guess that two asteroids in contact and two
asteroids orbiting one another have related origins.
Sometimes, we hear about an asteroid
making a “near miss.” (Why do we call it a “near miss”? A “near hit” is what we
really mean.) But then we read a little more carefully, and it turns out that
its closest approach to the Earth was several hundreds of thousands or millions
of kilometers. That doesn't count-that's too far away, farther even than the
Moon. If we had an inventory of all the near-Earth asteroids, including those
considerably smaller than a kilometer across, we could project their orbits
into the future and predict which ones are potentially dangerous. There are an
estimated 2,000 of them bigger than a kilometer across, of which we have
actually observed only a few percent. There are maybe 200,000 bigger than 100
meters in diameter.
The near-Earth asteroids have evocative
mythological names: Orpheus, Hathor, Icarus, Adonis, Apollo, Cerberus, Khufu,
Amor, Tantalus, Aten, Midas, Ra-Shalom, Phaethon, Toutatis, Quetzalcoatl. There
are a few of special exploratory potential-for example, Nereus. In general,
it's much easier to get onto and off of near-Earth asteroids than the Moon.
Nereus, a tiny world about a kilometer across, is one of the easiest. It would
be real exploration of a truly new world.
Some humans (all from the former Soviet
Union) have already been in space for periods longer than the entire roundtrip
time to Nereus. The rocket technology to get there already exists. It's a much
smaller step than going to Mars or even, in several respects, than returning to
the Moon. If something went wrong, though, we would be unable to run home to
safety in only a few days. In this respect, its level of difficulty lies
somewhere between a voyage to Mars and one to the Moon.
Of many possible future missions to
Nereus, there's one that takes 10 months to get there from Earth, spends 30
days there, and then requires only 3 weeks to return to home. We could visit
Nereus with robots, or-if we're up to it-with humans. We could examine this
little world's shape, constitution, interior, past history, organic chemistry,
cosmic evolution, and possible tie to comets. We could bring samples back for
examination at leisure in Earthbound laboratories. We could investigate whether
there really are commercially valuable resources-metals or minerals-there. If
we are ever going to send humans to Mars, near-Earth asteroids provide a
convenient and appropriate intermediate goal-to test out the equipment and
exploratory protocols while studying an almost wholly unknown little world.
Here's a way to get our feet wet again when we're ready to reenter the cosmic
ocean.
CHAPTER 18
The marsh of camarina
[I]t's too late to make any improvements
now. The universe is finished; the copestone is on, and the chips were carted
off a million years ago.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 2
(1851)
Camarina was a city in southern Sicily,
founded by colonists from Syracuse in 598 B. C. A generation or two later, it
was threatened by a pestilence-festering, some said, in the adjacent marsh.
(While the germ theory of disease was certainly not widely accepted in the
ancient world, there were hints-for example, Marcus Varro in the first century
B. C. advised explicitly against building cities near swamps “because there are
bred certain minute creatures which cannot be seen by the eyes, which float in
the air and enter the body through the mouth and nose and there cause serious
disease.”) The danger to Camarina was great. Plans were drawn to drain the
marsh. When the oracle was consulted, though, it forbade such a course of
action, counseling patience instead. But lives were at stake, the oracle was
ignored, and the marsh was drained. The pestilence was promptly halted. Too
late, it was recognized that the marsh had protected the city from its
enemies-among whom there had now to be counted their cousins the Syracusans. As
in America 2,300 years later, the colonists had quarreled with the mother
country. In 552 B. C., a Syracusan force crossed over the dry land where the
marsh had been, slaughtered every man, woman, and child, and razed the city.
The marsh of Camarina became proverbial for eliminating a danger in such a way
as to usher in another, much worse.
THE CRETACEOUS-TERTIARY COLLISION (or
collisions-there may have been more than one) illuminates the peril from
asteroids and comets. In sequence, a world-immolating fire burned vegetation to
a crisp all over the planet; a stratospheric dust cloud so darkened the sky
that surviving plants had trouble making a living from photosynthesis; there
were worldwide freezing temperatures, torrential rains of caustic acids,
massive depletion of the ozone layer, and, to top it off, after the Earth
healed itself from these assaults, a prolonged greenhouse warming (because the
main impact seems to have volatilized a deep layer of sedimentary carbonates,
pouring huge amounts of carbon dioxide into the air). It was not a single
catastrophe, but a parade of them, a concatenation of terrors. Organisms
weakened by one disaster were finished off by the next. It is quite uncertain
whether our civilization would survive even a considerably less energetic
collision.
Since there are many more small
asteroids than large ones, run-of-the-mill collisions with the Earth will be
made by the little guys. But the longer you're prepared to wait, the more devastating
the impact you can expect. On average, once every few hundred years the Earth
is hit by an object about 70 meters in diameter; the resulting energy released
is equivalent to the largest nuclear weapons explosion ever detonated. Every
10,000 years, we're hit by a 200-meter object that might induce serious
regional climatic effects. Every million years, an impact by a body over 2
kilometers in diameter occurs, equivalent to nearly a million megatons of
TNT-an explosion that would work a global catastrophe, killing (unless
unprecedented precautions were taken) a significant fraction of the human
species. A million megatons of TNT is 100 times the explosive yield of all the
nuclear weapons on the planet, if simultaneously blown up. Dwarfing even this,
in a hundred million years or so, you can bet on something like the
Cretaceous-Tertiary event, the impact of a world 10 kilometers across or
bigger. The destructive energy latent in a large near-Earth asteroid dwarfs
anything else the human species can get its hands on.
As first shown by the American planetary
scientist Christopher Chyba and his colleagues, little asteroids or comets, a
few tens of meters across, break and burn up on entering our atmosphere. They
arrive comparatively often but do no significant harm. Some idea of how
frequently they enter the Earth's atmosphere has been revealed by declassified
Department of Defense data obtained from special satellites monitoring the
Earth for clandestine nuclear explosions. There seem to have been hundreds of
small worldlets (and at least one larger body) impacting in the last 20 years.
They did no harm. But, we need to be very sure we can distinguish a small
colliding comet or asteroid from an atmospheric nuclear explosion.
Civilization-threatening impacts require
bodies several hundred meters across, or more. (A meter is about a yard; 100
meters is roughly the length of a football field.) They arrive something like
once every 200,000 years. Our civilization is only about 10,000 years old, so
we should have no institutional memory of the last such impact. Nor do we.
Comet Shoemaker-Levy 9, in its
succession of fiery explosions on Jupiter in July 1994, reminds us that such
impacts really do occur in our time-and that the impact of a body a few
kilometers across can spread debris over an area as big as the Earth. It was a
kind of portent.
In the very week of the Shoemaker-Levy
impact, the Science and Space Committee of the U. S. House of Representatives
drafted legislation that requires NASA “in coordination with the Department of
Defense and the space agencies of other countries” to identify and determine
the orbital characteristics of all Earth-approaching “comets and asteroids that
are greater than 1 kilometer in diameter.” The work is to be completed by the
year 2005. Such a search program had been advocated by many planetary
scientists. But it took the death throes of a comet to move it toward practical
implementation.
Spread out over the waiting time, the
dangers of asteroid collision do not seem very worrisome. But if a big impact
happens, it would be an unprecedented human catastrophe. There's something like
one chance in two thousand that such a collision will happen in the lifetime of
a newborn baby. Most of us would not fly in an airplane if the chance of
crashing were one in two thousand. (In fact for commercial flights the chance
is one in two million. Even so, many people consider this large enough to worry
about, or even to take out insurance for.) When our lives are at stake, we
often change our behavior to arrange more favorable odds. Those who don't tend
to be no longer with us.
Perhaps we should practice getting to
these worldlets and diverting their orbits, should the hour of need ever arise.
Melville notwithstanding, some of the chips of creation are still left, and
improvements evidently need to be made. Along parallel and only weakly
interacting tracks, the planetary science community and the U. S. and Russian
nuclear weapons laboratories, aware of the foregoing scenarios, have been
pursuing these questions: how to monitor all sizable near-Earth interplanetary
objects, how to characterize their physical and chemical nature, how to predict
which ones may be on a future collision trajectory with Earth, and, finally,
how to prevent a collision from happening.
The Russian spaceflight pioneer
Konstantin Tsiolkovsky argued a century ago that there must be bodies
intermediate ill size between the observed large asteroids and those asteroidal
fragments, the meteorites, that occasionally fall to Earth. He wrote about
living on small asteroids in interplanetary space. He did not have military
applications in mind. In the early 1980s, though, some in the U. S. weapons
establishment argued that the Soviets might use near-Earth asteroids as
first-strike weapons; the alleged plan was called “Ivan's Hammer.”
Countermeasures were needed. But, at the same time, it was suggested, maybe it
wasn't a bad idea for the United States to learn how to use small worlds as
weapons of its own. The Defense Department's Ballistic Missile Defense
Organization, the successor to the Star Wars office of the 1980s, launched an
innovative spacecraft called Clementine to orbit the Moon and fly by the
near-Earth asteroid Geographos. (After completing a remarkable reconnaissance
of the Moon in May 1994, the spacecraft failed before it could reach
Geographos.)
In principle, you could use big rocket
engines, or projectile impact, or equip tile asteroid with giant reflective
panels and shove it with sunlight or powerful Earth-based lasers. But with
technology that exists right now, there are only two ways. First, one or more
high-yield nuclear weapons might blast the asteroid or comet into fragments
that would disintegrate and atomize on entering the Earth's atmosphere. If the
offending worldlet is only weakly held together, perhaps only hundreds of
megatons would suffice. Since there is no theoretical upper limit to the
explosive yield of a thermonuclear weapon, there seem to be those in the
weapons laboratories who consider making bigger bombs not only as a stirring
challenge, but also as a way to mute pesky environmentalists by securing a seat
for nuclear weapons on the save-the-Earth bandwagon.
Another approach under more serious
discussion is less dramatic but still an effective way of maintaining the
weapons establishment-a plan to alter the orbit of any errant worldlet by
exploding nuclear weapons nearby. The explosions (generally near the asteroid's
closest point to the Sun) are arranged to deflect it away from the Earth. A flurry of low-yield nuclear weapons, each
giving a little push in the desired direction, is enough to deflect a
medium-sized asteroid with only a few weeks' warning. The method also offers,
it is hoped, a way to deal with a suddenly detected long-period comet on imminent
collision trajectory with the Earth: The comet would be intercepted with a
small asteroid. (Needless to say, this game of celestial billiards is even more
difficult and uncertain-and therefore even less practical in the near
future-than the herding of an asteroid on a known, well-behaved orbit with
months or years at our disposal.)
We don't know what a standoff nuclear
explosion would do to an asteroid. The answer may vary from asteroid to
asteroid. Some small worlds might be strongly held together; others might be
little more than self-gravitating gravel heaps. If an explosion breaks, let's
say, a 10kilometer asteroid up into hundreds of 1-kilometer fragments, the
likelihood that at least one of them impacts the Earth is probably increased,
and the apocalyptic character of the consequences may not be much reduced. On
the other hand, if the explosion disrupts the asteroid into a swarm of objects
a hundred meters in diameter or smaller, all of them might ablate away like
giant meteors on entering the Earth's atmosphere. In this case little impact
damage would be caused. Even if the asteroid were wholly pulverized into fine
powder, though, the resulting high-altitude dust layer might be so opaque as to
block the sunlight and change the climate. We do not yet know.
A vision of dozens or hundreds of
nuclear-armed missiles on ready standby to deal with threatening asteroids or
comets has been offered. However premature in this particular application, it
seems very familiar; only the enemy has been changed. It also seems very
dangerous.
The problem, Steven Ostro of JPL and I
have suggested, is that if you can reliably deflect a threatening worldlet so
it does not collide with the Earth, you can also reliably deflect a harmless
worldlet so it does collide with the Earth. Suppose you had a full inventory,
with orbits, of the estimated 300,000 near-Earth asteroids larger than 100
meters-each of them large enough, on impacting the Earth, to have serious
consequences. Then, it turns out, you also have a list of huge numbers of
inoffensive asteroids whose orbits could be altered with nuclear warheads so
they quickly collide with the Earth.
Suppose we restrict our attention to the
2,000 or so near-Earth asteroids that are a kilometer across or bigger-that is,
the ones most likely to cause a global catastrophe. Today, with only about 100
of these objects catalogued, it would take about a century to catch one when
it's easily deflectable to Earth and alter its orbit. We think we've found one,
an as-yet-unnamed asteroid so far denoted only as 1991OA. In 2070, this world,
about 1 kilometer in diameter, will come within 4. 5 million kilometers of the
Earth's orbit-only fifteen times the distance to the Moon. To deflect 1991OA so
it hits the Earth, only about 60 megatons of TNT equivalent needs to be
exploded in the right way-the equivalent of a small number of currently
available nuclear warheads.
Now imagine a time, a few decades hence,
when all such near-Earth asteroids are inventoried and their orbits compiled.
Then, as Alan Harris of JPL, Greg Canavan of the Los Alamos National
Laboratory, Ostro, and I have shown, it might take only a year to select a
suitable object, alter its orbit, and send it crashing into the Earth with
cataclysmic effect.
The technology required-large optical
telescopes, sensitive detectors, rocket propulsion systems able to lift a few
tons of payload and make precise rendezvous in nearby space, and thermonuclear
weapons-all exist today. Improvements in all but perhaps the last can be
confidently expected. If we're not careful, many nations may have these
capabilities in the next few decades. What kind of world will we then have
made?
We have a tendency to minimize the
dangers of new technologies. A year before the Chernobyl disaster, a Soviet
nuclear power industry deputy minister was asked about the safety of Soviet
reactors, and chose Chernobyl as a particularly safe site. The average waiting
time to disaster, he confidently estimated, was a hundred thousand years. Less
than a year later...devastation. Similar reassurances were provided by NASA
contractors the year before the Challenger disaster: You would have to wait ten
thousand years, they estimated, for a catastrophic failure of the shuttle. One
year later...heartbreak.
Chlorofluorocarbons (CFCs) were
developed specifically as a completely safe refrigerant-to replace ammonia and
other refrigerants that, on leaking out, had caused illness and some deaths.
Chemically inert, nontoxic (in ordinary concentrations), odorless, tasteless,
non-allergenic, nonflammable, CFCs represent a brilliant technical solution to
a well-defined practical problem. They found uses in many other industries
besides refrigeration and air conditioning. But, as I described above, the
chemists who developed CFCs overlooked one essential fact-that the molecules'
very inertness guarantees that they are circulated to stratospheric altitudes
and there cracked open by sunlight, releasing chlorine atoms which then attack
the protective ozone layer. Due to the work of a few scientists, the dangers
may have been recognized and averted in time. We humans have now almost stopped
producing CFCs. We won't actually know if we've avoided real harm for about a
century; that's how long it takes for all the CFC damage to be completed. Like
the ancient Camarinans, we make mistakes. Not only do we often ignore the
warnings of the Oracles; characteristically we do not even consult them.
The notion of moving asteroids into
Earth orbit has proved attractive to some space scientists and long-range
planners. They foresee mining the minerals and precious metals of these worlds
or providing resources for the construction of space infrastructure without
having to fight the Earth's gravity to get them up there. Articles have been
published on how to accomplish this end and what the benefits will be. In
modern discussions, the asteroid is inserted into orbit around the Earth by
first making it pass through and be braked by the Earth's atmosphere, a
maneuver with very little margin for error. For the near future we can, I
think, recognize this whole endeavor as unusually dangerous and foolhardy,
especially for metal worldlets larger than tens of meters across. This is the
one activity where errors in navigation or propulsion or mission design can
have the most sweeping and catastrophic consequences.
The foregoing are examples of
inadvertence. But there's another kind of peril: We are sometimes told that
this or that invention would of course not be misused. no sane person would be
so reckless. This is the “only a madman” argument. Whenever I hear it (and it's
often trotted out in such debates), I remind myself that madmen really exist.
Sometimes they achieve the highest levels of political power in modern
industrial nations. This is the century of Hitler and Stalin, tyrants who posed
the gravest dangers not just to the rest of the human family, but to their own
people as well. In the winter and spring of 1945, Hitler ordered Germany to be
destroyed-even “what the people need for elementary survival”-because the
surviving Germans had “betrayed” him, and at any rate were “inferior” to those
who had already died. If Hitler had had nuclear weapons, the threat of a
counterstrike by Allied nuclear weapons, had there been any, is unlikely to
have dissuaded him. It might have encouraged him.
Can we humans be trusted with
civilization-threatening technologies? If the chance is almost one in a
thousand that much of the human population will be killed by an impact in the
next century, isn't it more likely that asteroid deflection technology will get
into the wrong hands in another century-some misanthropic sociopath like a
Hitler or a Stalin eager to kill everybody, a megalomaniac lusting after
“greatness” and “glory,” a victim of ethnic violence bent on revenge, someone
in the grip of unusually severe testosterone poisoning, some religious fanatic
hastening the Day of judgment, or just technicians incompetent or
insufficiently vigilant in handling the controls and safeguards? Such people
exist. The risks seem far worse than the benefits, the cure worse than the
disease. The cloud of near-Earth asteroids through which the Earth plows may
constitute a modern Camarine marsh.
It's easy to think that all of this must
be very unlikely, mere anxious fantasy. Surely sober heads would prevail. Think
of how many people would be involved in preparing and launching warheads, in
space navigation, in detonating warheads, in checking what orbital perturbation
each nuclear explosion has made, in herding the asteroid so it is on an impact
trajectory with Earth, and so on. Isn't it noteworthy that although Hitler gave
orders for the retreating Nazi troops to burn Paris and to lay waste to Germany
itself, his orders were not carried out? Surely someone essential to the
success of the deflection mission will recognize the danger. Even assurances
that the project is designed to destroy some vile enemy nation would probably
be disbelieved, because the effects of collision are planet-wide (and anyway
it's very hard to make sure your asteroid excavates its monster crater in a
particularly deserving nation).
But now imagine a totalitarian state,
not overrun by enemy troops, but one thriving and self-confident. Imagine a
tradition in which orders are obeyed without question. Imagine that those
involved in the operation are supplied a cover story: The asteroid is about to
impact the Earth, and it is their job to deflect it-but in order not to worry
people needlessly, the operation must be performed in secret. In a military
setting with a command hierarchy firmly in place, compartmentalization of
knowledge, general secrecy, and a cover story, can we be confident that even
apocalyptic orders would be disobeyed? Are we really sure that in the next
decades and centuries and millennia, nothing like this might happen? How sure
are we?
It's no use saying that all technologies
can be used for good or for ill. That is certainly true, but when the “ill”
achieves a sufficiently apocalyptic scale, we may have to set limits on which
technologies may be developed. (In a way we do this all the time, because we
can't afford to develop all technologies. Some are favored and some are not.)
Or constraints may have to be levied by the community of nations on madmen and
autarchs and fanaticism.
Tracking asteroids and comets is
prudent, it's good science, and it doesn't cost much. But, knowing our
weaknesses, why would we even consider now developing the technology to deflect
small worlds? For safety, shall we imagine this technology in the hands of many
nations, each providing checks and balances against misuse by another? This is
nothing like the old nuclear balance of terror. It hardly inhibits some madman
intent on global catastrophe to know that if he does not hurry, a rival may
beat him to it. How confident can we be that the community of nations will be
able to detect a cleverly designed, clandestine asteroid deflection in time to
do something about it? If such a technology were developed, can any
international safeguards be envisioned that have a reliability commensurate
with the risk?
Even if we restrict ourselves merely to
surveillance, there's a risk. Imagine that in a generation we characterize the
orbits of 30,000 objects of 100-meter diameter or more, and that this
information is publicized, as of course it should be. Maps will be published
showing near-Earth space black with the orbits of asteroids and comets, 30,000
swords of Damocles hanging over our heads-ten times more than the number of
stars visible to the naked eye under conditions of optimum atmospheric clarity.
Public anxiety might be much greater in such a time of knowledge than in our
current age of ignorance. There might be irresistible public pressure to
develop means to mitigate even nonexistent threats, which would then feed the
danger that deflection technology would be misused. For this reason, asteroid
discovery and surveillance may not be a mere neutral tool of future policy, but
rather a kind of booby trap. To me, the only foreseeable solution is a
combination of accurate orbit estimation, realistic threat assessment, and
effective public education-so that in democracies at least, the citizens can
make their own, informed decisions. This is a job for NASA.
Near-Earth asteroids, and means of
altering their orbits, are being looked at seriously. There is some sign that
officials in the Department of Defense and the weapons laboratories are
beginning to understand that there may be real dangers in planning to push
asteroids around. Civilian and military scientists have met to discuss the
subject. On first hearing about the asteroid hazard, many people think of it as
a kind of Chicken Little fable; Goosey-Lucy, newly arrived and in great
excitement, is communicating the urgent news that the sky is falling. The
tendency to dismiss the prospect of any catastrophe that we have not personally
witnessed is in the long run very foolish. But in this case it may be an ally
of prudence.
MEANWHILE WE MUST STILL FACE the
deflection dilemma. If we develop and deploy this technology, it may do us in.
If we don't, some asteroid or comet may do us in. The resolution of the dilemma
hinges, I think, on the fact that the likely timescales of the two dangers are
very different-short for the former, long for the latter.
I like to think that our future
involvement with near-Earth asteroids will go something like this: From
ground-based observatories, we discover all the big ones, plot and monitor
their orbits, determine rotation rates and compositions. Scientists are
diligent in explaining the dangers-neither exaggerating nor muting the prospects.
We send robotic spacecraft to fly by a few selected bodies, orbit them, land on
them, and return surface samples to laboratories on Earth. Eventually we send
humans. (Because of the low gravities, they will be able to make standing broad
jumps of ten kilometers or more into the sky, and lob a baseball into orbit
around the asteroid.) Fully aware of the dangers, we make no attempts to alter
trajectories until the potential for misuse of world-altering technologies is
much less. That might take a while.
If we're too quick in developing the
technology to move worlds around, we may destroy ourselves; if we're too slow,
we will surely destroy ourselves. The reliability of world political
organizations and the confidence they inspire will have to make significant
strides before they can be trusted to deal with a problem of this seriousness.
At the same time, there seems to be no acceptable national solution. Who would
feel comfortable with the means of world destruction in the hands of some
dedicated (or even potential) enemy nation, whether or not our nation had
comparable powers? The existence of interplanetary collision hazards, when
widely understood, works to bring our species together. When facing a common
danger, we humans have sometimes reached heights widely thought impossible; we
have set aside our differences-at least until the danger passed.
But this danger never passes. The
asteroids, gravitationally churning, are slowly altering their orbits; without
warning, new comets come careening toward us from the transplutonian darkness.
There will always be a need to deal with them in a way that does not endanger
us. By posing two different classes of peril-one natural, the other
human-made-the small near-Earth worlds provide a new and potent motivation to create
effective transnational institutions and to unify the human species. It's hard
to see any satisfactory alternative.
In our usual jittery,
two-steps-forward-one-step-back mode, We are moving toward unification anyway.
There are powerful influences deriving from transportation and communications
technologies, the interdependent world economy, and the global environmental
crisis. The impact hazard merely hastens the pace.
Eventually, cautiously, scrupulously
careful to attempt nothing with asteroids that could inadvertently cause a
catastrophe on Earth, I imagine we will begin to learn how to change the orbits
of little nonmetallic worlds, smaller than 100 meters across. We begin with
smaller explosions and slowly work our way up. We gain experience in changing
the orbits of various asteroids and comets of different compositions and
strengths. We try to determine which ones can be pushed around and which
cannot. By the twenty-second century, perhaps, we move small worlds around the
Solar System, using (see next chapter) not nuclear explosions but nuclear
fusion engines or their equivalents. We insert small asteroids made of precious
and industrial metals into Earth orbit. Gradually develop a defensive
technology to deflect a large asteroid or comet that might in the foreseeable
future hit the Earth, while, with meticulous care, we build layers of
safeguards against misuse.
Since the danger of misusing deflection
technology seems so much greater than the danger of an imminent impact, we can
afford to wait, take precautions, rebuild political institutions-for decades
certainly, probably centuries. If we play our cards right and are not unlucky,
we can pace what we do up there by what progress we're making down here. The
two are in any case deeply connected.
The asteroid hazard forces our hand.
Eventually, we must establish a formidable human presence throughout the inner
Solar System. On an issue of this importance I do not think we will be content
with purely robotic means of mitigation. To do so safely we must make changes
in our political and international systems. While much about our future is
cloudy, this conclusion seems a little more robust, and independent of the
vagaries of human institutions.
In the long term, even if we were not
the descendants of professional wanderers, even if we were not inspired by
exploratory passions, some of us would still have to leave the Earth-simply to
ensure the survival of all of us. And once we're out there, we'll need bases,
infrastructures. It would not be very long before some of us were living in
artificial habitats and on other worlds. This is the first of two mussing
arguments, omitted in our discussion of missions to Mars, for a permanent human
presence in space.
OTHER PLANETARY SYSTEMS must face their
own impact hazards-because small primordial worlds, of which asteroids and
comets are remnants, are the stuff out of which planets form there as well.
After the planets are made, many of these planetesimals are left over. The
average time between civilization-threatening impacts on Earth is perhaps
200,000 years, twenty times the age of our civilization. Very different waiting
times may pertain to extraterrestrial civilizations, if they exist, depending
on such factors as the physical and chemical characteristics of the planet and
its biosphere, the biological and social nature of the civilization, and of
course the collision rate itself. Planets with higher atmospheric pressures
will be protected against somewhat larger 1mpactors, although the pressure
cannot be much greater before greenhouse warming and other consequences make
life improbable. If the gravity is much less than on Earth, impactors will make
less energetic collisions and the hazard will be reduced-although it cannot be
reduced very much before the atmosphere escapes to space.
The impact rate in other planetary
systems is uncertain. Our system contains two major populations of small bodies
that feed potential impactors into Earth-crossing orbits. Both the existence of
the source populations and the mechanisms that maintain the collision rate
depend on how worlds are distributed. For example, our Oort Cloud seems to have
been populated by gravitational ejections of icy worldlets from the vicinity of
Uranus and Neptune. If there are no planets that play the role of Uranus and
Neptune in systems otherwise like our own, their Oort Clouds may be much more
thinly populated. Stars in open and globular stellar clusters, stars in double
or multiple systems, stars closer to the center of the Galaxy, stars
experiencing more frequent encounters with Giant Molecular Clouds in
interstellar space, may all experience higher impact fluxes at their
terrestrial planets. The cometary flux might be hundreds or thousands of times
more at the Earth had the planet Jupiter never formed-according to a
calculation by George Wetherill of the Carnegie Institution of Washington. In
systems without Jupiter-like planets, the gravitational shield against comets
is down, and civilization-threatening impacts much more frequent.
To a certain extent, increased fluxes of
interplanetary objects might increase the rate of evolution, as the mammals
that flourished and diversified after the Cretaceous-Tertiary collision wiped
out the dinosaurs. But there must be a point of diminishing returns: Clearly, some
flux is too high for the continuance of any civilization.
One consequence of this train of
argument is that, even if civilizations commonly arise on planets throughout
the Galaxy, few of them will be both long-lived and non-technological. Since
hazards from asteroids and comets must apply to inhabited planets all over the
Galaxy, if there are such, intelligent beings everywhere will have to unify
their home worlds politically, leave their planets, and move small nearby
worlds around. Their eventual choice, as ours, is spaceflight or extinction.
CHAPTER 19
Remaking the planets
Who could deny that man could somehow
also make the heavens, could he only obtain the instruments and the heavenly
material?
—MARSILIO FICINO, “THE SOUL OF MAN” (CA.
1474)
In the midst of the Second World War, a
young American writer named Jack Williamson envisioned a populated Solar
System. In the twenty-second century, he imagined, Venus would be settled by
China, Japan, and Indonesia; Mars by Germany; and the moons of Jupiter by
Russia. Those who spoke English, the language in which Williamson was writing,
were confined to the asteroids-and of course the Earth.
The story, published in Astounding
Science Fiction in July 1942, was called “Collision Orbit” and written under
the pseudonym Will Stewart. Its plot hinged on the imminent collision of an
uninhabited asteroid with a colonized one, and the search for a means of
altering the trajectories of small worlds. Although no one on Earth was
endangered, this may have been the first appearance, apart from newspaper comic
strips, of asteroid collisions as a threat to humans. (Comets impacting the
Earth had been a staple peril.)
The environments of Mars and Venus were
poorly understood in the early 1940s; it was conceivable that humans could live
there without elaborate life-support systems. But the asteroids were another
matter. It was well known, even then, that asteroids were small, dry, airless
worlds. If they were to be inhabited, especially by large numbers of people,
these little worlds would somehow have to be fixed.
In “Collision Orbit,” Williamson
portrays a group of “spatial engineers,” able to render such barren outposts
clement. Coining a word, Williamson called the process of metamorphosis into an
Earth-like world “terraforming.” He knew that the low gravity on an asteroid
means that any atmosphere generated or transported there would quickly escape
to space. So his key terraforming technology was “paragravity,” an artificial
gravity that would hold a dense atmosphere.
As nearly as we can tell today,
paragravity is a physical impossibility. But we can imagine domed, transparent
habitats on the surfaces of asteroids, as suggested by Konstantin Tsiolkovsky,
or communities established in the insides of asteroids, as outlined in the
1920s by the British scientist J. D. Bernal. Because asteroids are small and
their gravities low, even massive subsurface construction might be
comparatively easy. If a tunnel were dug clean through, you could jump in at
one end and emerge some 45 minutes later at the other, oscillating up and down
along the toll diameter of this world indefinitely. Inside the right kind of
asteroid, a carbonaceous one, you can find materials for manufacturing stone,
metal, and plastic construction and plentiful water-all you might need to build
a subsurface closed ecological system, an underground garden. Implementation
would require a significant step beyond what we have today, but-unlike
“paragravity”-nothing in such a scheme seems impossible. All the elements can be
found in contemporary technology. If there were sufficient reason, a fair
number of us could be living on (or in asteroids by the twenty-second century.
They would of course need a source of
power, not just to sustain themselves, but, as Bernal suggested, to move their
asteroidal homes around. (It does not seem so big a step from explosive
alteration of asteroid orbits to a more gentle means of propulsion a century or
two later.) If an oxygen atmosphere were generated from chemically bound water,
then organics could be burned to generate power, just as fossil fuels are
burned on the Earth today. Solar power could be considered, although for the
main-belt asteroids the intensity of sunlight is only about 10 percent what it
is on Earth. Still, we could imagine vast fields of solar panels covering the
surfaces of inhabited asteroids and converting sunlight into electricity.
Photovoltaic technology is routinely used in Earth-orbiting spacecraft, and is
in increasing use on the surface of the Earth today. But while that might be
enough to warm and light the homes of these descendants, it does not seem
adequate to change asteroid orbits.
For that, Williamson proposed using
anti-matter. Antimatter is just like ordinary matter, with one significant
difference. Consider hydrogen: An ordinary hydrogen atom consists of a
positively charged proton on the inside and a negatively charged electron on
the outside. An atom of anti-hydrogen consists of a negatively charged proton
on the inside and a positively charged electron (also called a positron) on the
outside. The protons, whatever the sign of their charges, have the same mass;
and the electrons, whatever the sign of their charges, have the same mass.
Particles with opposite charges attract. A hydrogen atom and an anti-hydrogen
atom are both stable, because in both cases the positive and negative
electrical charges precisely balance.
Anti-matter is not some hypothetical
construct from the perfervid musings of science fiction writers or theoretical
physicists. Anti-matter exists. Physicists make it in nuclear accelerators; it
can be found in high-energy cosmic rays. So why don't we hear more about it?
Why has no one held up a lump of antimatter for our inspection? Because matter
and anti-matter, when brought into contact, violently annihilate each other,
disappearing in an intense burst of gamma rays. We cannot tell whether
something is made of matter or anti-matter just by looking at it. The
spectroscopic properties of, for example, hydrogen and anti-hydrogen are
identical.
Albert Einstein's answer to the question
of why we see only matter and not anti-matter was, “Matter won”-by which he
meant that in our sector of the Universe at least, after almost all the matter
and anti-matter interacted and annihilated each other long ago, there was some
of what we call ordinary matter left over.
As far as we can tell today, from gamma ray astronomy and other means,
the Universe is made almost entirely of matter. The reason for this engages the
deepest cosmological issues, which need not detain us here. But if there was
only a one-particle-in-a-billion difference in the preponderance of matter over
anti-matter at the beginning, even this would be enough to explain the Universe
we see today.
Williamson imagined that humans in the
twenty-second century would move asteroids around by the controlled mutual
annihilation of matter and anti-matter. All the resulting gamma rays, if
collimated, would make a potent rocket exhaust. The anti-matter would be
available in the main asteroid belt (between the orbits of Mars and Jupiter),
because this was his explanation for the existence of the asteroid belt. In the
remote past, he proposed, an intruder anti-matter worldlet arrived in the Solar
System from the depths of space, impacted, and annihilated what was then an
Earthlike planet, fifth from the Sun. The fragments of this mighty collision
are the asteroids, and some of them are still made of anti-matter. Harness an
anti-asteroid-Williamson recognized that this might be tricky-and you can move
worlds around at will.
At the time, Williamson's ideas were
futuristic, but far from foolish. Some of “Collision Orbit” can be considered
visionary. Today, however, we have good reason to believe that there are no
significant amounts of anti-matter in the Solar System, and that the asteroid
belt, far from being a fragmented terrestrial planet, is an enormous array of
small bodies prevented (by the gravitational tides of Jupiter) from forming an
Earthlike world.
However, we do generate (very) small
amounts of antimatter in nuclear accelerators today, and we will probably be
able to manufacture much larger amounts by the twenty-second century. Because
it is so efficient-converting all of the matter into energy, E = MC2, with 100
percent efficiency-perhaps anti-matter engines will be a practical technology
by then, vindicating Williamson Failing that, what energy sources can we
realistically expect to be available, to reconfigure asteroids, to light them
warm them, and move them around?
The Sun shines by jamming protons
together and turning them into helium nuclei. Energy is released in the
process, although with less than 1 percent the efficiency of the annihilation
of matter and anti-matter. But even proton-proton reactions are far beyond
anything we can realistically imagine for ourselves in the near future. The
required temperatures are much too high. Instead) of jamming protons together,
though, we might use heavier kinds of hydrogen. We already do so in
thermonuclear weapons. Deuterium is a proton bound by nuclear forces to a
neutron; tritium is a proton bound by nuclear forces to two neutrons. It seems
likely that in another century we will have practical power schemes that
involve the controlled fusion of deuterium and tritium, and of deuterium and
helium. Deuterium and tritium are present as minor constituents in water (on
Earth and other worlds). The kind of helium needed for fusion, 3He (two protons
and a neutron make up its nucleus), has been implanted over billions of years
by the solar wind in the surfaces of the asteroids. These processes are not
nearly as efficient as the proton-proton reactions in the Sun, but they could
provide enough power to run a small city for a year from a lode of ice only a
few meters in size.
Fusion reactors seem to be coming along
too slowly to play a major role in solving, or even significantly mitigating,
global warming. But by the twenty-second century, they ought to be widely
available. With fusion rocket engines, it will be possible to more asteroids
and comets around the inner Solar System taking a main-belt asteroid, for
example, and inserting it into orbit around the Earth. A world 10 kilometers
across could be transported from Saturn, say, to Mars through nuclear burning
of the hydrogen in an icy comet a kilometer across. (Again, I'm assuming a time
of much greater political stability and safety.)
PUT ASIDE FOR THE MOMENT any qualms you
might have about the ethics of rearranging worlds, or our ability to do so
without catastrophic consequences. Digging out the insides of worldlets,
reconfiguring them for human habitation, and moving them from one place in the
Solar System to another seems to be within our grasp in another century or two.
Perhaps by then we will have adequate international safeguards as well. But
what about transforming the surface environments not of asteroids or comets,
but of planets? Could we live on Mars?
If we wanted to set up housekeeping on
Mars, it's easy to see that, in principle at least, we could do it: There's
abundant sunlight. There's plentiful water in the rocks and in underground and
polar ice. The atmosphere is mostly carbon dioxide. It seems likely that in
self-contained habitats-perhaps domed enclosures-we could grow crops,
manufacture oxygen from water, recycle wastes.
At first we'd be dependent on
commodities resupplied from Earth, but in time we'd manufacture more and more
of them ourselves. We'd become increasingly self-sufficient. The domed
enclosures, even if made of ordinary glass, would let in the visible sunlight
and screen out the Sun's ultraviolet rays. With oxygen masks and protective
garments-but nothing as bulky and cumbersome as a spacesuit-we could leave
these enclosures to go exploring, or to build another domed village and farms.
It seems very evocative of the American
pioneering experience, but with at least one major difference: In the early
stages, large subsidies are essential. The technology required is too expensive
for some poor family, like my grandparents a century ago, to pay their own
passage to Mars. The early Martian pioneers will be sent by governments and
will have highly specialized skills. But in a generation or two, when children
and grandchildren are born there- and especially when self-sufficiency is
within reach-that will begin to change. Youngsters born on Mars will be given
specialized training in the technology essential for survival in this new
environment. The settlers will become less heroic and less exceptional. The
full range of human strengths and deficiencies will begin to assert themselves.
Gradually, precisely because of the difficulty of getting from Earth to Mars, a
unique Martian culture will begin to emerge-distinct aspirations and fears tied
to the environment they live in, distinct technologies, distinct social
problems, distinct solutions-and, as has occurred in every similar circumstance
throughout human history, a gradual sense of cultural and political
estrangement from the mother world.
Great ships will arrive carrying
essential technology from Earth, new families of settlers, scarce resources. It
is hard to know, on the basis of our limited knowledge of Mars, whether they
will go home empty-or whether they will carry with them something found only on
Mars, something considered very valuable on Earth. Initially much of the
scientific investigation of samples of the Martian surface will be done on
Earth. But in time the scientific study of Mars (and its moons Phobos and
Deimos) will be done from Mars.
Eventually-as has happened with
virtually every other form of human transportation-interplanetary travel will
become accessible to people of ordinary means: to scientists pursuing their own
research projects, to settlers fed up with Earth, even to venturesome tourists.
And of course there will be explorers.
If the time ever came when it was possible
to make the Martian environment much more Earth-like-so protective garments,
oxygen masks, and domed farmlands and cities could be dispensed with-the
attraction and accessibility of Mars would be increased many-fold. The same, of
course, would be true for any other world which could be engineered so that
humans could live there without elaborate contrivances to keep the planetary
environment out. We would feel much more comfortable in our adopted home if an
intact dome or spacesuit weren't all that stood between us and death. (But
perhaps I exaggerate the dangers. People who live in the Netherlands seem at
least as well adjusted and carefree as other inhabitants of Northern Europe;
vet their dikes are all that stand between them and the sea.
Recognizing the speculative nature of
the question and the limitations in our knowledge, is it nevertheless possible
to envision terraforming the planets?
We need look no further than our own
world to see that humans are now able to alter planetary environments in a profound
way. Depletion of the ozone layer, global warming from an increased greenhouse
effect, and global cooling from nuclear war are all ways in which present
technology can significantly alter the environment of our world-and in each
case as an inadvertent consequence of doing something else. If we had intended
to alter our planetary environment, we would be fully able to generate still
greater change. As our technology becomes more powerful, we will be able to
work still more profound changes.
But just as (in parallel parking) it's
easier to get out of a parking place than into one, it's easier to destroy a
planetary environment than to move it into a narrowly prescribed range of
temperatures, pressures, compositions, and so on. We already know of a multitude
of desolate and uninhabitable worlds, and-with very narrow margins-only one
green and clement one. This is a major conclusion from early in the era of
spacecraft exploration of the Solar System. In altering the Earth, or any world
with an atmosphere, we must be very careful about positive feedbacks, where we
nudge an environment a little bit and it takes of on its own-a little cooling
leading to runaway glaciation, as may have happened on Mars, or a little
warming to a runaway greenhouse effect, as happened on Venus. It is not at all
clear that our knowledge is sufficient to this purpose.
As far as I know, the first suggestion
in the scientific literature about terraforming the planets was made in a 1961
article I wrote about Venus. I was pretty sure then that Venus had a surface
temperature well above the normal boiling point of water, produced by a carbon
dioxide/water vapor greenhouse effect. I imagined seeding its high clouds with
genetically engineered microorganisms that would take CO2, N2, and H2O out of
the atmosphere and convert them into organic molecules. The more CO2 removed,
the smaller the greenhouse effect and the cooler the surface. The microbes
would be carried down through the atmosphere toward the ground, where they
would be fried, so water vapor would be returned to the atmosphere; but the
carbon from the CO2 would be converted irreversibly by the high temperatures
into graphite or some other involatile form of carbon. Eventually, the
temperatures would fall below the boiling point and the surface of Venus would
become habitable, dotted with pools and lakes of warm water.
The idea was soon taken up by a number
of science fiction authors in the continuing dance between science and science
fiction-in which the science stimulates the fiction, and the fiction stimulates
a new generation of scientists, a process benefiting both genres. But as the
next step in the dance, it is now clear that seeding Venus with special
photosynthetic microorganisms will not work. Since 1961 we've discovered that
the clouds of Venus are a concentrated solution of sulfuric acid, which makes
the genetic engineering rather more challenging. But that in itself is not a
fatal flaw. (There are microorganisms that live out their lives in concentrated
solutions of sulfuric acid.) Here's the fatal flaw: In 1961 I thought the
atmospheric pressure at the surface of Venus N\-as a few “bars,” a few times
the surface pressure on Earth. We now know it to be 90 bars, so that if the
scheme worked, the result would be a surface buried in hundreds of meters of
fine graphite, and an atmosphere made of 65 bars of almost pure molecular
oxygen. Whether we would first implode under the atmospheric pressure or
spontaneously burst into flames in all that oxygen is an open question. However,
long before so much oxygen could build up, the graphite would spontaneously
burn back into CO2, short-circuiting the process. At best, such a scheme can
carry the terraforming of Venus only partway.
Let's assume that by the early
twenty-second century we have comparatively inexpensive heavy-lift vehicles, so
we can carry large payloads to other worlds; abundant and powerful fusion
reactors; and well-developed genetic engineering. All three assumptions are
likely, given current trends. Could we terraform the planets? James Pollack of
NASA's Ames Research Center and I surveyed this problem. Here's a summary of
what we found:
VENUS: Clearly the problem with Venus is
its massive greenhouse effect. If we could reduce the greenhouse effect almost
to zero, the climate might be balmy. But a 90-bar CO2 atmosphere is
oppressively thick. Over every postage stamp-sized square inch of surface, the
air weighs as much as six professional football players, piled one on top of
another. Making all that go away will take some doing.
Imagine bombarding Venus with asteroids
and comets. Each impact would blow away some of the atmosphere. To blow away
almost all of it, though, would require using up more big asteroids and comets
than there are-at least in the planetary part of the Solar System. Even if that
many potential impactors existed, even if we could make them all collide with
Venus (this is the overkill approach to the impact hazard problem), think what
we would have lost. Who knows what wonders, what practical knowledge they
contain? We would also obliterate much of Venus' gorgeous surface geology-which
we've just begun to understand, and which may teach us much about the Earth.
This is an example of bruteforce terraforming. I suggest we want to steer
entirely clear of such methods, even if someday we'll be able to afford them
(which I very much doubt). We want something more elegant, more subtle, more
respectful of the environments of other worlds. A microbial approach has some
of those virtues, but does not do the trick, as we've just seen.
We can imagine pulverizing a dark
asteroid and spreading the powder through the upper atmosphere of Venus, or
carrying such dust up from the surface. This would be the physical equivalent
of nuclear winter or the Cretaceous-Tertiary post-impact climate. If the
sunlight reaching the ground is sufficiently attenuated, the surface
temperature must fall. But by its very nature, this option plunges Venus into
deep gloom, with daytime light levels perhaps only as bright as on a moonlit
night on Earth. The oppressive, crushing 90-bar atmosphere would remain
untouched. Since the emplaced dust would sediment out every few years, the
layer would have to be replenished in the same period of time. Perhaps such an
approach would be acceptable for short exploratory missions, but the
environment generated seems very stark for a self-sustaining human community on
Venus.
We could use a giant artificial sunshade
in orbit around Venus to cool the surface; but it would be enormously
expensive, as well as having many of the deficiencies of the dust layer.
However, if the temperatures could be lowered sufficiently, the CO2 in the
atmosphere would rain out. There would be a transitional time of CO2 oceans on
Venus. If those oceans could be covered over to prevent re-evaporation-for
example, with water oceans made by melting a large, icy moon transported from
the outer Solar System-then the CO2 might conceivably be sequestered away, and
Venus converted into a water (or low-fizz seltzer) planet. Ways have also been
suggested to convert the CO2 into carbonate rock.
Thus all proposals for terraforming
Venus are still brute-force, inelegant, and absurdly expensive. The desired
planetary metamorphosis may be beyond our reach for a very long time, even if
we thought it was desirable and responsible. The Asian colonization of Venus
that Jack Williamson imagined may have to be redirected somewhere else.
MARS: For Mars we have just the opposite
problem. There's not enough greenhouse effect. The planet is a frozen desert.
But the fact that Mars seems to have had abundant rivers, lakes, and perhaps
even oceans 4 billion years ago-at a time when the Sun was less bright than it
is today-makes you wonder if there's solve natural instability in the Martian
climate, something on hair trigger that once released would all by itself
return the planet to its ancient clement state. (Let's note from the start that
doing so would destroy Martian landforms that hold key data on the
past-especially the laminated polar terrain.)
As we know very well from Earth and
Venus, carbon dioxide is a greenhouse gas. There are carbonate minerals found
on Mars, and dry ice in one of the polar caps. They could be converted into CO2
gas. But to make enough of a greenhouse effect to generate comfortable temperatures
on Mars would require the entire surface of the planet to be plowed up and
processed to a depth of kilometers. Apart from the daunting obstacles in
practical engineering that this represents-fusion power or no fusion power-and
the inconvenience to whatever self-contained, closed ecological systems humans
had already established on the planet it would also constitute the
irresponsible destruction of a unique scientific resource and database, the
Martian surface.
What about other greenhouse gases?
Alternatively, we might take chlorofluorocarbons (CFCs or HCFCs) to Mars after
manufacturing them on Earth. These are artificial substances that, so far as we
know, are found nowhere else in the Solar System. We can certainly imagine
manufacturing enough CFCs on Earth to warm Mars, because by accident in a few
decades with present technology on Earth we've managed to synthesize enough to
contribute to global warming on our planet. Transportation to Mars would be
expensive, though: Even using Saturn V- or Energiya-class boosters, it would
require at least a launch a day for a century. But perhaps they could be
manufactured from fluorine-containing minerals on Mars.
There is, in addition, a serious
drawback: On Mars as on Earth, abundant CFCs would prevent formation of an
ozone layer. CFCs might bring Martian temperatures into a clement range, but
guarantee that the solar ultraviolet hazard would remain extremely serious.
Perhaps the solar ultraviolet light could be absorbed by an atmospheric layer
of pulverized asteroidal or surface debris injected in carefully titrated
amounts above the CFCs. But now we're in the troubling circumstance of having
to deal with propagating side effects, each of which requires its own
large-scale technological solution.
A third possible greenhouse gas for
warming Mars is ammonia (NH3). Only a little ammonia would be enough to warm
the Martian surface to above the freezing point of water. In principle, this
might be done by specially engineered microorganisms that would convert Martian
atmospheric N2 to NH3 as some microbes do on Earth, but do it under Martian
conditions. Or the same conversion might be done in special factories.
Alternatively, the nitrogen required could be carried to Mars from elsewhere in
the Solar System. (N2 is the principal constituent in the atmospheres of both
Earth and Titan.) Ultraviolet light would convert ammonia back into N2 in about
30 years, so there would have to be a continuous resupply of NH3.
A judicious combination of CO2, CFC, and
NH3 greenhouse effects on Mars looks as if it might be able to bring surface
temperatures close enough to the freezing point of water for the second phase
of Martian terraforming to begin-temperatures rising due to the pressure of
substantial water vapor in the air, widespread production of O2 by genetically
engineered plants, and fine-tuning the surface environment. Microbes and larger
plants and animals could be established on Mars before the overall environment
was suitable for unprotected human settlers.
Terraforming Mars is plainly much easier
than terraforming Venus. But it is still very expensive by present standards,
and environmentally destructive. If there were sufficient justification,
though, perhaps the terraforming of Mars could be under way by the
twenty-second century.
THE MOONS OF JUPITER AND SATURN:
Terraforming the satellites of the Jovian planets presents varying degrees of
difficulty. Perhaps the easiest to contemplate is Titan. It already has an
atmosphere, made mainly of N2 like the Earth's, and is much closer to
terrestrial atmospheric pressures than either Venus or Mars. Moreover,
important greenhouse gases, such as NH3 and H20, are almost certainly frozen
out on its surface. Manufacture of initial greenhouse gases that do not freeze
out at present Titan temperatures plus direct warming of the surface by nuclear
fusion could, it seems, be the key early steps to one day terraform Titan.
IF THERE WERE A COMPELLING REASON for
terraforming other worlds, this greatest of engineering projects might be
feasible on the timescale we've been describing-certainly for asteroids,
possibly for Mars, Titan, and other moons of the outer planets, and probably
not for Venus. Pollack and I recognized that there are those who feel a
powerful attraction to the idea of rendering other worlds in the Solar System
suitable for human habitation-in establishing observatories, exploratory bases,
communities, and homesteads there. Because of its pioneering history, this may
be a particularly natural and attractive idea in the United States.
In any case, massive alteration of the
environments of other worlds can be done competently and responsibly only when
we have a much better understanding of those worlds than is available today.
Advocates of terraforming must first become advocates of the long-term and
thorough scientific exploration of other worlds.
Perhaps when we really understand the
difficulties of terraforming, the costs or the environmental penalties will
prove too steep, and we will lower our sights to domed or subsurface cities or
other local, closed ecological systems, greatly improved versions of Biosphere
II, on other worlds. Perhaps we will abandon the dream of converting the
surfaces of other worlds to something approaching the Earth's. Or perhaps there
are much more elegant, cost-effective, and environmentally responsible ways of
terraforming that we have not yet imagined.
But if we are seriously to pursue the
matter, certain questions ought to be asked: Given that any terraforming scheme
entails a balance of benefits against costs, how certain must we be that key
scientific information will not thereby be destroyed before proceeding? How
much understanding of the world in question do we need before planetary
engineering can be relied upon to produce the desired end state? Can we
guarantee a long-term human commitment to maintain and replenish an engineered
world, when human political institutions are so short lived? If a world is even
conceivably inhabited-perhaps only by microorganisms-do humans have a right to
alter it? What is our responsibility to preserve the worlds of the Solar System
in their present wilderness states for future generations-who may contemplate
uses that today we are too ignorant to foresee? These questions may perhaps be
encapsulated into a final question: Can we, who have made such a mess of this
world, be trusted with others?
It is just conceivable that some of the
techniques that might eventually terraform other worlds might be applied to
ameliorate the damage we have done to this one. Considering the relative
urgencies, a useful indication of when the human species is ready to consider
terraforming seriously is when we have put our own world right. We can consider
it a test of the depth of our understanding and our commitment. The first step
in engineering the Solar System is to guarantee the habitability of the Earth.
Then we'll be ready to spread out to
asteroids, comets, Mars, the moons of the outer Solar System, and beyond. Jack
Williamson 's prediction that this will begin to come about by the twenty-second
century may not be far off the mark.
THE NOTION OF OUR DESCENDANTS living and
working on other worlds, and even moving some of them around for their
convenience, seems the most extravagant science fiction. Be realistic, a voice
inside my head counsels. But this is realistic. We're on the cusp of the
technology, near the midpoint between impossible and routine. It's easy to be
conflicted about it. If we don't do something awful to ourselves in the
interim, in another century terraforming may seem no more impossible than a
human-tended space station does today.
I think the experience of living on
other worlds is bound to change us. Our descendants, born and raised elsewhere,
will naturally begin to owe primary loyalty to the worlds of their birth,
whatever affection they retain for the Earth. Their physical needs, their
methods of supplying those needs, their technologies, and their social
structures will all have to be different.
A blade of grass is a commonplace on
Earth; it would be a miracle on Mars. Our descendants on Mars will know the
value of a patch of green. And if a blade of grass is priceless, what is the
value of a human being? The American revolutionary Tom Paine, in describing his
contemporaries, had thoughts along these lines:
The wants which necessarily accompany
the cultivation of a wilderness produced among them a state of society which
countries long harassed by the quarrels and intrigues of governments had
neglected to cherish. In such a situation man becomes what he ought to be. He
sees his species...as kindred.
Having seen at first hand a procession
of barren and desolate worlds, it will be natural for our spacefaring
descendants to cherish life. Having learned something from the tenure of our
species on Earth, they may wish to apply those lessons to other worlds-to spare
generations to come the avoidable suffering that their ancestors were obliged
to endure, and to draw upon our experience and our mistakes as we begin our
open-ended evolution into space.
CHAPTER 20
Darkness
Far away, hidden from the eyes of
daylight, there are watchers in the skies.
—EURIPIDES, THE BACCHAE /CA. 406 B. C.)
As children, we fear the dark. Anything
might be out. here. The unknown troubles us. Ironically, it is our fate to live
in the dark. This unexpected finding of science is only about three centuries
old. Head out from the Earth in any direction you choose, and-after an initial
flash of blue and a longer wait while the Sun fades-you are surrounded by
blackness, punctuated only here and there by the faint and distant stars.
Even after we are grown, the darkness
retains its power to frighten us. And so there are those who say we should not
inquire too closely into who else might be living in that darkness. Better not
to know, they say.
There are 400 billion stars in the Milky
Way Galaxy. Of this immense multitude, could it be that our humdrum Sun is the
only one with an inhabited planet? Maybe. Maybe the origin of life or
intelligence is exceedingly improbable. Or maybe civilizations arise all the time,
but wipe themselves out as soon as they are able.
Or, here and there, peppered across
space, orbiting other suns, maybe there are worlds something like our own, on
which other beings gaze up and wonder as we do about who else lives in the
dark. Could the Milky Way be rippling with life and intelligence-worlds calling
out to worlds-while we on Earth are alive at the critical moment when we first
decide to listen?
Our species has discovered a way to
communicate through the dark, to transcend immense distances. no means of
communication is faster or cheaper or reaches out farther. It's called radio.
After billions of years of biological
evolution-on their planet and ours-an alien civilization cannot be in
technological lockstep with us. There have been humans for more than twenty
thousand centuries, but we've had radio only for about one century. If alien
civilizations are behind us, they're likely to be too far behind to have radio.
And if they're ahead of us, they're likely to be far ahead of us. Think of the
technical advances on our world over just the last few centuries. What is for
us technologically difficult or impossible, what might seem to us like magic,
might for them be trivially easy. They might use other, very advanced means to
communicate with their peers, but they would know about radio as an approach to
newly emerging civilizations. Even with no more than our level of technology at
the transmitting and receiving ends, we could communicate today across much of
the Galaxy. They should be able to do much better.
If they exist.
But our fear of the dark rebels. The
idea of alien beings troubles us. We conjure up objections:
“It's too expensive.” But, in its
fullest modern technological expression, it costs less than one attack
helicopter a year.
“We'll never understand what they're
saying.” But, because the message -is transmitted by radio, we and they must
have radio physics, radio astronomy, and radio technology in common. The laws
of Nature are the same everywhere; so science itself provides a means and
language of communication even between very different kinds of beings-provided
they both have science. Figuring out the message, if we're fortunate enough to
receive one, may be much easier than acquiring it.
“It would be demoralizing to learn that
our science is primitive.” But by the standards of the next few centuries, at
least some of our present science will be considered primitive,
extraterrestrials or no extraterrestrials. (So will some of our present
politics, ethics, economics, and religion.) To go beyond present science is one
of. the chief goals of science. Serious students are not commonly plunged into
fits of despair on turning the pages of a textbook and discovering that some
further topic is known to the author but not yet to the student. Usually the
students struggle a little, acquire the new knowledge, and, following an
ancient human tradition, continue to turn the pages.
“All through history advanced
civilizations have ruined civilizations just slightly more backward.”
Certainly. But malevolent aliens, should they exist, will not discover our
existence from the fact that we listen. The search programs only receive; they
do not send.
THE DEBATE IS, for the moment, moot. We
are now, on an unprecedented scale, listening for radio signals from possible
other civilizations in the depths of space. Alive today is the first generation
of scientists to interrogate the darkness. Conceivably it might also be the
last generation before contact is made-and this the last moment before we
discover that someone in the darkness is calling out to us.
This quest is called the Search for
Extraterrestrial Intelligence (SETI). Let me describe how far we've come.
The first SETI program was carried out
by Frank Drake at the National Radio Astronomy Observatory in Greenbank, West
Virginia, in 1960. He listened to two nearby Sun-like stars for two weeks at
one particular frequency. (“Nearby” is a relative term: The nearest was 12
light-years-70 trillion miles-away.)
Almost at the moment Drake pointed the
radio telescope and turned the system on, he picked up a very strong signal.
Was it a message from alien beings? Then it went away. If the signal
disappears, you can't scrutinize it. You can't see if, because of the Earth's
rotation, it moves with the sky. If it's not repeatable, you've learned almost
nothing from it-it might be terrestrial radio interference, or a failure of
your amplifier or detector...or an alien signal. Unrepeatable data, no matter
how illustrious the scientist reporting them, are not worth much.
Weeks later, the signal was detected
again. It turned out to be a military aircraft broadcasting on an unauthorized
frequency. Drake reported negative results. But in science a negative result is
not at all the same thing as a failure. His great achievement was to show that
modern technology is fully able to listen for signals from hypothetical
civilizations on the planets of other stars.
Since then there've been a number of
attempts, often on time borrowed from other radio telescope observing programs,
and almost never for longer than a few months. There've been some more false
alarms, at Ohio State, in Arecibo, Puerto Rico, in France, Russia, and
elsewhere, but nothing that could pass muster with the world scientific
community.
Meanwhile, the technology for detection
has been getting cheaper; the sensitivity keeps improving; the scientific
respectability of SETI has continued to grow; and even NASA and Congress have
become a little less afraid to support it. Diverse, complementary search
strategies are possible and necessary. It was clear years ago that if the trend
continued, the technology for a comprehensive SETI effort would eventually fall
within the reach even of private organizations (or wealthy individuals); and
sooner or later, the government would be willing to support a major program.
After 30 years of work, for some of us it's been later rather than sooner. But
at last the time has come.
THE PLANETARY SOCIETY-a nonprofit
membership organization that Bruce Murray, then the Director of JPL, and I
founded in 1980-is devoted to planetary exploration and the search for
extraterrestrial life. Paul Horowitz, a physicist at Harvard University, had
made a number of important innovations for SETI and was eager to try them out.
If we could find the money to get him started, we thought we could continue to
support the program by donations from our members.
In 1983 Ann Druyan and I suggested to
the filmmaker Steven Spielberg that this was an ideal project for him to
support. Breaking with Hollywood tradition, he had in two wildly successful
movies conveyed the idea that extraterrestrial beings might not be hostile and
dangerous. Spielberg agreed. With his initial support through The Planetary
Society, Project META began.
META is an acronym for “Megachannel ExtraTerrestrial
Assay.” The single frequency of Drake's first system grew to 8. 4 million. But
each channel, each “station,” we tune to has an exceptionally narrow frequency
range. There are no known processes out among the stars and galaxies that can generate
such sharp radio “lines.” If we pick up anything falling into so narrow a
channel, it must, we think, be a token of intelligence and technology.
What's more, the Earth turns-which means
that any distant radio source will have a sizable apparent motion, like the
rising and setting of the stars. Just as the steady tone of a car's horn dips
as it drives by, so any authentic extraterrestrial radio source will exhibit a
steady drift in frequency due to the Earth's rotation. In contrast, any source
of radio interference at the Earth's surface will be rotating at the same speed
as the META receiver. META's listening frequencies are continuously changed to
compensate for the Earth's rotation, so that any narrow-band Signals from the
sky will always appear in a single channel. But any radio interference down
here on Earth will give itself away by racing through adjacent channels.
The META radio telescope at Harvard,
Massachusetts, is 26 meters (84 feet) in diameter. Each day, as the Earth
rotates the telescope beneath the sky, a swath of stars narrower than the full
moon is swept out and examined. Next day, it's an adjacent swath. Over a year,
all of the northern sky and part of the southern is observed. An identical
system, also sponsored by The Planetary Society, is in operation just outside
Buenos Aires, Argentina, to examine the southern sky. So together the two META
systems have been exploring the entire sky.
The radio telescope, gravitationally
glued to the spinning Earth, looks at any given star for about two minutes.
Then it's on to the next. 8. 4 million channels sounds like a lot, but
remember, each channel is very narrow. All of them together constitute only a
few parts in 100,000 of the available radio spectrum. So we have to park our 8.
4 million channels somewhere in the radio spectrum for each year of
observation, near some frequency that an alien civilization, knowing nothing
about us, might nevertheless conclude we're listening to.
Hydrogen is by far the most abundant
kind of atom in the Universe. It's distributed in clouds and as diffuse gas
throughout interstellar space. When it acquires energy, it releases some of it
by giving off radio waves at a precise frequency of 1420. 405751768 megahertz.
(One hertz means the crest and trough of a wave arriving at your detection
instrument each second. So 1420 megahertz means 1. 420 billion waves entering
your detector every second. Since the wavelength of light is just the speed of
light divided by the frequency of the wave, 1420 megahertz corresponds to a wavelength
of 21 centimeters.) Radio astronomers anywhere in the Galaxy will be studying
the Universe at 1420 megahertz and can anticipate that other radio astronomers,
no matter how different they may look, will do the same.
It's as if someone told you that there's
only one station on your home radio set's frequency band, but that no one knows
its frequency. Oh yes, one other thing: Your set's frequency dial, kith its
thin marker you adjust by turning a knob, happens to reach from the Earth to
the Moon. To search systematically through this vast radio spectrum, patiently
turning the knob, is going to be very time-consuming. Your problem is to set
the dial correctly from the beginning, to choose the right frequency. If you
can correctly guess what frequencies that extraterrestrials are broadcasting to
us on-the “magic” frequencies-then you can save yourself much time and trouble.
These are the sorts of reasons that we first listened, as Drake did, at
frequencies near 1420 megahertz, the hydrogen “magic” frequency.
Horowitz and I have published detailed
results from five years of full-time searching with Project META and two years
of follow-up. We can't report that we found a signal from alien beings. But we
did find something puzzling, something that for me in quiet moments, every now
and then, raises goose bumps:
Of course, there's a background level of
radio noise from Earth-radio and television stations, aircraft, portable
telephones, nearby and more distant spacecraft. Also, as with all radio
receivers, the longer you wait, the more likely it is that there'll be some
random fluctuation in the electronics so strong that it generates a spurious
signal. So we ignore anything that isn't much louder than the background.
Any strong narrow-band signal that
remains in a single channel we take very seriously. As it logs in the data,
META automatically tells the human operators to pay attention to certain
signals. Over five years we made some 60 trillion observations at various
frequencies, while examining the entire accessible sky. A few dozen signals
survive the culling. These are subjected to further scrutiny, and almost all of
them are rejected-for example, because an error has been found by
fault-detection microprocessors that examine the signal-detection microprocessors.
What's left-the strongest candidate
signals after three surveys of the sky-are 11 “events.” They satisfy all but
one of our criteria for a genuine alien signal. But the one failed criterion is
supremely important: Verifiability. We've never been able to find any of them
again. We look back at that part of the sky three minutes later and there's
nothing there. We look again the following day: nothing. Examine it a year
later, or seven years later, and still there's nothing.
It seems unlikely that every signal we
get from alien civilizations would turn itself off a couple of minutes after we
begin listening, and never repeat. (How would they know we're paying
attention?) But, just possibly, this is the effect of twinkling. Stars twinkle
because parcels of turbulent air are moving across the line of sight between
the star and us. Sometimes these air parcels act as a lens and cause the light
rays from a given star to converge a little, making it momentarily brighter.
Similarly, astronomical radio sources may also twinkle-owing to clouds of
electrically charged (or “ionized”) gas in the great near-vacuum between the
stars. We observe this routinely with pulsars.
Imagine a radio signal that's a little
below the strength that we could otherwise detect on Earth. Occasionally the
signal will by chance be temporarily focused, amplified, and brought within the
detectability range of our radio telescopes. The interesting thing is that the
lifetimes of such brightening, predicted from the physics of the interstellar gas,
are a few minutes-and the chance of reacquiring the signal is small. We should
really be pointing steadily at these coordinates in the sky, watching them for
months.
Despite the fact that none of these
signals repeats, there's an additional fact about them that, every time I think
about it, sends a chill down my spine: 8 of the 11 best candidate signals lie
in or near the plane of the Milky Way Galaxy. The five strongest are in the
constellations Cassiopeia, Monoceros, Hydra, and two in Sagittarius-in the
approximate direction of the center of the Galaxy. The Milky Way is a flat,
wheel-like collection of gas and dust and stars. Its flatness is why we see it
as a band of diffuse light across the night sky. That's where almost all the
stars in our galaxy are. If our candidate signals really were radio
interference from Earth or some undetected glitch in the detection electronics,
we shouldn't see them preferentially when we're pointing at the Milky Way.
But maybe we had an especially unlucky
and misleading run of statistics. The probability that this correlation with
the galactic plane is due merely to chance is less than half a percent. Imagine
a wall-size map of the sky, ranging from the North Star at the top to the
fainter stars toward which the Earth's south pole points at the bottom. Snaking
across this wall map are the irregular boundaries of the Milky Way. Now suppose
that you were blindfolded and asked to throw five darts at random at the map
(with much of the southern sky, inaccessible from Massachusetts, declared off
limits). You'd have to throw the set of five darts more than 200 times before,
by accident, you got them to fall as closely within the precincts of the Milky
Way as the five strongest META signals did. Without repeatable signals, though,
there's no way we can conclude that we've actually found extraterrestrial
intelligence.
Or maybe the events we've found are
caused by some new kind of astrophysical phenomenon, something that nobody has
thought of yet, by which not civilizations, but stars or gas clouds (or
something) that do lie in the plane of the Milky Way emit strong signals in
bafflingly narrow frequency bands.
Let's permit ourselves, though, a moment
of extravagant speculation. Let's imagine that all our surviving events are in
fact due to radio beacons of other civilizations. Then we can estimate-from how
little time we've spent watching each piece of sky-how many such transmitters
there are in the entire Milky Way. The answer is something approaching a
million. If randomly strewn through space, the nearest of them would be a few
hundred light years away, too far for them to have picked up our own TV or
radar signals yet. They would not know for another few centuries that a
technical civilization has emerged on Earth. The Galaxy would be pulsing with
life and intelligence, but-unless they're busily exploring huge numbers of
obscure star systems-wholly oblivious of what has been happening down here
lately. A few centuries from now, after they do hear from us, things might get
very interesting. Fortunately, we'd have many generations to prepare.
If, on the other hand, none of our
candidate signals is an authentic alien radio beacon, then we're forced to the
conclusion that very few civilizations are broadcasting, maybe none, at least
at our magic frequencies and strongly enough for us to hear:
Consider a civilization like our own,
but which dedicated all its available power (about 10 trillion watts) to
broadcasting a beacon signal at one of our magic frequencies and to all
directions in space. The META results would then imply that there are no such
civilizations out to 25 light-years-a volume that encompasses perhaps a dozen
Sun-like stars. This is not a very stringent limit. If, in contrast, that
civilization were broadcasting directly at our position in space, using an
antenna no more advanced than the Arecibo Observatory, then if META has found
nothing, it follows that there are no such civilizations anywhere in the Milky
Way Galaxy-out of 400 billion stars, not one. But even assuming they would want
to, how would they know to transmit in our direction?
Now consider, at the opposite
technological extreme, a very advanced civilization omnidirectionally and
extravagantly broadcasting at a power level 10 trillion times greater (1026
watts, the entire energy output of a star like the Sun). Then, if the META
results are negative, we can conclude not only that there are no such
civilizations in the Milky Way, but none out to 70 million light-years-none in
M31, the nearest galaxy like our own, none in M33, or the Fornax system, or
M81, or the Whirlpool Nebula, or Centaurus A, or the Virgo cluster of galaxies,
or the nearest Seyfert galaxies; none among any of the hundred trillion stars
in thousands of nearby galaxies. Stake through its heart or not, the geocentric
conceit stirs again.
Of course, it might be a token not of
intelligence but of stupidity to pour so much energy into interstellar (and
intergalactic) communication. Perhaps they have good reasons not to hail all
comers. Or perhaps they don't care about civilizations as backward as we are.
But still-not one civilization in a hundred trillion stars broadcasting with
such power on such a frequency? If the META results are negative, we have set
an instructive limit-but whether on the abundance of very advanced
civilizations or their communications strategy we have no way of knowing. Even
if META has found nothing, a broad middle range remains open-of abundant
civilizations, more advanced than we and broadcasting omnidirectionally at
magic frequencies. We would not have heard from them yet.
ON OCTOBER 12, 1992-auspiciously or
otherwise the 500th anniversary of the “discovery” of America by Christopher
Columbus-NASA turned on its new SETI program. At a radio telescope in the
Mojave Desert, a search was initiated intended to cover the entire sky
systematically-like META, making no guesses about which stars are more likely,
but greatly expanding the frequency coverage. At the Arecibo Observatory, an
even more sensitive NASA study began that concentrated on promising nearby star
systems. When fully operational, the NASA searches would have been able to
detect much fainter signals than META, and look for kinds of signals that META
could not.
The META experience reveals a thicket of
background static and radio interference. Quick reobservation and confirmation
of the signal-specially at other, independent radio telescopes-is the key to
being sure. Horowitz and I gave NASA scientists the coordinates of our fleeting
and enigmatic events. Perhaps they would be able to confirm and clarify our
results. The NASA program was also developing new technology, stimulating
ideas, and exciting schoolchildren. In the eyes of many it was well worth the
$10 million a year being spent on it. But almost exactly a year after
authorizing it, Congress pulled the plug on NASA's SETI program. It cost too
much, they said. The post-Cold War U. S. defense budget is some 30,000 times
larger.
The chief argument of the principal
opponent of the NASA SETI program-Senator Richard Bryan of Nevada-was this
[from the Congressional Record for September 22, 1993]:
So far, the NASA SETI Program has found
nothing. In fact, all the decades of SETI research have found no confirmable
signs of extraterrestrial life.
Even with the current NASA version of
SETI, I do not think many of its scientists would be willing to guarantee that
we are likely to see any tangible results in the [foreseeable] future...
Scientific research rarely, if ever,
offers guarantees of success-and I understand that-and the full benefits of
such research are often unknown until very late in the process. And I accept
that, as well.
In the case of SETI, however, the
chances of success are so remote, and the likely benefits of the program are so
limited, that there is little justification for 12 million taxpayer dollars to
be expended for this program.
But how, before we have found
extraterrestrial intelligence, can we “guarantee” that we will find it? How, on
the other hand, can we know that the chances of success are “remote”? And if we
find extraterrestrial intelligence, are the benefits really likely to be “so
limited”? As in all great exploratory ventures, we do not know what we will
find and we don't know the probability of finding it. If we did, we would not
have to look.
SETI is one of those search programs
irritating to those who want well-defined cost/benefit ratios. Whether ETI can
be found; how long it would take to find it; and what it would cost to do so
are all unknown. The benefits might be enormous, but we can't really be sure of
that either. It would of course be foolish to spend a major fraction of the
national treasure on such ventures, but I wonder if civilizations cannot be
calibrated by whether they pay some attention to trying to solve the great
problems.
Despite these setbacks, a dedicated band
of scientists and engineers, centered at the SETI Institute in Palo Alto,
California, has decided to go ahead, government or no government. NASA has
given them permission to use the equipment already paid for; captains of the
electronics industry have donated a few million dollars; at least one
appropriate radio telescope is available; and the initial stages of this
grandest of all SETI programs is on track. If it can demonstrate that a useful
sky survey is possible without being swamped by background noise-and especially
if, as is very likely from the META experience, there are unexplained candidate
signals-perhaps Congress will change its mind once more and fund the project.
Meanwhile, Paul Horowitz has come up
with a new program-different from META, different from what NASA was
doing-called BETA. BETA stands for “Billion-channel ExtraTerrestrial Assay.” It
combines narrow-band sensitivity, wide frequency coverage, and a clever way to
verify signals as they're detected. If The Planetary Society can find the
additional support, this system-much cheaper than the former NASA
program-should be on the air soon.
WOULD I LIKE TO BELIEVE that with META
we've detected transmissions from other civilizations out there in the dark, sprinkled
through the vast Milky Way Galaxy? You bet. After decades of wondering and
studying this problem, of course I would. To me, such a discovery would be
thrilling. It would change everything. We would be hearing from other beings,
independently evolved over billions of years, viewing the Universe perhaps very
differently, probably much smarter, certainly not human. How much do they know
that we don't?
For me, no signals, no one calling out
to us is a depressing prospect. “Complete silence,” said Jean-Jacques Rousseau
in a different context, “induces melancholy; it is an image of death.” But I'm
with Henry David Thoreau: “Why should I feel lonely? Is not our planet in the
Milky Way?”
The realization that such beings exist
and that, as the evolutionary process requires, they must be very different
from us, would have a striking implication: Whatever differences divide us down
here on Earth are trivial compared to the differences between any of us and any
of them. Maybe it's a long shot, but the discovery of extraterrestrial
intelligence might play a role in unifying our squabbling and divided planet.
It would be the last of the Great Demotions, a rite of passage for our species
and a transforming event in the ancient quest to discover our place in the Universe.
In our fascination with SETI, we might
be tempted, even without good evidence, to succumb to belief but this would be
self-indulgent and foolish. We must surrender our skepticism only in the face
of rock-solid evidence. Science demands a tolerance for ambiguity. Where we are
ignorant, we withhold belief. Whatever annoyance the uncertainty engenders
serves a higher purpose: It drives us to accumulate better data. This attitude
is the difference between science and so much else. Science offers little in
the way of cheap thrills. The standards of evidence are strict. But when
followed they allow us to see far, illuminating even a great darkness.
CHAPTER 21
To the sky!
The stairs of the sky are let down for
him that he may ascend thereon to heaven. O gods, put your arms under the king:
raise him, lift him to the sky. To the
sky! To the sky!
—HYMN FOR A DEAD PHARAOH (EGYPT, CA.
2600 B. C.)
When my grandparents were children, the
electric light, the automobile, the airplane, and the radio were Stupefying
technological advances, the wonders of the age. You might hear wild stories
about them, but you could not find a single exemplar in that little village in
Austria-Hungary, near the banks of the river Bug. But in that same time, around
the turn of the last century, there were two men who foresaw other, far more
ambitious, inventions-Konstantin Tsiolkovsky, the theoretician, a nearly deaf
schoolteacher in the obscure Russian town of Kaluga, and Robert Goddard, the
engineer, a professor at an equally obscure American college in Massachusetts.
They dreamt of using rockets to journey to the planets and the stars. Step by
step, they worked out the fundamental physics and many of the details.
Gradually, their machines took shape. Ultimately, their dream proved infectious.
In their time, the very idea was
considered disreputable, or even a symptom of some obscure derangement. Goddard
found that merely mentioning a voyage to other worlds subjected him to
ridicule, and he dared not publish or even discuss in public his long-term
vision of flights to the stars. As teenagers, both had epiphanal visions of
spaceflight that never left them. “I still have dreams in which I fly up to the
stars in my machine,” Tsiolkovsky wrote in middle age. “It is difficult to work
all on your own for many years, in adverse conditions without a gleam of hope,
without any help.” Many of his contemporaries thought he was truly mad. Those
who knew physics better than Tsiolkovsky and Goddard-including The New York
Times in a dismissive editorial not retracted until the eve of Apollo
11-insisted that rockets could not work in a vacuum, that the Moon and the
planets were forever beyond human reach.
A generation later, inspired by
Tsiolkovsky and Goddard, Wernher von Braun was constructing the first rocket
capable of reaching the edge of space, the V-2. But in one of those ironies
with which the twentieth century is replete, von Braun was building it for the
Nazis-as an instrument of indiscriminate slaughter of civilians, as a
“vengeance weapon” for Hitler, the rocket factories staffed with slave labor,
untold human suffering exacted in the construction of every booster, and von
Braun himself made an officer in the SS. He was aiming at the Moon, he joked
unselfconsciously, but hit London instead.
Another generation later, building on
the work of Tsiolkovsky and Goddard, extending von Braun's technological
genius, we were up there in space, silently circumnavigating the Earth,
treading the ancient and desolate lunar surface. Our machines-increasingly competent
and autonomous-were spreading through the Solar System, discovering new worlds,
examining them closely, searching for life, comparing them with Earth.
This is one reason that in the long
astronomical perspective there is something truly epochal about “now”-which we
can define as the few centuries centered on the year you're reading this book.
And there's a second reason: This is the first moment in the history of our
planet when any species, by its own voluntary actions, has become a danger to
itself-as well as to vast numbers of others. Let me recount the ways:
•We've been burning fossil fuels for
hundreds of thousands of years. By the 1960s, there were so many of us burning
wood, coal, oil, and natural gas on so large a scale, that scientists began to
worry about the increasing greenhouse effect; the dangers of global warming
began slowly slipping into public consciousness.
•CFCs were invented in the 1920s and
1930s; in 1974 they were discovered to attack the protective ozone layer.
Fifteen years later a worldwide ban on their production was going into effect.
• Nuclear weapons were invented in 1945.
It took until 1983 before the global consequences of thermonuclear war were
understood. By 1992, large numbers of warheads were being dismantled.
•The first asteroid was discovered in
1801. More or less serious proposals to move them around were floated beginning
in the 1980s. Recognition of the potential dangers of asteroid deflection
technology followed shortly after.
•Biological warfare has been with us for
centuries, but its deadly mating with molecular biology has occurred only
lately.
•We humans have already precipitated
extinctions of species on a scale unprecedented since the end of the Cretaceous
Period. But only in the last decade has the magnitude of these extinctions
become clear, and the possibility raised that in our ignorance of the
interrelations of life on Earth we may be endangering our own future.
Look at the dates on this list and
consider the range of new technologies currently under development. Is it not
likely that other dangers of our own making are yet to be discovered, some
perhaps even more serious?
In the littered field of discredited
self-congratulatory chauvinisms, there is only one that seems to hold up, one
sense in which we are special: Due to our own actions or inactions, and the
misuse of our technology, we live at an extraordinary moment, for the Earth at
least-the first time that a species has become able to wipe itself out. But
this is also, we may note, the first time that a species has become able to
journey to the planets and the stars. The two times, brought about by the same
technology, coincide-a few centuries in the history of a 4. 5-billion-year-old
planet. If you were somehow dropped down on the Earth randomly at any moment in
the past (or future), the chance of arriving at this critical moment would be
less than 1 in 10 million. Our leverage on the future is high just now.
It might be a familiar progression,
transpiring on many worlds-a planet, newly formed, placidly revolves around its
star; life slowly forms; a kaleidoscopic procession of creatures evolves;
intelligence emerges which, at least up to a point, confers enormous survival
value; and then technology is invented. It dawns on them that there are such things
as laws of Nature, that these laws can be revealed by experiment, and that
knowledge of these laws can be made both to save and to take lives, both on
unprecedented scales. Science, they recognize, grants immense powers. In a
flash, they create world-altering contrivances. Some planetary civilizations
see their way through, place limits on what may and what must not be done, and
safely pass through the time of perils. Others are not so lucky or so prudent,
perish.
Since, in the long run, every planetary
society will be endangered by impacts from space, every surviving civilization
is obliged to become spacefaring—not because of exploratory or romantic zeal,
but for the most practical reason imaginable: staying alive. And once you're
out there in space for centuries and millennia, moving little worlds around and
engineering planets, your species has been pried loose from its cradle. If they
exist, many other civilizations will eventually venture far from home.
A MEANS HAS BEEN OFFERED of estimating
how precarious our circumstances are-remarkably, without in any way addressing
the nature of the hazards. J. Richard Gott III is an astrophysicist at
Princeton University. He asks us to adopt a generalized Copernican principle,
something I've described elsewhere as the Principle of Mediocrity. Chances are
that we do not live in a truly extraordinary time. Hardly anyone ever did. The
probability is high that we're born, live out our days, and die somewhere in
the broad middle range of the lifetime of our species (or civilization, or
nation). Almost certainly, Gott says, we do not live in first or last times. So
if your species is very young, it follows that it's unlikely to last
long-because if it were to last long, you (and the rest of us alive today)
would be extraordinary in living, proportionally speaking, so near the
beginning.
What then is the projected longevity of
our species? Gott concludes, at the 97. 5 percent confidence level, that there
will be humans for no more than 8 million years. That's his upper limit, about
the same as the average lifetime of many mammalian species. In that case, our
technology neither harms nor helps. But Gott's lower limit, with the same
claimed reliability, is only 12 years. He will not give you 40-to-1 odds that
humans will still be around by the time babies now alive become teenagers. In
everyday life we try very hard not to take risks so large, not to board
airplanes, say, with 1 chance in 40 of crashing. We will agree to surgery in
which 95 percent of patients survive only if our disease has a greater than 5
percent chance of killing us. Mere 40-to-1 odds on our species surviving
another 12 years Would be, if valid, a cause for supreme concern. If Gott is
right, not only may we never be out among the stars; there's a fair chance we
may not be around long enough even to make the first footfall on another
planet.
To me, this argument has a strange,
vaporish quality. Knowing nothing about our species except how old it is, we
make numerical estimates, claimed to be highly reliable, about its future
prospects. How? We go with the winners. Those who have been around are likely
to stay around. Newcomers tend to disappear. The only assumption is the quite
plausible one that there is nothing special about the moment at which we inquire
into the matter. So why is the argument unsatisfying? Is it just that we are
appalled by its implications?
Something like the Principle of
Mediocrity must have very broad applicability. But we are not so ignorant as to
imagine that everything is mediocre. There is something special about our
time-not just the temporal chauvinism that those who reside in any epoch
doubtless feel, but something, as outlined above, clearly unique and strictly
relevant to our species' future chances: This is the first time that (a) our
exponentiating technology has reached the precipice of self-destruction, but
also the first time that (b) we can postpone or avoid destruction by going
somewhere else, somewhere off the Earth.
These two clusters of capabilities, (a)
and (b), make our time extraordinary in directly contradictory ways-which both
(a) strengthen and (b) weaken Gott's argument. I don't know how to predict
whether the new destructive technologies will hasten, more than the new
spaceflight technologies will delay, human extinction. But since never before
have we contrived the means of annihilating ourselves, and never before have w
e developed the technology for settling other worlds, I think a compelling case
can be made that our time is extraordinary precisely in the context of Gott's
argument. If this is true, it significantly increases the margin of error in
such estimates of future longevity. The worst is worse, and the best better:
Our short-term prospects are even bleaker and-if we can survive the
short-term-our long-term chances even brighter than Gott calculates.
But the former is no more cause for
despair than the latter is for complacency. Nothing forces us to be passive
observers, clucking in dismay as our destiny inexorably works itself out. If we
cannot quite seize fate by the neck, perhaps we can misdirect it, or mollify
it, or escape it.
Of course we must keep our planet
habitable-not on a leisurely timescale of centuries or millennia, but urgently,
on a timescale of decades or even years. This will involve changes in
government, in industry, in ethics, in economics, and in religion. We've never
done such a thing before, certainly not on a global scale. It may be too
difficult for us. Dangerous technologies may be too widespread. Corruption may
be too pervasive. Too many leaders may be focused on the short term rather than
the long. There may be too many quarreling ethnic groups, nation-states, and
ideologies for the right kind of global change to be instituted. We may be too
foolish to perceive even what the real dangers are, or that much of what we
hear about them is determined by those with a vested interest in minimizing
fundamental change.
However, we humans also have a history
of making long-lasting social change that nearly everyone thought impossible. Since
our earliest days, we've worked not just for our own advantage but for our
children and our grandchildren. My grandparents and parents did so for me. We
have often, despite our diversity, despite endemic hatreds, pulled together to
face a common enemy. We seem, these days, much more willing to recognize the
dangers before us than we were even a decade ago. The newly recognized dangers
threaten all of us equally. no one can say how it will turn out down here.
THE MOON WAS WHERE the tree of
immortality grew in ancient Chinese myth. The tree of longevity if not of
immortality, it seems, indeed grows on other worlds. If we were up there among
the planets, if there were self-sufficient human communities oil many worlds,
our species would be insulated from catastrophe. The depletion of the
ultraviolet-absorbing shield on one world would, if anything, be a warning to
take special care of the shield on another. A cataclysmic impact on one world
would likely leave all the others untouched. The more of us beyond the Earth,
the greater the diversity of worlds we inhabit, the more varied the planetary
engineering, the greater the range of societal standards and values-then the
safer the human species will be.
If you grow up living underground in a
world with a hundredth of an Earth gravity and black skies through the portals,
you have a very different set of perceptions, interests, prejudices, and
predispositions than someone who lives on the surface of the home planet.
Likewise if you live on the surface of Mars in the throes of terraforming, or
Venus, or Titan. This strategy-breaking up into many smaller self-propagating
groups, each with somewhat different strengths and concerns, but all marked by
local pride-has been widely employed in the evolution of life on Earth, and by
our own ancestors in particular. It may, in fact, be key to understanding why
we humans are the way we are. This is
the second of the missing justifications for a permanent human presence in
space: to improve our chances of surviving, not just the catastrophes we can
foresee, but also the ones we cannot. Gott also argues that establishing human
communities on other worlds may offer us our best chance of beating the odds.
To take out this insurance policy is not
very expensive, not on the scale on which we do things on Earth. It would not
even require doubling the space budgets of the present spacefaring nations
(which, in all cases, are only a small fraction of the military budgets and
many other voluntary expenditures that might be considered marginal or even
frivolous). We could soon be setting humans down on near-Earth asteroids and
establishing bases on Mars. We know how to do it, even with present technology,
in less than a human lifetime. And the technologies will quickly improve. We
will get better at going into space.
A serious effort to send humans to other
worlds is relatively so inexpensive on a per annum basis that it cannot
seriously compete with urgent social agendas on Earth. If we take this path,
streams of images from other worlds will be pouring down on Earth at the speed
of light. Virtual reality will make the adventure accessible to millions of
stay-on-Earths. Vicarious participation will be much more real than at any
earlier age of exploration and discovery. And the more cultures and people it
inspires and excites, the more likely it will happen.
But by what right, we might ask
ourselves, do we inhabit, alter, and conquer other worlds? If anyone else were
living in the Solar System, this would be an important question. If, though,
there's no one else in this system but us, don't we have a right to settle it?
Of course, our exploration and
homesteading should be enlightened by a respect for planetary environments and
the scientific knowledge they hold. This is simple prudence. Of course,
exploration and settlement ought to be done equitably and transnationally, by
representatives of the entire human species. Our past colonial history is not
encouraging in these regards; but this time we are not motivated by gold or
spices or slaves or a zeal to convert the heathen to the One True Faith, as
were the European explorers of the fifteenth and sixteenth centuries. Indeed,
this is one of the chief reasons we're experiencing such intermittent progress,
so many fits and starts in the manned space programs of all nations.
Despite all the provincialisms I
complained about early in this book, here I find myself an unapologetic human
chauvinist. If there were other life in this solar system, it would be in
imminent danger because the humans are coming. In such a case, I might even be
persuaded that safeguarding our species by settling certain other worlds is
offset, in part at least, by the danger we would pose to everybody else. But as
nearly as we can tell, so far at least, there is no other life in this system,
not one microbe. There's only Earthlife.
In that case, on behalf of Earthlife, I
urge that, with full knowledge of our limitations, we vastly increase our
knowledge of the Solar System and then begin to settle other worlds.
These are the missing practical
arguments: safeguarding 'the Earth from otherwise inevitable catastrophic
impacts and hedging our bets on the many other threats, known and unknown, to
the environment that sustains us. Without these arguments, a compelling case
for sending humans to Mars and elsewhere might be lacking. But with them-and
the buttressing arguments involving science, education, perspective, and hope-I
think a strong case can be made. If our long-term survival is at stake, we have
a basic responsibility to our species venture to other worlds.
Sailors on a becalmed sea, we sense the
stirring of a breeze.
CHAPTER 22
Tiptoeing through the Milky Way
I swear by the shelters of the stars (a
mighty oath, if you but knew it)...
—THE QUR'AN, SURA 56 (7TH CENTURY)
Of course, it is strange to inhabit the
earth no longer, To give up customs one barely had time to learn...
—RAINER MARIA RILKE, “THE FIRST ELEGY”
(1923)
The prospect of scaling heaven, of
ascending to the sky, of altering other worlds to suit our purposes-no matter
how well intentioned we may be-sets the warning flags flying: We remember the
human inclination toward overweening pride; we recall our fallibility and
misjudgments when presented with powerful new technologies. We recollect the
story of the Tower of Babel, a building “whose top may reach unto heaven,” and
God's fear . shout our species, that now “nothing will be restrained from them
which they have imagined to do.”
We come upon Psalm 15, which stakes a
divine claim to other worlds: “[T]he heavens are the Lord's, but the Earth lath
he given to the children of men.” Or Plato's retelling of the Greek analogue of
Babel-the tale of Otys and Ephialtes. They were mortals who “dared to scale
heaven.” The gods were faced with a choice. Should they kill the upstart humans
“and annihilate [their] race with thunderbolts”? On the one Band, “this would
be the end of the sacrifices and worship which mien offered” the gods and which
gods craved. “But, on the other hand, the gods could not suffer [such]
insolence to be unrestrained.''
If, in the long term, though, we have no
alternative, if our choice really is many worlds or none, we are in need of
other sorts of myths of encouragement. They exist. Many religions, from
Hinduism to Gnostic Christianity to Mormon doctrine, teach that-as impious as
it may sound-it is the goal of humans to become gods. Or consider a story in
the Jewish Talmud left out of the Book of Genesis. (It is in doubtful accord
with the account of the apple, the Tree of Knowledge, the Fall, and the
expulsion from Eden.) In the Garden, God tells Eve and Adam that He has
intentionally left the Universe unfinished. It is the responsibility of humans,
over countless generations, to participate with God in a “glorious”
experiment-” completing the Creation.”
The burden of such a responsibility is
heavy, especially on so weak and imperfect a species as ours, one with so
unhappy a history. Nothing remotely like “completion” can be attempted without
vastly more knowledge than we have today. But perhaps, if our very existence is
at stake, we will find ourselves Able to rise to this supreme challenge.
ALTHOUGH HE DID NOT quite use any of the
arguments of the preceding chapter, it was Robert Goddard's intuition that “the
navigation of interplanetary space must be effected to ensure the continuance
of the race.” Konstantin Tsiolkovsky made a similar judgment:
There are countless planets, like many
island Earths...Man occupies one of them. But why could he not avail himself of
others, and of the might of numberless suns?...When the Sun has exhausted its
energy, it would be logical to leave it and look for another, newly kindled,
star still in its prime.
This might be done earlier, he
suggested, long before the Sun dies, “by adventurous souls seeking fresh worlds
to conquer.”
But as I rethink this whole argument,
I'm troubled. Is it too much Buck Rogers? Does it demand an absurd confidence
in future technology? Does it ignore my own admonitions about human
fallibility? Surely in the short term it's biased against technologically
less-developed nations. Are there no practical alternatives that avoid these
pitfalls?
All our self-inflicted environmental
problems, all our weapons of mass destruction are products of science and
technology. So, you might say, let's just back off from science and technology.
Let's admit that these tools are simply too hot to handle. Let's create a
simpler society, in which no matter how careless or short-sighted we are, we're
incapable of altering the environment on a global or even on a regional scale.
Let's throttle back to a minimal, agriculturally intensive technology, with
stringent controls on new knowledge. An authoritarian theocracy is a
tried-and-true way to enforce the controls.
Such a world culture is unstable,
though, in the long run if not the short-because of the speed of technological
advance. Human propensities for self-betterment, envy, and competition will
always be throbbing subsurface; opportunities for short-term, local advantage
will sooner or later be seized. Unless there are severe constraints on thought
and action, in a flash we'll be back to where we are today. So controlled a
society must grant great powers to the elite that does the controlling,
inviting flagrant abuse and eventual rebellion. It's very hard-once we've seen
the riches, conveniences, and lifesaving medicines that technology offers-to
squelch human inventiveness and acquisitiveness. And while such a devolution of
the global civilization, were it possible, might conceivably address the
problem of self-inflicted technological catastrophe, it would also leave us
defenseless against eventual asteroidal and cometary impacts.
Or you might imagine throttling back
much further, back to hunter-gatherer society, where we live off the natural
products of the land and abandon even agriculture. Javelin, digging stick, bow,
arrow, and fire would then be technology enough. But the Earth could support at
the very most a few tens of millions of hunter-gatherers. How could we get down
to such low population levels without instigating the very catastrophes we are
trying to avoid? Besides, we hardly know how to live the hunter-gatherer life
anymore: We've forgotten their cultures, their skills, their tool-kits. We've
killed off almost all of them, and we've destroyed much of the environment that
sustained them. Except for a tiny remnant of us, we might not be able, even if
we gave it high priority, to go back. And again, even if we could return, we
would be helpless before the impact catastrophe that inexorably will come.
The alternatives seem worse than cruel:
They are ineffective. Many of the dangers we face indeed arise from science and
technology-but, more fundamentally, because we leave become powerful without
becoming commensurately wise. The world-altering powers that technology has
delivered into our hands now require a degree of consideration and foresight
that has never before been asked of us.
Science cuts two ways, of course; its
products can be used for both good and evil. But there's no turning back from
science. The early warnings about technological dangers also come from science.
The solutions may well require more of us than just a technological fix. Many
will have to become scientifically literate. We may have to change institutions
and behavior. But our problems, whatever their origin, cannot be solved apart
from science. The technologies that threaten us and the circumvention of those
threats both issue from the same font. They are racing neck and neck.
In contrast, with human societies on
several worlds, our prospects would be far more favorable. Our portfolio would
be diversified. Our eggs would be, almost literally, in many baskets. Each
society would tend to be proud of the virtues of its world, its planetary
engineering, its social conventions, its hereditary predispositions.
Necessarily, cultural differences would be cherished and exaggerated. This
diversity would serve as a tool of survival.
When the off Earth settlements are
better able to fend for themselves, they will have every reason to encourage
technological advance, openness of spirit, and adventure-even if those left on
Earth are obliged to prize caution, fear new knowledge, and institute Draconian
social controls. After the first few self-sustaining communities are
established on other worlds, the Earthlings might also be able to relax their
strictures and lighten up. The humans in space would provide those on Earth
with real protection against rare but catastrophic collisions by asteroids or
comets on rogue trajectories. Of course, for this very reason, humans in space
would hold the upper hand in any serious dispute with those on Earth.
The prospects of such a time contrast
provocatively with forecasts that the progress of science and technology is now
near some asymptotic limit; that art, literature, and music are never to
approach, much less exceed, the heights our species has, on occasion, already
touched; and that political life on Earth is about to settle into some
rock-stable liberal democratic world government, identified, after Hegel, as “the
end of history.” Such an expansion into space also contrasts with a different
but likewise discernible trend in recent times-toward authoritarianism,
censorship, ethnic hatred, and a deep suspicion of curiosity and learning.
Instead, I think that, after some debugging, the settlement of the Solar System
presages an open-ended era of dazzling advances in science and technology;
cultural flowering; and wide-ranging experiments, up there in the sky, in
government and social organization. In more than one respect, exploring the
Solar System and homesteading other worlds constitutes the beginning, much more
than the end, of history.
IT'S IMPOSSIBLE, for us humans at least,
to look into our future, certainly not centuries ahead. no one has ever done so
with any consistency and detail. I certainly do not imagine that I can. I have,
with some trepidation, gone as far as I have to this point in the book, because
we are just recognizing the truly unprecedented challenges brought on by our
technology. These challenges have, I think, occasional straightforward
implications, some of which I've tried briefly to lay out. There are also less
straightforward, much longer-term implications about which I'm even less
confident. Nevertheless, I'd like to present them too for your consideration:
Even when our descendants are
established on near-Earth asteroids and Mars and the moons of the outer Solar
System and the Kuiper Comet Belt, it still won't be entirely safe. In the long
run, the Sun may generate stupendous X-ray and ultraviolet outbursts; the Solar
System will enter one of the vast interstellar clouds lurking nearby and the
planets will darken and cool; a shower of deadly comets will come roaring out
of the Oort Cloud threatening civilizations on many adjacent worlds; we will
recognize that a nearby star is about to become a supernova. In the really long
run, the Sun-on its way to becoming a red giant star-will get bigger and
brighter, the Earth will begin to lose its air and water to space, the soil
will char, the oceans will evaporate and boil, the rocks will vaporize, and our
planet may even be swallowed up into the interior of the Sun.
Far from being made for us, eventually
the Solar System will become too dangerous for us. In the long run, putting all
our eggs in a single stellar basket, no matter how reliable the Solar System
has been lately, may be too risky. In the long run, as Tsiolkovsky and Goddard
long ago recognized, we need to leave the Solar System.
If that's true for us, you might very
well ask, why isn't it true for others? And if it is true for others, why
aren't they here? There are many possible answers, including the contention
that they have come here-although the evidence for that is pitifully slim. Or
there may be no one else out there, because they destroy themselves, with
almost no exceptions, before they achieve interstellar flight; or because in a
galaxy of 400 billion suns ours is the first technical civilization.
A more likely explanation, I think,
issues from the simple fact that space is vast and the stars are far apart.
Even if there were civilizations much older and more advanced than we-expanding
out from their home worlds, reworking new worlds, and then continuing onward to
other stars-they would be unlikely, according to calculations performed by
William I. Newman of UCLA and me, to be here. Yet. And because the speed of
light is finite, the TV and radar news that a technical civilization has arisen
on some planet of the Sun has not reached them. Yet.
Should optimistic estimates prevail and
one in every million stars shelters a nearby technological civilization, and if
as well they're randomly strewn through the Milky Way-were these provisos to
hold-then the nearest one, we recall, would be a few hundred light-years
distant: at the closest, maybe 100 lightyears, more likely a thousand
light-years-and, of course, perhaps nowhere, no matter how far. Suppose the
nearest civilization on a planet of another star is, say, 200 light-years away.
Then, some 150 years from now they'll begin to receive our feeble post-World
War II television and radar emission. What will they make of it? With each
passing year the signal will get louder, more interesting, perhaps more
alarming. Eventually, they may respond: by returning a radio message, or by
visiting. In either case, the response will likely be limited by the finite
value of the speed of light. With these wildly uncertain numbers, the answer to
our unintentional mid-century call into the depths of space will not arrive
until around the year 2350. If they're farther away, of course, it will take
longer; and if much farther away, much longer. The interesting possibility
arises that our first receipt of a message from an alien civilization, a
message intended for us (not just an all-points bulletin), will occur in a time
when we are well situated on many worlds in our solar system and preparing to
move on.
With or without such a message, though,
we will have reason to continue outward, seeking other solar systems. Or-even
safer in this unpredictable and violent sector of the Galaxy-to sequester some
of us in self-sufficient habitations in interstellar space, far from the
dangers constituted by the stars. Such a future would, I think, naturally
evolve, by slow increments, even without any grand goal of interstellar travel:
For safety, some communities may wish to
sever their ties with the rest of humanity-uninfluenced by other societies,
other ethical codes, other technological imperatives. In a time when comets and
asteroids are being routinely repositioned, we will be able to populate a small
world and then cut it loose. In successive generations, as this world sped
outward, the Earth would fade from bright star to pale dot to invisibility; the
Sun would appear dimmer, until it was no more than a vaguely yellow point of
light, lost among thousands of others. The travelers would approach
interstellar night. Some such communities may be content with occasional radio
and laser traffic with the old home worlds. Others, confident of the
superiority of their own survival chances and wary of contamination, may try to
disappear. Perhaps all contact with them will ultimately be lost, their very
existence forgotten.
Even the resources of a sizable asteroid
or comet are finite, though, and eventually more resources must be sought
elsewhere-especially water, needed for drink, for a breathable oxygen
atmosphere, and for hydrogen to power fusion reactors. So in the long run these
communities must migrate from world to world, with no lasting loyalty to any.
We might call it “pioneering,” or “homesteading.” A less sympathetic observer
might describe it as sucking dry the resources of little world after little
world. But there are a trillion little worlds in the Oort Comet Cloud.
Living in small numbers on a modest
stepmother world far from the Sun, we will know that every scrap of food and
every drop of water is dependent on the smooth operation of a farsighted
technology; but these conditions are not radically unlike those to which we are
already accustomed. Digging resources out of the ground and stalking passing
resources seem oddly familiar, like a forgotten memory of childhood: It is,
with a few significant changes, the strategy of our hunter-gatherer ancestors.
For 99. 9 percent of the tenure of humans on Earth, we lived such a life.
Judging from some of the last surviving hunter-gatherers just before they were
engulfed by the present global civilization, we may have been relatively happy.
It's the kind of life that forged us. So after a brief, only partially
successful sedentary experiment, we may become wanderers again-more
technological than last time, but even then our technology, stone tools and
fire, was our only hedge against extinction.
If safety lies in isolation and
remoteness, then some of our descendants will eventually emigrate to the outer
comets of the Oort Cloud. With a trillion cometary nuclei, each separated from
the next by about as much as Mars is from Earth, there will be a great deal to
do out there.
The outer edge of the Sun's Oort Cloud
is perhaps halfway to the nearest star. Not every other star has an Oort Cloud,
but many probably do. As the Sun passes nearby stars, our Oort Cloud will
encounter, and partially pass through, other comet clouds, like two swarms of
gnats interpenetrating but not colliding. To occupy a comet of another star
will then be not much more difficult than to occupy one of our own. From the
frontiers of some other solar system the children of the blue dot may peer
longingly at the moving points of light denoting substantial (and well-lit)
planets. Some communities-feeling the ancient human love for oceans and
sunlight stirring within them-may begin the long journey down to the bright,
warm, and clement planets of a new sun.
Other communities may consider this last
strategy a weakness. Planets are associated with natural catastrophes. Planets
may have pre-existing life and intelligence. Planets are easy for other beings
to find. Better to remain in the darkness. Better to spread ourselves among
many “small and obscure worlds. Better to stay hidden.
ONCE WE CAN SEND our machines and
ourselves far from home, far from the planets-once we really enter the theater
of the Universe-we are bound to come upon phenomena unlike anything we've ever
encountered. Here are three possible examples:
First: Starting some 550 astronomical
units (AU) out-about ten times farther from the Sun than Jupiter, and therefore
much more accessible than the Oort Cloud-there's something extraordinary. Just
as an ordinary lens focuses far-off images, so does gravity. (Gravitational
lensing by distant stars and galaxies is now being detected.) Five hundred
fifty AU from the Sun-only a year away if we could travel at 1 percent the
speed of light-is where the focus begins (although when effects of the solar
corona, the halo of ionized gas surrounding the Sun, are taken into account,
the focus may be considerably farther out). There, distant radio signals are
enormously enhanced, amplifying whispers. The magnification of distant images
would allow us (with a modest radio telescope) to resolve a continent at the
distance of the nearest star and the inner Solar System at the distance of the
nearest spiral galaxy. If you are free to roam an imaginary spherical shell at
the appropriate focal distance and centered on the Sun, you are free to explore
the Universe in stupendous magnification, to peer at it with unprecedented
clarity, to eavesdrop on the radio signals of distant civilizations, if any,
and to glimpse the earliest events in the history of the Universe.
Alternatively, the lens could be used the other way, to amplify a very modest
signal of ours so it could be heard over immense distances. There are reasons
that draw us to hundreds and thousands of AU. Other civilizations will have
their own regions of gravitational focusing, depending on the mass and radius
of their star, some a little closer, some a little farther away than ours.
Gravitational lensing may serve as a common inducement for civilizations to
explore the regions just beyond the planetary parts of their solar systems.
Second: Spend a moment thinking about
brown dwarfs, hypothetical very low temperature stars, considerably more
massive than Jupiter, but considerably less massive than the Sun. Nobody knows
if brown dwarfs exist. Some experts, using nearer stars as gravitational lenses
to detect the presence of more distant ones, claim to have found evidence of
brown dwarfs. From the tiny fraction of the whole sky that has so far been
observed by this technique, an enormous number of brown dwarfs is inferred.
Others disagree. In the 1950s, it was suggested by the astronomer Harlow
Shapley of Harvard that brown dwarfs-he called them “Lilliputian stars”-were
inhabited. He pictured their surfaces as warm as a June day in Cambridge, with
lots of area. They would be stars that humans could survive on and explore.
Third: The physicists B. J. Carr and
Stephen Hawking of Cambridge University have shown that fluctuations in the
density of matter in the earliest stages of the Universe could have generated a
wide variety of small black holes. Primordial black holes-if they exist-must
decay by emitting radiation to space, a consequence of the laws of quantum
mechanics. The less massive the black hole, the faster it dissipates. Any
primordial black hole in the final stages of decay today would have to weigh
about as much as a mountain. All the smaller ones are gone. Since the
abundance-to say nothing of the existence-of primordial black holes depends on
what happened in the earliest moments after the Big Bang, no one can be sure
that there are any to be found; we certainly can't be sure that any lie nearby.
Not very restrictive upper limits on their abundance have been set by the
failure so far to find short gamma ray pulses, a component of the Hawking
radiation.
In a separate study, G. E. Brown of
Caltech and the pioneering nuclear physicist Hans Bethe of Cornell suggest that
about a billion non-primordial black holes are strewn through the Galaxy,
generated in the evolution of stars. If so, the nearest may be only 10 or 20
lightyears away.
If there are black holes within
reach-whether they're as massive as mountains or as stars-we will have amazing
physics to study firsthand, as well as a formidable new source of energy. By no
means do I claim that brown dwarfs or primordial black holes are likely within
a few light-years, or anywhere. But as we enter interstellar space, it is
inevitable that we will stumble upon whole new categories of wonders and
delights, some with transforming practical applications.
I do not know where my train of argument
ends. As more time passes, attractive new denizens of the cosmic zoo will draw
us farther outward, and increasingly improbable and deadly catastrophes must
come to pass. The probabilities are cumulative. But, as time goes on,
technological species will also accrue greater and greater powers, far
surpassing any we can imagine today. Perhaps, if we are very skillful (lucky, I
think, won't be enough), we will ultimately spread far from home, sailing
through the starry archipelagos of the vast Milky Way Galaxy. If we come upon
anyone else-or, more likely, if they come upon us-we will harmoniously
interact. Since other spacefaring civilizations are likely to be much more
advanced than we, quarrelsome humans in interstellar space are unlikely to last
long.
Eventually, our future may be as
Voltaire, of all people, imagined:
Sometimes by the help of a sunbeam, and
sometimes by the convenience of a comet, [they] glided from sphere to sphere,
as a bird hops from bough to bough. In a very little time [they] posted through
the Milky Way...
We are, even now, discovering vast
numbers of gas and dust disks around young stars-the very structures out of
which, in our solar system four and a half billion years ago, the Earth and the
other planets formed. We're beginning to understand how fine dust grains slowly
grow into worlds; how big Earthlike planets accrete and then quickly capture
hydrogen and helium to become the hidden cores of gas giants; and how small
terrestrial planets remain comparatively bare of atmosphere. We are
reconstructing the histories of worlds-how mainly ices and organics collected
together in the chilly outskirts of the early Solar System, and mainly rock and
metal in the inner regions warmed by the young Sun. We have begun to recognize
the dominant role of early collisions in knocking worlds over, gouging huge
craters and basins in their surfaces and interiors, spinning them up, making
and obliterating moons, creating rings, carrying, it may be, whole oceans down
from the skies, and then depositing a veneer of organic matter as the neat
finishing touch in the creation of worlds. We are beginning to apply this
knowledge to other systems.
In the next few decades we have a real
chance of examining the layout and something of the composition of many other
mature planetary systems around nearby stars. We will begin to know which
aspects of our system are the rule and which the exception. What is more
common-planets like Jupiter, planets like Neptune, or planets like Earth? Or do
all other systems have Jupiters and Neptunes and Earths? What other categories
of worlds are there, currently unknown to us? Are all solar systems embedded in
a vast spherical cloud of comets? Most stars in the sky are not solitary suns
like our own, but double or multiple systems in which the stars are in mutual
orbit. Are there planets in such systems? If so, what are they like? If, as we
now think, planetary systems are a routine consequence of the origin of suns,
have they followed very different evolutionary paths elsewhere? What do elderly
planetary systems, billions of years more evolved than ours, look like? In the
next few centuries our knowledge of other systems will become increasingly
comprehensive. We will begin to know which to visit, which to seed, and which
to settle.
Imagine we could accelerate continuously
at 1 g-what we're comfortable with on good old terra firma-to the midpoint of
our voyage, and decelerate continuously at 1 g until we arrive at our
destination. It would then take a day to get to Mars, a week and a half to
Pluto, a year to the Oort Cloud, and a few years to the nearest stars.
Even a modest extrapolation of our
recent advances in transportation suggests that in only a few centuries we will
be able to travel close to the speed of light. Perhaps this is hopelessly
optimistic. Perhaps it will really take millennia or more. But unless we
destroy ourselves first we will be inventing new technologies as strange to us
as Voyager might be to our hunter-gatherer ancestors. Even today we can think
of ways-clumsy, ruinously expensive, inefficient to be sure-of constructing a
starship that approaches light speed. In time, the designs will become more
elegant, more affordable, more efficient. The day will come when we overcome
the necessity of jumping from comet to comet. We will begin to soar through the
light-years and, as St. Augustine said of the gods of the ancient Greeks and
Romans, colonize the sky.
Such descendants may be tens or hundreds
of generations removed from anyone who ever lived on the surface of a planet.
Their cultures will be different, their technologies far advanced, their
languages changed, their association with machine intelligence much more
intimate, perhaps their very appearance markedly altered from that of their
nearly mythical ancestors who first tentatively set forth in the late twentieth
century into the sea of space. But they will be human, at least in large part;
they will be practitioners of high technology; they will have historical
records. Despite Augustine's judgment on Lot's wife, that “no one who is being
saved should long for what he is leaving,” they will not wholly forget the
Earth.
But we're not nearly ready, you may be
thinking. As Voltaire put it in his Memnon, “our little terraqueous globe is
the madhouse of those hundred thousand millions of worlds.” We, who cannot even
put our own planetary home in order, riven with rivalries and hatreds,
despoiling our environment, murdering one another through irritation and
inattention as well as on deadly purpose, and moreover a species that until
only recently was convinced that the Universe was made for its sole benefit-are
we to venture out into space, move worlds, reengineer planets, spread to
neighboring star systems?
I do not imagine that it is precisely
we, with our present customs and social conventions, who will be out there. If
we continue to accumulate only power and not wisdom, we will surely destroy
ourselves. Our very existence in that distant time requires that we will have
changed our institutions and ourselves. How can I dare to guess about humans in
the far future? It is, I think, only a matter of natural selection. If we
become even slightly more violent, shortsighted, ignorant, and selfish than we
are now, almost certainly we will have no future.
If you're young, it's just possible that
we will be taking our first steps on near-Earth asteroids and Mars during your
lifetime. To spread out to the moons of the Jovian planets and the Kuiper Comet
Belt will take many generations more. The Oort Cloud will require much longer
still. By the time we're ready to settle even the nearest other planetary
systems, we will have changed. The simple passage of so many generations will
have changed us. The different circumstances we will be living under will have
changed us. Prostheses and genetic engineering will have changed us. Necessity
will have changed us. We're an adaptable species.
It will not be we who reach Alpha
Centauri and the other nearby stars. It will be a species very like us, but
with more of our strengths and fewer of our weaknesses, a species returned to
circumstances more like those for which it was originally evolved, more
confident, farseeing, capable, and prudent-the sorts of beings we would want to
represent us in a Universe that, for all we know, is filled with species much
older, much more powerful, and very different.
The vast distances that separate the
stars are providential. Beings and worlds are quarantined from one another. The
quarantine is lifted only for those with sufficient self-knowledge and judgment
to have safely traveled from star to star.
ON IMMENSE TIMESCALES, in hundreds of
millions to billions of years, the centers of galaxies explode. We see,
scattered across deep space, galaxies with “active nuclei,” quasars, galaxies
distorted by collisions, their spiral arms disrupted, star systems blasted with
radiation or gobbled up by black holes-and we gather that on such timescales
even interstellar space, even galaxies may not be safe.
There is a halo of dark matter
surrounding the Milky Way, extending perhaps halfway to the distance of the
next spiral galaxy (M31 in the constellation Andromeda, which also contains
hundreds of billions of stars). We do not know what this dark matter is, or how
it is arranged-but some of it may be in worlds untethered to individual stars.
If so, our descendants of the remote future will have an opportunity, over
unimaginable intervals of time, to become established in intergalactic space,
and to tiptoe to other galaxies.
But on the timescale for populating our
galaxy, if not long before, we must ask: How immutable is this longing for
safety that drives us outward? Will we one day feel content with the time our
species has had and our successes, and willingly exit the cosmic stage?
Millions of years from now-probably much sooner-we will have made ourselves
into something else. Even if we do nothing intentionally, the natural process
of mutation and selection will have worked our extinction or evolved us into
some other species on just such a timescale (if we may judge by other mammals).
Over the typical lifetime of a mammalian species, even if we were able to
travel close to the speed of light and were dedicated to nothing else, we could
not, I think, explore even a representative fraction of the Milky Way Galaxy.
There's just too much of it. And beyond are a hundred billion galaxies more.
Will our present motivations remain unchanged over geological, much less
cosmological, timescales-when we ourselves have been transfigured? In such
remote epochs, we may discover outlets for our ambitions far grander and more
worthy than merely populating an unlimited number of worlds.
Perhaps, some scientists have imagined,
we will one day create new forms of life, link minds, colonize stars,
reconfigure galaxies, or prevent, in a nearby volume of space, the expansion of
the Universe. In a 1993 article in the journal Nuclear Physics, the physicist
Andrei Linde-conceivably, in a playful mood-suggests that laboratory
experiments (it would have to be quite a laboratory) to create separate, closed-off,
expanding universes might ultimately be possible. “However,” he writes to me,
“I myself do not know whether [this suggestion] is simply a joke or something
else.” In such a list of projects for the far future, we will have no
difficulty in recognizing a continuing human ambition to arrogate powers once
considered godlike-or, in that other more encouraging metaphor, to complete the
Creation.
FOR MANY PAGES NOW, we have left the
realm of plausible conjecture for the heady intoxication of nearly unconstrained
speculation. It is time to return to our own age.
My grandfather, born before radio waves
were even a laboratory curiosity, almost lived to see the first artificial
satellite beeping down at us from space. There are people who were born before
there was such a thing as an airplane, and who in old age saw four ships
launched to the stars. For all our failings, despite our limitations and
fallibilities, we humans are capable of greatness. This is true of our science
and some areas of our technology, of our art, music, literature, altruism, and
compassion, and even, on rare occasion, of our statecraft. What new wonders
undreamt of in our time will we have wrought in another generation? And
another? How far will our nomadic species have wandered by the end of the next
century? And the next millennium?
Two billion years ago our ancestors were
microbes; a halfbillion years ago, fish; a hundred million years ago, something
like mice; ten million years ago, arboreal apes; and a million years ago,
proto-humans puzzling out the taming of fire. Our evolutionary lineage is
marked by mastery of change. In our time, the pace is quickening.
When we first venture to a near-Earth
asteroid, we will have entered a habitat that may engage our species forever.
The first voyage of men and women to Mars is the key step in transforming us
into a multiplanet species. These events are as momentous as the colonization
of the land by our amphibian ancestors and the descent from the trees by our
primate ancestors.
Fish with rudimentary lungs and fins
slightly adapted for walking must have died in great numbers before
establishing a permanent foothold on the land. As the forests slowly receded,
our upright apelike forebears often scurried back into the trees, fleeing the
predators that stalked the savannahs. The transitions were painful, took
millions of years, and were imperceptible to those involved. In our case the
transition occupies only a few generations, and with only a handful of lives
lost. The pace is so swift that we are still barely able to grasp what is
happening.
Once the first children are born off
Earth; once we have bases and homesteads on asteroids, comets, moons, and
planets; once we're living off the land and bringing up new generations on
other worlds, something will have changed forever in human history. But
inhabiting other worlds does not imply abandoning this one, any more than the
evolution of amphibians meant the end of fish. For a very long time only a
small fraction of us will be out there.
“In modern Western society,” writes the
scholar Charles Lindholm, the erosion of tradition and the collapse of accepted
religious belief leaves us without a telos [an end to which we strive], a
sanctified notion of humanity's potential. Bereft of a sacred project, we have only
a demystified image of a frail and fallible humanity no longer capable of
becoming god-like.
I believe it is healthy-indeed,
essential-to keep our frailty and fallibility firmly in mind. I worry about
people who aspire to be “god-like.” But as for a long-term goal and a sacred
project, there is one before us. On it the very survival of our species
depends. If we have been locked and bolted into a prison of the self, here is
an escape hatch-something worthy, something vastly larger than ourselves, a crucial
act on behalf of humanity. Peopling other worlds unifies nations and ethnic
groups, binds the generations, and requires us to be both smart and wise. It
liberates our nature and, in part, returns us to our beginnings. Even now, this
new telos is within our grasp.
The pioneering psychologist William
James called religion a “feeling of being at home in the Universe.” Our
tendency has been, as I described in the early chapters of this book, to
pretend that the Universe is how we wish our home would be, rather than to
revise our notion of what's homey so it embraces the Universe. If, in
considering James' definition, we mean the real Universe, then we have no true
religion yet. That is for another time, when the sting of the Great Demotions
is well behind us, when we are acclimatized to other worlds and they to us,
when we are spreading outward to the stars.
The Cosmos extends, for all practical
purposes, forever. After a brief sedentary hiatus, we are resuming our ancient
nomadic way of life. Our remote descendants, safely arrayed on many worlds
through the Solar System and beyond, will be unified by their common heritage,
by their regard for their home planet, and by the knowledge that, whatever
other life may be, the only humans in all the Universe come from Earth.
They will gaze up and strain to find the
blue dot in their skies. They will love it no less for its obscurity and
fragility. They will marvel at how vulnerable the repository of all our
potential once was, how perilous our infancy, how humble our beginnings, how
many rivers we had to cross before we found our way.
RAINER MARIA RILKE, “THE FIFTH ELEGY”
(1923)
We were wanderers from the beginning. We
knew every stand of tree for a hundred miles. When the fruits or nuts were
ripe, we were there. We followed the herds in their annual migrations. We
rejoiced in fresh meat. through stealth, feint, ambush, and main-force assault,
a few of us cooperating accomplished what many of us, each hunting alone, could
not. We depended on one another. Making it on our own was as ludicrous to
imagine as was settling down.
Working together, we protected our
children from the lions and the hyenas. We taught them the skills they would
need. And the tools. Then, as now, technology was the key to our survival.
When the drought was prolonged, or when
an unsettling chill lingered in the summer air, our group moved on-sometimes to
unknown lands. We sought a better place. And when we couldn't get on with the
others in our little nomadic band, we left to find a more friendly bunch
somewhere else. We could always begin again.
For 99. 9 percent of the time since our
species came to be, we were hunters and foragers, wanderers on the savannahs
and the steppes. There were no border guards then, no customs officials. The
frontier was everywhere. We were bounded only by the Earth and the ocean and
the sky-plus occasional grumpy neighbors.
When the climate was congenial, though,
when the food was plentiful, we were willing to stay put. Unadventurous.
Overweight. Careless. In the last ten thousand years-an instant in our long
history—we've abandoned the nomadic fife. We've domesticated the plants and
animals. Why chase the food when you can make it come to you?
For all its material advantages, the
sedentary life has left us edgy, unfulfilled. Even after 400 generations in
villages and cities, we haven't forgotten. The open road still softly calls,
like a nearly forgotten song of childhood. We invest far-off places with a
certain romance. This appeal, I suspect, has been meticulously crafted by
natural selection as an essential element in our survival. Long summers, mild
winters, rich harvests, plentiful game-none of them lasts forever. It is beyond
our powers to predict the future. Catastrophic events have a way of sneaking up
on us, of catching us unaware. Your own life, or your band's, or even your
species' might be owed to a restless few-drawn, by a craving they can hardly
articulate or understand, to undiscovered lands and new worlds.
Herman Melville, in Moby Dick, spoke for
wanderers in all epochs and meridians: “I am tormented with an everlasting itch
for things remote. I love to sail forbidden seas...”
To the ancient Greeks and Romans, the
known world comprised Europe and an attenuated Asia and Africa, all surrounded
by an impassable World Ocean. Travelers might encounter inferior beings called
barbarians or superior beings called gods. Every tree had its dryad, every
district its legendary hero. But there were not very many gods, at least at
first, perhaps only a few dozen. They lived on mountains, under the Earth, in
the sea, or up there in the sky. They sent messages to people, intervened in
human affairs, and interbred with us.
As time passed, as the human exploratory
capacity hit its stride, there were surprises: Barbarians could be fully as
clever as Greeks and Romans. Africa and Asia were larger than anyone had
guessed. The World Ocean was not impassable. There were Antipodes. Three new
continents existed, had been settled by Asians in ages past, and the news had
never reached Europe. Also the gods were disappointingly hard to find.
The first large-scale human migration
from the Old World to the New happened during the last ice age, around 11,500
years ago, when the growing polar ice caps shallowed the oceans and made it
possible to walk on dry land from Siberia to Alaska. A thousand years later, we
were in Tierra del Fuego, the southern tip of South America. Long before
Columbus, Indonesian argonauts in outrigger canoes explored the western
Pacific; people from Borneo settled Madagascar; Egyptians and Libyans
circumnavigated Africa; and a great fleet of ocean going junks from Ming
Dynasty China crisscrossed the Indian Ocean, established a base in Zanzibar,
rounded the Cape of Good Hope, and entered the Atlantic Ocean. In the fifteenth
through seventeenth centuries, European sailing ships discovered new continents
(new, at any rate, to Europeans) and circumnavigated the planet. In the
eighteenth and nineteenth centuries, American and Russian explorers, traders,
and settlers raced west and east across two vast continents to the Pacific.
This zest to explore and exploit, however thoughtless its agents may have been,
has clear survival value. It is not restricted to any one nation or ethnic
group. It is an endowment that all members of the human species hold in common.
Since we first emerged, a few million
years ago in East Africa, we have meandered our way around the planet. There
are now people on every continent and the remotest islands, from pole to pole,
from Mount Everest to the Dead Sea, on the ocean bottoms and even,
occasionally, in residence 200 miles up-humans, like the gods of old, living in
the sky.
These days there seems to be nowhere
left to explore, at least on the land area of the Earth. Victims of their very
success the explorers now pretty much stay home.
Vast migrations of people-some
voluntary, most not- have shaped the human condition. More of us flee from war,
oppression, and famine today than at any other time in human history. As the
Earth's climate changes in the coming decade. there are likely to be far
greater numbers of environmental refugees. Better places will always call to
us. Tides of people will continue to ebb and flow across the planet. But the
lands we run to now have already been settled. Other people, often
unsympathetic to our plight, are there before us.
LATE IN THE NINETEENTH CENTURY, Leib
Gruber was growing up in Central Europe, in an obscure town in the immense,
polyglot, ancient Austro-Hungarian Empire. His father sold fish when he could.
But times were often hard. As a young man, the only honest employment Leib
could find was carrying people across the nearby river Bug. The customer, male
or female, would mount Leib's back; in his prized boots, the tools of his
trade, he would wade out in a shallow stretch of the river and deliver his
passenger to the opposite bank. Sometimes the water reached his waist. There
were no bridges here, no ferryboats. Horses might have served the purpose, but
they had other uses. That left Leib and a few other young men like him. They
had no other uses. no other work was available. They would lounge about the
riverbank, calling out their prices, boasting to potential customers about the
superiority of their drayage. They hired themselves out like four-footed
animals. My grandfather was a beast of burden
I don't think that in all his young
manhood Leib had ventured more than a hundred kilometers from his little
hometown of Sassow. But then, in 1904, he suddenly ran away to the New World to
avoid a murder rap, according to one family legend. He left his young wife
behind. How different from his tiny back-water hamlet the great German port
cities must have seemed, how vast the ocean, how strange the lofty skyscrapers
and endless hub-bub of his new land. We know nothing of his crossing, but have
found the ship's manifest for the journey undertaken later by his wife, Chaiya
joining Leib after he had saved enough to bring her over. She traveled in the
cheapest class on the Batavia, a vessel of Hamburg registry. There's something
heartbreakingly terse about the document: Can she read or write? No. Can she
speak English? No. How much money does she have? I can imagine her
vulnerability and her shame as she replies, “One dollar.”
She disembarked in New York, was
reunited with Leib, lived just long enough to give birth to my mother and her
sister, and then died from “complications” of childbirth. In those few years in
America, her name had sometimes been anglicized to Clara. A quarter century
later, my mother named her own firstborn, a son, after the mother she never
knew.
OUR DISTANT ANCESTORS, watching the
stars, noted five that did more than rise and set in stolid procession, as the
so-called “fixed” stars did. These five had a curious and complex motion. Over
the months they seemed to wander slowly among the stars. Sometimes they did
loops. Today we call them planets, the Greek word for wanderers. It was, I
imagine, a peculiarity our ancestors could relate to.
We know now that the planets are not
stars, but other worlds, gravitationally lashed to the Sun. Just as the
exploration of the Earth was being completed, we began to recognize it as one
world among an uncounted multitude of others, circling the Sun or orbiting the
other stars that make up the Milky Way galaxy. Our planet and our solar system
are surrounded by a new world ocean the depths of space. It is no more
impassable than the last.
Maybe it's a little early. Maybe the
time is not quite yet. But those other worlds-promising untold
opportunities-beckon.
In the last few decades, the United
States and the former Soviet Union have accomplished something stunning and
historic-the close-up examination of all those points of light, from Mercury to
Saturn, that moved our ancestors to wonder and to science. Since the advent of
successful interplanetary flight in 1962, our machines have flown by, orbited,
or landed on more than seventy new worlds. We have wandered among the
wanderers. We have found vast volcanic eminences that dwarf the highest
mountain on Earth; ancient river valleys on two planets enigmatically one too
cold and the other too hot for running water; a giant planet with an interior
of liquid metallic hydrogen into which a thousand Earths would fit; whole moons
that have melted; a cloud-covered place with an atmosphere of corrosive raids,
where even the high plateaus are above the melting point of lead ancient
surfaces on which a faithful record of the violent formation of the Solar
System is engraved; refugee ice worlds from the transplutonian depths;
exquisitely patterned ring systems, marking the subtle harmonies of gravity;
and a world surrounded by clouds of complex organic molecules like those that m
the earliest history of our planet led to the origin of life. Silently, they
orbit the Sun, waiting.
We have uncovered wonders undreamt by
our ancestors who first speculated on the nature of those wandering lights in
the night sky. We have probed the origins of our planet and ourselves. By
discovering what else is possible, by coming face to face with alternative
fates of worlds more or less like our own, we have begun to better understand
the Earth. Every one of these worlds is lovely and instructive. But, so far as
we know, they are also, every one of them, desolate and barren. Out there,
there are no “better places.” So far, at least.
During the Viking robotic mission,
beginning in July 1976, in a certain sense I spent a year on Mars. I examined
the boulders and sand dunes, the sky red even at high noon, the ancient river
valleys, the soaring volcanic mountains, the fierce wind erosion, the laminated
polar terrain, the two dark potato-shaped moons. But there was no life-not a
cricket or a blade of grass, or even, so far as we can tell for sure, a
microbe. These worlds have not been graced, as ours has, by life. Life is a
comparative rarity. You can survey dozens of worlds and find that on only one
of them does life arise and evolve and persist.
Having in all their lives till then
crossed nothing wider than a layer, Leib and Chaiya graduated to crossing
oceans. They had one great advantage: On the other side of the waters there
would be-invested with outlandish customs, it is true-other human beings
speaking their language and sharing at least some of their values, even people
to whom they were closely related.
In our time we've crossed the Solar
System and sent four ships to the stars. Neptune lies a million times farther
from Earth than New York City is from the banks of the Bug. But there are no
distant relatives, no humans, and apparently no life waiting for us on those other
worlds. no letters conveyed by recent emigres help us to understand the new
land only digital data transmitted at the speed of light by unfeeling, precise
robot emissaries. They tell us that these new worlds are not much like home.
But we continue to search for inhabitants. We can't help it. Life looks for
life.
No one on Earth, not the richest among
us, can afford the passage; so we can't pick up and leave for Mars or Titan on
a whim, or because we're bored, or out of work, or drafted into the army, or
oppressed, or because, justly or unjustly, we've been accused of a crime. There
does not seem to be sufficient short-term profit to motivate private industry.
If we humans ever go to these worlds, then, it will be because a nation or a
consortium of them believes it to be to its advantage-or to the advantage of
the human species. Just now, there are a great many matters pressing in on us
that compete for the money it takes to send people to other worlds.
That's what this book is about: other
worlds, what awaits us on them, what they tell us about ourselves, and-given
the urgent problems our species now faces-whether it makes sense to go. Should
we solve those problems first? Or are they a reason for going?
This book is, in many ways, optimistic
about the human prospect. The earliest chapters may at first sight seem to
revel overmuch in our imperfections. But they lay an essential spiritual and
logical foundation for the development of my argument.
I have tried to present more than one
facet of an issue. There will be places where I seem to be arguing with myself.
I am. Seeing some merit to more than one side, I often argue with myself. I
hope by the last chapter it will be clear where I come out.
The plan of the book is roughly this: We
first examine the widespread claims made over all of human history that our
world and our species are unique, and even central to the workings and purpose
of the Cosmos. We venture through the Solar System in the footsteps of the
latest voyages of exploration and discovery, and then assess the reasons
commonly offered for sending humans into space. In the last and most
speculative part of the book, I trace how I imagine that our long-term future
in space will work itself out.
Pale Blue Dot is about a new
recognition, still slowly overtaking us, of our coordinates, our place in the
Universe and how, even if the call of the open road is muted in our time, a
central element of the human future lies far beyond the Earth.
CHAPTER 1
You are here
The entire Earth is but a point, and the
place of our own habitation but a minute corner of it.
—MARCUS AURELIUS, ROMAN EMPEROR,
MEDITATIONS, BOOK 4 (CA. 170)
As the astronomers unanimously teach,
the circuit of the whole earth, which to us seems endless, compared with the
greatness of the universe has the likeness of a mere tiny point.
—AMMIANUS MARCELLINUS ACA. 330-395, THE
LAST MAJOR ROMAN HISTORIAN, IN THE CHRONICLE OF EVENTS
The spacecraft was a long way from home,
beyond the orbit of the outermost planet and high above the ecliptic plane-which
is an imaginary flat surface that we can think of as something like a racetrack
in which the orbits of the planets are mainly confined. The ship was speeding
away from the Sun at 40,000 miles per hour. But in early February of 1990, it
was overtaken by an urgent message from Earth.
Obediently, it turned its cameras back
toward the now-distant planets. Slewing its scan platform from one spot in the
sky to another, it snapped 60 pictures and stored them in digital form on its
tape recorder. Then, slowly, in March, April, and May, it radioed the data back
to Earth. Each image was composed of 640,000 individual picture elements
(“pixels”), like the dots in a newspaper wire-photo or a pointillist painting.
The spacecraft was 3. 7 billion miles away from Earth, so far away that it took
etch pixel 51/2 hours, traveling at the speed of light, to reach us. The
pictures might have been returned earlier, but the big radio telescopes in
California, Spain, and Australia that receive these whispers from the edge of
the Solar System had responsibilities to other ships that ply the sea of space
among them, Magellan, bound for Venus, and Galileo on its tortuous passage to
Jupiter.
Voyager 1 was so high above the ecliptic
plane because, in 1981, it had made a close pass by Titan, the giant moon of
Saturn. Its sister ship, Voyager 2, was dispatched on a different trajectory,
within the ecliptic plane, and so she was able to perform her celebrated
explorations of Uranus and Neptune The two Voyager robots have explored four
planets and nearly sixty moons. They are triumphs of human engineering an. one
of the glories of the American space program. They will he in the history books
when much else about our time forgotten.
The Voyagers were guaranteed to work
only until the Saturn encounter. I thought it might be a good idea, just after
Saturn, to have them take one last glance homeward. From Saturn, I knew the
Earth would appear too small for Voyager to make out any detail. Our planet
would be just a point of light, a lonely pixel, hardly distinguishable from the
many other points of light Voyager could see, nearby planets and far-off suns.
But precise because of the obscurity of our world thus revealed, such picture
might be worth having.
Mariners had painstakingly mapped the
coastlines of the continents. Geographers had translated these findings into
charts and globes. Photographs of tiny patches of the Earth had been obtained
first by balloons and aircraft, then by rockets in brief ballistic flight, and
at last by orbiting spacecraft-giving a perspective like the one you achieve by
positioning your eyeball about an inch above a large globe. While almost
everyone is taught that the Earth is a sphere with all of us somehow glued to
it by gravity, the reality of our circumstance did not really begin to sink in
until the famous frame-filling Apollo photograph of the whole Earth-the one
taken by the Apollo 17 astronauts on the last journey of humans to the Moon.
It has become a kind of icon of our age.
There's Antarctica at what Americans and Europeans so readily regard as the
bottom, and then all of Africa stretching up above it: You can see Ethiopia,
Tanzania, and Kenya, where the earliest humans lived. At top right are Saudi
Arabia and what Europeans call the Near East. Just barely peeking out at the
top is the Mediterranean Sea, around which so much of our global civilization
emerged. You can make out the blue of the ocean, the yellow-red of the Sahara
and the Arabian desert, the brown-green of forest and grassland.
And yet there is no sign of humans in
this picture, not our reworking of the Earth's surface, not our machines, not
ourselves: We are too small and our statecraft is too feeble to be seen by a
spacecraft between the Earth and the Moon. From this vantage point, our obsession
with nationalism is nowhere in evidence. The Apollo pictures of the whole Earth
conveyed to multitudes something well known to astronomers: On the scale of
worlds-to say nothing of stars or galaxies-humans are inconsequential, a thin
film of life on an obscure and solitary lump of rock and metal.
It seemed to me that another picture of
the Earth, this one taken from a hundred thousand times farther away, might
help in the continuing process of revealing to ourselves our true circumstance
and condition. It had been well understood by the scientists and philosophers
of classical antiquity that the Earth was a mere point in a vast encompassing
Cosmos, but no one had ever seen it as such. Here was our first chance (and
perhaps also our last for decades to come).
Many in NASA's Voyager Project were
supportive. But from the outer Solar System the Earth lies very near the Sun,
like a moth enthralled around a flame. Did we want to aim the camera so close
to the Sun as to risk burning out the spacecraft's vidicon system? Wouldn't it
be better to delay until all the scientific images from Uranus and Neptune, if
the spacecraft lasted that long, were taken?
And so we waited- and a good thing
too-from 1981 at Saturn, to 1986 at Uranus, to 1989, when both spacecraft had
passed the orbits of Neptune and Pluto. At last the time came But there were a
few instrumental calibrations that needed to be done first, and we waited a
little longer. Although the spacecraft were in the right spots, the instruments
were still working beautifully, and there were no other pictures to take, a few
project personnel opposed it. It wasn't science, they said. Then we discovered
that the technicians who devise and transmit the radio commands to Voyager
were, in a cash-strapped NASA, to be laid off immediately or transferred to
other jobs. If the picture were to be taken, it had to be done right then. At
the last minute actually, in the midst of the Voyager 2 encounter with Neptune,
the then NASA Administrator, Rear Admiral Richard Truly, stepped in and made
sure that these images were obtained. The space scientists Candy Hansen of
NASA's Jet Propulsion Laboratory (JPL) and Carolyn Porco of University of
Arizona designed the command sequence and calculated the camera exposure times.
So here they are-a mosaic of squares
laid down on top of the planets and a background smattering of more distant
stars. We were able to photograph not only the Earth, but also five other of
the Sun's nine known planets. Mercury, the innermost, was lost in the glare of
the Sun, and Mars and Pluto were too small, too dimly lit, and/or too far away.
Uranus and Neptune are so dim that to record their presence required long
exposures; accordingly, their images were smeared because of spacecraft motion.
This is how the planets would look to an alien spaceship approaching the Solar
System after a long interstellar voyage.
From this distance the planets seem only
points of light, smeared or unsmeared-even through the high-resolution
telescope aboard Voyager. They are like the planets seen with the naked eye
from the surface of the Earth-luminous dots, brighter than most of the stars.
Over a period of months the Earth, like the other planets, would seem to move
among the stars. You cannot tell merely by looking at one of these dots what
it's like, what's on it, what its past has been, and whether, n this particular
epoch, anyone lives there.
Because of the reflection of sunlight
off the spacecraft, the Earth seems to be sitting in a beam of light, as if
there were some special significance to this small world. But it's just an
accident of geometry and optics. The Sun emits its radiation equitably in all
directions. Had the picture been taken a little earlier or a little later,
there would have been no sunbeam highlighting the Earth.
And why that cerulean color? The blue
comes partly from the sea, partly from the sky. While water in a glass is
transparent, It absorbs slightly more red light than blue. If you have tens of
meters of the stuff or more, the red light is absorbed out and what gets
reflected back to space is mainly blue. In the same way, a short line of sight
through air seems perfectly transparent. Nevertheless-something Leonardo da
Vinci excelled at portraying-the more distant the object, the bluer it seems.
Why? Because the air scatters blue light around much better than it does red.
So the bluish cast of this dot comes from its thick but transparent atmosphere
and its deep oceans of liquid water. And the white? The Earth on an average day
is about half covered with white water clouds.
We can explain the wan blueness of this
little world because we know it well. Whether an alien scientist newly arrived
at the outskirts of our solar system could reliably deduce oceans and clouds
and a thickish atmosphere is less certain. Neptune, for instance, is blue, but
chiefly for different reasons. From this distant vantage point, the Earth might
not seem of any particular interest.
But for us, it's different. Look again
at that dot. That's here. That's home. That's us. On it everyone you love,
everyone you know, everyone you ever heard of, every human being who ever was,
lived out their lives. The aggregate of our joy and suffering, thousands of
confident religions, ideologies, and economic doctrines, every hunter and
forager, every hero and coward, every creator and destroyer of civilization,
ever king and peasant, every young couple in love, every moth and father,
hopeful child, inventor and explorer, every teacher of morals, every corrupt
politician, every “superstar,” every “supreme leader,” every saint and sinner
in the history of our species lived there-on a mote of dust suspended in a
sunbeam.
The Earth is a very small stage in a
vast cosmic arena. Think of the rivers of blood spilled by all those generals
and emperors so that, in glory and triumph, they could become momentary masters
of a fraction of a dot. Think of the endless visited by the inhabitants of one
corner of this pixel the scarcely distinguishable inhabitants of some other
corner, how frequent their misunderstandings, how eager they are to kill one
another, how fervent their hatreds.
Our posturings, our imagined
self-importance, the delusion that we have some privileged position in the
Universe, are challenged by this point of pale light. Our planet is a lonely
speck in the great enveloping cosmic dark. In our obscurity, in all this
vastness, there is no hint that help will come from elsewhere to save us from
ourselves.
The Earth is the only world known so far
to harbor life. There is nowhere else, at least in the near future, to which
our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the
moment the Earth is where we make our stand.
It has been said that astronomy is
a humbling and character-building experience. There is perhaps no better demonstration
of the folly of human conceits than this distant image of our tiny world. To
me, it underscores our responsibility to deal more kindly with one another, and
to preserve and cherish the pale blue dot, the only home we've ever known.
CHAPTER 2
Aberrations of light
If man were taken away from the world,
the rest would seem to be all astray, without aim or purpose...and to be
leading to nothing.
—FRANCIS BACON, WISDOM OF THE ANCIENTS
(1619)
Ann Druyan suggests an experiment: Look
back again at the pale blue dot of the preceding chapter. Take a good long look
at it. Stare at the dot for any length of time and then try to convince
yourself that God created the whole Universe for one of the 10 million or so
species of life that inhabit that speck of dust. Now take it a step further:
Imagine that everything was made just for a single shade of that species, or
gender, or ethnic or religious subdivision. If this doesn't strike you as
unlikely, pick another dot. Imagine it to be inhabited by a different form of
intelligent life. They, too, cherish the notion of a God who has created
everything for their benefit. How seriously do you take their claim?
“SEE THAT STAR?”
“You mean the bright red one?” his
daughter asks in return.
“Yes. You know, it might not be there
anymore. It might be gone by now-exploded or something. Its light is still
crossing space, just reaching our eyes now. But we don't see it as it is. We
see it as it was.”
Many people experience a stirring sense
of wonder when they first confront this simple truth. Why? Why should it be so
compelling? On our little world light travels, for all practical purposes,
instantaneously. If a lightbulb is glowing, then of course it's physically
where we see it, shining away. We reach out our hand and touch it: It's there
all right, and unpleasantly hot. If the filament fails, then the light goes
out. We don't see it in the same place, glowing, illuminating the room years
after the bulb breaks and it's removed from its socket. The very notion seems
nonsensical. But if we're far enough away, an entire sun can go out and we'll
continue to see it shining brightly; we won't learn of its death, it may be,
for ages to come-in fact, for how long it takes light, which travels fast but
not infinitely fast, to cross the intervening vastness.
The immense distances to the stars and
the galaxies mean that we see everything in space in the past-some as they were
before the Earth came to be. Telescopes are time machines. Long ago, when an
early galaxy began to pour light out into the surrounding darkness, no witness
could have known that billions of years later some remote clumps of rock and
metal, ice and organic molecules would fall together to make a place called
Earth; or that life would arise and thinking beings evolve who would one day
capture a little of that galactic light, and try to puzzle out what had sent it
on its way.
And after the Earth dies, some 5 billion
years from now, after it is burned to a crisp or even swallowed by the Sun,
there will be other worlds and stars and galaxies coming into being-and they
will know nothing of a place once called Earth.
IT ALMOST NEVER FEELS like prejudice.
Instead, it seems fitting and just-the idea that, because of an accident of
birth, our group (whichever one it is) should have a central position in the
social universe. Among Pharaonic princelings and Plantagenet pretenders,
children of robber barons and Central Committee bureaucrats, street gangs and
conquerors of nations, members of confident majorities, obscure sects, and reviled
minorities, this self-serving attitude seems as natural as breathing. It draws
sustenance from the same psychic wellsprings as sexism, racism, nationalism,
and the other deadly chauvinisms that plague our species. Uncommon strength of
character is needed to resist the blandishments of those who assure us that we
have an obvious, even God-given, superiority over our fellows. The more
precarious our self-esteem, the greater our vulnerability to such appeals.
Since scientists are people, it is not
surprising that comparable pretensions have insinuated themselves into the
scientific worldview. Indeed, many of the central debates in the history of
science seem to be, in part at least, contests over whether humans are special.
Almost always, the going-in assumption is that we are special. After the
premise is closely examined, though, it turns out-in dishearteningly many
cases-that we are not.
Our ancestors lived out of doors. They
were as familiar with the night sky as most of us are with our favorite television
programs. The Sun, the Moon, the stars, and the planets all rose in the east
and set in the west, traversing the sky overhead in the interim. The motion of
the heavenly bodies was not merely a diversion, eliciting a reverential nod and
grunt; it was the only way to tell the time of day and the seasons. For hunters
and gatherers, as well as for agricultural peoples, knowing about the sky was a
matter of life and death.
How lucky for us that the Sun, the Moon,
the planets, and the stars are part of some elegantly configured cosmic
clockwork! It seemed to be no accident. They were put here for a purpose, for
our benefit. Who else makes use of them? What else are they good for?
And if the lights in the sky rise and
set around us, isn't it evident that we're at the center of the Universe? These
celestial bodies-so clearly suffused with unearthly powers, especially the Sun
on which we depend for light and heat-circle us like courtiers fawning on a
king. Even if we had not already guessed, the most elementary examination of
the heavens reveals that we are special. The Universe seems designed for human
beings. It's difficult to contemplate these circumstances without experiencing
stirrings of pride and reassurance. The entire Universe, made for us! We must
really be something.
This satisfying demonstration of our
importance, buttressed by daily observations of the heavens, made the
geocentrist conceit a transcultural truth-taught in the schools, built into the
language, part and parcel of great literature and sacred scripture. Dissenters
were discouraged, sometimes with torture and death. It is no wonder that for
the vast bulk of human history, no one questioned it.
It was doubtless the view of our
foraging and hunting ancestors. The great astronomer of antiquity, Claudius
Ptolemaeus (Ptolemy), in the second century knew that the Earth was a sphere,
knew that its size was “a point” compared to the distance of the stars, and
taught that it lay “right in the middle of the heavens.” Aristotle, Plato, St.
Augustine, St. Thomas Aquinas, and almost all the great philosophers and
scientists of all cultures over the 3,000 years ending in the seventeenth
century bought into this delusion. Some busied themselves figuring out how the
Sun, the Moon, the stars, and the planets could be cunningly attached to
perfectly transparent, crystalline spheres-the big spheres, of course, centered
on the Earth-that would explain the complex motions of the celestial bodies so
meticulously chronicled by generations of astronomers. And they succeeded: With
later modifications, the geocentric hypothesis adequately accounted for the
facts of planetary motion as known in the second century, and in the sixteenth.
From there it was only a slight
extrapolation to an even more grandiose claim-that the “perfection” of the
world would be incomplete without humans, as Plato asserted in the Timaeus.
“Man...is all,” the poet and cleric John Donne wrote in 1625. “He is not a
piece of the world, but the world itself; and next to the glory of God, the
reason why there is a world.”
And yet-never mind how many kings,
popes, philosophers, scientists, and poets insisted on the contrary-the Earth
through those millennia stubbornly persisted in orbiting the Sun. You might
imagine an uncharitable extraterrestrial observer looking down on our species
over all that time-with us excitedly chattering, “The Universe is created for
us! We're at the center! Everything pays homage to us!”-and concluding that our
pretensions are amusing, our aspirations pathetic, that this must be the planet
of the idiots.
But such a judgment is too harsh. We did
the best we could. There was an unlucky coincidence between everyday
appearances and our secret hopes. We tend not to be especially critical when
presented with evidence that seems to confirm our prejudices. And there was
little countervailing evidence.
In muted counterpoint, a few dissenting
voices, counseling humility and perspective, could be heard down through the
centuries. At the dawn of science, the atomist philosophers of ancient Greece
and Rome- those who first suggested that matter is made of atoms-Democritus,
Epicurus, and their followers (and Lucretius, the first popularizer of science)
scandalously proposed many worlds and many alien life forms, all made of the
same kinds of atoms as we. They offered for our consideration infinities in
space and time. But in the prevailing canons of the West, secular and
sacerdotal, pagan and Christian, atomist ideas were reviled. Instead, the
heavens were not at all like our world. They were unalterable and “perfect.”
The Earth was mutable and “corrupt.” The Roman statesman and philosopher Cicero
summarized the common view: “In the heavens...there is nothing of chance or
hazard, no error, no frustration, but absolute order, accuracy, calculation and
regularity.”
Philosophy and religion cautioned that
the gods (or God) were far more powerful than we, jealous of their prerogatives
and quick to mete out justice for insufferable arrogance. At the same time,
these disciplines had not a clue that their own teaching of how the Universe is
ordered was a conceit and a delusion.
Philosophy and religion presented mere
opinion-opinion that might be overturned by observation and experiment-as
certainty. This worried them not at all. That some of their deeply held beliefs
might turn out to be mistakes was a possibility hardly considered. Doctrinal
humility was to be practiced by others. Their own teachings were inerrant and
Infallible. In truth, they had better reason to be humble than they knew.
BEGINNING WITH COPERNICUS in the middle
sixteenth century, the issue was formally joined. The picture of the Sun rather
than the Earth at the center of the Universe was understood to be dangerous.
Obligingly, many scholars were quick to assure the religious hierarchy that
this newfangled hypothesis represented no serious challenge to conventional
wisdom. In a kind of split-brain compromise, the Sun-centered system was
treated as a mere computational convenience, not an astronomical reality that
is, the Earth was really at the center of the Universe, as everybody knew; but
if you wished to predict where Jupiter would be on the second Tuesday of
November the year after next, you were permitted to pretend that the Sun was at
the center. Then you could calculate away and not affront the Authorities.
“This has no danger in it,” wrote Robert
Cardinal Bellarmine, the foremost Vatican theologian in the early seventeenth
century, and suffices for the
mathematicians. But, to affirm that the Sun is really fixed in the center of
the heavens and that the Earth revolves very swiftly around the Sun is a
dangerous thing, not only irritating the theologians and philosophers, but
injuring our holy faith and making the sacred scripture false.”
“Freedom of belief is pernicious,”
Bellarmine wrote on another occasion. “It is nothing but the freedom to be
wrong.”
Besides, if the Earth was going around
the Sun, nearby stars should seem to move against the background of more
distant stars as, every six months, we shift our perspective from one side of
the Earth's orbit to the other. no such “annual parallax” had been found. The
Copernicans argued that this was because the stars were extremely far
away-maybe a million times more distant than the Earth is from the Sun. Perhaps
better telescopes, in future times, would find an annual parallax. The
geocentrists considered this a desperate attempt to save a flawed hypothesis,
and ludicrous on the face of it.
When Galileo turned the first
astronomical telescope to the sky, the tide began to turn. He discovered that
Jupiter had a little retinue of moons circling it, the inner ones orbiting
faster than the outer ones, just as Copernicus had deduced for the motion of
the planets about the Sun. He found that Mercury and Venus went through phases
like the Moon (showing they orbited the Sun). Moreover, the cratered Moon and
the spotted Sun challenged the perfection of the heavens. This may in part
constitute the sort of trouble Tertullian was worried about thirteen hundred
years earlier, when he pleaded, “If you have any sense or modesty, have done
with prying into the regions of the sky, into the destiny and secrets of the
universe.”
In contrast, Galileo taught that we can
interrogate Nature by observation and experiment. Then, “facts which at first
sight seem improbable will, even on scant explanation, drop the cloak which had
hidden them and stand forth in naked and simple beauty.” Are not these facts,
available even for skeptics to confirm, a surer insight into God's Universe
than all the speculations of the theologians? But what if these facts
contradict the beliefs of those who hold their religion incapable of making
mistakes? The princes of the Church threatened the aged astronomer with torture
if he persisted in teaching the abominable doctrine that the Earth moved. He
was sentenced to a kind of house arrest for the remainder of his life.
A generation or two later, by the time
Isaac Newton demonstrated that simple and elegant physics could quantitatively
explain-and predict-all the observed lunar and planetary motions (provided you
assumed the Sun at the center of the Solar System), the geocentrist conceit
eroded further.
In 1725, in an attempt to discover
stellar parallax, the painstaking English amateur astronomer James Bradley
stumbled on the aberration of light. The term “aberration,” I suppose, conveys
something of the unexpectedness of the discovery. When observed over the course
of a year, stars were found to trace little ellipses against the sky. But all
the stars were found to do so. This could not be stellar parallax, where we
would expect a big parallax for nearby stars and an indetectible one for
faraway stars. Instead, aberration is similar to how raindrop falling directly
down on a speeding auto seem to the passenger, to be falling at a slant; the faster
the car goes, the steeper the slant. If the Earth were stationary at the center
of the Universe. and not speeding in its orbit around the Sun, Bradley would
nor have found the aberration of light. It was a compelling demonstration that
the Earth revolved about the Sun. It convinced most astronomers and some others
but not, Bradley thought, the “Anti-Copernicans.”
But not until 1837 did direct
observations of the stars prove in the clearest way that the Earth is indeed
circling the Sun. The long-debated annual parallax was at last discovered-not
by better arguments, but by better instruments. Because explaining what it
means is much more straightforward than explaining the aberration of light, its
discovery was very important. It pounded the final nail into the coffin of
geocentrism. You need only look at your finger with your left eye and then with
your right and see it seem to move. Everyone can understand parallax.
By the nineteenth century, all
scientific geocentrists had been converted or rendered extinct. Once most
scientists had been convinced, informed public opinion had swiftly changed, in
some countries in a mere three or four generations. Of course, in the time of
Galileo and Newton and even much later, there were still some who objected, who
tried to prevent the new Sun-centered Universe from becoming accepted, or even
known. And there were many who at least harbored secret reservations.
By the late twentieth century, just in
case there were any holdouts, we have been able to settle the matter directly.
We've been able to test whether we live in an Earth-centered system with
planets affixed to transparent crystal spheres, or in a Sun-centered system
with planets controlled at a distance by the gravity of the Sun. We have, for
example, probed the planets with radar. When we bounce a signal off a moon of
Saturn, we receive no radio echo from a nearer crystal sphere attached to
Jupiter. Our spacecraft arrive at their appointed destinations with astonishing
precision, exactly as predicted by Newtonian gravitation. When our ships fly to
Mars, say, their instruments do not hear a tinkling sound or detect shards of
broken crystal as they crash through the “spheres” that-according to the
authoritative opinions that prevailed for millennia-propel Venus or the Sun ill
their dutiful motions about the central Earth.
When Voyager 1 scanned the Solar System
from beyond the outermost planet, it saw, just as Copernicus and Galileo had
said we would, the Sun in the middle and the planets in concentric orbits about
it. Far from being the center of the Universe, the Earth is just one of the
orbiting dots. no longer confined to a single world, we are now able to reach
out to others and determine decisively what kind of planetary system we
inhabit.
EVERY OTHER PROPOSAL, and their number
is legion, to displace us from cosmic center stage has also been resisted, in
part for similar reasons. We seem to crave privilege, merited not by our work,
but by our birth, by the mere fact that, say, we are humans and born on Earth.
We might call it the anthropocentric-the “human-centered”-conceit.
This conceit is brought close to culmination
in the notion that we are created in God's image: The Creator and Ruler of the
entire Universe looks just like me. My, what a coincidence How convenient and
satisfying! The sixth-century-B. C. Green philosopher Xenophanes understood the
arrogance of the perspective:
“The Ethiopians make their gods black
and snub-nosed; the Thracians say theirs have blue eyes and red hair...Yes, and
if oxen and horses or lions had hands, and could paint with their hands, and
produce works of art as men do, horses would paint the forms of the gods like
horses, and oxen like oxen...”
Such attitudes were once described as
“provincial”-the naive expectation that the political hierarchies and social
conventions of an obscure province extend to a vast empire composed of many
different traditions and cultures; that the familiar boondocks, our boondocks,
are the center of the world. The country bumpkins know almost nothing about
what else is possible. They fail to grasp the insignificance of their province
or the diversity of the Empire. With ease, they apply their own standards and
customs to the rest of the planet. But plopped down in Vienna, say, or Hamburg,
or New York, ruefully they recognize how limited their perspective has been.
They become “deprovincialized.”
Modern science has been a voyage into the
unknown, with a lesson in humility waiting at every stop. Many passengers would
rather have stayed home.
CHAPTER 3
The great demotions
[One philosopher] asserted that he knew
the whole secret...[H]e surveyed the two celestial strangers from top to toe,
and maintained to their faces that their persons, their worlds, their suns, and
their stars, were created solely for the use of man. At this assertion our two
travelers let themselves fall against each other, seized with a fit
of...inextinguishable laughter.
—VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL
HISTORY (1752)
In the seventeenth century there was
still some hope that, even if the Earth was not the center of the Universe, it
might be the only “world.” But Galileo's telescope revealed that “the Moon
certainly does not possess a smooth and polished surface” and that other worlds
might look “just like the face of the Earth itself.” The Moon and the planets
showed unmistakably that they had as much claim to being worlds as the Earth
does-with mountains, craters, atmospheres, polar ice caps, clouds, and, in the
case of Saturn, a dazzling, unheard-of set of circumferential rings. After
millennia of philosophical debate, the issue was settled decisively in favor of
“the plurality of worlds.” They might be profoundly different from our planet.
None of them might be as congenial for life. But the Earth was hardly the only
one.
This was the next in the series of Great
Demotions, downlifting experiences, demonstrations of our apparent
insignificance, wounds that science has, in its search for Galileo's facts,
delivered to human pride.
WELL, SOME HOPED, even if the Earth
isn't at the center of the Universe, the Sun is. The Sun is our Sun. So the
Earth is approximately at the center of the Universe. Perhaps some of our pride
could in this way be salvaged. But by the nineteenth century, observational
astronomy had made it clear that the Sun is but one lonely star in a great
self-gravitating assemblage of suns called the Milky Way Galaxy. Far from being
at the center of the Galaxy, our Sun with its entourage of dim and tiny planets
lies in an undistinguished sector of an obscure spiral arm. We are thirty
thousand light years from the Center.
Well, our Milky Way is the only galaxy.
The Milky Way Galaxy is one of billions, perhaps hundreds of billions of
galaxies notable neither in mass nor in brightness nor in how its stars are
configured and arrayed. Some modern deep sky photographs show more galaxies
beyond the Milky Way than stars within the Milky Way. Every one of them is an
island universe containing perhaps a hundred billion suns. Such an image is a
profound sermon on humility.
Well, then, at least our Galaxy is at
the center of the Universe. No, this is wrong too. When the expansion of the
Universe was first discovered, many people naturally gravitated to the notion
that the Milky Way was at the center of the expansion, and all the other
galaxies running away from us. We now recognize that astronomers on any galaxy
would see all the others running away . from them; unless they were very
careful, they would all conclude that they were at the center of the Universe.
There is, in fact, no center to the expansion, no point of origin of the Big
Bang, at least not in ordinary three-dimensional space.
Well, even if there are hundreds of
billions of galaxies, each with hundreds of billions of stars, no other star
has planets. If there are n other planets beyond our Solar System, perhaps
there's no other life in the Universe. Our uniqueness might then be saved. Sing
planets are small and feebly shine by reflected sunlight, they're hard to find.
Although applicable technology is improving wit breathtaking speed, even a
giant world like Jupiter, orbiting the nearest star, Alpha Centauri, would
still be difficult to detect. Iii our ignorance, the geocentrists find hope.
There was once a scientific
hypothesis-not just well received but prevailing-that supposed our solar system
to have formed through the near collision of the ancient Sun with another star;
the gravitational tidal interaction pulled out tendrils of sunstuff that
quickly condensed into planets. Since space is mainly empty and near stellar collisions
most rare, it was concluded that few other planetary systems exist-perhaps only
one, around that other star that long ago co-parented the worlds of our solar
system. Early in my studies, I was amazed and disappointed that such a view had
ever been taken seriously, that for planets of other stars, absence of evidence
had been considered evidence of absence.
Today we have firm evidence for at least
three planets orbiting an extremely dense star, the pulsar designated B1257+12,
about which I'll say more later. And we've found, for more than half the stars
with masses like the Sun's, that early in their careers they're surrounded by
great disks of gas aid dust out of which planets seem to form. Other planetary
systems now look to be a cosmic commonplace, maybe even worlds something like
the Earth. We should be able, in the next few decades, to inventory at least
the larger planets, if they exist, of hundreds of nearby stars.
Well, if our position in space doesn't
reveal our special role, our position in time does: We've been in the Universe
since The Beginning (give or take a few days). We've been given special
responsibilities by the Creator. It once seemed very reasonable to think of the
Universe as beginning just a little before our collective memory is obscured by
the passage of time and the illiteracy of our ancestors. Generally speaking,
that's hundreds or thousands of years ago. Religions that purport to describe
the origin of the Universe often specify-implicitly or explicitly-a date of
origin of roughly such vintage, a birthday for the world.
If you add up all the “begats” in
Genesis, for example, you get an age for the Earth: 6,000 years old, plus or
minus a little. The universe is said to be exactly as old as the Earth. This is
still the standard of Jewish, Christian, and Moslem fundamentalists and is
clearly reflected in the Jewish calendar.
But so young a Universe raises an
awkward question: How is it that there are astronomical objects more than 6,000
light-years away? It takes light a year to travel a light-year, 10,000 years to
travel 10,000 light-years, and so on. When we look at the center of the Milky
Way Galaxy, the light we see left its source 30,000 years ago. The nearest
spiral galaxy like our own, M31 in the constellation Andromeda, is 2 million
light-years away, so we are seeing it as it was when the light from it set out
on its long journey to Earth-2 million years ago. And when we observe distant
quasars 5 billion light-years away, we are seeing them as they were 5 billion
years ago, before the Earth was formed. (They are, almost certainly, very
different today.)
If, despite this, we were to accept the
literal truth of such religious books, how could we reconcile the data? The
only plausible conclusion, I think, is that God recently made all the photons
of light arriving on the Earth in such a coherent format as to mislead
generations of astronomers into the misapprehension that there are such things
as galaxies and quasars, and intentionally driving them to the spurious
conclusion that the Universe is vast and old. This is such a malevolent
theology I still have difficulty believing that anyone, no matter how devoted
to the divine inspiration of any religious book, could seriously entertain it.
Beyond this, the radioactive dating of rocks,
the abundance of impact craters on many worlds, the evolution of the stars, and
the expansion of the Universe each provides compelling and independent evidence
that our Universe is many billions of years old-despite the confident
assertions of revered theologians that a world so old directly contradicts the
word of God, and that at any rate information on the antiquity of the world is
inaccessible except to faith. These
lines of evidence, as well, would have to be manufactured by a deceptive and
malicious deity-unless the world is much older than the literalists in the
Judeo-Christian-Islamic religion suppose. Of course, no such problem arises for
those many religious people who treat the Bible and the Qur'an as historical
and moral guides and great literature, but who recognize that the perspective
of these scriptures on the natural world reflects the rudimentary science of
the time in which they were written.
Ages rolled by before the Earth began.
More ages will run their course before it is destroyed. A distinction needs to
be drawn between how old the Earth is (around 4. 5 billion years) and how old
the Universe is (about 15 billion years since the Big Bang). The immense
interval of time between the origin of the Universe and our epoch was two-thirds
over before the Earth came to be. Some stars and planetary systems are billions
of years younger, others billions of years older. But in Genesis, chapter 1,
verse 1, the Universe and the Earth are created on the same day. The
Hindu-Buddhist-Jain religion tends not to confound the two events.
As for humans, we're latecomers. We
appear in the last instant of cosmic time. The history of the Universe till now
was 99. 998 percent over before our species arrived on the scene. In that vast
sweep of aeons, we could not have assumed any special responsibilities for our
planet, or life, or anything else. We were not here.
Well, if we can't find anything special
about our position or our epoch, maybe there's something special about our
motion. Newton and all the other great classical physicists held that the
velocity of the Earth in space constituted a “privileged frame of reference.”
That's actually what it was called. Albert Einstein, a keen critic of prejudice
and privilege all his life, considered this “absolute” physics a remnant of an
increasingly discredited Earth chauvinism. It seemed to him that the laws of
Nature must be the same no matter what the velocity or frame of reference of
the observer. With this as his starting point, he developed the Special Theory
of Relativity. Its consequences are bizarre, counter intuitive, and grossly
contradict common sense-but only at very high speeds. Careful and repeated
observations show that his justly celebrated theory is an accurate description
of how the world is made. Our commonsense intuitions can be mistaken. Our
preferences don't count. We do not live in a privileged reference frame.
One consequence of special relativity is
time dilation-the slowing down of time as the observer approaches light speed.
You can still find claims that time dilation applies to watches and elementary
particles-and, presumably, to circadian and other rhythms in plants, animals,
and microbes-but not to human biological clocks. Our species has been granted,
it is suggested, special immunity from the laws of Nature, which must
accordingly be able to distinguish deserving from undeserving collections of
matter. (In fact, the proof Einstein gave for special, relativity admits no
such distinctions.) The idea of humans as exceptions to relativity seems
another incarnation of the notion of special creation:
Well, even if our position, our epoch,
our motion, and our world are not unique, maybe we are. We're different from
the other animals. We're specially created. The particular devotion of the Creator
of the Universe is evident in us. This position was passionately defended on
religious and other grounds. But in the middle nineteenth century Charles
Darwin showed convincingly how one species can evolve into another by entirely
natural processes, which come down to the heartless business of Nature saving
the heredities that work and rejecting those that don't. “Man in his arrogance
thinks himself a great work worthy [of] the interposition of a deity,” Darwin
wrote telegraphically in his notebook. “More humble and I think truer to
consider him created from animals.” The profound and intimate connections of
humans with the other life forms, on Earth have been compellingly demonstrated
in the late twentieth century by the new science of molecular biology.
IN EACH AGE the self-congratulatory
chauvinisms are challenged in yet another arena of scientific debate-in this
century, for example, in attempts to understand the nature of human sexuality,
the existence of the unconscious mind, and the fact that many psychiatric
illnesses and character “defects” have a molecular origin. But also:
Well, even if we're closely related to
some of the other animals, we're different-not just in degree, but in kind-on
what really matters: reasoning, selfconsciousness, tool making, ethics,
altruism, religion, language, nobility of character. While humans, like all
animals, have traits that set them apart-otherwise, how could we distinguish
one species from another? -human uniqueness has been exaggerated, sometimes
grossly so. Chimps reason, are self-conscious, make tools, show devotion, and
so on. Chimps and humans have 99. 6 percent of their active genes in common.
(Ann Druyan and I run through the evidence in our book Shadows of Forgotten
Ancestors.)
In popular culture, the very opposite
position is also embraced, although it too is driven by human chauvinism (plus
a failure of the imagination): Children's stories and cartoons make animals
dress in clothes, live in houses, use knives and forks, and speak. The three
bears sleep in beds. The owl and the pussycat go to sea in a beautiful
pea-green boat. Dinosaur mothers cuddle their young. Pelicans deliver the mail.
Dogs drive cars. A worm catches a thief. Pets have human names. Dolls,
nutcrackers, cups, and saucers dance and have opinions. The dish runs away with
the spoon. In the Thomas the Tank Engine series, we even have anthropomorphic
locomotives and railway cars, charmingly portrayed. no matter what we're
thinking about, animate or inanimate, we tend to invest it with human traits.
We can't help ourselves. The images come readily to mind. Children are clearly
fond of them.
When we talk about a “threatening” sky,
a “troubled” sea diamonds “resisting” being scratched, the Earth “attracting”
passing asteroid, or an atom being “excited,” we are again drawn to a kind of
animist worldview. We reify. Some ancient level of our thinking endows
inanimate Nature with life, passions, and forethought.
The notion that the Earth is self-aware
has lately been growing at the fringes of the “Gaia” hypothesis. But this was
commonplace belief of both the ancient Greeks and the earl Christians. Origen
wondered whether “the earth also, according to its own nature, is accountable
for some sin.” A host of ancient scholars thought the stars alive. This was
also the position of Origen, of St. Ambrose (the mentor of St. Augustine), and
even, in a more qualified form, of St. Thomas Aquinas. The Stoic philosophical
position on the Sun's nature was stated by Cicero, in the first century B. C. :
“Since the Sun resembles those fires which are contained in the bodies of
living creatures, the Sun must also be alive.”
Animist attitudes in general seem to
have been spreading recently. In a 1954 American survey, 75 percent of people
polled were willing to state that the Sun is not alive; in 1989, only 30
percent would support so rash a proposition. On whether an automobile tire can
feel anything, 90 percent of respondents denied it emotions in 1954, but only
73 percent in 1989.
We can recognize here a shortcoming-in some
circumstances serious-in our ability to understand the world.
Characteristically, willy-nilly, we seem compelled to project our own nature
onto Nature. Although this may result in a consistently distorted view of the
world, it does have one great virtue-projection is the essential precondition
for compassion.
Okay, maybe we're not much, maybe we're
humiliatingly related t0 apes, but at least we're the best there is. God and
angels aside, we're the only intelligent beings in the Universe. One
correspondent writes to me, “I am as sure of this as anything in my experience.
There is no conscious life anywhere else in the Universe. Mankind thus returns
to its rightful position as center of the universe.” However, partly through
the influence of science acid science fiction, most people today, in the United
States at least, reject this proposition-for reasons essentially stated by the
ancient Greek philosopher Chrysippus: “For any human being in existence to
think that there is nothing in the whole world superior to himself would be an
insane piece of arrogance.”
But the simple fact is that we have not
yet found extraterrestrial life. We are in the earliest stages of looking. The
question is wide open. If I had to guess-especially considering our long
sequence of failed I would guess that the Universe is filled with beings far
more intelligent, tar more advanced than we are. But of course I might be
wrong. Such a conclusion is at best based on a plausibility argument, derived
from the numbers of planets, the ubiquity of organic matter, the immense
timescales available for evolution, and so on. It is not a scientific
demonstration. The question is among the most fascinating in all of science. As
described in this book, we are just developing the tools to treat it seriously.
What about the related matter of whether
we are capable of creating intelligences smarter than ourselves? Computers
routinely do mathematics that no unaided human can manage, outperform world
champions in checkers and grand masters in chess, speak and understand English
and other languages, write presentable short stories and musical compositions,
learn from their mistakes, and competently pilot ships, airplanes, and
spacecraft. Their abilities steadily improve. They're getting smaller, faster, and
cheaper. Each year, the tide of scientific advance laps a little further ashore
on the island of human intellectual uniqueness with its embattled castaways.
If, at so early a stage in our technological evolution, we have been able to go
so far in creating intelligence out of silicon and metal, what will be possible
in the following decades and centuries? What happens when smart machines are
able to manufacture smarter machines?
PERHAPS THE CLEAREST INDICATION that the
search for an unmerited privileged position for humans will never be wholly
abandoned is what in physics and astronomy is called the Anthropic Principle.
It would be better named the Anthropocentric Principle. It comes in various
forms. The “Weak” Anthropic Principle merely notes that if the laws of Nature
and the physical constants-such as the speed of light, the electrical charge of
the electron, the Newtonian gravitational constant, or Planck's quantum
mechanical constant had been different, the course of events leading to the
origin of humans would never have transpired. Under other laws and constants,
atoms would not hold together, stars would evolve too quickly to leave
sufficient time for life to evolve on nearby planets, the chemical elements of
which life is made would never have been generated, and so on. Different laws,
no humans.
There is no controversy about the Weak
Anthropic Principle: Change the laws and constants of Nature, if you could, and
a very different universe may emerge-in many cases, a universe incompatible
with life. The mere fact that we exist
implies (but does not impose) constraints on the laws of Nature. In contrast,
the various “Strong” Anthropic Principles go much farther; some of their
advocates come close to deducing that the laws of Nature and the values of the
physical constants were established (don't ask how or by Whom) so that humans
would eventually come to be. Almost all of the other possible universes, they
say, are inhospitable. In this way, the ancient conceit that the Universe was
made for us is resuscitated.
To me it echoes Dr. Pangloss in
Voltaire's Candide, convinced that this world, with all its imperfections, is
the best possible. It sounds like playing my first hand of bridge, winning,
knowing that there are 54 billion billion billion (5. 4 X 1028) possible other
hands that I was equally likely to have been dealt...and then foolishly
concluding that a god of bridge exists and favors me, a god who arranged the
cards and the shuffle with my victory foreordained from The Beginning. We do
not know how many other winning hands there are in the cosmic deck, how many
other kinds of universes, laws of Nature, and physical constants: that could
also lead to life and Intelligence and perhaps even delusions of
self-importance. Since we know next to nothing about how the Universe was
made-or even if it was made-it's difficult to pursue these notions
productively.
Voltaire asked “Why is there anything?”
Einstein's formulation was to ask whether God had any choice in creating the
Universe. But if the Universe is infinitely old-if the Big Bang some 15 billion
years ago is only the most recent cusp in an infinite series of cosmic
contractions and expansions-then it was never created and the question of why
it is as it is is rendered meaningless.
If, on the other hand, the Universe has
a finite age, why is it the way it is? Why wasn't it given a very different
character? Which laws of Nature go with which others? Are there meta-laws
specifying the connections? Can we possibly discover them? Of all conceivable
laws of gravity, say, which ones can exist simultaneously with which
conceivable laws of quantum physics that determine the very existence of
macroscopic matter? Are all laws we can think of possible, or is there only a
restricted number that can somehow be brought into existence? Clearly we have
not a glimmering of how to determine which laws of Nature are “possible” and
which are not. Nor do we have more than the most rudimentary notion of what
correlations of natural laws are “permitted.”
For example, Newton's universal law of
gravitation specifies that the mutual gravitational force attracting two bodies
towards each other is inversely proportional to the square of how far they are
apart. You move twice as far from the center of the Earth and you weigh a quarter
as much; ten times farther and you weigh only a hundredth of your ordinary
weight; etc. It is this inverse square law that permits the exquisite circular
and elliptical orbits of planets around the Sun, and moons around the
planets-as well as the precision trajectories of our interplanetary spacecraft.
If r is the distance between the centers of two masses, we say that the
gravitational force varies as 1/r2.
But if this exponent were different-if
the gravitational law were 1/r4, say, rather than 1/r2 -then the orbits would
not close; over billions of revolutions, the planets would spiral in and be
consumed in the fiery depths of the Sun, or spiral out and be lost to
interstellar space. If the Universe were constructed with an inverse fourth
power law rather than an inverse square law, soon there would be no planets for
living beings to inhabit.
So of all the possible gravitational
force laws, why are we so lucky as to live in a universe sporting a law
consistent with life? First of course, we're so “lucky,” because if we weren't,
we wouldn't be here to ask the question. It is no mystery that inquisitive
beings who evolve on planets can be found only in universes that admit planets.
Second, the inverse square law is not is the only one consistent with stability
over billions of years. Any power law less steep than 1/r3 (1/r2. 99 or 1/r,
for example) will keep a planet in the vicinity of a circular orbit even if
it's given a shove. We have a tendency to overlook the possibility that other
conceivable laws of Nature might also be consistent with life.
But there's a further point: It's not
arbitrary that we have an inverse square law of gravitation. When Newton's
theory is understood in terms of the more encompassing general theory of
relativity, we recognize that the exponent of the gravity law is 2 because the
number of physical dimensions we live in is 3. All gravity laws aren't
available, free for a Creator's choosing. Even given an infinite number of
three-dimensional universes for some great god to tinker with, the gravity law
would always lave to be the law of the inverse square. Newtonian gravity, we
might say, is not a contingent facet of our universe, but a necessary one.
In general relativity, gravity is due to
the dimensionality and curvature of space. When we talk about gravity we are
talking about local dimples in space-time. This is by no means obvious and even
affronts commonsense notions. But when examined deeply, the ideas of gravity
and mass are not separate matters, but ramifications of the underlying geometry
of space-time.
I wonder if something like this doesn't
apply generally to all anthropic hypotheses. The laws or physical constants on
which our lives depend turn out to be members of a class, perhaps even a vast
class, of other laws and other physical constants-but some of these are also
compatible with a kind of life. Often we do not (or cannot) work through what
those other universes allow. Beyond that, not every arbitrary choice of a law
of Nature or a physical constant may be available, even to a maker of
universes. Our understanding of which laws of Nature and which physical
constants are up for grabs is fragmentary at best.
Moreover, we have no access to any of
those putative alternative universes. We have no experimental method by which anthropic
hypotheses may be tested. Even if the existence of such universes were to
follow firmly from well-established theories-of quantum mechanics or
gravitation, say-we could not be sure that there weren't better theories that
predict no alternative universes. Until that time comes, if it ever does, it
seems to me premature to put faith in the Anthropic Principle as an argument
for human centrality or uniqueness.
Finally, even if the Universe were
intentionally created to allow for the emergence of life or intelligence, other
beings may exist on countless worlds. If so, it would be cold comfort to
anthropocentrists that we inhabit one of the few universes that allow life and
intelligence.
There is something stunningly narrow
about how the Anthropic Principle is phrased. Yes, only certain laws and
constants of nature are consistent with our kind of life. But essentially the
same laws and constants are required to make a rock. So why not talk about a
Universe designed so rocks could one day come to be, and strong and weak Lithic
Principles? If stones could philosophize, I imagine Lithic Principles would be
at the intellectual frontiers.
There are cosmological models being
formulated today in which even the entire Universe is nothing special. Andrei
Linde, formerly of the Lebedev Physical Institute in Moscow and now at Stanford
University, has incorporated current understanding of the strong and weak
nuclear forces and quantum physics into a new cosmological model. Linde
envisions a vast Cosmos, much larger than our Universe-perhaps extending to
infinity both in space and time-not the paltry 15 billion light-years or so in
radius and 15 billion years in age which are the usual understanding. In this
Cosmos there is, as here, a kind of quantum fluff in which tiny structures-
much smaller than an electron-are everywhere forming, reshaping, and
dissipating; in which, as here, fluctuations in absolutely empty space create
pairs of elementary particles-an electron and a positron, for example. In the
froth of quantum bubbles, the vast majority remain submicroscopic. But a tiny
fraction inflate, grow, and achieve respectable universehood. They are so far
away from us, though-much farther than the 15 billion light-years that is the
conventional scale of our universe-that, if they exist, they appear to be
wholly inaccessible and undetectable.
Most of these other universes reach a
maximum size and then collapse, contract to a point, and disappear forever.
Others may oscillate. Still others may expand without limit. In different
universes there will be different laws of nature. We live, Linde argues, in one
such universe-one in which the physics is congenial for growth, inflation,
expansion, galaxies, stars, worlds, life. We imagine our universe to be unique,
but it is one of an immense number-perhaps an infinite number-of equally valid,
equally independent, equally isolated universes. There will be life in some,
and not in others. In this view the observable Universe is just a newly formed
backwater of a much vaster, infinitely old, and wholly unobservable Cosmos. If
something like this is right, even our residual pride, pallid as it must be, of
living in the only universe is denied to us.
Maybe someday, despite current evidence,
a means will be devised to peer into adjacent universes, sporting very
different laws of nature, and we will see what else is possible. Or perhaps
inhabitants of adjacent universes can peer into ours. Of course, in such
speculations we have far exceeded the bounds of knowledge. But if something
like Linde's Cosmos is true, there is-amazingly-still another devastating
deprovincialization awaiting us.
Our powers are far from adequate to be
creating universes anytime soon. Strong Anthropic Principle ideas are not
amenable to proof (although Linde's cosmology does have some testable
features). Extraterrestrial life aside, if self-congratulatory pretensions to
centrality have now retreated to such bastions impervious to experiment, then
the sequence of scientific battles with human chauvinism would seem to have
been, at least largely, won.
THE LONG-STANDING VIEW, as summarized by
the philosopher Immanuel Kant, that “without man...the whole of creation would
be a mere wilderness, a thing in vain, and have no final end” is revealed to be
self-indulgent folly. A Principle of Mediocrity seems to apply to all our
circumstances. We could not have known beforehand that the evidence would be,
so repeatedly and thoroughly, incompatible with the proposition that human
beings are at center stage in the Universe. But most of the debates have now
been settled decisively in favor of a position that, however painful, can be
encapsulated in a single sentence: We have not been given the lead in the
cosmic drama.
Perhaps someone else has. Perhaps no one
else has. In either case, we have good reason for humility.
CHAPTER 4
A universe not made for us
The Sea of Faith
Was once, too, at the full, and round
earth's shore
Lay like the folds of a bright girdle
furl'd.
But now I only hear
Its melancholy, long, withdrawing roar,
Retreating, to the breath
Of the night-wind, down the vast edges
drear
And naked shingles of the world.
—MATTHEW ARNOLD, “DOVER BEACH” (1867)
What a beautiful sunset,” we say, or
“I'm up before sunrise.” no matter what the scientists allege, in everyday
speech we often ignore their findings. We don't talk about the Earth turning,
but about the Sun rising and setting. Try formulating it in Copernican
language. Would you say, “Billy, be home by the time the Earth has rotated
enough so as to occult the Sun below the local horizon”? Billy would be long
gone before you're finished. We haven't been able even to find a graceful
locution that accurately the Heliocentric insight. We at the center and
everything else circling us is built into our languages; we teach it to our
children. We are unreconstructed geocentrists hiding behind a Copernican
veneer.
In 1633 the Roman Catholic Church
condemned Galileo for teaching that the Earth goes around the Sun. Let's take a
closer look at this famous controversy. In the preface to his book comparing
the two hypotheses-an Earth-centered and a Sun-centered universe-Galileo had
written,
The celestial phenomena will be
examined, strengthening the Copernican hypothesis until it might seem that this
must triumph absolutely.
And later in the book he confessed,
Nor can I ever sufficiently admire
[Copernicus and his followers]; they have through sheer force of intellect done
such violence to their own senses as to prefer what reason told them over what
sensible experience plainly showed them...
The Church declared, in its indictment
of Galileo,
The doctrine that the earth is neither
the center of the universe nor immovable, but moves even with a daily rotation,
is absurd, and both psychologically and theologically false, and at the least
an error of faith.
Galileo replied,
The doctrine of the movements of the
earth and the fixity of the sun is condemned on the ground that the Scriptures
speak in many places of the sun moving and the earth standing still...It is
piously spoken that the Scriptures cannot lie. But none will deny that they are
frequently abstruse and their true meaning difficult to discover, and more than
the bare words signify. I think that in the discussion of natural problems we
ought to begin not with the Scriptures, but with experiments and
demonstrations.
But in his recantation (June 22, 1633)
Galileo was made to say,
Having been admonished by the Holy
Office entirely to abandon the false opinion that the Sun was the center of the
universe and immovable, and that the Earth was not the center of the same and
that it moved...I have been...suspected of heresy, that is, of having held and
believed that the Sun is the center of the universe and immovable, and that the
Earth is not the center of the same, and that it does move...1 abjure with a
sincere heart and unfeigned faith, I curse and detest the same errors and
heresies, and generally all and every error and sect contrary to the Holy
Catholic Church.
It took the Church until 1832 to remove
Galileo's work from its list of books which Catholics were forbidden to read at
the risk of dire punishment of their immortal souls.
Pontifical disquiet with modern science
has ebbed and flowed since the time of Galileo. The high-water mark in recent
history is the 1864 Syllabus of Errors of Pius IX, the pope who also convened
the Vatican Council at which the doctrine of papal infallibility was, at his
insistence, first proclaimed. Here are a few excerpts:
Divine revelation is perfect and,
therefore, it is not subject to continual and indefinite progress in order to
correspond with the progress of human reason...no man is free to embrace and
profess that religion which he believes to be true, guided by the light of
reason...The Church has power to define dogmatically the religion of the Catholic
Church to be the only true religion...It is necessary even in the present day
that the Catholic religion shall be held as the only religion of the state, to
the exclusion of all other forms of worship...The civil liberty of every mode
of worship, and full power given to all of openly and publicly manifesting
their opinions and their ideas conduce more easily to corrupt the morals and
minds of the people...The Roman Pontiff cannot and ought not to reconcile
himself or agree with, progress, liberalism and modern civilization.
To its credit, although belatedly and
reluctantly, the Church in 1992 repudiated its denunciation of Galileo. It
still cannot quite bring itself, though, to see the significance of its
opposition. In a 1992 speech Pope John Paul II argued,
From the beginning of the Age of
Enlightenment down to our own day, the Galileo case has been a sort of “myth”
in which the image fabricated out of the events is quite far removed from
reality. In this perspective, the Galileo case was a symbol of the Catholic
Church's supposed rejection of scientific progress, or of “dogmatic”
obscurantism opposed to the free search for truth.
But surely the Holy Inquisition ushering
the elderly and infirm Galileo in to inspect the instruments of torture in the
dungeons of the Church not only admits but requires just such an
interpretation. This was not mere scientific caution and restraint, a
reluctance to shift a paradigm until compelling evidence, such as the annual
parallax, was available. This was fear of discussion and debate. Censoring
alternative views and threatening to torture their proponents betray a lack of
faith in the very doctrine and parishioners that are ostensibly being
protected. Why were threats and Galileo's house arrest needed? Cannot truth
defend itself in its confrontation with error?
The Pope does, though, go on to add:
The error of the theologians of the
time, when they maintained the centrality of the earth, was to think that our
understanding of the physical world's structure was in soiree way imposed by
the literal sense of Sacred Scriptures.
Here indeed considerable progress has
been made-although proponents of fundamentalist faiths will be distressed to
hear from the Pontiff that Sacred Scripture is not always literally true.
But if the Bible is not everywhere
literally true, which parts are divinely inspired and which are merely fallible
and human? As soon as we admit that there are scriptural mistakes (or
concessions to the ignorance of the times), then how can the Bible be an
inerrant guide to ethics and morals? Might sects and individuals now accept as
authentic the parts of the Bible they like, and reject those that are
inconvenient or burdensome? Prohibitions against murder, say, are essential for
a society to function, but if divine retribution for murder is considered
implausible, won't more people think they can get away with it?
Many felt that Copernicus and Galileo
were up to no good and erosive of the social order. Indeed any challenge from
any source, to the literal truth of the Bible might have such consequences. We
can readily see how science began to make people nervous. Instead of
criticizing those who perpetuated the myths, public rancor was directed at
those who discredited them.
OUR ANCESTORS UNDERSTOOD origins by
extrapolating from their own experience. How else could they have done it? So
the Universe was hatched from a cosmic egg, or conceived in the sexual congress
of a mother god and a father god, or was a kind of product of the Creator's
workshop-perhaps the latest of many flawed attempts. And the Universe was not
much bigger than we see, and not much older than our written or oral records,
and nowhere very different from places that we know.
We've tended in our cosmologies to make
things familiar. Despite all our best efforts, we've not been very inventive.
In the West, Heaven is placid and fluffy, and Hell is like the inside of a
volcano. In many stories, both realms are governed by dominance hierarchies
headed by gods or devils. Monotheists talked about the king of kings. In every
culture we imagined something like our own political system running the
Universe. Few found the similarity suspicious.
Then science came along and taught us
that we are not the measure of all things, that there are wonders unimagined,
that the Universe is not obliged to conform to what we consider comfortable or
plausible. We have learned something about the idiosyncratic nature of our
common sense. Science has carried human self-consciousness to a higher level.
This is surely a rite of passage, a step towards maturity. It contrasts starkly
with the childishness and narcissism of our pre-Copernican notions.
But why should we want to think that the
Universe was made for us? Why is the idea so appealing? Why do we nurture it?
Is our self-esteem so precarious that nothing short of a universe custom-made
for us will do?
Of course it appeals to our vanity.
“What a man desires, he also imagines to be true,” said Demosthenes. “The light
of faith makes us see what we believe,” cheerfully admitted St. Thomas Aquinas.
But I think there may be something else. There's a kind of ethnocentrism among
primates. To whichever little group we happen to be born, we owe passionate
love and loyalty. Members of other groups are beneath contempt, deserving of
rejection and hostility. That both groups are. of the same species, that to an
outside observer they are virtually indistinguishable, makes no difference.
This is certainly the pattern among the chimpanzees, our closest relatives in
the animal kingdom. Ann Druyan and I have described how this way of viewing the
world may have made enormous evolutionary sense a few million years ago,
however dangerous it has become today. Even members of hunter-gatherer
groups-as far from the technological feats of our present global civilization
as it is possible for humans to be-solemnly describe their little band,
whichever it is, as “the people.” Everyone else is something different,
something less than human.
If this is our natural way of viewing
the world, then it should occasion no surprise that every time we make a naive
judgment about our place in the Universe-one untempered by careful and
skeptical scientific examination-we almost always opt for the centrality of our
group and circumstance. We want to believe, moreover, that these are objective
facts, and not our prejudices finding a sanctioned vent.
So it's not much fun to have a gaggle of
scientists incessantly haranguing us with “You're ordinary, you're unimportant
your privileges are undeserved, there's nothing special about you.” Even
unexcitable people might, after a while, grow annoyed at this incantation and
those who insist on chanting it. It almost seems that the scientists are
getting some weird satisfaction out of putting humans down. Why can't they find
a way in which we're superior? Lift our spirits! Exalt us! In such debates
science, with its mantra of discouragement, feels cold and remote,
dispassionate, detached, unresponsive to human needs.
And, again, if we're not important, not
central, not the apple of God's eye, what is implied for our theologically
based moral codes? The discovery of our true bearings in the Cosmos was
resisted for so long and to such a degree that many traces of the debate
remain, sometimes with the motives of the geocentrists laid bare. Here, for
example, is a revealing unsigned commentary in the British review The Spectator
in 1892:
[I]t is certain enough that the
discovery of the heliocentric motion of the planets which reduced our earth to
its proper “insignificance” in the solar system, did a good deal to reduce to a
similar but far from proper “insignificance” the moral principles by which the
predominant races of the earth had hitherto been guided and restrained. Part of
this effect was no doubt due to the evidence afforded that the physical science
of various inspired writers was erroneous instead of being infallible, -a
conviction which unduly shook the confidence felt even in their moral and
religious teaching. But a good deal of it was due only to the mere sense of
“insignificance” with which man has contemplated himself, since he has
discovered that he inhabits nothing but a very obscure corner of the universe,
instead of the central world round which sun, moon, and stars alike revolved.
There can be no doubt that man may feel himself, and has often felt himself, a
great deal too insignificant to be the object of any particular divine training
or care. If the earth be regarded as a sort of ant-hill, and the life and death
of human beings as the life and death of so many ants which run into and out of
so many holes in search of food and sunlight, it is quite certain that no
adequate importance will be attached to the duties of human life, and that a
profound fatalism and hopelessness, instead of new hopefulness, will attach to
human effort...
[F]or the present at least, our
horizons are quite vast enough...; till we can get used to the infinite
horizons we already have, and not lose our balance so much as we usually do in
contemplating them, the yearning for still wider horizons is premature.
WHAT DO WE REALLY WANT from philosophy
and religion? Palliatives? Therapy? Comfort? Do we want reassuring fables or an
understanding of our actual circumstances? Dismay that the Universe does not
conform to our preferences seems childish. You might think that grown-ups would
be ashamed to put such disappointments into print. The fashionable way of doing
this is not to blame the Universe-which seems truly pointless-but rather to
blame the means by which we know the Universe, namely science.
George Bernard Shaw, in the preface to
his play St. Joan, described a sense of science preying on our credulity,
forcing on us an alien worldview, intimidating belief:
In the Middle Ages, people believed that
the Earth was flat, for which they had at least the evidence of their senses:
we believe it to be round, not because as many as one per cent of us could give
the physical reason for so quaint a belief, but because modern science has
convinced us that nothing that is obvious is true, and that everything that is
magical, improbable, extraordinary, gigantic, microscopic, heartless, or
outrageous is scientific.
A more recent and very instructive
example is Understanding the Present: Science and the Soul of Modern Man, by
Bryan Appleyard, a British journalist. This book makes explicit what many
people feel, all over the world, but are too embarrassed to say. Appleyard's
candor is refreshing. He is a true believer and will not let us slough over the
contradictions between modern science and traditional religion:
“Science has taken away our religion,”
he laments. And what sort of religion is it that he longs for? One in which
“the human race was the point, the heart, the final cause of the whole system.
It placed our selves definitively upon the universal map.”...“We were the end,
the purpose, the rational axle around which the great aetherian shells
rotated.” He longs for “the universe of Catholic orthodoxy” in which “the
cosmos is shown to be a machine constructed around the drama of salvation”-by
which Appleyard means that, despite explicit orders to the contrary, a woman
and a man once ate of an apple, and that this act of insubordination
transformed the Universe into a contrivance for operant-conditioning their
remote descendants.
By contrast, modern science “presents us
as accidents. We are caused by the cosmos, but we are not the cause of it.
Modem man is not finally anything, he has no role in creation.” Science is
“spiritually corrosive, burning away ancient authorities and traditions. It
cannot really co-exist with anything.”...“Science, quietly and inexplicitly, is
talking us into abandoning our selves, our true selves.” It reveals “the mute,
alien spectacle of nature.”...“Human beings cannot live with such a revelation.
The only morality left is that of the consoling lie.” Anything is better than
grappling with the unbearable burden of being tiny.
In a passage reminiscent of Plus IX,
Appleyard even decries the fact that “a modern democracy can be expected to
include a number of contradictory religious faiths which are obliged to agree
on a certain limited number of general injunctions, but no more. They must not
burn each other's places of worship, but they may deny, even abuse each other's
God. This is the effective, scientific way of proceeding.”
But what is the alternative? Obdurately
to pretend to certainty in an uncertain world? To adopt a comforting belief
system, no matter how out of kilter with the facts it is? If we don't know
what's real, how can we deal with reality? For practical reasons, we cannot
live too much in fantasyland. Shall we censor one another's religions and burn
down one another's places of worship? How can we be sure which of the thousands
of human belief systems should become unchallenged, ubiquitous, mandatory?
These quotations betray a failure of
nerve before the Universe its grandeur and magnificence, but especially its
indifference. Science has taught us that, because we have a talent for
deceiving ourselves, subjectivity may not freely reign. This is one reason
Appleyard so mistrusts science: It seems too reasoned, measured, and
impersonal. Its conclusions derive from the interrogation of Nature, and are
not in all cases predesigned to satisfy our wants. Appleyard deplores
moderation. He yearns for inerrant doctrine, release from the exercise of
judgment, and an obligation to believe but not to question. He has not grasped
human fallibility. He recognizes no need to institutionalize error-correcting
machinery either in our social institutions or in our view of the Universe.
This is the anguished cry of the infant
when the Parent does not come. But most people eventually come to grips with
reality, and with the painful absence of parents who will absolutely guarantee
that no harm befalls the little ones so long as they do what they are told.
Eventually most people find ways to accommodate to the Universe-especially when
given the tools to think straight.
“All that we pass on to our children” in
the scientific age, Appleyard complains, “is the conviction that nothing is
true, final or enduring, including the culture from which they sprang.” How
right he is about the inadequacy of our legacy. But would it be enriched by
adding baseless certainties? He scorns “the pious hope that science and
religion are independent realms which can easily be separated.” Instead,
“science, as it is now, is absolutely not compatible with religion.”
But isn't Appleyard really saying that
some religions now find it difficult to make unchallenged pronouncements on the
nature of the world that are straight-out false? We recognize that even revered
religious leaders, the products of their time as we are of ours, may have made
mistakes. Religions contradict one another on small matters, such as whether we
should put on a hat or take one off on entering a house of worship, or whether
we should eat beef and eschew pork or the other way around, all the way to the
most central issues, such as whether there are no gods, one God, or many gods.
Science has brought many of us to that
state in which Nathaniel Hawthorne found Herman Melville: “He can neither
believe, nor be comfortable in his unbelief.” Or Jean-Jacques Rousseau: “They
had not persuaded me, but they had troubled me. Their arguments had shaken me
without ever convincing me...It is hard to prevent oneself from believing what
one so keenly desires.” As the belief systems taught by the secular and
religious authorities are undennined, respect for authority in general probably
does erode. The lesson is clear: Even politics] leaders must be wary of
embracing false doctrine. This is not a failing of science, but one of its
graces.
Of course, worldview consensus is
comforting, while clashes of opinion may be unsettling, and demand more of us.
But unless we insist, against all evidence, that our ancestors were perfect,
the advance of knowledge requires us to unravel and then restitch the consensus
they established.
In some respects, science has far
surpassed religion in delivering awe. How is it that hardly any major religion
has looked at science and concluded, “This is better than we thought! The
Universe is much bigger than our prophets said, grander, more subtle, more
elegant. God must be even greater than we dreamed”? Instead they say, “No, no,
no! My god is a little god, and I want him to stay that way.” A religion, old or
new, that stressed the magnificence of the Universe as revealed by modern
science might be able to draw forth reserves of reverence and awe hardly tapped
by the conventional faiths. Sooner or later, such a religion will emerge.
IF YOU LIVED two or three millennia ago,
there was no shame in holding that the Universe was made for us. It was an
appealing thesis consistent with everything we knew; it was what the most
learned among us taught without qualification. But we have found out much since
then. Defending such a position today amounts to willful disregard of the
evidence, and a flight from self-knowledge.
Still, for many of us, these
deprovincializations rankle. Even if they do not fully cant' the day, they
erode confidence-unlike the happy anthropocentric certitudes, rippling with
social utility, of an earlier age. We long to be here for a purpose, even
though, despite much self-deception, none is evident. “The meaningless
absurdity of life,” wrote Leo Tolstoy, “is the only incontestable knowledge
accessible to man.” Our time is burdened under the cumulative weight of
successive debunkings of our conceits: We're Johnny-come-latelies. We live in
the cosmic boondocks. We emerged from microbes and muck. Apes are our cousins.
Our thoughts and feelings are not fully under our own control. There may be
much smarter and very different beings elsewhere. And on top of all this, we're
making a mess of our planet and becoming a danger to ourselves.
The trapdoor beneath our feet swings
open. We find ourselves in bottomless free fall. We are lost in a great
darkness, and there's no one to send out a search party. Given so harsh a
reality, of course we're tempted to shut our eyes and pretend that we're safe
and snug at home, that the fall is only a bad dream.
We lack consensus about our place in the
Universe. There is no generally agreed upon long-term vision of the goal of our
species-other than, perhaps, simple survival. Especially when times are hard,
we become desperate for encouragement, unreceptive to the litany of great
demotions and dashed hopes, and much more willing to hear that we're special,
never mind if the evidence is paper-thin. If it takes a little myth and ritual
to get us through a night that seems endless, who among us cannot sympathize
and understand?
But if our objective is deep knowledge
rather than shallow reassurance, the gains from this new perspective far
outweigh the losses. Once we overcome our fear of being tiny, we find ourselves
on the threshold of a vast and awesome Universe that utterly dwarfs-in time, in
space, and in potential-the tidy anthropocentric proscenium of our ancestors.
We gaze across billions of light-years of space to view the Universe shortly
after the Big Bang, and plumb the fine structure of matter. We peer down into
the core of our planet, and the blazing interior of our star. We read the
genetic language in which is written the diverse skills and propensities of
every being on Earth. We uncover hidden chapters in the record of our own
origins, and with some anguish better understand our nature and prospects. We
invent and refine agriculture, without which almost all of us would starve to
death. We create medicines and vaccines that save the lives of billions. We
communicate at the speed of light, and whip around the Earth in an hour and a
half. We have sent dozens of ships to more than seventy worlds, and four
spacecraft to the stars. We are right to rejoice in our accomplishments, to be
proud that our species has been able to see so far, and to judge our merit in
part by the very science that has so deflated our pretensions.
To our ancestors there was much in
Nature to be afraid of-lightning, storms, earthquakes, volcanos, plagues,
drought, long winters. Religions arose in part as attempts to propitiate and
control, if not much to understand, the disorderly aspect of Nature. The
scientific revolution permitted us to glimpse an underlying ordered Universe in
which there was a literal harmony of the worlds (Johannes Kepler's phrase). If
we understand Nature, there is a prospect of controlling it or at least
mitigating the harm it may bring. In this sense, science brought hope.
Most of the great deprovincializing
debates were entered into with no thought for their practical implications.
Passionate and curious humans wished to understand their actual circumstances,
how unique or pedestrian they and their world are, their ultimate origins and
destinies, how the Universe works. Surprisingly, some of these debates have
yielded the most profound practical benefits. The very method of mathematical
reasoning that Isaac Newton introduced to explain the motion of the planets
around the Sun has led to most of the technology Of our modern world. The
Industrial Revolution, for all its shortcomings, is still the global model of
how an agricultural nation can emerge from poverty. These debates have
bread-and-butter consequences.
It might have been otherwise. It might
have been that the balance lay elsewhere, that humans by and large did not want
to yaw about a disquieting Universe, that we were unwilling to hermit
challenges to the prevailing wisdom. Despite determined resistance in every
age, it is very much to our credit that we have allowed ourselves to follow the
evidence, to draw conclusions that at first seem daunting: a Universe so much larger
and older that our personal and historical experience is dwarfed and humbled, a
Universe in which, every day, suns are born and worlds obliterated, a Universe
in which humanity, newly arrived, clings to an obscure clod of matter.
How much more satisfying had we been
placed in a garden custom-made for us, its other occupants put there for us to
use as we saw fit. There is a celebrated story in the Western tradition like
this, except that not quite everything was there for us. There was one
particular tree of which we were not to partake, a tree of knowledge. Knowledge
and understanding and wisdom were forbidden to us in this story. We were to be
kept ignorant. But we couldn't help ourselves. We were starving for
knowledge-created hungry, you might say. This was the origin of all our
troubles. In particular, it is why we no longer live in a garden: We found out
too much. So long as we were incurious and obedient, I imagine, we could
console ourselves with our importance and centrality, and tell ourselves that
we were the reason the Universe was made. As we began to indulge our curiosity,
though, to explore, to learn how the Universe really is, we expelled ourselves
from Eden. Angels with a flaming sword were set as sentries at the gates of
Paradise to bar our return. The gardeners became exiles and wanderers.
Occasionally we mourn that lost world, but that, it seems to me, is maudlin and
sentimental. We could not happily have remained ignorant forever.
There is in this Universe much of what
seems to he design. Every time we come upon it, we breathe a sigh of relief. We
are forever hoping to find, or at least safely deduce, a Designer. But instead,
we repeatedly discover that natural processes-collisional selection of worlds,
say, or natural selection of gene pools, or even the convection pattern in a
pot of boiling water-can extract order out of chaos, and deceive us into
deducing purpose where there is none. In everyday life, we often sense-in the
bedrooms of teenagers, or in national politics-that chaos is natural, and order
imposed from above. While there are deeper regularities in the Universe than
the simple circumstances we generally describe as orderly, all that order,
simple and complex, seems to derive from laws of Nature established at the Big
Bang (or earlier), rather than as a consequence of belated intervention by an
imperfect deity. “God is to be found in the details” is the famous dictum of
the German scholar Abu Warburg. But, amid much elegance and precision, the
details of life and the Universe also exhibit haphazard, jury-rigged
arrangements and much poor planning. What shall we make of this: an edifice
abandoned early in construction by the architect?
The evidence, so far at least and laws
of Nature aside, does not require a Designer. Maybe there is one hiding,
maddeningly unwilling to be revealed. Sometimes it seems a very slender hope.
The significance of our lives and our
fragile planet is then determined only by our own wisdom and courage. We are
the custodians of life's meaning. We long for a Parent to care for us, to
forgive us our errors, to save us from our childish mistakes. But knowledge is
preferable to ignorance. Better by far to embrace the hard truth than a
reassuring fable.
If we crave some cosmic purpose, then
let us find ourselves a worthy goal.
CHAPTER 5
Is there intelligent life on Earth?
They journeyed a long time and found
nothing. At length they discerned a small light, which was the Earth...[But]
they could not find the smallest reason to suspect that we and our fellow-citizens
of this globe have the honor to exist.
—VOLTAIRE, MICROMEGAS. A PHILOSOPHICAL
HISTORY (1752)
There are places, in and around our
great cities, where the natural world has all but disappeared. You can make out
streets and sidewalks, autos, parking garages, advertising billboards,
monuments of glass and steel, but not a tree or a blade of grass or any
animal-besides, of course, the humans. There are lots of humans. Only when you
look straight up through the skyscraper canyons can you make out a star or a
patch of blue-reminders of what was there long before humans came to be. But
the bright lights of the big cities bleach out the stars, and even that patch
of blue is sometimes gone, tinted brown by industrial technology.
It's not hard, going to work every
day in such a place, to be impressed with ourselves. How we've transformed the
Earth for our benefit and convenience! But a few hundred miles up or down there
are no humans. Apart from a thin film of life at the very surface of the Earth,
an occasional intrepid spacecraft, and some radio static, our impact on the
Universe is nil. It knows nothing of us.
YOU'RE AN ALIEN EXPLORER entering the
Solar System after a long journey through the blackness of interstellar space.
You examine the planets of this humdrum star from afar-a pretty handful, some
gray, some blue, some red, some yellow. You're interested in what kinds of
worlds these are, whether their environments are static or changing, and
especially whether there are life and intelligence. You have no prior knowledge
of the Earth. You've just discovered its existence.
There's a galactic ethic, let's imagine:
Look but don't touch. You can fly by these worlds; you can orbit them; but you
are strictly forbidden to land. Under such constraints, could you figure out
what the Earth's environment is like and whether anyone lives there?
As you approach, your first impression
of the whole Earth is white clouds, white polar caps, brown continents, and
some bluish substance that covers two thirds of the surface. When you take the
temperature of this world from the infrared radiation it emits, you find that
most latitudes are above the freezing point of water, while the polar caps are
below freezing. Water is a very abundant material in the Universe; polar caps
made of solid water would be a reasonable guess, as well as clouds of solid and
liquid water.
You might also he tempted by the idea
that the blue stuff is enormous quantities-kilometers deep-of liquid water. The
suggestion is bizarre, though, at least as far as this solar system is
concerned, because surface oceans of liquid water exist nowhere else. When you
look in the visible and near-infrared spectrum for telltale signatures of
chemical composition, sure enough you discover water ice in the polar caps, and
enough water vapor in the air to account for the clouds; this is also just the
right amount that must exist because of evaporation if the oceans are in fact
made of liquid water. The bizarre hypothesis is confirmed.
The spectrometers further reveal that
the air on this world is one fifth oxygen, O2. no other planet in the Solar
System has anything close to so much oxygen. Where does it all come from? The
intense ultraviolet light from the Sun breaks water, H20, down into oxygen and
hydrogen, and hydrogen, the lightest gas, quickly escapes to space. This is a
source of O2, certainly, but it doesn't easily account for so much oxygen.
Another possibility is that ordinary
visible light, which the Sun pours out in vast amounts, is used on Earth to
break water apart-except that there's no known way to do this without life.
There would have to be plants, life-forms colored by a pigment that strongly
absorbs visible light, that knows how to split a water molecule by saving up
the energy of two photons of light, that retains the H and excretes the O, and
that uses the hydrogen thus liberated to synthesize organic molecules. The
plants would have to be spread over much of the planet. All this is asking a
lot. If you're a good skeptical scientist, so much Oz would not be proof of
life. But it certainly might be cause for suspicion.
With all that oxygen you're not
surprised to discover ozone (O3) in the atmosphere, because ultraviolet light
makes ozone out of molecular oxygen (O2). The ozone then absorbs dangerous
ultraviolet radiation. So if the oxygen is due to life, there's a curious sense
in which the life is protecting itself. But this life “night be mere
photosynthetic plants. A high level of intelligence is not implied.
When you examine the continents more
closely, you find there are, crudely speaking, two kinds of regions. One shows
the spectrum of ordinary rocks and minerals as found on many worlds. The other
reveals something unusual: a material, covering vast areas, that strongly
absorbs red light. (The Sun, of course, shines in light of all colors, with a
peak in the yellow.) This pigment might be just the agent needed if ordinary
visible light is being used to break water apart and account for the oxygen in
the air. It's another hint, this time a little stronger, of life, not a bug
here and there, but a planetary surface overflowing with life. The pigment is
in fact chlorophyll: It absorbs blue light as well as red, and is responsible
for the fact that plants are green. What you're seeing is a densely vegetated
planet.
So the Earth is revealed to possess
three properties unique at least in this solar system-oceans, oxygen, life.
It's hard not to think they're related, the oceans being the sites of origin,
and the oxygen the product, of abundant life.
When you look carefully at the infrared
spectrum of the Earth, you discover the minor constituents of the air. In
addition to water vapor, there's carbon dioxide (CO2), methane (CH4), and other
gases that absorb the heat that the Earth tries to radiate away to space at night.
These gases warm the planet. Without them, the Earth would everywhere be below
the freezing point of water. You've discovered this world's greenhouse effect.
Methane and oxygen together in the same
atmosphere is peculiar. The laws of chemistry are very clear: In an excess of
O2, CH4 should be entirely converted into H2O and CO2, The process is so
efficient that not a single molecule in all the Earth's atmosphere should be
methane. Instead, you find that one out of every million molecules is methane, ail
immense discrepancy. What could it mean?
The only possible explanation is that
methane is being injected into the Earth's atmosphere so quickly that its
chemical reaction with Oz can't keep pace. Where does all this methane come
from? Maybe it seeps out of the deep interior of the Earth-but quantitatively
this doesn't seem to work, and Mars and Venus don't have anything like this
much methane. The only alternatives are biological, a conclusion that makes no
assumptions about the chemistry of life, or what it looks like, but follows
merely from how unstable methane is in an oxygen atmosphere. In fact, the
methane arises from such sources as bacteria in bogs, the cultivation of rice,
the burning of vegetation, natural gas from oil wells, and bovine flatulence.
In an oxygen atmosphere, methane is a sign of life.
That the intimate intestinal activities
of cows should be detectable from interplanetary space is a little
disconcerting, especially when so much of what we hold dear is not. But an
alien scientist flying by the Earth would, at this point, be unable to deduce
bogs, rice, fire, oil, or cows. Just life.
All the signs of life that we've
discussed so far are due to comparatively simple forms (the methane in the
rumens of cows is generated by bacteria that homestead there). Had your
spacecraft flown by the Earth a hundred million years ago, in the age of the
dinosaurs when there were no humans and no technology, you would still have
seen oxygen and ozone, they chlorophyll pigment, and far too much methane. At
present, though, your instruments are finding signs not just of life, but of
high technology-something that couldn't possibly have been detected even a
hundred years ago:
You are detecting a particular kind of
radio wave emanating from Earth. Radio waves don't necessarily signify life and
intelligence Many natural processes generate them. You've already found radio
emissions from other, apparently uninhabited worlds-generated by electrons
trapped in the strong magnetic fields of planets, by chaotic motions at the
shock front that separates these magnetic fields from the interplanetary
magnetic field, and by lightning. (Radio “whistlers” usually sweep from high
notes to low, and then begin again.) Some of these radio emissions are
continuous; some come in repetitive bursts; some last a few minutes and then
disappear.
But this is different: A portion of the
radio transmission from Earth is at just the frequencies where radio waves
begin to leak out of the planet's ionosphere, the electrically charged region
above the stratosphere that reflects and absorbs radio waves. There is a
constant central frequency for each transmission, added to which is a modulated
signal (a complex sequence of ons and offs). no electrons in magnetic fields,
no shock waves, no lightning discharges can generate something like this.
Intelligent life seems to be the only possible explanation. Your conclusion
that the radio transmission is due to technology on Earth holds no matter what
the ons and offs mean: You don't have to decode the message to be sure it is a
message. (This signal is really, let us suppose, communications from the U. S.
Navy to its distant nuclear-armed submarines.)
So, as an alien explorer, you would know
that at least one species on Earth has achieved radio technology. Which one is
it? The beings that make methane? Those that generate oxygen? The ones whose
pigment colors the landscape green? Or somebody else, somebody more subtle,
someone not otherwise detectable to a spacecraft plummeting by? To search for
this technological species, you might want to examine the Earth at finer and
finer resolution-seeking, if not the beings themselves, at least their
artifacts.
You look first with a modest telescope,
so the finest detail you can resolve is one or two kilometers across. You can
make out no monumental architecture, no strange formations, no unnatural
reworking of the landscape, no signs of life. You see a dense atmosphere in
motion. The abundant water must evaporate and then rain back down. Ancient
impact craters, apparent on the Earth's nearby Moon, are almost wholly absent.
There must, then, be a set of processes whereby new land is created and then
eroded away in much less time than the age of this world. Running water is
implicated. As you look with finer and finer definition you find mountain
ranges, river valleys, and many other indications that the planet is
geologically active. There are also odd places surrounded by vegetation, but
which are themselves denuded of plants. They look like discolored smudges on
the landscape.
When you examine the Earth at about
100-meter resolution, everything changes. The planet is revealed to be covered
with straight lines, squares, rectangles, circles-sometimes huddling along
river banks or nestling on the lower slopes of mountains, sometimes stretching
over plains, but rarely in deserts or high mountains, and absolutely never in
the oceans. Their regularity, complexity, and distribution would be hard to
explain except by life and intelligence, although a deeper understanding of function
and purpose might be elusive. Perhaps you would conclude only that the dominant
life-forms have a simultaneous passion for territoriality and Euclidean
geometry. At this resolution you could not see them, much less know them.
Many of the devegetated smudges are
revealed to have an underlying checkerboard geometry. These are the planets
cities. Over much of the landscape, and not just in the cities, there is a
profusion of straight lines, squares, rectangles, circles. The dark smudges of
the cities are revealed to be highly geometrized, with only a few patches of
vegetation-themselves with highly regular boundaries-left intact. There are
occasional triangles, and in one city there is even a pentagon.
When you take pictures at a meter
resolution or better, you find that the crisscrossing straight lines within the
cities and the long straight lines that join them with other cities are filled
with streamlined, multicolored beings a few meters in length, politely running
one behind the other, in long, slow orderly procession. They are very patient.
One stream of beings stops so another stream can continue at right angles.
Periodically, the favor is returned. At night, they turn on two bright lights
in front so they can see where they're going. Some, a privileged few, go into
little houses when their workday is done and retire for the night. Most are
homeless and sleep in the streets.
At last! You've detected the source of
all the technology. the dominant life-forms on the planet. The streets of the
cities and the roadways of the countryside are evidently built for their
benefit. You might believe that you were really beginning to understand life on
Earth. And perhaps you'd be right.
If the resolution improved just a little
further, you'd discover tiny parasites that occasionally enter and exit the
dominant organisms. They play some deeper role, though, because a stationary
dominant organism will often start up again just after it's reinfected by a
parasite, and stop again just before the parasite is expelled. This is
puzzling. But no one said life on Earth would be easy to understand.
All the images you've taken so far are
in reflected sunlight-that is, on the day side of the planet. Something most
interesting is revealed when you photograph the Earth at light: The planet is
lit up. The brightest region, near the Arctic Circle, is illuminated by the
aurora borealis-generated not by life , but by electrons and protons from the
Sun, beamed down by the Earth's magnetic field. Everything else you see is due
to life. The lights recognizably outline the same continents you can make out
in daytime; and many correspond to cities you've already mapped. The cities are
concentrated near coastlines. They tend to be sparser in continental interiors.
Perhaps the dominant organisms are desperate for seawater (or maybe oceangoing
ships were once essential for commerce and emigration).
Some of the lights, though, are not due
to cities. In North Africa, the Middle East, and Siberia, for example, there
are very bright lights in a comparatively barren landscape due, it turns out,
to burnoff in oil and natural gas wells. In the Sea of Japan on the day you
first look, there is a strange, triangular-shaped area of light. In daytime it
corresponds to open ocean. This is no city. What could it be? It is in fact the
Japanese squid fishing fleet, using brilliant illumination to attract schools
of squid up to their deaths. On other days, this pattern of light wanders all
over the Pacific Ocean, seeking prey. In effect, what you have discovered here
is sushi.
It seems sobering to me that from space
you can so readily detect some of the odds and ends of life on Earth-the
gastrointestinal habits of ruminants, Japanese cuisine, the means of
communicating with nomadic submarines that carry death for 200 cities-while so
much of our monumental architecture, our greatest engineering works, our
efforts to care for one another, are almost wholly invisible. It's a kind of
parable.
BY THIS POINT your expedition to the
Earth must be considered highly successful. You've characterized the
environment; you've detected life; you've found manifestations of intelligent
beings; you may even have identified the dominant species, the one transfixed
with geometry and rectilinearity. Surely this planet is worth a longer and more
detailed study. That's why you've now inserted your spacecraft into orbit
around the Earth.
Looking down on the planet, you uncover
new puzzles. All over the Earth, smokestacks are pouring carbon dioxide and
toxic chemicals into the air. So are the dominant beings who run on the
roadways. But carbon dioxide is a greenhouse gas. As you watch, the amount of
it in the atmosphere increases steadily, year after year. The same is true of
methane and other greenhouse gases. If this keeps up, the temperature of the
planet is going to increase. Spectroscopically, you discover another class of
molecules being injected into the air, the chlorofluorocarbons. Not only are
they greenhouse gases, but they are also devastatingly effective in destroying
the protective ozone layer.
You look more closely at the center of
the South American continent, which-as you know by now-is a vast rain forest.
Every night you see thousands of fires. In the daytime, you find the region
covered with smoke. Over the years, all over the planet, you find less and less
forest and more and more scrub desert.
You look down on the large island of
Madagascar. The rivers are colored brown, generating a vast stain in the
surrounding ocean. This is topsoil being washed out to sea at a rate so high
that in another few decades there will be none left. The same thing is
happening, you note, at the mouths of rivers all over the planet.
But no topsoil means no agriculture. In
another century, what will they eat? What will they breathe? How will they cope
with a changing and more dangerous environment?
From your orbital perspective, you can
see that something has unmistakably gone wrong. The dominant organisms, whoever
they are-who have gone to so much trouble to rework the surface-are
simultaneously destroying their ozone layer and their forests, eroding their
topsoil, and performing massive, uncontrolled experiments on their planet's
climate. Haven't they noticed what's happening? Are they oblivious to their
fate? Are they unable to work together on behalf of the environment that
sustains them all?
Perhaps, you think, it's time to
reassess the conjecture that there's intelligent life on Earth.
LOOKING FOR LIFE ELSEWHERE:A CALIBRATION
Spacecraft from the Earth have now flown
by dozens of planets, moons, comets, and asteroids-equipped with cameras,
instruments for measuring heat and radio waves, spectrometers to determine
composition, and a host of other devices. We have found not a hint of life
anywhere else in the Solar System. But you might be skeptical about our ability
to detect life elsewhere, especially life different from the kind we know.
Until recently we had never performed the obvious calibration test: to fly a
modern interplanetary spacecraft by the Earth and see whether we could detect
ourselves. This all changed on December 8, 1990.
Galileo is a NASA spacecraft designed to
explore the giant planet Jupiter, its moons, and its rings. It's named after
the heroic Italian scientist who played so central a role in toppling the
geocentric pretension. It is he who first saw Jupiter as a world, and who
discovered its four big moons. To get to Jupiter, the spacecraft had to fly
close by Venus (once) and the Earth (twice) and be accelerated by the gravities
of these planets-otherwise there wasn't enough oomph to get it where it was
going. This necessity of trajectory design permitted us, for the first time, to
look systematically at the Earth from an alien perspective.
Galileo passed only 960 kilometers
(about 600 miles) above the Earth's surface. With some exceptions-including
pictures showing features finer than 1 kilometer across, and the images of the
Earth at night-much of the spacecraft data described in this chapter were
actually obtained by Galileo. With Galileo we were able to deduce an oxygen atmosphere,
water, clouds, oceans, polar ice, life, and intelligence. The use of
instruments and protocols developed to explore the planets to monitor the
environmental health of our own-something NASA is now doing in earnest-was
described by the astronaut Sally Ride as “Mission to Planet Earth.”
Other members of the NASA scientific
team who worked with me on Galileo's detection of life on Earth were Drs. W.
Reid Thompson, Cornell University; Robert Carlson, JPL; Donald Gurnett,
University of Iowa; and Charles Hord, University of Colorado.
Our success in detecting life on Earth
with Galileo, without making any assumptions beforehand about what kind of life
it must be, increases our confidence that when we fail to find life on other
planets, that negative result is meaningful. Is this judgment anthropocentric,
geocentric, provincial? I don't think so. We're not looking only for our kind
of biology. Any widespread photosynthetic pigment, any gas grossly out of
equilibrium with the rest of the atmosphere, any rendering of the surface into
highly geometrized patterns, any steady constellation of lights on the night
hemisphere, any non-astrophysical sources of radio emission would betoken the
presence of life. On Earth we have found of course only our type, but many other
types would have been detectable elsewhere. We have not found them. This
examination of the third planet strengthens our tentative conclusion that, of
all the worlds in the Solar System, only ours is graced by life.
We have just begun to search. Maybe life
is hiding on Mars or Jupiter, Europa or Titan. Maybe the Galaxy is filled with
worlds as rich in life as ours. Maybe we are on the verge of making such
discoveries. But in terms of actual knowledge, at this moment the Earth is
unique. no other world is yet known to harbor even a microbe, much less a
technical civilization.
CHAPTER 6
The triumph ov Voyger
They that go down to the sea in ships,
that do business in great waters; these see the works of the Lord, and his
wonders in the deep.
—PSALMS, 107 (CA. 150 B. C)
The visions we offer our children shape
the future. It matters what those visions are. Often they become
self-fulfilling prophecies. Dreams are maps.
I do not think it irresponsible to
portray even the direst futures; if we are to avoid them, we must understand
that they are possible. But where are the alternatives? Where are the dreams
that motivate and inspire? We long for realistic maps of a world we can be
proud to give to our children. Where are the cartographers of human purpose? Where
are the visions of hopeful futures, of technology as a tool for human
betterment and not a gun on hair trigger pointed at our heads?
NASA, in its ordinary course of doing
business, offers such a vision. But in the late 1980s and early '90s, many
people saw the U. S. space program as, instead, a succession of
catastrophes-seven brave Americans killed on a mission whose main function was
to put up a communications satellite that could have been launched at less cost
without risking anybody; a billion-dollar telescope sent up with a bad case of
myopia; a spacecraft to Jupiter whose main antenna-essential for returning data
to Earth-did not unfurl; a probe lost just as it was about to orbit Mars. Some
people cringe every time NASA describes as exploration sending a few astronauts
200 miles up in a small capsule that endlessly circles the Earth and goes
nowhere. Compared to the brilliant achievements of robotic missions, it is
striking how rarely fundamental scientific findings emerge from manned
missions. Except for repairing ineptly manufactured or malfunctioning
satellites, or launching a satellite that could just as well have been sent up
in an unmanned booster, the manned program has, since the 1970s, seemed unable
to generate accomplishments commensurate with the cost. Others looked at NASA
as a stalking horse for grandiose schemes to put weapons into space, despite
the fact that an orbiting weapon is in many circumstances a sitting duck. And
NASA showed many symptoms of an aging, arteriosclerotic, overcautious,
unadventurous bureaucracy. The trend is perhaps beginning to be reversed.
But these criticisms-many of them surely
valid-should not blind us to NASA triumphs in the same period: the first
exploration of the Uranus and Neptune systems, the in-orbit repair of the
Hubble space telescope, the proof that the existence of galaxies is compatible
with the Big Bang, the first close-up observations of asteroids, mapping Venus
pole to pole, monitoring ozone depletion, demonstrating the existence of a
black hole with the mass of a billion suns at the center of a nearby galaxy,
and a historic commitment to joint space endeavors by the U. S. and Russia.
There are far-reaching, visionary, and
even revolutionary implications to the space program. Communications satellites
link up the planet, are central to the global economy, and, through television,
routinely convey the essential fact that We live in a global community.
Meteorological satellites predict the weather, save lives in hurricanes and
tornados, and avoid many billions of dollars in crop losses every year.
Military-reconnaissance and treaty-verification satellites make nations and the
global civilization more secure; in a world with tens of thousands of nuclear
weapons, they calm the hotheads and paranoids on all sides; they are essential
tools for survival on a troubled and unpredictable planet.
Earth-observing satellites, especially a
new generation soon to be deployed, monitor the health of the global
environment: greenhouse warming, topsoil erosion, ozone layer depletion, ocean
currents, acid rain, the effects of floods and droughts, and new dangers we
haven't yet discovered. This is straightforward planetary hygiene.
Global positioning systems are now in
place so that your locale is radio-triangulated by several satellites. Holding
a small instrument the size of a modern shortwave radio, you can read out to
high precision your latitude and longitude. no crashed airplane, no ship in fog
and shoals, no driver in an unfamiliar city need ever be lost again.
Astronomical satellites peering outward
from Earth's orbit observe with unsurpassed clarity-studying questions ranging
from the possible existence of planets around nearby stars to the origin and
fate of the Universe. Planetary probes from close range explore the gorgeous
array of other worlds in our solar system comparing their fates with ours.
All of these activities are
forward-looking, hopeful, stirring and cost-effective. None of them requires
“manned" spaceflight. A key issue facing the future of NASA and addressed
in this book is whether the purported justifications for human spaceflight are
coherent and sustainable. Is it worth the cost?
But first, let's consider the visions of
a hopeful future vouchsafed by robot spacecraft out among the planets.
VOYAGER 1 AND VOYAGER 2 are the ships
that opened the Solar System for the human species, trailblazing a path for
future generations. Before their launch, in August and September 1977, we were
almost wholly ignorant about most of the planetary part of the Solar System. In
the next dozen years, they provided our first detailed, close-up information on
many new worlds-some of them previously known only as fuzzy disks in the
eyepieces of ground-based telescopes, some merely as points of light, and some
whose very existence was unsuspected. They are still returning reams of data.
These spacecraft have taught us about
the wonders of other worlds, about the uniqueness and fragility of our own,
about beginnings and ends. They have given us access to most of the Solar System-both
in extent and in mass. They are the ships that first explored what may be
homelands of our remote descendants.
U. S. launch vehicles are these days too
feeble to get such a spacecraft to Jupiter and beyond in only a few years by
rocket propulsion alone. But if we're clever (and lucky), there's something
else we can do: We can (as Galileo also did, years later) fly close to one
world, and have its gravity fling us on to the next. A gravity assist, it's
called. It costs us almost nothing but ingenuity. lt's something like grabbing
hold of a post on a moving merry-go-round as it passes-to speed you up and
fling you in some new direction. The spacecraft's acceleration is compensated
for by a deceleration in the planet's orbital motion around the Sun. But
because the planet is so massive compared to the spacecraft, it slows down
hardly at all. Each Voyager spacecraft picked up a velocity boost of nearly
40,000 miles per hour from Jupiter's gravity. Jupiter in turn was slowed down
in its motion around the Sun. By how much? Five billion years from now, when
our Sun becomes a swollen red giant, Jupiter will be one millimeter short of
where it would have been had Voyager not flown by it in the late twentieth
century.
Voyager 2 took advantage of a rare
lining-up of the planets: A close flyby of Jupiter accelerated it on to Saturn,
Saturn to Uranus, Uranus to Neptune, and Neptune to the stars. But you can't do
this anytime you like: The previous opportunity for such a game of celestial
billiards presented itself during the presidency of Thomas Jefferson. We were
then only at the horseback, canoe, and sailing ship stage of exploration.
(Steamboats were the transforming new technology just around the corner.)
Since adequate funds were unavailable,
NASA's Jet Propulsion Laboratory (JPL) could afford to build spacecraft that
would work reliably only as far as Saturn. Beyond that, all bets were off.
However, because of the brilliance of the engineering design and the fact that
the JPL engineers who radioed instructions up to the spacecraft got smarter
faster than the spacecraft got stupid-both spacecraft went on to explore Uranus
and Neptune. These days they are broadcasting back discoveries from beyond the
most distant known planet of the Sun.
We tend to hear much more about the
splendors returned than the ships that brought them, or the shipwrights. It has
al, ways been that way. Even those history books enamored of the voyages of
Christopher Columbus do not tell us much about the builders of the Nina, the
Pinta, and the Santa Maria, or about the principle of the caravel. These
spacecraft, their designers, builders, navigators, and controllers are examples
of what science and engineering, set free for well-defined peaceful purposes,
can accomplish. Those scientists and engineers should be role models for an
America seeking excellence and international competitiveness. They should be on
our stamps.
At each of the four giant
planets-Jupiter, Saturn, Uranus, and Neptune-one or both spacecraft studied the
planet itself, its rings, and its moons. At Jupiter, in 1979, they braved a
dose of trapped charged particles a thousand times more intense than what it
takes to kill a human; enveloped in all that radiation, they discovered the
rings of the largest planet, the first active volcanos outside Earth, and a
possible underground ocean on an airless world-among a host of surprising
discoveries. At Saturn, in 1980 and 1981, they survived a blizzard of ice and
found not a few new rings, but thousands. They examined frozen moons mysteriously
melted in the comparatively recent past, and a large world with a putative
ocean of liquid hydrocarbons surmounted by clouds of organic matter.
On January 25, 1986, Voyager 2 entered
the Uranus system and reported a procession of wonders. The encounter lasted
only a few hours, but the data faithfully relayed back to Earth have
revolutionized our knowledge of the aquamarine planet, its 15 moons. its
pitch-black rings, and its belt of trapped high-energy charged particles. On
August 25, 1989, Voyager 2 swept through the Neptune system and observed, dimly
illuminated by the distant Sun, kaleidoscopic cloud patterns and a bizarre moon
on which plumes of fine organic particles were being blown about by the
astonishingly thin air. And in 1992, having flown beyond the outermost known
planet, both Voyagers picked up radio emission thought to emanate from the
still remote heliopause-the place where the wind from the Sun gives way to the
wind from the stars.
Because we're stuck on Earth, we're
forced to peer at distant worlds through an ocean of distorting air. Much of
the ultraviolet, infrared, and radio waves they emit do not penetrate our
atmosphere. It's easy to see why our spacecraft have revolutionized the study
of the Solar System: We ascend to stark clarity in the vacuum of space, and
there approach our objectives, flying past them, as did Voyager, or orbiting
them, or landing on their surfaces.
These spacecraft have returned four
trillion bits of information to Earth, the equivalent of about 100,000 encyclopedia
volumes. I described the Voyagers 1 and 2 encounters with the Jupiter system in
Cosmos. In the following pages, I'll say something about the Saturn, Uranus,
and Neptune encounters.
JUST BEFORE VOYAGER 2 was to encounter
the Uranus system, the mission design had specified a final maneuver, a brief
firing of the on-board propulsion system to position the spacecraft correctly
so it could thread its way on a preset path among the hurtling moons. But the
course correction proved unnecessary. The spacecraft was already within 200
kilometers of its designed trajectory-after a journey along an arcing path 5
billion kilometers long. This is roughly the equivalent of throwing a pin
through the eye of a needle 50 kilometers away, or firing your rifle in Washington
and hitting the bull's-eye in Dallas.
Mother lodes of planetary treasure were
radioed back to Earth. But Earth is so far away that by the time the signal
frog Neptune was gathered in by radio telescopes on our planet, the received
power was only 10-16 watts (fifteen zeros between the decimal point and the
one). This weak signal bears the same pro, portion to the power emitted by an
ordinary reading lamp as the diameter of an atom bears to the distance from the
Earth to the Moon. It's like hearing an amoeba's footstep.
The mission was conceived during the
late 1960s. It was first funded in 1972. But it was not approved in its final
form (including the encounters with Uranus and Neptune) until after the ships
had completed their reconnaissance of Jupiter. The two spacecraft were lifted
off the Earth by a nonreusable Titan/Centaur booster configuration. Weighing
about a ton, a Voyager would fill a small house. Each draws about 400 watts of
power-considerably less than an average American home-from a generator that
converts radioactive plutonium into electricity. (If it had to rely on solar
energy, the available power would diminish quickly as the ship ventured farther
and farther from the Sun Were it not for nuclear power, Voyager would have
returned no data at all from the outer Solar System, except perhaps a little
from Jupiter.)
The flow of electricity through the
innards of the spacecraft would generate enough magnetism to overwhelm the
sensitive instrument that measures interplanetary magnetic fields. So the
magnetometer is placed at the end of along boom, far from the offending
electrical currents. With other projections, it gives Voyager alter a slightly
porcupine appearance. Cameras, infrared and ultraviolet spectrometers, and an
instrument called a photopolarimeter are on a scan platform that swivels on
command so these device can be aimed at a target world. The spacecraft must
know where Earth is if the antenna is to be pointed properly and the data
rereceived back home. It also needs to know where the Sun is and at least one
bright star, so it can orient itself in three dimensions and point properly
toward any passing world. If you can't point the cameras, it does no good to be
able to return pictures over billions of miles.
Each spacecraft cost about as much as a
single modern strategic bomber. But unlike bombers, Voyager cannot, once
launched, be returned to the hangar for repairs. The ship's computers and
electronics are therefore designed redundantly. Much key machinery, including
the essential radio receiver, had at least one backup-waiting to be called upon
should the hour of need ever arrive. When either Voyager finds itself in
trouble, the computers use branched contingency tree logic to work out the
appropriate course of action. If that doesn't work, the ship radios home for
help.
As the spacecraft journeys increasingly
far from Earth, the roundtrip radio travel time also increases, approaching
eleven hours by the time Voyager is at the distance of Neptune. Thus, in case
of emergency, the spacecraft needs to know how to put itself into a safe
standby mode while awaiting instructions from Earth. As it ages, more and more
failures are expected, both in its mechanical parts and in its computer system,
although there is no sign, even now, of a serious memory deterioration, some
robotic Alzheimer's disease.
This is not to say that Voyager is
perfect. Serious mission-threatening, white-knuckle mishaps did occur. Each
time, special teams of engineers-some of whom had been with the Voyager program
since its inception-were assigned to “work” the problem. They would study the
underlying science and draw upon their previous experience with the failed
subsystems. They would experiment with identical Voyager spacecraft equipment
that had never been launched, or even manufacture a large number of components
of the sort that failed in order to gain some statistical understanding of the
failure mode.
In April 1978, almost eight months after
launch, and while the ship was approaching the asteroid belt, an omitted ground
command-a human error-caused Voyager 2's on-board computer to switch from the
prime radio receiver to its backup, During the next ground transmission to the
spacecraft, the backup receiver refused to lock onto the signal from Earth. A
component called a tracking loop capacitor had failed. After seven days in
which Voyager 2 was entirely out of contact, its fault protection software
suddenly commanded the backup receiver to be switched off and the prime
receiver to be switched back on. Mysteriously-to this day, no one knows why-the
prime receiver failed moments later. It was never heard from again. To top it
off, the on-board computer now foolishly insisted on using the failed primary
receiver. Through an unlucky concatenation of human and robotic error, the
spacecraft was now in real jeopardy. no one could think of a way to get Voyager
2 to revert to the backup receiver. Even if it did, the backup receiver
couldn't receive the commands from Earth, because of that failed capacitor.
There were many project personnel who feared that all was lost.
But after a week of obdurate
unresponsiveness to all commands, instructions to switch automatically between
receivers were accepted and programmed into the skittish onboard computer.
During that same week the JPL engineers designed an innovative command
frequency control procedure to make sure that essential orders would be
understood by the damaged backup receiver.
The engineers were now able to
recommunicate, at least in a rudimentary way, with the spacecraft. Unfortunately
the backup receiver now turned giddy, becoming extremely sensitive to stray
heat dumped when various components of the spacecraft powered up or down. In
the following months the JPL engineers devised and conducted tests that let
them thoroughly understand the thermal implications of most spacecraft
operational modes: What would prevent and what would permit receipt of commands
from Earth?
With this information, the backup
receiver problem was entirely circumvented. It subsequently acquired all the
commands sent from Earth on how to gather data in the Jupiter, Saturn, Uranus,
and Neptune systems. The engineers had saved the mission. (To be on the safe
side, during most of Voyager 2's subsequent flight a nominal data-taking
sequence for the next planet to be encountered was always sitting in the
on-board computers-should the spacecraft again become deaf to entreaties from
home.)
Another heart-wrenching failure occurred
just after Voyager 2 emerged from behind Saturn (as seen from the Earth) in
August 1981. The scan platform had been moving feverishly-pointing here and
there among the rings, moons, and the planet itself during the all-too-brief
moments of close approach. Suddenly, the platform jammed. A stuck scan platform
is a maddening predicament: knowing that the spacecraft is flying past wonders
that have never been witnessed, that we will not see again for years or
decades, and the incurious spacecraft staring fixedly off into space, ignoring
everything.
The scan platform is driven by actuators
containing gear trains. So first the JCL engineers ran an identical copy of a
flight actuator in a simulated mission. This actuator failed after 348 turns;
the actuator on the spacecraft had failed after 352 turns. The problem turned
out to be a lubrication failure. Good to know, but what to do about it?
Plainly, it would be impossible to overtake Voyager with ail oilcan.
The engineers wondered whether they
could restart the tailed actuator by alternate heating and cooling; maybe the
resulting thermal stresses would induce the components of the actuator to
expand and contract at different rates and unjam the system. They tested this
notion with specially manufactured actuators in the laboratory, and then
jubilantly found that in this way they could start the scan platform up again
in space. Project personnel also devised ways to diagnose any additional trend
toward actuator failure early enough to work around the problem. Thereafter,
Voyager 2's scan platform worked perfectly. All the pictures taken in the Uranus
and Neptune systems owe their existence to this work. The engineers had saved
the day again.
Voyagers 1 and 2 were designed to
explore the Jupiter and Saturn systems only. It is true that their trajectories
would carry them on past Uranus and Neptune, but officially these planets were
never contemplated as targets for Voyager exploration: The spacecraft were not
supposed to last that long. Because of our wish to fly close to the mystery
world Titan, Voyager 9 was flung by Saturn on a path that could never encounter
any other known world; it is Voyager 2 that flew on to Uranus and Neptune with
brilliant success. At these immense distances, sunlight is getting
progressively dimmer, and the radio signals transmitted to Earth are getting
progressively fainter. These were predictable but still very serious problems
that the JPL engineers and scientists also had to solve.
Because of the low light levels at
Uranus and Neptune, the Voyager television cameras were obliged to take long
time exposures. But the spacecraft was hurtling so fast through, say, the
Uranus system (at about 35,000 miles per hour) that the image would have been
smeared or blurred. To compensate, the entire spacecraft had to be moved during
the time exposures to cancel out the motion, like panning in the direction
opposite yours while taking a photograph of a street scene from a moving car.
This may sound easy, but it's not: You have to neutralize the most innocent of
motions. At zero gravity, the mere start and stop of the on-board tape recorder
can jiggle the spacecraft enough to smear the picture.
This problem was solved by sending up
commands to the spacecraft's little rocket engines (called thrusters), machines
of exquisite sensitivity. With a little puff of gas at the start and stop of
each data-taking sequence, the thrusters compensated for the tape-recorder
jiggle by turning the entire spacecraft just a little. To deal with the low
radio power received at Earth, the engineers devised a new and more efficient
way to record and transmit the data, and the radio telescopes on Earth were
electronically linked together with others to increase their sensitivity.
Overall, the imaging system worked, by many criteria, better at Uranus and
Neptune than it did at Saturn or even at Jupiter.
Voyager may not yet be done exploring.
There is, of course, a chance that some vital subsystem will fail tomorrow, but
as far as the radioactive decay of the plutonium power source is concerned, the
two Voyager spacecraft should be able to return data to Earth roughly through
the year 2015.
Voyager is an intelligent being-part
robot, part human. It extends the human senses to far-off worlds. For simple
tasks and short-term problems, it relies on its own intelligence; but for more
complex tasks and longer-term problems, it turns to the collective intelligence
and experience of the JPL engineers. This trend is sure to grow. The Voyagers
embody the technology of the early 1970s; if spacecraft were designed for such
a mission today, they would incorporate stunning advances in artificial
intelligence, in miniaturization, in data-processing speed, in the ability to
self-diagnose and repair, and in the propensity to learn from experience They
would also be much cheaper.
In the many environments too dangerous
for people, on Earth as well as in space, the future belongs to robot-human
partnerships that will recognize the two Voyagers as antecedents and pioneers.
For nuclear accidents, mine disasters, undersea exploration and archaeology,
manufacturing, prowling the interiors of volcanos, and household help, to name
only a few potential applications, it could make an enormous difference to have
a ready corps of smart, mobile, compact, commandable robots that can diagnose
and repair their own malfunctions. There are likely to be many more of this
tribe in the near future.
It is conventional wisdom now that
anything built by the government will be a disaster. But the two Voyager
spacecraft were built by the government (in partnership with that other
bugaboo, academia). They came in at cost, on time, and vastly exceeded their
design specifications-as well as the fondest dreams of their makers. Seeking
not to control, threaten, wound, or destroy, these elegant machines represent
the exploratory part of our nature set free to roam the Solar System and
beyond. This kind of technology, the treasures it uncovers freely available to
all humans everywhere, has been, over the last few decades, one of the few
activities of the United States admired as much by those who abhor many of its
policies as by those who agree with it on every issue. Voyager cost each
American less than a penny a year from launch to Neptune encounter. Missions to
the planets are one of those things-and I mean this not just for the United
States, but for the human species-that we do best.
CHAPTER 7
Among the moons of Saturn
Seat thyself sultanically among the
moons of Saturn.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 107
(1851)
There is a world, midway in size between
the Moon and Mars, where the upper air is rippling with electricity-streaming
in from the archetypical ringed planet next door, where the perpetual brown
overcast is tinged with an odd burnt orange, and where the very stuff of life
falls out of the skies onto the unknown surface below. It is so far away that
light takes more than an hour to get there from the Sun. Spacecraft take years.
Much about it is still a mystery-including whether it holds great oceans. We
know just enough, though, to recognize that within reach may be a place where
certain processes ate today working themselves out that aeons ago on Earth led
to the origin of life.
On our own world a long-standing-and in
some respects quite promising-experiment has been under way on the evolution of
matter. The oldest known fossils are about 3. 6 billion years old. Of course,
the origin of life had to have happened well before that. But 4. 2 or 4. 3
billion years ago the Earth was being so ravaged by the final stages of its
formation that life could not yet have come into being: Massive collisions were
melting the surface, turning the oceans into steam and driving any atmosphere
that had accumulated since the last impact off into space. So around 4 billion
years ago, there was a fairly narrow window-perhaps only a hundred million
years wide in which our most distant ancestors came to be. Once conditions
permitted, life arose fast. Somehow. The first living things very likely were
inept, far less capable than the most humble microbe alive today-perhaps just
barely able to make crude copies of themselves. But natural selection, the key
process first coherently described by Charles Darwin, is an instrument of such
enormous power that from the most modest beginnings there can emerge all the
richness and beauty of the biological world.
Those first living things were made of
pieces, parts, building blocks which had to come into being on their own-that
is, driven by the laws of physics and chemistry on a lifeless Earth. The
building blocks of all terrestrial life are called organic molecules, molecules
based on carbon. Of the stupendous ]lumber of possible organic molecules, very
few are used at the heart of life. The two most important classes are the amino
acids, the building blocks of proteins, and the nucleotide bases, the building
blocks of the nucleic acids. Mist before the origin of life, where did these
molecules come from? There are only two possibilities: from the outside or from
the inside. We know that vastly more comets and asteroids were hitting the
Earth than do so today, that these small worlds are rich storehouses of complex
organic molecules, and that some of these molecules escaped being fried on
impact. Here I'm describing homemade, not imported, goods: the organic
molecules generated in the air and waters of the primitive Earth.
Unfortunately, we don't know very much
about the composition of the early air, and organic molecules are far easier to
make in some atmospheres than in others. There couldn't have been much oxygen,
because oxygen is generated by green plants and there weren't any green plants
yet. There was probably more hydrogen, because hydrogen is very abundant in the
Universe and escapes from the upper atmosphere of the Earth into space better
than any other atom (because it's so light). If we can imagine various possible
early atmospheres, we can duplicate them in the laboratory, supply some energy,
and see which organic molecules are made and in what amounts. Such experiments
have over the years proved provocative and promising. But our ignorance of
initial conditions limits their relevance.
What we need is a real world whose
atmosphere still retains some of those hydrogen-rich gases, a world in other
respects something like the Earth, a world in which the organic building blocks
of life are being massively generated in our own time, a world we can go to to
seek our own beginnings. There is only one such world in the Solar System. That
world is Titan, the big moon of Saturn. It's about 5,150 kilometers (3,200
miles) in diameter, a little less than half the size of the Earth. It takes 16
of our days to complete one orbit of Saturn.
No world is a perfect replica of any
other, and in at least one important respect Titan is very different from the
primitive Earth: Being so far from the Sun, its surface is extremely cold, far
below the freezing point of water, around 180° below zero Celsius. So while the
Earth at the time of the origin of life was, as now, mainly ocean-covered,
plainly there can be no oceans of liquid water on Titan. (Oceans made of other
stuff are a different story, as we shall see.) The low temperatures provide an
advantage, though, because once molecules are synthesized on Titan, they tend
to stick around: The higher the temperature, the faster molecules fall to
pieces. On Titan the molecules that have been raining down like manna from
heaven for the last 4 billion years might still be there, largely unaltered,
deep-frozen, awaiting the chemists from Earth.
THE INVENTION OF THE TELESCOPE In the
seventeenth century led to the discovery of many new worlds. In 1610 Galileo
first spied the four large satellites of Jupiter. It looked like a miniature
solar system, the little moons racing around Jupiter as the planets were
thought by Copernicus to orbit the Sun. It was another blow to the
geocentrists. Forty-five years later, the celebrated Dutch physicist
Christianus Huygens discovered a moon moving about the planet Saturn and named
it Titan. † It was a dot of light a billion miles away, gleaming in reflected
sunlight. From the time of its discovery, when European men wore long curly
wigs, to world War II, when American men cut their hair down to stubble, almost
nothing more was discovered about Titan except the fact it had a curious, tawny
color. Ground-based telescopes could, even in principle, barely make out some
enigmatic detail. The Spanish astronomer J. Comas Sola reported at the turn of
the twentieth century some faint and indirect evidence of an atmosphere.
In a way, I grew up with Titan. I did my
doctoral dissertation at the University of Chicago under the guidance of Gerard
P. Kuiper, the astronomer who made the definitive discovery that Titan has an
atmosphere. Kuiper was Dutch and in a direct line of intellectual descent from
Christianus Huygens. In 1914, while making a spectroscopic examination of
Titan, Kuiper was astonished to find the characteristic spectral features of
the gas methane. When he pointed the telescope at Titan, there was the
signature of methane. When he pointed it away, not a hint of methane. But moons
were not supposed to hold onto sizable atmospheres, and the Earth's Moon
certainly doesn't. Titan could retain an atmosphere, Kuiper realized, even
though its gravity was less than Earth's, because its upper atmosphere is very
cold. The molecules simply aren't moving fast enough for significant numbers to
achieve escape velocity and trickle away to space.
Daniel Harris, a student of Kuiper's,
showed definitively that Titan is red. Maybe we were looking at a rusty
surface, like that of Mars. If you wanted to learn more about Titan, you could
also measure the polarization of sunlight reflected off it. Ordinary sunlight
is unpolarized. Joseph Veverka, now a fellow faculty member at Cornell
University, was my graduate student at Harvard University, and therefore, so to
speak, a grandstudent of Kuiper's. In his doctoral work, around 1970, he
measured the polarization of Titan and found that it changed as the relative
positions of Titan, the Sun, and the Earth changed. But the change was very
different from that exhibited by, say, the Moon. Veverka concluded that the
character of this variation was consistent with extensive clouds or haze on
Titan. When we looked at it through the telescope, we weren't seeing its
surface. We knew nothing about what the surface was like. We had no idea how
fat below the clouds the surface was.
So, by the early 1970s, as a kind of
legacy from Huygens and his line of intellectual descent, we knew at least that
Titan has a dense methane-rich atmosphere, and that it's probable enveloped by
a reddish cloud veil or aerosol haze. But what kind of cloud is red? By the
early 1970s my colleague Bishun Khare and I had been doing experiments at
Cornell in which we irradiated various methane-rich atmospheres with
Ultraviolet light or electrons and were generating reddish or brownish solids;
the stuff would coat the interiors of our reaction vessels. It seemed to me
that, if methane-rich Titan had red-brown clouds, those clouds might very well
be similar to what we were making in the laboratory. We called this material
tholin, after a Greek word for “muddy.” At the beginning we had yen little idea
what it was made of. It was some organic stew made by breaking apart our
starting molecules, and allowing the atoms-carbon, hydrogen, nitrogen-and
molecular fragments to recombine.
The word “organic” carries no imputation
of biological origin; following long-standing chemical usage dating back mots
than a century, it merely describes molecules built out of car bon atoms
(excluding a few very simple ones such as carbon monoxide, CO, and carbon
dioxide, CO2). Since life on Earth is based oil organic molecules, and since
there was a time before there was life on Earth, some process must have made
organic molecules on our planet before the time of the first organism.
Something sitar, I proposed, might be happening on Titan today.
The epochal event in our understanding
of Titan was the arrival in 1980 and 1981 of the Voyager 1 and 2 spacecraft in
the Saturn system. The ultraviolet, infrared, and radio instruments revealed
the pressure and temperature through the atmosphere-from the hidden surface to
the edge of space. We learned how high the cloud tops are. We found that the
air on Titan is composed mainly of nitrogen, N2, as on the Earth today. The
other principal constituent is, as Kuiper found, methane. CH4 the starting material
from which carbon-based organic molecules are generated there.
A variety of simple organic molecules
was found, present as gases, mainly hydrocarbons and nitriles. The most complex
of them have four “heavy” (carbon and/or nitrogen) atoms. Hydrocarbons are
molecules composed of carbon and hydrogen atoms only, and are familiar to us as
natural gas, petroleum, and waxes. (They're quite different from carbohydrates,
such as sugars and starch, which also have oxygen atoms.) Nitriles are
molecules with a carbon and nitrogen atom attached in a particular way. The
best known nitrile is HCN, hydrogen cyanide, a deadly gas for humans. But
hydrogen cyanide is implicated in the steps that on Earth led to the origin of
life.
Finding these simple organic molecules in
Titan's upper atmosphere-even if present only in a part per million or a part
per billion-is tantalizing. Could the atmosphere of the primeval Earth have
been similar? There's about ten times more air oil Titan than there is on Earth
today, but the early Earth may well have had a denser atmosphere.
Moreover, Voyager discovered an
extensive region of energetic electrons and protons surrounding Saturn, trapped
by the planet's magnetic field. During the course of its orbital motion around
Saturn, Titan bobs in and out of this magnetosphere. Beams of electrons (plus
solar ultraviolet light) fall on the upper air of Titan, just as charged
particles (plus solar ultraviolet light) were intercepted by the atmosphere of
the primitive Earth.
So it's a natural thought to irradiate
the appropriate mixture of nitrogen and methane with ultraviolet light or
electrons at very low pressures, and find out what more complex molecules can
be made. Can we simulate what's going on in Titan's high atmosphere? In our
laboratory at Cornell-with my colleague W. Reid Thompson playing a key
role-we've replicated some of Titan's manufacture of organic gases. The
simplest hydrocarbons on Titan are manufactured by ultraviolet light from the
Sun. But for all the other gas products, those made most readily by electrons
in the laboratory correspond to those discovered by Voyager on Titan, and in
the same proportions. The correspondence is one to one. The next most abundant
gases that we've found in the laboratory will be looked for in future studies
of Titan. The most complex organic gases we make have six or seven carbon
and/or nitrogen atoms. These product molecules are on their way to forming
tholins.
WE HAD HOPED FOR A BREAK In the weather
as Voyager 1 approached Titan. A long distance away, it appeared as a tiny
disk; at closest approach, our camera's field of view was filled by a small
province of Titan. If there had been a break in the haze and clouds, even only
a few miles across, as we scanned the disk we would have seen something of its
hidden surface. But there was no hint of a break. This world is socked in. no
one on Earth knows what's on Titan's surface. And an observer there, looking up
ill ordinary visible light, would have no idea of the glories that await upon
ascending through the haze and beholding Saturn and its magnificent rings.
From measurements by Voyager, by the
International Ultraviolet Explorer observatory in Earth orbit, and by
ground-based telescopes, we know a fair amount about the orange-brown haze
particles that obscure the surface: which colors of light they like to absorb,
which colors they pretty much let pass through them, how much they bend, the
light that does pass through them, and how big they are. (They're mostly the
size of the particles in cigarette smoke.) The “optical properties” will
depend, of course, on the composition of the haze particles.
In collaboration with Edward Arakawa of
Oak Ridge National Laboratory in Tennessee, Khare and I have measured the
optical properties of Titan tholin. It turns out to be a dead ringer for the
real Titan haze. no other candidate material, mineral or organic, matches the
optical constants of Titan. So we can fairly claim to have bottled the haze of
Titan-formed high in its atmosphere, slowly falling out, and accumulating in
copious amounts on its surface. What is this stuff made of?
It's very hard to know the exact
composition of a complex organic solid. For example, the chemistry of coal is
still not fully understood, despite a long-standing economic incentive. But
we've found out some things about Titan tholin. It contains many of the
essential building blocks of life on Earth. Indeed, if you drop Titan tholin
into water you make a large number of amino acids, the fundamental constituents
of proteins, and nucleotide bases also, the building blocks of DNA and RNA.
Some of the amino acids so formed are widespread in living things on Earth.
Others are of a completely different sort. A rich array of other organic
molecules is present also, some relevant to life, some not. During the past
four billion years, immense quantities of organic molecules sedimented out of
the atmosphere onto the surface of Titan. If it's all deep-frozen and unchanged
in the intervening aeons, the amount accumulated should be at least tens of meters
(a hundred feet) thick; outside estimates put it at a kilometer deep.
But at 180°C below the freezing point of
water, you might very well think that amino acids will never be made. Dropping
tholins into water may be relevant to the early Earth, but not, it would seem,
to Titan. However, comets and asteroids must on occasion come crashing into the
surface of Titan. (The other nearby moons of Saturn show abundant impact
craters, and the atmosphere of Titan isn't thick enough to prevent large,
high-speed objects from reaching the surface.) Although we've never seen the
surface of Titan, planetary scientists nevertheless know something about its
composition. The average density of Titan lies between the density of ice and
the density of rock. Plausibly it contains both. Ice and rock are abundant on
nearby worlds, some of which are made of nearly pure rice. If the surface of
Titan is icy, a high-speed cometary impact will temporarily melt the ice.
Thompson and I estimate that any given spot on Titan's surface has a better
than 50-50 chance of having once been melted, with an average lifetime of the
impact melt and slurry of almost a thousand years.
This makes for a very different story.
The origin of life on Earth seems to have occurred in oceans and shallow tidepools.
Life on Earth is made mainly of water, which plays an essential physical and
chemical role. Indeed, it's hard, for us water-besotted creatures to imagine
life without water. If on our planet the origin of life took less than a
hundred million years, is there any chance that on Titan it took a thousand?
With tholins mixed into liquid water-even for only a thousand years the surface
of Titan may be much further along toward the origin of life than we thought.
DESPITE ALL THIS we understand pitifully
little about Titan. This was brought home forcefully to me at a scientific
symposium on Titan held in Toulouse, France, and sponsored by the European
Space Agency (ESA). While oceans of liquid water are impossible on Titan,
oceans of liquid hydrocarbons are not. Clouds of methane (CH4), the most
abundant hydrocarbon, are expected not far above the surface. Ethane (C2H6),
the next most abundant hydrocarbon, must condense out at the surface in the
same way that water vapor becomes a liquid near the surface of the Earth, where
the temperature is generally between the freezing and melting points. Vast
oceans of liquid hydrocarbons should have accumulated over the lifetime of
Titan. They would lie far beneath the haze and clouds. But that doesn't mean
they would be wholly inaccessible to us-because radio waves readily penetrate
the atmosphere of Titan and its suspended, slowly falling fine particles.
In Toulouse, Duane O. Muhleman of the
California Institute of Technology described to us the very difficult technical
feat of transmitting a set of radio pulses from a radio telescope in
California's Mojave Desert, so they reach Titan, penetrate through the haze and
clouds to its surface, are reflected back into space, and then returned to
Earth. Here, the greatly enfeebled signal is picked up by an array of radio
telescopes near Socorro, New Mexico. Great. If Titan has a rocky or icy
surface, a radar pulse reflected off its surface should be detectable on Earth.
But if Titan were covered with hydrocarbon oceans, Muhleman shouldn't see a
thing: Liquid hydrocarbons are black to these radio waves, and no echo would
have been returned to Earth. In fact, Muhleman's giant radar system sees a
reflection when some longitudes of Titan are turned toward Earth, and not at
other longitudes. All right, you might say, so Titan has oceans and continents,
and it was a continent that reflected the signals back to Earth. But if Titanic
in this respect like the Earth-for some meridians (through Europe and Africa,
say) mainly continent, and for others (through the central Pacific, say) mainly
ocean-then we must confront another problem:
The orbit of Titan around Saturn is not
a perfect circle. it's noticeably squashed out, or elliptical. If Titan has
extensive oceans, though, the giant planet Saturn around which it orbits will
raise substantial tides on Titan, and the resulting tidal friction will
circularize Titan's orbit in much less than the age of the Solar System. In a
1982 scientific paper called “The Tide in the Seas of Titan,” Stanley Dermott,
now at the University of Florida, and I argued that for this reason Titan must
be either an all-ocean or an all-land world. Otherwise the tidal friction in
places where the ocean is shallow would have taken its toll. Lakes and islands
might be permitted, but anything more and Titan would have a very different
orbit than the one we see.
We have, then, three scientific
arguments-one concluding that this world is almost entirely covered with
hydrocarbon oceans, another that it's a mix of continents and oceans, and a
third requiring us to choose, counseling that Titan can't have extensive oceans
and extensive continents at the same time. It will be interesting to see what
the answer turns out to be.
What I've just told you is a kind of
scientific progress report. Tomorrow there might be a new finding that clears
up these mysteries and contradictions. Maybe there's something wrong with
Muhleman's radar results, although it's hard to see what it might be: His
system tells him he's seeing Titan when it's nearest, when he ought to be
seeing Titan. Maybe there's something wrong with Dermott's and my calculation
about the tidal evolution of the orbit of Titan, but no one has been able to
find any errors so far. And it's Bard to see how ethane can avoid condensing
out at the surface of Titan. Maybe, despite the low temperatures, over billions
of years there's been a change in the chemistry; maybe some combination of
comets impacting from the sky and volcanoes and other tectonic events, helped
along by cosmic rays, can congeal liquid hydrocarbons, turning them into some
complex organic solid that reflects radio waves back to space. Or maybe
something reflective to radio waves is floating on the ocean surface. But
liquid hydrocarbons are very underdense: Every known organic solid, unless
extremely frothy, would sink like a stone in the seas of Titan.
Dermott and I now wonder whether, when
we imagined continents and oceans on Titan, we were too transfixed by our
experience on our own world, too Earth-chauvinist in our thinking. Battered,
cratered terrain and abundant impact basins cover other moons in the Saturn
system. If we pictured liquid hydrocarbons slowly accumulating on one of those
worlds, we would wind up not with global oceans, but with isolated large craters
filled, although not to the brim, with liquid hydrocarbons. Many circular seas
of petroleum, some over a hundred miles across, would be splattered across the
surface-but no perceptible waves would be stimulated by distant Saturn and, it
is conventional to think, no ships, no swimmers, no surfers, and no fishing.
Tidal friction should, we calculate, be negligible in such a case, and Titan's
stretched-out, elliptical orbit would not have become so circular. We can't
know for sure until we start getting radar or near-infrared images of the
surface. But perhaps this is the resolution of our dilemma: Titan as a world of
large circular hydrocarbon lakes, more of them in some longitudes than in
others.
Should we expect an icy surface covered
with deep tholin sediments, a hydrocarbon ocean with at most a few organic
encrusted islands poking up here and there, a world of crater lakes, or
something more subtle that we haven't yet figured out? This isn't just an
academic question, because there's a real spacecraft being designed to go to
Titan. In a joint NASA/ESA program, a spacecraft called Cassini will be
launched in October 1997-if all goes well. With two flybys of Venus, one of
Earth, and one of Jupiter for gravitational assists, the ship will, after a
seven-year voyage, be injected into orbit around Saturn. Each time the
spacecraft comes close to Titan, the moon will be examined by an array of
instruments, including radar. Because Cassini will be so much closer to Titan,
it will be able to resolve many details on Titan's surface indetectable to
Muhleman's pioneering Earth-based system. It's also likely that the surface can
be viewed in the near infrared. Maps of the hidden surface of Titan may be in
our hands sometime in the summer of 2004.
Cassini is also carrying an entry probe,
fittingly called Huygens, which will detach itself from the main spacecraft and
plummet into Titan's atmosphere. A great parachute will be deployed. The
instrument package will slowly settle through the organic haze down into the
lower atmosphere, through the methane clouds. It will examine organic chemistry
as it descends, and-if it survives the landing-on the surface of this world as
well.
Nothing is guaranteed. But the mission
is technically feasible, hardware is being built, an impressive coterie of
specialists, including many young European scientists, are hard at work on it,
and all the nations responsible seem committed to the project. Perhaps it will
actually come about. Perhaps winging across the billion miles of intervening
interplanetary space will be, in the not too distant future. news about how far
along the path to life Titan has come.
CHAPTER 8
The first new planet
I implore you, you do not hope to be
able to give the reasons for the number of planets, do you? This worry has been
resolved...
— JOHANNES KEPLER, EPITOME OF COPERNICAN
ASTRONOMY, BOOK 4 / 1621
Before we invented civilization, our
ancestors lived mainly in the open, out under the sky. Before we devised
artificial lights and atmospheric pollution and modern forms of nocturnal
entertainment, we watched the stars. There were practical calendrical reasons,
of course, but there was more to it than that. Even today, the most jaded city
dweller can be unexpectedly moved upon encountering a clear night sky studded
with thousands of twinkling stars. When it happens to me after all these years,
it still takes my breath away.
In every culture, the sky and the
religious impulse are intertwined. I lie back in an open field and the sky
surround me. I'm overpowered by its scale. It's so vast and so far away that my
own insignificance becomes palpable. But I don't feel rejected by the sky. I'm
a part of it, tiny, to be sure, but everything is tiny compared to that
overwhelming immensity, And when I concentrate on the stars, the planets, and
their motions, I have an irresistible sense of machinery, clockwork, elegant
precision working on a scale that, however lofty our aspirations, dwarfs and
humbles us.
Most of the great inventions in human
history-from stone tools and the domestication of fire to written language-were
made by unknown benefactors. Our institutional memory of long-gone events is
feeble. We do not know the name of that ancestor who first noted that planets
were different from stars. She or he must have lived tens, perhaps even
hundreds of thousands of years ago. But eventually people all over the world
understood that five, no more, of the bright points of light that grace the
night sky break lockstep with the others over a period of months, moving
strangely-almost as if they had minds of their own.
Sharing the odd apparent motion of these
planets were the Sun and Moon, making seven wandering bodies in all. These
seven were important to the ancients, and they named them after gods not any
old gods, but the main gods, the chief gods, the ones who tell other gods (and
mortals) what to do. One of the planets, bright and slow-moving, was named by
the Babylonians after Marduk, by the Norse after Odin, by the Greeks after
Zeus, and by the Romans after Jupiter, in each case the king of the gods. The
faint, fast-moving one that was never far from the Sun the Romans named
Mercury, after the messenger of the gods; the most brilliant of them was named
Venus, after the goddess of love and beauty; the blood red one Mars, after the god
of war; and the most sluggish of the bunch Saturn, after the god of time. These
metaphors and allusions were the best our ancestors could do: They possessed no
scientific instruments beyond the naked eye, they were confined to the Earth,
and they had no idea that it, too, is a planet.
When it got to be time to design the
week-a period of time, unlike the day, month, and year, with no intrinsic
astronomical significance-it was assigned seven days, each named after one of
the seven anomalous lights in the night sky. We can readily make out the
remnants of this convention. In English, Saturday is Saturn's day. Sunday and
Mo[o]nday are clear enough. Tuesday through Friday are named after the gods of
the Saxon and kindred Teutonic invaders of Celtic/Roman Britain: Wednesday, for
example, is Odin's (or Wodin's) day, which would be more apparent if we
pronounced it as it's spelled, “Wedn's Day”; Thursday is Thor's day; Friday is
the day of Freya, goddess of love. The last day of the week stayed Roman, the
rest of it became German.
In all Romance languages, such as
French, Spanish, and Italian, the connection is still more obvious, because
they 4 derive from ancient Latin, in which the days of the week were named (in
order, beginning with Sunday) after the Sun, the Moon, Mars, Mercury, Jupiter,
Venus, and Saturn. (The Sun's day became the Lord's day.) They could have named
the days in order of the brightness of the corresponding astronomical
bodies-the Sun, the Moon, Venus, Jupiter, Mars, Saturn, Mercury (and thus Sunday,
Monday, Friday, Thursday, Tuesday, Saturday, Wednesday)-but they did not. If
the days of the week in Romance languages had been ordered by distance from the
Sun, the sequence would be Sunday, Wednesday, Friday, Monday, Tuesday,
Thursday, Saturday. no one knew the order of the planets, though, back when we
were naming planets, gods, and days of the week. The ordering of the days of
the week seems arbitrary, although perhaps it does acknowledge the primacy of
the Sun.
This collection of seven gods, seven
days, and seven worlds the Sun, the Moon, and the five wandering planets
entered the perceptions of people everywhere. The number seven began to acquire
supernatural connotations. There were seven “heavens,” the transparent
spherical shells, centered on the Earth, that were imagined to make these
worlds move. The outermost-the seventh heaven-is where the “fixed” stars were
imagined to reside. There are Seven Days of Creation (if we include God's day
of rest), seven orifices to the head, seven virtues, seven deadly sins, seven
evil demons in Sumerian myth. seven vowels in the Greek alphabet (each
affiliated with a planetary god), Seven Governors of Destiny according to the
Hermetists, Seven Great Books of Manichaeism, Seven Sacraments, Seven Sages of
Ancient Greece, and seven alchemical “bodies” (gold, silver, iron, mercury,
lead, tin, and copper-gold still associated with the Sun, silver with the Moon,
iron with Mars, etc.). The seventh son of a seventh son is endowed with
supernatural powers. Seven is a “lucky” number. In the New Testament's Book of
Revelations, seven seals on a scroll are opened, seven trumpets are sounded,
seven bowls are filled. St. Augustine obscurely argued for the mystic
importance of seven on the grounds that three “is the first whole number that
is odd” (what about one?), “four the first that is even” (what about two?), and
“of these...seven is composed.” And so on. Even in our time these associations
linger.
The existence even of the four
satellites of Jupiter that Galileo discovered-hardly planets-was disbelieved on
the grounds that it challenged the precedence of the number seven. As
acceptance of the Copernican system grew, the Earth was added to the list of
planets, and the Sun and Moon were removed. Thus, there seemed to be only six
planets (Mercury, Venus, Earth, Mars, Jupiter, and Saturn). So learned academic
arguments were invented showing why there had to be six. For example, six is
the first “perfect” number, equal to the sum of its divisors (1 + 2 + 3). Q. E.
D. And anyway, there were only six days of creation, not seven. People found
ways to accommodate from seven planets to six.
As those adept at numerological
mysticism adjusted to the Copernican system, this self-indulgent mode of
thinking spilled over from planets to moons. The Earth had one moon; Jupiter
had the four Galilean moons. That made five. Clearly one was missing. (Don't
forget: Six is the first perfect number.) When Huygens discovered Titan in
1655, he and many others convinced themselves that it was the last: Six
planets, six moons, and God's in His Heaven.
The historian of science I. Bernard
Cohen of Harvard University has pointed out that Huygens actually gave up
searching for other moons because it was apparent, from such arguments, that no
more were to be found. Sixteen years later, ironically with Huygens in
attendance, G. D. Cassim of the Paris Observatory discovered a seventh
moon-Iapetus, a bizarre world with one hemisphere black and the other white, in
an orbit exterior to Titan's. Shortly after, Cassim discovered Rhea, the next
Saturnian moon interior to Titan.
Here was another opportunity for
numerology, this time harnessed to the practical task of flattering patrons.
Cassim added up the number of planets (six) and the number of satellites
(eight) and got fourteen. Now it so happened that the man who built Cassim's
observatory for him and paid his salary was Louis XIV of France, the Sun King.
The astronomer promptly “presented” these two new moons to his sovereign and
proclaimed that Louis's “conquests” reached to the ends of the Solar System.
Discreetly, Cassim then backed off from looking for more moons; Cohen suggests
he was afraid one more might now offend Louis-a monarch not to be trifled with,
who would shortly be throwing his subjects into dungeons for the crime of being
Protestants. Twelve years later, though, Cassim returned to the search and
found-doubtless with a measure of trepidation-another two moons. (It is
probably a good thing that we have not continued in this vein; otherwise France
would have been burdened by seventy-some-odd Bourbon kings named Louis.)
WHEN CLAIMS OF NEW WORLDS WERE MADE In
the late eighteenth century, the force of such numerological arguments had much
dissipated. Still, it was with a real sense of surprise that people heard in
1781 about a new planet, discovered through the telescope. New moons were
comparatively unimpressive, especially after the first six or eight. But that
there were new planets to be found and that humans had devised the means to do
so were both considered astonishing, and properly so. If there is one
previously unknown planet, there may be many more-in this solar system and in
others. Who can tell what might be found if a multitude of new worlds are
hiding in the dark?
The discovery was made not even by a
professional astronomer but by William Herschel, a musician whose relatives had
come to Britain with the family of another anglified German, the reigning
monarch and future oppressor of the American colonists, George III. It became
Herschel's wish to call the planet George (“George's Star,” actually), after
his patron. but, providentially, the name didn't stick. (Astronomers seem to
have been very busy buttering up kings.) Instead, the planet that Herschel
found is called Uranus (an inexhaustible source of hilarity renewed in each
generation of English-speaking nine-year-olds). It is named after the ancient
sky god who, according to Greek myth, was Saturn's father and thus the
grandfather of the Olympian gods.
We no longer consider the Sun and Moon to
be planets, and ignoring the comparatively insignificant asteroids and comets,
count Uranus as the seventh planet in order from the Sun (Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto). It is the first planet
unknown to the ancients. The four outer, Jovian, planets turn out to be very
different from the four inner, terrestrial, planets. Pluto is a separate case.
As the years passed and the quality of
astronomical instruments unproved, we began to learn more about distant Uranus.
What reflects the dim sunlight back to us is no solid surface, but atmosphere
and clouds just as for Titan, Venus, Jupiter Saturn, and Neptune. The air on
Uranus is made of hydrogen and helium, the two simplest gases. Methane and
other hydrocarbons are also present. Just below the clouds visible to
Earthbound observers is a massive atmosphere with enormous quantities of
ammonia, hydrogen sulfide, and, especially, water.
At depth on Jupiter and Saturn, the
pressures are so great that atoms sweat electrons, and the air becomes a metal.
That does not seem to happen on less massive Uranus, because the pressures at
depth are less. Still deeper, discovered only by its subtle tugs on Uranus'
moons, wholly inaccessible to view, under the crushing weight of the overlying
atmosphere, is a rocky surface. A big Earthlike planet is hiding down there,
swathed in an immense blanket of air.
The Earth's surface temperature is due
to the sunlight it intercepts. Turn off the Sun and the planet soon chills-not
to trifling Antarctic cold, not just so cold that the oceans freeze, but to a
cold so intense that the very air precipitates out, forming a ten-meter-thick
layer of oxygen and nitrogen snows covering the whole planet. The little bit of
energy that trickles up from the Earth's hot interior would be insufficient to
melt these snows. For Jupiter, Saturn, and Neptune it's different. There's
about as much heat pouring out from their interiors as they acquire from the
warmth of the distant Sun. Turn off the Sun, and they would be only a little
affected.
But Uranus is another story. Uranus is
an anomaly among the Jovian planets. Uranus is like the Earth: There's very
little intrinsic heat pouring out. We have no good understanding of why this
should be, why Uranus-which in many respects is so similar to Neptune-should
lack a potent source of internal heat. For this reason, among others, we cannot
say we understand what is going on in the deep interiors of these mighty
worlds.
Uranus is lying on its side as it goes
around the Sun. In the 1990s. the south pole is heated by the Sun, and it is
this pole that Earthbound observers at the end of the twentieth century see
when they look at Uranus. It takes Uranus 84 Earth years to make one circuit of
the Sun. So in the 2030s, the north pole will be sunward (and Earthward). In
the 2070s the south pole will be pointing to the Sun once again. In between,
Earthbound astronomers will be looking mainly at equatorial latitudes.
All the other planets spin much more
upright in their orbits. no one is sure of the reason for Uranus' anomalous
spin; the most promising suggestion is that sometime in its early history,
billions of years ago, it was struck by a rogue planet, about the size of the
Earth, in a highly eccentric orbit. Such a collision, if it ever happened, must
have worked much tumult in the Uranus system; for all we know, there may be
other vestiges of ancient havoc still left for us to find. But Uranus'
remoteness tends to guard its mysteries.
In 1977 a team of scientists led by
James Elliot, then of Cornell University, accidentally discovered that, like
Saturn, Uranus has rings. The scientists were flying over the Indian Ocean in a
special NASA airplane-the Kuiper Airborne Observatory-to witness the passage of
Uranus in front of a star. (Such passages, or occultations as they're called,
happen from time to time, precisely because Uranus slowly moves with respect to
the distant stars.) The observers were surprised to find that the star winked
on and off several times just before it passed behind Uranus and its
atmosphere, then several times more just after it emerged. Since the patterns
of winking on and off were the same before and after occultation, this finding
(and much subsequent work) has led to the discovery of nine very thin, very
dark circumplanetary rings, giving Uranus the appearance of a bull's-eye in the
sky.
Surrounding the rings, Earthbound
observers understood were the concentric orbits of the five moons then known:
Miranda, Ariel, Umbriel, Titania, and Oberon. They're named after characters in
Shakespeare's A Midsummer Night's Dream and The Tempest, and in Alexander
Pope's The Rape of the Lock. Two of them were found by Herschel himself. The
innermost of the five, Miranda, was discovered as recently as 1948, by my
teacher G. P. Kuiper. †† I remember how great an achievement the discovery of a
new moon of Uranus was considered back then. The near-infrared light reflected
by all five moons subsequently revealed the spectral signature of ordinary
water ice on their Surfaces. And no wonder-Uranus is so far from the Sun that
it is no brighter there at noontime than it is after sunset on Earth. The
temperatures are frigid. Any water must be frozen.
A REVOLUTION IN OUR UNDERSTANDING of the
Uranus system-the planet, its rings, and its moons-began on January 24, 1986.
On that day, after a journey of 81/2 years, the Voyager 2 spacecraft sailed
very near to Miranda, and hit the bull's-eye in the sky. Uranus' gravity then
flung it on to Neptune. The spacecraft returned 4,300 close-up pictures of the Uranus
system and a wealth of other data.
Uranus was found to be surrounded by an
intense radiation belt, electrons and protons trapped by the planet's magnetic
field. Voyager flew through this radiation belt, measuring the magnetic field
and the trapped charged particles as it went. It also detected-in changing
timbres, harmonies, and nuance, but mainly in fortissimo-a cacophony of radio
waves generated by the speeding, trapped particles. Something similar was
discovered on Jupiter and Saturn and would be later found at Neptune-but always
with a theme and counterpoint characteristic of each world.
On Earth the magnetic and geographical
poles are quite close together. On Uranus the magnetic axis and the axis of
rotation are tilted away from each other by some 60 degrees. no one yet
understands why: Some have suggested that we are catching Uranus in a reversal
of its north and south magnetic poles, as periodically happens on Earth. Others
propose that this too is the consequence of that mighty, ancient collision that
knocked the planet over. But we do not know.
Uranus is emitting much more ultraviolet
light than it's receiving from the Sun, probably generated by charged particles
leaking out of the magnetosphere and striking its upper atmosphere. From a
vantage point in the Uranus system, the spacecraft examined a bright star
winking on and off as the rings of Uranus passed by. New faint dust bands were
found. From the perspective of Earth, the spacecraft passed behind Uranus; so
the radio signals it was transmitting back home passed tangentially through the
Uranian atmosphere, probing it-to below its methane clouds. A vast and deep
ocean, perhaps 8,000 kilometers thick, of super-heated liquid water floating in
the air is inferred by some.
Among the principal glories of the
Uranus encounter were the pictures. With Voyager's two television cameras, we
discovered ten new moons, determined the length of the day in the clouds of
Uranus (about 17 hours), and studied about a dozen rings. The most spectacular
pictures were those returned from the five larger, previously known moons of
Uranus, especially the smallest of them, Kuiper's Miranda. Its surface is a
tumult of fault valleys, parallel ridges, sheer cliffs, low mountains, impact
craters, and frozen floods of once-molten surface material. This turmoiled
landscape is unexpected for a small, cold, icy world so distant from the Sun.
Perhaps the surface was melted and reworked in some long-gone epoch when a
gravitational resonance between Uranus, Miranda, and Ariel pumped energy from
the nearby planet into Miranda's interior. Or perhaps we are seeing the results
of the primordial collision that is thought to have knocked Uranus over. Or,
just conceivably, maybe Miranda was once utterly destroyed, dismembered,
blasted into smithereens by a wild careening world, with many collision
fragments still left in Miranda's orbit. The shards and remnants, slowly
colliding, gravitationally attracting one another, may have re-aggregated into
just such a jumbled, patchy, unfinished world as Miranda is today.
For me, there's something eerie about
the pictures of dusky Miranda, because I can remember so well when it `vas only
a faint point of light almost lost in the glare of Uranus, discovered through
great difficulty by dint of the astronomer's skills and patience. In only half
a lifetime it has gone from an undiscovered world to a destination whose
ancient and idiosyncratic secrets have been at least partially revealed.
CHAPTER 9
An American ship at the frontiers of the solar
system
... by the shore Of Triton's Lake... I
will clear my breast of secrets.
—EURIPIDES, ION (CA. 413 B. C.)
Neptune was the final port of call in
Voyager 2's grand tour of the Solar System. Usually, it is thought of as the
penultimate planet, with Pluto the outermost. But because of Pluto's
stretched-out, elliptical orbit, Neptune has lately been the outermost planet,
and will remain so until 1999. Typical temperatures in its upper clouds are
about -240°C, because it is so far from the warming rays of the Sun. It would
be colder still except for the heat welling up from its interior. Neptune
glides along the hem of interstellar night. It is so far away that, in its sky,
the Sun appears as little more than an extremely bright star.
How far? So far away that it has
yet to complete a single trip around the Sun, a Neptunian year, since its
discovery in 1846. It's so far away that it cannot be seen with the naked eye.
It's so far away that it takes light-faster than which nothing can go-more than
five hours to get from Neptune to Earth.
When Voyager 2 raced through the Neptune
system in 1989, its cameras, spectrometers, particle and field detectors, and
other instruments were feverishly examining the planet, its moons, and its
rings. The planet itself, like its cousins Jupiter, Saturn, and Uranus, is a
giant. Every planet is an Earthlike world at heart-but the four gas giants wear
elaborate, cumbersome disguises. Jupiter and Saturn are great gas worlds with
relatively small rocky and icy cores. But Uranus and Neptune are fundamentally
rock and ice worlds swaddled in dense atmospheres that hide them from view.
Neptune is four times bigger than the
Earth. When we look down on its cool, austere blueness, again we are seeing
only atmosphere and clouds-no solid surface. Again, the atmosphere is made
mainly of hydrogen and helium, with a little methane and traces of other
hydrocarbons. There may also be some nitrogen. The bright clouds, which seem to
be methane crystals, float above thick, deeper clouds of unknown composition. From
the motion of the clouds we discovered fierce winds, approaching the local
speed of sound. A Great Dark Spot was found, curiously at almost the same
latitude as the Great Red Spot on Jupiter. The azure color seems appropriate
for a planet named after the god of the sea.
Surrounding this dimly lit, chilly,
stormy, remote world is-here also-a system of rings, each composed of
innumerable orbiting objects ranging in size from the fine particles in
cigarette smoke to small trucks. Like the rings of the other Jovian planets,
those of Neptune seem to be evanescent-it is calculated that gravity and solar
radiation will disrupt them in much less than the age of the Solar System. If
they are destroyed quickly, we must see them only because they were made recently.
But how can rings be made?
The biggest moon in the Neptune system
is called Triton. Nearly six of our days are required for it to orbit Neptune,
which-alone among big moons in the Solar System-it does in the opposite
direction to which its planet spins (clockwise if we say Neptune rotates
counterclockwise). Triton has a nitrogen-rich atmosphere, somewhat similar to
Titan's; but, because the air and haze are much thinner, we can see its
surface. The landscapes are varied and splendid. This is a world of ices-nitrogen
ice, methane ice, probably underlain by more familiar water ice and rock. There
are impact basins, which seem to have been flooded with liquid before
refreezing (so there once were lakes on Triton); impact craters; long
crisscrossing valleys; vast plains covered by freshly fallen nitrogen snow;
puckered terrain that resembles the skin of a cantaloupe; and more or less
parallel, long, dark streaks that seem to have been blown by the wind and then
deposited on the icy surface despite how sparse Triton's atmosphere is (about
1/10,000 the thickness of the Earth's).
All the craters on Triton are
pristine-as if stamped out by some vast milling device. There are no slumped
walls or muted relief. Even with the periodic falling and evaporation of snow,
it seems that nothing has eroded the surface of Triton in billions of years. So
the craters that were gouged out during the formation of Triton must have all
been filled in and covered over by some early global resurfacing event. Triton
orbits Neptune in the opposite direction to Neptune's rotation-unlike the
situation with the Earth and its moon, and with most of the large moons in the
Solar System. If Triton had formed out of the same spinning disk that made
Neptune, it ought to be going around Neptune in the same direction that Neptune
rotates. So Triton was not made from the original local nebula around Neptune,
but arose somewhere else-perhaps far beyond Pluto-and was by chance
gravitationally captured when it passed too close to Neptune. This event should
have raised enormous solid-body tides in Triton, melting the surface and
sweeping away all the past topography.
In some places the surface is as bright
and white as freshly fallen Antarctic snows (and may offer a skiing experience
unrivaled in all the Solar System). Elsewhere there's a tint, ranging from pink
to brown. One possible explanation: Freshly fallen snows of nitrogen, methane,
and other hydrocarbons are irradiated by solar ultraviolet light and by
electrons trapped in the magnetic field of Neptune, through which Triton plows.
We know that such irradiation will convert the snows (like the corresponding
gases) to complex, dark, reddish organic sediments, ice tholins-nothing alive,
but here too composed of some of the molecules implicated in the origin of life
on Earth four billion years ago.
In local winter, layers of ice and snow
build up on the surface. (Our winters, mercifully, are only 4 percent as long.)
Through the spring, they are slowly transformed, more and more reddish organic
molecules accumulating. By summertime, the ice and snow have evaporated; the
gases so released migrate halfway across the planet to the winter hemisphere
and there cover the surface with ice and snow again. But the reddish organic
molecules do not vaporize and are not transported-a lag deposit, they are next
winter covered over by new snows, which are in turn irradiated, and by the
following summer the accumulation is thicker. As time goes on, substantial
amounts of organic matter are built up on the surface of Triton, which may
account for its delicate color markings.
The streaks begin in small, dark source
regions, perhaps when the warmth of spring and summer heats subsurface volatile
snows. As they vaporize, gas comes gushing out as in a geyser, blowing off
less-volatile surface snows and dark organics. Prevailing low-speed winds carry
away the dark organics, which slowly sediment out of the thin air, are
deposited on the ground, and generate the appearance of the streaks. This, at
least, is one reconstruction of recent Tritonian history.
Triton may have large, seasonal polar
caps of smooth nitrogen ice underlying layers of dark organic materials.
Nitrogen snows seem recently to have fallen at the equator. Snowfalls, geysers,
windblown organic dust, and high-altitude hazes were entirely unexpected on a
world with so thin an atmosphere.
Why is the air so thin? Because Triton
is so far from the Sun. Were you somehow to pick this world up and move it into
orbit around Saturn, the nitrogen and methane ices would quickly evaporate, a
much denser atmosphere of gaseous nitrogen and methane would form, and
radiation would generate an opaque tholin haze. It would become a world very
like Titan. Conversely, if you moved Titan into orbit about Neptune, almost all
its atmosphere would freeze out as snows and ices, the tholin would fall out
and not be replaced, the air would clear, and the surface would become visible
in ordinary light. It would become a world very like Triton.
These two worlds are not identical. The
interior of Titan seems to contain much more ice than that of Triton, and much
less rock. Titan's diameter is almost twice that of Triton. Still, if placed at
the same distance from the Sun they would look like sisters. Alan Stern of the
Southwest Research Institute suggests that they are two members of a vast
collection of small worlds rich in nitrogen and methane that formed in the
early Solar System. Pluto, yet to be visited by a spacecraft, appears to be
another member of this group. Many more may await discovery beyond Pluto. The
thin atmospheres and icy surfaces of all these worlds are being irradiated-by
cosmic rays, if nothing else and nitrogen-rich organic compounds are being
formed. It looks as if the stuff of life is sitting not just on Titan, but
throughout the cold, dimly lit outer reaches of our planetary system.
Another class of small objects has
recently been discovered, whose orbits take them-at least part of the
time-beyond Neptune and Pluto. Sometimes called minor planets or asteroids,
they are more likely to be inactive comets (with no tails, of course; so far
from the Sun, their ices cannot readily vaporize). But they are much bigger
than the run-of-the-mill comets we know. They may be the vanguard of a vast
array of small worlds that extends from the orbit of Pluto halfway to the
nearest star. The innermost province of the Oort Comet Cloud, of which these
new objects may be members, is called the Kuiper Belt, after my mentor Gerard
Kuiper, who first suggested that it should exist. Short-period comets-like
Halley's-arise in the Kuiper Belt, respond to gravitational tugs, sweep into
the inner part of the Solar System, grow their tails, and grace our skies.
Back in the late nineteenth century,
these building blocks of worlds-then mere hypotheses-were called “planetesimals.”
The flavor of the word is, I suppose, something like that of “infinitesimals”:
You need an infinite number of them to make anything. It's not quite that
extreme with planetesimals, although a very large number of them would be
required to make a planet. For example, trillions of bodies each a kilometer in
size would be needed to coalesce to make a planet with the mass of the Earth.
Once there were much larger numbers of worldlets in the planetary part of the
Solar System. Most of them are now gone-ejected into interstellar space, fallen
into the Sun, or sacrificed in the great enterprise of building moons and
planets. But out beyond Neptune and Pluto the discards, the leftovers that were
never aggregated into worlds, may be waiting-a few largish ones in the
100-kilometer range, and stupefying numbers of kilometer-sized and smaller
bodies peppering the outer Solar System all the way out to the Oort Cloud.
In this sense there are planets beyond
Neptune and Pluto-but they are not nearly as big as the Jovian planets, or even
Pluto. Larger worlds may, for all we know, also be hiding in the dark beyond
Pluto, worlds that can properly be called planets. The farther away they are,
the less likely it is that we would have detected them. They cannot lie just
beyond Neptune, though; their gravitational tugs would have perceptibly altered
the orbits of Neptune and Pluto, and the Pioneer 10 and 11 and Voyager 1 and 2
spacecraft.
The newly discovered cometary bodies
(with names like 1992QB and 1993FW) are not planets in this sense. If our
detection threshold has just encompassed them, many more of them probably
remain to be discovered in the outer Solar System-so far away that they're hard
to see from Earth, so distant that it's a long journey to get to them. But
small, quick ships to Pluto and beyond are within our ability. It would make
good sense to dispatch one by Pluto and its moon Charon, and then, if we can,
to make a close pass by one of the denizens of the Kuiper Comet Belt.
The rocky Earthlike cores of Uranus and
Neptune seem to have accreted first, and then gravitationally attracted massive
amounts of hydrogen and helium gas from the ancient nebula out of which the
planets formed. Originally, they lived in a hailstorm. Their gravities were
just sufficient to eject icy worldlets, when they came too close, far out
beyond the realm of the planets, to populate the Oort Comet Cloud. Jupiter and
Saturn became gas giants by the same process. But their gravities were too
strong to populate the Oort Cloud: Ice worlds that came close to them were
gravitationally pitched out of the Solar System entirely-destined to wander
forever in the great dark between the stars.
So the lovely comets that on occasion
rouse us humans to wonder and to awe, that crater the surfaces of inner planets
and outer moons, and that now and then endanger life on Earth would be unknown
and unthreatening had Uranus and Neptune not grown to be giant worlds four and
a half billion years ago.
THIS IS THE PLACE for a brief interlude
on planets far beyond Neptune and Pluto, planets of other stars.
Many nearby stars are surrounded by thin
disks of orbiting gas and dust, often extending to hundreds of astronomical
units (AU) from the local star (the outermost planets, Neptune and Pluto, are
about 40 AU from our Sun). Younger Sun-like stars are much more likely to have
disks than older ones. In some cases, there's a hole in the center of the disk
as in a phonograph record. The hole extends out from the star to perhaps 30 or
40 AU. This is true, for example, for the disks surrounding the stars Vega and
Epsilon Eridam. The hole in the disk surrounding Beta Pictoris extends to only
15 AU from the star. There is a real possibility that these inner, dust-free
zones have been cleaned up by planets that recently formed there. Indeed, this
sweeping-out process is predicted for the early history of our planetary
system. As observations improve, perhaps we will see telltale details in the
configuration of dust and dust-free zones that will indicate the presence of
planets too small and dark to be seen directly. Spectroscopic data suggest that
these disks are churning and that matter is falling in on the central
stars-perhaps from comets formed in the disk, deflected by the unseen planets,
and evaporating as they approach too close to the local sun.
Because planets are small and shine by
reflected light, they tend to be washed out in the glare of the local sun.
Nevertheless, many efforts are now under way to find fully formed planets
around nearby stars-by detecting a faint brief dimming of starlight as a dark
planet interposes itself between the star and the observer on Earth; or by
sensing a faint wobble in the motion of the star as it's tugged first one way
and then another by an otherwise invisible orbiting companion. Spaceborne
techniques will be much more sensitive. A Jovian planet going around a nearby
star is about a billion times fainter than its sun; nevertheless, a new
generation of ground-based telescopes that can compensate for the twinkling in
the Earth's atmosphere may soon be able to detect such planets in only a few
hours' observing time. A terrestrial planet of a neighboring star is a hundred
times fainter still; but it now seems that comparatively inexpensive
spacecraft, above the Earth's atmosphere, might be able to detect other Earths.
None of these searches has succeeded yet, but we are clearly on the verge of
being able to detect at least Jupiter-sized planets around the nearest stars-if
there are any to be found.
A most important and serendipitous
recent discovery is of a bona fide planetary system around an unlikely star,
some 1,300 light-years away, found by a most unexpected technique: The pulsar
designated B1257+12 is a rapidly rotating neutron star, an unbelievably dense
sun, the remnant of a massive star that suffered a supernova explosion. It
spins, at a rate measured to impressive precision, once every 0.
0062185319388187 seconds. This pulsar is pushing 10,000 rpm.
Charged particles trapped in its intense
magnetic field generate radio waves that are cast across the Earth, about 160
flickers a second. Small but discernible changes in the flash rate were
tentatively interpreted by Alexander Wolszczan, now at Pennsylvania State
University, in 1991-as a tiny reflex motion of the pulsar in response to the
presence of planets. In 1994 the predicted mutual gravitational interactions of
these planets were confirmed by Wolszczan from a study of timing residuals at
the microsecond level over the intervening years. The evidence that these are
truly new planets and not starquakes on the neutron star surface (or something)
is now overwhelming-or, as Wolszczan put it, “irrefutable”; a new solar system
is “unambiguously identified.” Unlike all the other techniques, the pulsar
timing method makes close-in terrestrial planets comparatively easy and more
distant Jovian planets comparatively difficult to detect.
Planet C, some 2. 8 times more massive
than the Earth, orbits the pulsar every 98 days at a distance of 0. 47
astronomical units (AU); Planet B, with about 3. 4 Earth masses, has a
67-Earth-day year at 0. 36 AU. A smaller world, Planet A, still closer to the
star, with about 0. 015 Earth masses, is at 0. 19 AU. Crudely speaking, Planet
B is roughly at the distance of Mercury from our Sun; Planet C is midway
between the distances of Mercury and Venus; and interior to both of them is
Planet A, roughly the mass of the Moon at about half Mercury's distance from
our Sun. Whether these planets are the remnants of an earlier planetary system
that somehow survived the supernova explosion that produced the pulsar, or
whether they formed from the resulting circumstellar accretion disk subsequent
to the supernova explosion, we do not know. But in either case, we have now
learned that there are other Earths.
The energy put out by B1257+12 is about
4. 7 times that of gun. But, unlike the Sun, most of this is not in visible
light, but in a fierce hurricane of electrically charged particles. Suppose
that these particles impinge on the planets and heat them. Then, even a planet
at 1 AU would have a surface around 280 Celsius degrees above the normal
boiling point of water, greater than the temperature of Venus.
These dark and broiling planets do not
seem hospitable for life. But there may be others, farther from B1257+12, that
are. (Hints of at least one cooler, outer world in the B1257+12 system exist.)
Of course, we don't even know that such worlds would retain their atmospheres;
perhaps any atmospheres were stripped away in the supernova explosion, if they
date back that far. But we do seem to be detecting a recognizable planetary
system. Many more are likely to become known in coming decades, around ordinary
Sun-like stars as well as white dwarfs, pulsars, and other end states of
stellar evolution.
Eventually, we will have a list of
planetary systems-each perhaps with terrestrials and Jovians and maybe new
classes of planets. We will examine these worlds, spectroscopically and in
other ways. We will be searching for new Earths and other life.
ON NONE OF THE WORLDS In the outer Solar
System did Voyager find signs of life, much less intelligence. There was
organic Matter galore-the stuff of life, the premonitions of life, perhaps but
as far as we could see, no life. There was no oxygen in their atmospheres, and
no gases profoundly out of chemical equilibrium, as methane is in the Earth's
oxygen. Many of the worlds were painted with subtle colors, but none with such
distinctive, sharp absorption features as chlorophyll provides over much of the
Earth's surface. On very few worlds was Voyager able to resolve details as
small as a kilometer across. By this standard, it would not have detected even
our own technical civilization had it been transplanted to the outer Solar
System. But for what it's worth, we found no regular patterning, no
geometrization, no passion for small circles, triangles, squares, or
rectangles. There were no constellations of steady points of light on the night
hemispheres. There were no signs of a technical civilization reworking the
surface of any of these worlds.
The Jovian planets are prolific
broadcasters of radio waves-generated in part by the abundant trapped and
beamed charged particles in their magnetic fields, in part by lightning, and in
part by their hot interiors. But none of this emission has the character of
intelligent life-or so it seems to the experts in the field.
Of course our thinking may be too
narrow. We may be missing something. For example, there is a little carbon
dioxide in the atmosphere of Titan, which puts its nitrogen/methane atmosphere
out of chemical equilibrium. I think the CO2 is provided by the steady
pitter-patter of comets falling into Titan's atmosphere-but maybe not. Maybe
there's something on the surface unaccountably generating CO2 in the face of
all that methane.
The surfaces of Miranda and Triton are
unlike anything else we know. There are vast chevron-shaped landforms and
crisscrossing straight lines that even sober planetary geologists once
mischievously described as “highways.” We think we (barely) understand these landforms
in terms of faults and collisions, but of course we might be wrong.
The surface stains of organic
matter-sometimes, as on Triton, delicately hued-are attributed to charged
particles producing chemical reactions in simple hydrocarbon ices, generating
more complex organic materials, and all this having nothing to do with the
intermediation of life. But of course we might be wrong.
The complex pattern of radio static,
bursts, and whistles that we receive from all four Jovian planets seems, in a
general way, explicable by plasma physics and thermal emission. (Much of the
detail is not yet well understood.) But of course we might be wrong.
We have found nothing on dozens of
worlds so clear and striking as the signs of life found by the Galileo
spacecraft in its passages by the Earth. Life is a hypothesis of last resort.
You invoke it only when there's no other way to explain what you see. If I had
to judge, I would say that there's no life on any of the worlds we've studied,
except of course our own. But I might be wrong, and, right or wrong, my
judgment is necessarily confined to this Solar System. Perhaps on some new
mission we'll find something different, something striking, something wholly
inexplicable with the ordinary tools of planetary science-and tremulously,
cautiously, we will inch toward a biological explanation. However, for now
nothing requires that we go down such a path. So far, the only life in the
Solar System is that which comes from Earth. In the Uranus and Neptune systems,
the only sign of life has been Voyager itself.
As we identify the planets of other
stars, as we find other worlds of roughly the size and mass of the Earth, we
will scrutinize them for life. A dense oxygen atmosphere may be detectable even
on a world we've never imaged. As for the Earth, that may by itself be a sign
of life. An oxygen atmosphere with appreciable quantities of methane would
almost certainly be a sign of life, as would modulated radio emission. Someday,
from observations of our planetary system or another, the news of life
elsewhere may be announced over the morning coffee.
THE VOYAGER SPACECRAFT are bound for the
stars. They are on escape trajectories from the Solar System, barreling along
at almost a million miles a day. The gravitational fields of Jupiter, Saturn,
Uranus, and Neptune have flung them at such high speeds that they have broken
the bonds that once tied them to the Sun.
Have they left the Solar System yet? The
answer depends very much on how you define the boundary of the Sun's realm. If
it's the orbit of the outermost good-sized planet, then the Voyager spacecraft
are already long gone; there are probably no undiscovered Neptunes. If you mean
the outermost planet, it may be that there are other-perhaps
Triton-like-planets far beyond Neptune and Pluto; if so, Voyager 1 and Voyager
2 are still within the Solar System. If you define the outer limits of the
Solar System as the heliopause-where the interplanetary particles and magnetic
fields are replaced by their interstellar counterparts-then neither Voyager has
yet left the Solar System, although they may do so in the next few decades. †††
But if your definition of the edge of the Solar System is the distance at which
our star can no longer hold worlds in orbit about it, then the Voyagers will not
leave the Solar System for hundreds of centuries.
Weakly grasped by the Sun's gravity, in
every direction in the sky, is that immense horde of a trillion comets or more,
the port Cloud. The two spacecraft will finish their passage through the Oort
cloud in another 20,000 years or so. Then, at last, completing their long
good-bye to the Solar System, broken free of the gravitational shackles that
once bound them to the Sun, the Voyagers will make for the open sea of
interstellar space. only then will Phase Two of their mission begin.
Their radio transmitters long dead, the
spacecraft will wander for ages in the calm, cold interstellar blackness-where
there is almost nothing to erode them. Once out of the Solar System, they will
remain intact for a billion years or more, as they circumnavigate the center of
the Milky Way galaxy.
We do not know whether there are other
space-faring civilizations in the Milky Way. If they do exist, we do not know
how abundant they are, much less where they are. But there is at least a chance
that sometime in the remote future one of the Voyagers will be intercepted and
examined by an alien craft.
Accordingly, as each Voyager left Earth
for the planets and the stars, it carried with it a golden phonograph record
encased in a golden, mirrored jacket containing, among other things; greetings
in 59 human languages and one whale language; a 12-minute sound essay including
a kiss, a baby's cry, and an EEG record of the meditations of a young woman in
love; 116 encoded pictures, on our science, our civilization, and ourselves;
and 90 minutes of the Earth's greatest hits-Eastern and Western, classical and
folk, including a Navajo night chant, a Japanese shakuhachi piece, a Pygmy
girl's initiation song, a Peruvian wedding song, a 3,000-year-old composition
for the ch'in called “Flowing Streams,” Bach, Beethoven, Mozart, Stravinsky,
Louis Armstrong, Blind Willie Johnson, and Chuck Berry's “Johnny B. Goode.”
Space is nearly empty. There is
virtually no chance that one of the Voyagers will ever enter another solar
system-and this is true even if every star in the sky is accompanied by
planets. The instructions on the record jackets, written in what we believe to
be readily comprehensible scientific hieroglyphics, can be read, and the
contents of the records understood, only if alien beings, somewhere in the
remote future, find Voyager in the depths of interstellar space. Since both
Voyagers will circle the center of the Milky Way Galaxy essentially forever,
there is plenty of time for the records to be found-if there's anyone out there
to do the finding.
We cannot know how much of the records
they would understand. Surely the greetings will be incomprehensible, but their
intent may not be. (We thought it would be impolite not to say hello.) The hypothetical
aliens are bound to be very different from us-independently evolved on another
world. Are we really sure they could understand anything at all of our message?
Every time I feel these concerns stirring, though, I reassure myself. Whatever
the incomprehensibilities of the Voyager record, any alien ship that finds it
will have another standard by which to judge us. Each Voyager is itself a
message. In their exploratory intent, in the lofty ambition of their
objectives, in their utter lack of intent to do harm, and in the brilliance of
their design and performance, these robots speak eloquently for us.
But being much more advanced scientists
and engineers than we-otherwise they would never be able to find and retrieve
the small, silent spacecraft in interstellar space-perhaps the aliens would
have no difficulty understanding what is encoded on these golden records.
Perhaps they would recognize the tentativeness of our society, the mismatch
between our technology and our wisdom. Have we destroyed ourselves since
launching Voyager, they might wonder, or have we gone on to greater things?
Or perhaps the records will never be
intercepted. Perhaps no one in five billion years will ever come upon them.
Five billion years is a long time. In five billion years, all humans will have
become extinct or evolved into other beings, none of our artifacts will have
survived on Earth, the continents will have become unrecognizably altered or
destroyed, and the evolution of the Sun will have burned the Earth to a crisp or
reduced it to a whirl of atoms.
Far from home, untouched by these remote
events, the Voyagers, bearing the memories of a world that is no more, will fly
on.
CHAPTER 10
Sacred black
Deep sky is, of all visual impressions,
the nearest akin to a feeling.
—SAMUEL TAYLOR COLERIDGE, NOTEBOOKS
(1805)
The blue of a cloudless May morning, or
the reds and oranges of a sunset at sea, have roused humans to wonder, to
poetry, and to science. no matter where on Earth we live, no matter what our
language, customs, or politics, we share a sky in common. Most of us expect
that azure blue and would, for good reason, be stunned to wake up one sunrise
to find a cloudless sky that was black or yellow or green. (Inhabitants of Los
Angeles and Mexico City have grown accustomed to brown skies, and those of
London and Seattle to gray ones-but even they still consider blue the planetary
norm.)
And yet there are worlds with black or
yellow skies, and maybe even green. The color of the sky characterizes the
world. Plop me down on any planet in the Solar System; without sensing the
gravity, without glimpsing the ground, let me take a quick look at the Sun and
sky, and I can, I think, pretty well tell you where I am. That familiar shade
of blue, interrupted here and there by fleecy white clouds, is a signature of
our world. The French have an expression, sacre-bleu!, which translates roughly
as “Good heavens!" Literally, it means “sacred blue!” Indeed. If there
ever is a true flag of Earth, this should be its color.
Birds fly through it, clouds are
suspended in it, humans admire and routinely traverse it, light from the Sun
and stars flutters through it. But what is it? What is it made of? Where does
it end? How much of it is there? Where does all that blue come from? If it's a
commonplace for all humans, if it typifies our world, surely we should know
something about it. What is the sky?
In August 1957, for the first time, a
human being rose above the blue and looked around-when David Simons, a retired
Air Force officer and a physician, became the highest human in history. Alone,
he piloted a balloon to an altitude of over 100,000 feet (30 kilometers) and
through his thick windows glimpsed a different sky. Now a professor at the
University of California Medical School in Irvine, Dr. Simons recalls it was a
dark, deep purple overhead. He had reached the transition region where the blue
of ground level is being overtaken by the perfect black of space.
Since Simons' almost forgotten flight,
people of many nations have flown above the atmosphere. It is now clear from
repeated and direct human (and robotic) experience that in space the daytime
sky is black. The Sun shines brightly on your ship. The Earth below you is
brilliantly illuminated. But the sky above is black as night.
Here is the memorable description by
Yuri Gagarin of what he saw on the first spaceflight of the human species,
aboard Vostok 1, on April 12, 1961:
The sky is completely black; and against
the background of this black sky the stars appear somewhat brighter and more
distinct. The Earth has a very characteristic, very beautiful blue halo, which
is seen well when you observe the horizon. There is a smooth color transition
from tender blue, to blue, to dark blue and purple, and then to the completely
black color of the sky. It is a very beautiful transition.
Clearly, the daylit sky-all that blue-
is somehow connected with the air. But as you look across the breakfast table,
your companion is not (usually) blue; the color of the sky must be a property
not of a little air, but of a great deal. If you look closely at the Earth from
space, you see it surrounded by a thin band of blue, as thick as the lower
atmosphere; indeed, it is the lower atmosphere. At the top of that band you can
make out the blue sky fading into the blackness of space. This is the
transition zone that Simons was the first to enter and Gagarin the first to
observe from above. In routine spaceflight, you start at the bottom of the
blue, penetrate entirely through it a few minutes after liftoff; and then enter
that boundless realm where a simple breath of air is impossible without
elaborate life-support systems. Human life depends for its very existence on
that blue sky. We are right to consider it tender and sacred.
We see the blue in daytime because
sunlight is bouncing off the air around and above us. On a cloudless night, the
sky is black because there is no sufficiently intense source of light to be
reflected off the air. Somehow, the air preferentially bounces blue light down
to us. How?
The visible light from the Sun comes in
many colors-violet, blue, green, yellow, orange, red-corresponding to light of
different wavelengths. (A wavelength is the distance front crest to crest as
the wave travels through air or space.) Violet and blue light waves have the
shortest wavelengths; orange and red the longest. What we perceive as color is
how our eyes and brains read the wavelengths of light. (We Might just as
reasonably translate wavelengths of light into, say, heard tones rather than
seen colors-but that's not how our senses evolved.)
When all those rainbow colors of the
spectrum are mixed together, as in sunlight, they seem almost white. These
waves travel together in eight minutes across the intervening 93 million miles
(150 million kilometers) of space from the Sun to the Earth. They strike the
atmosphere, which is made mostly of nitrogen and oxygen molecules. Some waves
are reflected by the air back into space. Some are bounced around before the
light reaches the ground and they can be detected by a passing eyeball. (Also,
some bounce off clouds or the ground back into space.) This bouncing around of
light waves in the atmosphere is called “scattering.”
But not all waves are equally well
scattered by the molecules of air. Wavelengths that are much longer than the size
of the molecules are scattered less; they spill over the molecules, hardly
influenced by their presence. Wavelengths that are closer to the size of the
molecules are scattered snore. And waves have trouble ignoring obstacles as big
as they are. (You can see this in water waves scattered by the pilings of
piers, or bathtub waves from a dripping faucet encountering a rubber duck.) The
shorter wavelengths, those that we sense as violet and blue light, are more
efficiently scattered than the longer wavelengths, those that we sense as
orange and red light. When we look up on a cloudless day and admire the blue
sky, we are witnessing the preferential scattering of the short waves in
sunlight. This is called Rayleigh scattering, after the English physicist who offered
the first coherent explanation for it. Cigarette smoke is blue for just the
same reason: The particles that make it up are about as small as the wavelength
of blue light.
So why is the sunset red? The red of the
sunset is what's left of sunlight after the air scatters the blue away. Since
the atmosphere is a thin shell of gravitationally bound gas surrounding the
solid Earth, sunlight must pass through a longer slant path of air at sunset
(or sunrise) than at noon. Since the violet and blue waves are scattered even
more during their now-longer path through the air than when the Sun is
overhead, what we see when we look toward the Sun is the residue-the waves of
sunlight that are hardly scattered away at all, especially the oranges and
reds. A blue sky makes a red sunset. (The noontime Sun seems yellowish partly
because it emits slightly more yellow light than other colors, and partly
because, even with the Sun overhead, some blue light is scattered out of the
sunbeams by the Earth's atmosphere.)
It is sometimes said that scientists are
unromantic, that their passion to figure out robs the world of beauty and
mystery. But is it not stirring to understand how the world actually works-that
white light is made of colors, that color is the way we perceive the
wavelengths of light, that transparent air reflects light, that in so doing it
discriminates among the waves, and that the sky is blue for the same reason
that the sunset is red? It does no harm to the romance of the sunset to know a
little bit about it.
Since most simple molecules are about
the same size (roughly a hundred millionth of a centimeter), the blue of the
Earth's sky doesn't much depend on what the air is made of-as long as the air
doesn't absorb the light. Oxygen and nitrogen molecules don't absorb visible
light; they only bounce it away in some other direction. Other molecules,
though, can gobble up the light. Oxides of nitrogen-produced in automotive
engines and in the fires of industry-are a source of the murky brown coloration
of smog. Oxides of nitrogen (made from oxygen and nitrogen) do absorb light.
Absorption, as well as scattering, can color a sky.
OTHER WORLDS, OTHER SKIES: Mercury, the
Earth's Moon, and most satellites of the other planets are small worlds;
because of their feeble gravities, they are unable to retain their
atmospheres-which instead trickle of into space. The near-vacuum of space then
reaches the ground. Sunlight strikes their surfaces unimpeded, neither
scattered nor absorbed along the way. The skies of these worlds are black, even
at noon. This has been witnessed firsthand so far by only 12 humans, the lunar
landing crews of Apollos 11, 12, and 14-17.
A full list of the satellites in the
Solar System, known as of this writing, is given in the accompanying table.
(Nearly half of them were discovered by Voyager.) All have black skies-except
Titan of Saturn and perhaps Triton of Neptune, which are big enough to have
atmospheres. And all asteroids as well.
Venus has about 90 times more air than
Earth. It isn't mainly oxygen and nitrogen as here-it's carbon dioxide. But
carbon dioxide doesn't absorb visible light either. What would the sky look
like from the surface of Venus if Venus had no clouds. With so much atmosphere
in the way, not only are violet and blue waves scattered, but all the other
colors as well-green yellow, orange, red. The air is so thick, though, that
hardly any blue light makes it to the ground; it's scattered back to space by
successive bounces higher up. Thus, the light that does reach the ground should
be strongly reddened-like an Earth sunset all over the sky. Further, sulfur in
the high clouds will stain the sky yellow. Pictures taken by the Soviet Venera
landers confirm that the skies of Venus are a kind of yellow-orange.
SIXTY-TWO WORLDS FOR THE THIRD
MILLENNIUM: KNOWN MOONS OF THE PLANETS (AND ONE ASTEROID)—LISTED IN ORDER OF
DISTANCE FROM THEIR PLANET
EARTH, 1
MARS, 2
IDA, 1
JUPITER, 16
SATURN, 18
URANUS, 15
NEPTUNE, 8
PLUT0, 1
Moon
Phobos
Dactyl
Metis
Pan
Cordelia
Naiad
Charon
Deimos
Adrastea
Atlas
Ophelia
Thalassa
Amalthea
Prometheus
Bianca
Despina
Thebe
Pandora
Cressida
Galatea
Epimetheus
Desdemona
Larissa
Europa
Janus
Juliet
Proteus
Ganymede
Mimas
Portia
Triton
Callisto
Enceladus
Rosalind
Nereid
Leda
Tethys
Belinda
Himalia
Telesto
Puck
Lysithea
Calypso
Miranda
Elara
Diane
Ariel
Ananke
Helene
Umbriel
Carme
Rhea
Titania
Pasiphae
Titan
Oberon
Sinope
Hyperion
Iapetus
Phoebe
Mars is a different story. It is a
smaller world than Earth, with a much thinner atmosphere. The pressure at the
surface of Mars is, in fact, about the same as the altitude in the Earth's
stratosphere to which Simons rose. So we might expect the Martian sky to be
black or purple-black. The first color picture from the surface of Mars was
obtained in July 1976 by the American Viking 1 lander-the first spacecraft to
touch down successfully on the surface of the Red Planet. The digital data were
dutifully radioed from Mars back to Earth, and the color picture assembled by
computer. To the surprise of all the scientists and nobody else, that first
image, released to the press, showed the Martian sky to be a comfortable, homey
blue-impossible for a planet with so insubstantial an atmosphere. Something had
gone wrong.
The picture on your color television set
is a mixture of three monochrome images, each in a different color of
light-red, green, and blue. You can see this method of color compositing in
video projection systems, which project separate beams of red, green, and blue
light to generate a full-color picture (including yellows). To get the right
color, your set needs to mix or balance these three monochrome images
correctly. If you turn up the intensity of, say, blue, the picture will appear
too blue. Any picture returned from space requires a similar color balance.
Considerable discretion is sometimes left to the computer analysts in deciding
this balance. The hiking analysts were not planetary astronomers, and with this
first color picture from Mars they simply mixed the colors until it looked
“right.” We are so conditioned by our experience on Earth that “right,” of
course, means a blue sky. The color of the picture was soon corrected-using
color calibration standards placed for this very purpose on board the
spacecraft-and the resulting composite showed no blue sky at all; rather it was
something between ochre and pink. Not blue, but hardly purple-black either.
This is the right color of the Martian
sky. Much of the surface of Mars is desert-and red because the sands are rusk.
There are occasional violent sandstorms that lift fine particles from the
surface high into the atmosphere. It takes a long time for them to fall out,
and before the sky has fully cleaned itself, there's always another sandstorm.
Global or near-global sandstorms occur almost every Martian year. Since rusty
particles are always suspended in this sky, future generations of humans, born
and living out their lives on Mars, will consider that salmon color to be as
natural and familiar as we consider our homey blue. From a single glance at the
daytime sky, they'll probably be able to tell how long it's been since the last
big sandstorm.
The planets in the outer Solar System-Jupiter,
Saturn, Uranus, and Neptune- are of a different sort. These are huge worlds
with giant atmospheres made mainly of hydrogen and helium. Their solid surfaces
are so deep inside that no sunlight penetrates there at all. Down there, the
sky is black, with no prospect of a sunrise-not ever. The perpetual starless
night is perhaps illuminated on occasion by a bolt of lightning. But higher in
the atmosphere, where the sunlight reaches, a much more beautiful vista awaits.
On Jupiter, above a high-altitude haze
layer composed of ammonia (rather than water) ice particles, the sky is almost
black. Farther down, in the blue sky region, are multicolored clouds-in various
shades of yellow-brown, and of unknown composition. (The candidate materials
include sulfur, phosphorus, and complex organic molecules.) Even farther down,
the sky will appear red-brown, except that the clouds there are of varying thicknesses,
and where they are thin, you might see a patch of blue. Still deeper, we
gradually return to perpetual night. Something similar is true on Saturn, but
the colors there are more muted.
Uranus and especially Neptune have an
uncanny, austere blue color through which clouds-some of them a little
whiter-are carried by high-speed winds. Sunlight reaches a comparatively clean
atmosphere composed mainly of hydrogen and helium but also rich in methane.
Long paths of methane absorb yellow and especially red light and let the green
and blue filter through. A thin hydrocarbon haze removes a little blue. There
may be a depth where the sky is greenish.
Conventional wisdom holds that the
absorption by methane and the Rayleigh scattering of sunlight by the deep atmosphere
together account for the blue colors on Uranus and Neptune. But analysis of
Voyager data by Kevin Baines of JPL seems to show that these causes are
insufficient. Apparently very deep-maybe in the vicinity of hypothesized clouds
of hydrogen sulfide-there is an abundant blue substance. So far no one has been
able to figure out what it might be. Blue materials are very rare in Nature. As
always happens in science, the old mysteries are dispelled only to be replaced
by new ones. Sooner or later we'll find out the answer to this one, too.
ALL WORLDS WITH NONBLACK SKIES have
atmospheres. If you're standing on the surface and there's an atmosphere thick
enough to see, there's probably a way to fly through it. We're now sending our
instruments to fly in the variously colored skies of other worlds. Someday we
will go ourselves.
Parachutes have already been used in the
atmospheres of Venus and Mars, and are planned for Jupiter and Titan. In 1985
two French-Soviet balloons sailed through the yellow skies of Venus. The Vega 9
balloon, about 4 meters across, dangled an instrument package 13 meters below.
The balloon inflated in the night hemisphere, floated about 54 kilometers above
the surface, and transmitted data for almost two Earth days before its batteries
failed. In that time it traveled 11,600 kilometers (nearly 7,000 miles) over
the surface of Venus, far below. The Vega 2 balloon had an almost identical
profile. The atmosphere of Venus has also been used for aerobraking-changing
the Magellan spacecraft's orbit by friction with the dense air; this is a key
future technology for converting flyby spacecraft to Mars into orbiters and
landers.
A Mars mission, scheduled to be launched
in 1998, and led by Russia, includes an enormous French hot air balloon-looking
something like a vast jellyfish, a Portuguese man-of-war. It's designed to sink
to the Martian surface every chilly twilight and rise high when heated by
sunlight the next day. The winds are so fast that, if all goes well, it will be
carried hundreds of kilometers each day, hopping and skipping over the north
pole. In the early morning, when close to the ground, it will obtain very high
resolution pictures and other data. The balloon has an instrumental guide-rope,
essential for its stability, conceived and designed by a private membership
organization based in Pasadena, California, The Planetary Society.
Since the surface pressure on Mars is
approximately that at an altitude of 100,000 feet on Earth, we know we can fly
airplanes there. The U-2, for example, or the SR-71 Blackbird routinely
approaches such low pressures. Aircraft with even larger wingspans have been
designed for Mars.
The dream of flight and the dream of
space travel are twins, conceived by similar visionaries, dependent on allied
technologies, and evolving more or less in tandem. As certain practical and
economic limits to flight on Earth are reached, the possibility arises of
flying through the multihued skies of other worlds.
IT IS NOW ALMOST POSSIBLE to assign
color combinations, based on the colors of clouds and sky, to every planet in
the Solar System-from the sulfur-stained skies of Venus and the rusty skies of
Mars to the aquamarine of Uranus and the hypnotic and unearthly blue of
Neptune. Sacre-jaunt, sacre-rouge, sacre-vert. Perhaps they will one day adorn
the flags of distant human outposts in the Solar System, in that time when the
new frontiers are sweeping out from the Sun to the stars, and the explorers are
surrounded by the endless black of space. Sacre-noir.
CHAPTER 11
Evening and morning star
This is another world
Which is not of men.
—LI BAI, “QUESTION AND ANSWER IN THE
MOUNTAINS”
(CHINA, TANG DYNASTY, CA. 730)
You can see it shining brilliantly in
the twilight, chasing the Sun down below the western horizon. Upon first
glimpsing it each night, people were accustomed to make a wish (“upon a star”).
Sometimes the wish came true.
Or you can spy it in the east before
dawn, fleeing the rising Sun. In these two incarnations, brighter than anything
else in the sky except only the Sun and the Moon it was known as the evening
and the morning star. Our ancestors did not recognize it was a world, the same
world, never too far from the Sun because it is in an orbit about it interior
to the Earth's. Just before sunset or just after sunrise, we can sometimes see
it near some fluffy white cloud, and then discover by the comparison that Venus
has a color, a pale lemon-yellow.
You peer through the eyepiece of a
telescope-even a big telescope, even the largest optical telescope on Earth-and
you can make out no detail at all. Over the months, you see a featureless disk
methodically going through phases, like the Moon: crescent Venus, full Venus,
gibbous Venus, new Venus. There is not a hint of continents or oceans.
Some of the first astronomers to see
Venus through the telescope immediately recognized that they were examining a
world enshrouded by clouds. The clouds, we now know, are droplets of
concentrated sulfuric acid, stained yellow by a little elemental sulfur. They
lie high above the ground. In ordinary visible light there's no hint of what
this planet's surface, some 50 kilometers below the cloud tops, is like, and
for centuries the best we had were wild guesses.
You might conjecture that if we could
take a much finer look there might be breaks in the clouds, revealing day by
day, in bits and pieces, the mysterious surface ordinarily hidden from our
view. Then the time of guesses would be over. The Earth is on average half
cloud-covered. In the early days of Venus exploration, we saw no reason that
Venus should be 100 percent overcast. If instead it was only 90 percent, or
even 99 percent, cloud-covered, the transient patches of clearing might tell us
much.
In 1960 and 1961, Mariners 1 and 2, the
first American spacecraft designed to visit Venus, were being prepared. There
were those, like me, who thought the ships should carry video cameras so they
could radio pictures back to Earth. The same technology would be used a few
years later when Rangers 7, 8, and 9 would photograph the Moon on the way to
their crash landings-the last making a bull's-eye in the crater Alphonsus. But
time was short for the Venus mission, and cameras were heavy. There were those
who maintained that cameras weren't really scientific instruments, but rather
catch-as-catch-can, razzle-dazzle, pandering to the public, and unable to
answer a single straightforward, well-posed scientific question. I thought
myself that whether there are breaks in the clouds was one such question. I
argued that cameras could also answer questions that we were too dumb even to
pose. I argued that pictures were the only way to show the public-who were,
after all, footing the bill-the excitement of robotic missions. At any rate, no
camera was flown, and subsequent missions have, for this particular world, at
least partly vindicated that judgment: Even at high resolution from close
flybys, in visible light it turns out there are no breaks in the clouds of
Venus, any more than in the clouds of Titan. These worlds are permanently
overcast.
In the ultraviolet there is detail, but
due to transient patches of high-altitude overcast, far above the main cloud
deck. The high clouds race around the planet much faster than the planet itself
turns: super-rotation. We have an even smaller chance of seeing the surface in
the ultraviolet.
When it became clear that the atmosphere
of Venus was much thicker than the air on Earth-as we now know, the pressure at
the surface is ninety times what it is here-it immediately followed that in
ordinary visible light we could not possibly see the surface, even if there
were breaks in the clouds. What little sunlight is able to make its tortuous
way through the dense atmosphere to the surface would be reflected back, all
right; but the photons would be so jumbled by repeated scattering off molecules
in the lower air that no image of surface features could be retained. It would
be like a “whiteout” in polar snowstorm. However, this effect, intense Rayleigh
scattering, declines rapidly with increasing wavelength; in the near-infrared,
it was easy to calculate, you could see the surface if there were breaks in the
clouds or if the clouds were transparent there.
So in 1970 Jim Pollack, Dave Morrison,
and I went to the McDonald Observatory of the University of Texas to try to
observe Venus in the near-infrared. We “hypersensitized” our emulsions; the
good old-fashioned glass photographic plates were treated with ammonia, and
sometimes heated or briefly illuminated, before being exposed at the telescope
to light from Venus. For a time the cellars of McDonald Observatory reeked of
ammonia. We took many pictures. None showed any detail. We concluded that
either we hadn't gone far enough into the infrared or the clouds of Venus were
opaque and unbroken in the near infrared.
More than 20 years later, the Galileo
spacecraft, making a close flyby of Venus, examined it with higher resolution
and sensitivity, and at wavelengths a little further into the infrared than we
were able to reach with our crude glass emulsions. Galileo photographed great
mountain ranges. We already knew of their existence, though; a much more
powerful technique had earlier been employed: radar. Radio waves effortlessly
penetrate the clouds and thick atmosphere of Venus, bounce off the surface, and
return to Earth, where they are gathered in and used to make a picture. The
first work had been done, chiefly, by. American ground-based radar at JPL's
Goldstone tracking station in the Mojave Desert and at the Arecibo Observatory
in Puerto Rico, operated by Cornell University.
Then the U. S. Pioneer 12, the Soviet
Venera 15 and ?6 and the U. S. Magellan missions inserted radar telescopes into
orbit around Venus and mapped the place pole to pole. Each spacecraft would
transmit a radar signal to the surface and then catch it as it bounced back.
From how reflective each patch of surface was and how long it took the signal
to return (shorter from mountains, longer from valleys), a detailed map of the
entire surface was slowly and painstakingly constructed.
The world so revealed turns out to be
uniquely sculpted by lava flows (and, to a much lesser degree, by wind), as
described in the next chapter. The clouds and atmosphere of Venus have now
become transparent to us, and another world has been visited by the doughty
robot explorers from Earth. Our experience with Venus is now being applied
elsewhere-especially to Titan, where once again impenetrable clouds hide an
enigmatic surface, and radar is beginning to give us hints of what might lie
below.
VENUS HAD LONG BEEN THOUGHT of as our
sister world. It is the nearest planet to the Earth. It has almost the same
mass, size, density, and gravitational pull as the Earth does. It's a little
closer to the Sun than the Earth, but its bright clouds reflect more sunlight
back to space than our clouds do. As a first guess you might very well imagine
that, under those unbroken clouds, Venus was rather like Earth. Early
scientific speculation included fetid swamps crawling with monster amphibians,
like the Earth in the Carboniferous Period; a world desert; a global petroleum
sea; and a seltzer ocean dotted here and there with limestone-encrusted
islands. While based on some scientific data, these “models” of Venus-the first
dating from the beginnings of the century, the second from the 1930s, and the
last two from the raid-1950s-were little more than scientific romances, hardly
constrained by the sparse data available.
Then, in 1956, a report was published in
The Astrophysical Journal by Cornell H. Mayer and his colleagues. They had
pointed a newly completed radio telescope, built in part for classified
research, on the roof of the Naval Research Laboratory in Washington, D. C., at
Venus and measured the flux of radio waves arriving at Earth. This was not
radar: no radio waves were bounced off Venus. This was listening to radio waves
that Venus on its own emits to space. Venus turned out to be much brighter than
the background of distant stars and galaxies. This in itself was not very
surprising. Every object warmer than absolute zero (-273°C) gives off radiation
throughout the electromagnetic spectrum, including the radio region. You, for
example, emit radio waves at an effective or “brightness” temperature of about
35°C, and if you were in surroundings colder than you are, a sensitive radio
telescope could detect the faint radio waves you are transmitting in all
directions. Each of us is a source of cold static.
What was surprising about Mayer's
discovery was that the brightness temperature of Venus is more than 300°C, far
higher than the surface temperature of the Earth or the measured infrared
temperature of the clouds of Venus. Some places on Venus seemed at least 200°
hotter than the normal boiling point of water. What could this mean?
Soon there was a deluge of explanations.
I argued that the high radio brightness temperature was a direct indication of
a hot surface, and that the high temperatures were due to a massive carbon
dioxide/water vapor greenhouse effect-in which some sunlight is transmitted
through the clouds and heats the Surface, but the surface experiences enormous
difficulty in radiating back to space because of the high infrared opacity of
carbon dioxide and water vapor. Carbon dioxide absorbs at a range of
wavelengths through the infrared, but there seemed to be “windows” between the CO2
absorption bands through which the surface could readily cool off to space.
Water vapor, though, absorbs at infrared frequencies that correspond in part to
the windows in the carbon dioxide opacity. The two gases together, it seemed to
me, could pretty well absorb almost all the infrared emission, even if there
was very little water vapor-something like two picket fences, the slats of one
being fortuitously positioned to cover the gaps of the other.
There was another very different
category of explanation, in which the high brightness temperature of Venus had
nothing to do with the ground. The surface could still be temperate, clement,
congenial. It was proposed that some region in the atmosphere of Venus or in
its surrounding magnetosphere emitted these radio waves to space. Electrical
discharges between water droplets in the Venus clouds were suggested. A glow
discharge in which ions and electrons recombined at twilight and dawn in the
upper atmosphere was offered. A very dense ionosphere had its advocates, in
which the mutual acceleration of unbound electrons (“free-free emission”) gave
off radio waves. (One proponent of this idea even suggested that the high
ionization required was due to an average of 10,000 times greater radioactivity
on Venus than on Earth-perhaps from a recent nuclear war there.) And, in the
light of the discovery of radiation from Jupiter's magnetosphere, it was
natural to suggest that the radio emission came from an immense cloud of
charged particles trapped by some hypothetical very intense Venusian magnetic
field.
In a series of papers I published in the
middle 1960s, many in collaboration with Jim Pollack, these conflicting models
of a high hot emitting region and a cold surface were subjected to a critical
analysis. By then we had two important new clues: the radio spectrum of Venus,
and the Mariner 2 evidence that the radio emission was more intense at the
center of the disk of Venus than toward its edge. By 1967 we were able to
exclude the alternative models with some confidence, and conclude that the
surface of Venus was at a scorching and un-Earthlike temperature, in excess of
400°C. But the argument was inferential, and there were many intermediate
steps. We longed for a more direct measurement.
In October 1967-commemorating the tenth
anniversary of Sputnik 1-the Soviet Venera 4 spacecraft dropped an entry
capsule into the clouds of Venus. It returned data from the hot lover
atmosphere, but did not survive to the surface. One day later, the United
States spacecraft Mariner 5 flew by Venus, its radio transmission to Earth
skimming the atmosphere at progressively greater depths. The rate of fading of
the signal gave information about atmospheric temperatures. Although there
seemed to be some discrepancies (later resolved) between the two sets of
spacecraft data, both clearly indicated that the surface of Venus is very hot.
Since then a progression of Soviet
Venera spacecraft and one cluster of American spacecraft from the Pioneer 12
mission have entered the deep atmosphere or landed on the surface and measured
directly-essentially by sticking out a thermometer-the surface and near-surface
temperatures. They turn out to be about 470°C, almost 900°F. When such factors
as calibration errors of terrestrial radio telescopes and surface emissivity
are taken into account, the old radio observations and the new direct
spacecraft measurements turn out to be in good accord.
Early Soviet landers were designed for
an atmosphere somewhat like our own. They were crushed by the high pressures
like a tin can in the grasp of a champion arm wrestler, or a World War II
submarine in the Tonga Trench. Thereafter, Soviet Venus entry vehicles were
heavily reinforced, like modern submarines, and successfully landed on the
searing surface. When it became clear how deep the atmosphere is and how thick
the clouds, Soviet designers became concerned that the surface might be
pitch-black. Veneras 9 and 10 were equipped with floodlights. They proved
unnecessary. A few percent of the sunlight that falls on the top of the clouds
makes it through to the surface, and Venus is about as bright as on a cloudy
day on Earth.
The resistance to the idea of a hot
surface on Venus can, I suppose, be attributed to our reluctance to abandon the
notion that the nearest planet is hospitable for life, for future exploration,
and perhaps even, in the longer term, for human settlement. As it turns out
there are no Carboniferous swamps no global oil or seltzer oceans. Instead,
Venus is a stifling, brooding inferno. There are some deserts, but it's mainly
a world of frozen lava seas. Our hopes are unfulfilled. The call of this world
is now more muted than in the early days of spacecraft exploration, when almost
anything was possible and our most romantic notions about Venus might, for all
we then knew, be realized.
MANY SPACECRAFT CONTRIBUTED to our
present understanding of Venus. But the pioneering mission was Mariner 2.
Mariner 1 failed at launch and-as they say of a racehorse with a broken leg-had
to be destroyed. Mariner 2 worked beautifully and provided the key early radio
data on the climate of Venus. It made infrared observations of the properties
of the clouds. On its way from Earth to Venus, it discovered and measured the
solar wind-the stream of charged particles that flows outward from the Sun,
filling the magnetospheres of any planets in its way, blowing back the tails of
comets, and establishing the distant heliopause. Mariner 2 was the first
successful planetary probe, the ship that ushered in the age of planetary
exploration.
It's still in orbit around the Sun,
every few hundred days still approaching, more or less tangentially, the orbit
of Venus. Each time that happens, Venus isn't there. But if we wait long
enough, Venus will one day be nearby and Mariner 2 will be accelerated by the
planet's gravity into some quite different orbit. Ultimately, Mariner 2, like
some planetesimal from ages past, will be swept up by another planet, fall into
the Sun, or be ejected from the Solar System.
Until then, this harbinger of the age of
planetary exploration, this minuscule artificial planet, will continue silently
orbiting the Sun. It's a little as if Columbus's flagship, the Santa Maria,
were still making regular runs with a ghostly crew across the Atlantic between
Cadiz and Hispaniola. In the vacuum of interplanetary space, Mariner 2 should
be in mint condition for many generations.
My wish on the evening and morning star
is this: that late in the twenty-first century some great ship, on its regular
gravity-assisted transit to the outer Solar System, intercepts this ancient
derelict and heaves it aboard, so it can be displayed in a museum of early
space technology-on Mars, perhaps, or Europa, or Iapetus.
CHAPTER 12
The ground melts
Midway between Thera and Therasia, fires broke forth from the sea
and continued for four days, so that the
whole sea boiled and blazed, and the fires cast up an island which was
gradually elevated as though by levers... After the cessation of the eruption,
the Rhodians, at the time of their
maritime supremacy, were first to venture upon
the scene and to erect on the island a temple.
—STRABO, GEOGRAPHY (CA. 7 B. C)
All over the Earth, you can find a kind
of mountain with one striking and unusual feature. Any child can recognize it:
The top seems sheared or squared off: If you climb to the summit or fly over
it, you discover that the mountain has a hole or crater at its peak. In some
mountains of this sort, the craters are small; in others, they are almost as
big as the mountain itself. Occasionally, the craters are filled with water.
Sometimes they're filled with a more amazing liquid: You tiptoe 10 the edge,
and see vast, glowing lakes of yellow-red liquid and fountains of fire. These
holes in the tops of mountains are called calderas, after the word “caldron,”
and the mountains on which they sit are known, of course, as volcanos-after
Vulcan, the Roman god of fire. There are perhaps 600 active volcanos discovered
on Earth. Some, beneath the oceans, are yet to be found.
A typical volcanic mountain looks safe
enough. Natural vegetation runs up its sides. Terraced fields decorate its
flanks. Hamlets and shrines nestle at its base. And yet, without warning, after
centuries of lassitude, the mountain may explode. Barrages of boulders,
torrents of ash drop out of the sky. Rivers of molten rock come pouring down
its sides. All over the Earth people imagined that an active volcano was an
imprisoned giant or demon struggling to get out.
The eruptions of Mt. St. Helens and Mt.
Pinatubo are recent reminders, but examples can be found throughout history. In
1902 a hot, glowing volcanic cloud swept down the slopes of Mt. Pelee and
killed 35,000 people in the city of St. Pierre on the Caribbean island of
Martinique. Massive mudflows from the eruption of the Nevado del Ruiz volcano
in 1985 killed more than 25,000 Colombians. The eruption of Mt. Vesuvius in the
first century buried in ash the hapless inhabitants of Pompeii and Herculaneum
and killed the intrepid naturalist Pliny the Elder as he made his way up the
side of the volcano, intent on arriving at a better understanding of its
workings. (Pliny was hardly the last: Fifteen volcanologists have been killed
in sundry volcanic eruptions between 1979 and 1993.) The Mediterranean island
of Santorin (also called Thera) is in reality the only part above water of the
rim of a volcano now inundated by the sea. The explosion of the Santorin
volcano in 1623 B. C. may, some historians think, have helped destroy the great
Minoan civilization on the nearby island of Crete and changed the balance of
power in early classical civilization. This disaster may be the origin of the
Atlantis legend as related by Plato, in which a civilization was destroyed “in
a single day and night of misfortune.” It must have been easy back then to
think that a god was angry.
Volcanos have naturally been regarded
with fear and awe. When medieval Christians viewed the eruption of Mt. Hekla in
Iceland and saw churning fragments of soft lava suspended over the summit, they
imagined they were seeing the souls of the damned awaiting entrance to Hell.
“Fearful howlings, weeping and gnashing of teeth,” “melancholy cries and loud
wailings” were dutifully reported. The glowing red lakes and sulfurous gases
within the Hekla caldera were thought to be a real glimpse into the underworld
and a confirmation of folk beliefs in Hell sand, by symmetry, in its partner,
Heaven).
A volcano is, in fact, an aperture to an
underground realm much vaster than the thin surface layer that humans inhabit,
and far more hostile. The lava that erupts from a volcano is liquid rock-rock
raised to its melting point, generally around 1000°C. The lava emerges from a
hole in the Earth; as it cools and solidifies, it generates and later remakes
the flanks of a volcanic mountain.
The most volcanically active locales on
Earth tend to be along ridges on the ocean floor and island arcs-at the
junction of two great plates of oceanic crust-either separating from each
other, or one slipping under the other. On the seafloor there are long zones of
volcanic eruptions-accompanied by swarms of earthquakes and plumes of abyssal
smoke and hot water-that we a are just beginning to observe with robot and
manned submersible vehicles.
Eruptions of lava must mean that the
Earth's interior is extremely hot. Indeed, seismic evidence shows that, only a
few hundred kilometers beneath the surface, nearly the entire body of the Earth
is at least slightly molten. The interior of the Earth is hot, in part, because
radioactive elements there, such as uranium, give off heat as they decay; and
in part because the Earth retains some of the original heat released in its
formation, when many small worlds fell together by their mutual gravity to make
the Earth, and when iron drifted down to form our planet's core.
The molten rock, or magma, rises through
fissures in the surrounding heavier solid rocks. We can imagine vast
subterranean caverns filled with glowing, red, bubbling, viscous liquids that
shoot up toward the surface if a suitable channel is by chance provided. The
magma, called lava as it pours out of the summit caldera, does indeed arise
from the underworld. The souls of the damned have so far eluded detection.
Once the volcano is fully built from
successive outpourings, and the lava is no longer spewing up into the caldera,
then it becomes just like any other mountain-slowly eroding because of rainfall
and windblown debris and, eventually, the movement of continental plates across
the Earth's surface. “How many years can a mountain exist before it is washed
to the sea?” asked Bob Dylan in the ballad “Blowing in the Wind.” The answer
depends on which planet we're talking about. For the Earth, it's typically
about ten million years. So mountains, volcanic and otherwise, must be built on
the same timescale; otherwise the Earth would be everywhere smooth as Kansas.
§§§§ Volcanic explosions can punch vast quantities of matter-mainly fine
droplets of sulfuric acid-into the stratosphere. There, for a year or two, they
reflect sunlight back to space and cool the Earth. This happened recently with
the Philippine volcano, Mt. Pinatubo, and disastrously in 1815-16 after the
eruption of the Indonesian volcano Mt. Tambora, which resulted in the
famine-ridden “year without a summer.” A volcanic eruption in Taupo, New
Zealand, in the year 177 cooled the climate of the Mediterranean, half a world
away, and dropped fine particles onto the Greenland ice cap. The explosion of
Mt. Mazama in Oregon (which left the caldera now called Crater Lake) in 4803 B.
C. had climatic consequences throughout the northern hemisphere. Studies of
volcanic effects on the climate were on the investigative path that eventually
led to the discovery of nuclear winter. They provide important tests of our use
of computer models to predict future climate change. Volcanic particles
injected into the upper air are also an additional cause of thinning of the
ozone layer.
So a large volcanic explosion in some
unfrequented and obscure part of the world can alter the environment on a
global scale. Both in their origins and in their effects, volcanos remind us of
how vulnerable we are to minor burps and sneezes in the Earth's internal
metabolism, and how important it is for us to understand how this subterranean
heat engine works.
IN THE FINAL STAGES of formation of the
Earth-as well as the Moon, Mars, and Venus- impacts by small worlds are thought
to have generated global magma oceans. Molten rock flooded the pre-existing
topography. Great floods, tidal waves kilometers high, of flowing, red-hot
liquid magma welled up from the interior and poured over the surface of the
planet, burying everything in their path: mountains, channels, craters, perhaps
even the last evidence of much earlier, more clement times. The geological
odometer was reset. All accessible records of surface geology begin with the
last global magma flood. Before the, cool and solidify, oceans of lava may be
hundreds or even thousands of kilometers thick. In our time, billions of years
later, the surface of such a world may be quiet, inactive, with no hint of
current vulcanism. Or there may be-as on Earth-a few small-scale but active
reminders of an epoch when the entire surface was flooded with liquid rock.
In the early days of planetary geology,
ground-based telescopic observations were all the data we had. A fervent debate
had been running for half a century on whether the craters of the Moon were due
to impacts or volcanos. A few low mounds with summit calderas were found-almost
certainly lunar volcanos. But the big craters-bowl or pan-shaped and sitting on
the flat ground and not the tops of mountains-were a different story. Some
geologists saw in them similarities with certain highly eroded volcanos on
Earth. Others did not. The best counter-argument was that we know there are
asteroids and comets that fly past the Moon; they must hit it sometimes; and
the collisions must make craters. Over the history of the Moon a large number
of such craters should have been punched out. So if the craters we see are not
due to impacts, where then are the impact craters? We now know from direct
laboratory examination of lunar craters that they are almost entirely of impact
origin. But 4 billion years ago this little world, nearly dead today, was
bubbling and churning away, driven by primeval vulcanism from sources of
internal heat now long gone.
In November 1971, NASA's Mariner 9
spacecraft arrived at Mars to find the planet completely obscured by a global
dust storm. Almost the only features to be seen were four circular spots rising
out of the reddish murk. But there was something peculiar about them: They had
holes in their tops. As the storm cleared, we were able to see unmistakably
that we had been viewing four huge volcanic mountains penetrating through the
dust cloud, each capped by a great summit caldera.
After the storm dissipated, the true
scale of these volcanos became clear. The largest-appropriately named Olympus
Mons, or Mt. Olympus, after the home of the Greek gods-is more than 25
kilometers (roughly 15 miles) high, dwarfing not only the largest volcano on
Earth but also the largest mountain of any sort, Mt. Everest, which stands 9
kilometers above the Tibetan plateau. There are some 20 large volcanos on Mars,
but none so massive as Olympus Mons, which has a volume about 100 times that of
the largest volcano on Earth, Mauna Loa in Hawaii.
By counting the accumulated impact
craters (made by small impacting asteroids, and readily distinguished from
summit calderas) on the flanks of the volcanos, estimates of their ages can be
derived. Some Martian volcanos turn out to be a few billion years old, although
none dates back to the very origin of Mars, about 4. 5 billion years ago. Some,
including Olympus Mons, are comparatively new-perhaps only a few hundred
million years old. It is clear that enormous volcanic explosions occurred early
in Martian history, perhaps providing an atmosphere much denser than the one
Mars holds today. What would the place have looked like if we had visited it
then?
Some volcanic flows on Mars (for
example, in Cerberus) formed as recently as 200 million years ago. It is, I
suppose, even possible-although there is no evidence either way-that Olympus
Mons, the largest volcano we know about for certain in the Solar System, will
be active again. Volcanologists, a patient sort, would doubtless welcome the
event.
In 1990-93 the Magellan spacecraft
returned surprising radar data about the landforms of Venus. Cartographers
prepared maps of almost the entire planet, with fine detail down to about 100
meters, the goal-line-to-goal-line distance in an American football stadium.
More data were radioed home by Magellan than by all other planetary missions
combined. Since much of the ocean floor remains unexplored (except perhaps for
still-classified data acquired by the U. S. and Soviet navies), we may know
more about the surface topography of Venus than about any other planet, Earth
included. Much of the geology of Venus is unlike anything seen on Earth or
anywhere else. Planetary geologists have given these landforms names, but that
doesn't mean we fully understand how they're formed.
Because the surface temperature of Venus
is almost 470°C (900°F), the rocks there are much closer to their melting
points than are those at the surface of the Earth. Rocks begin to soften and
flow at much shallower depths on Venus than on Earth. This is very likely the
reason that many geological features on Venus seem to be plastic and deformed.
The planet is covered by volcanic plains
and highland plateaus. The geological constructs include volcanic cones,
probable shield volcanos, and calderas. There are many places where we can see
that lava has erupted in vast floods. Some plains features ranging to over 200
kilometers in size are playfully called “ticks” and “arachnoids” (which
translates roughly as “spiderlike things”)-because they are circular
depressions surrounded by concentric rings, while long, spindly surface cracks
extend radially out from the center. Odd, flat “pancake domes”-a geological
feature unknown on Earth, but probably a kind of volcano-are perhaps formed by
thick, viscous lava slowly flowing uniformly in all directions. There are many
examples of more irregular lava flows. Curious ring structures called “coronae”
range up to some 2,000 kilometers across. The distinctive lava flows on
stifling hot Venus offer up a rich menu of geological mysteries.
The most unexpected and peculiar
features are the sinuous channels-with meanders and oxbows, looking just like
river valleys on Earth. The longest are longer than the greatest rivers on
Earth. But it is much too hot for liquid water on Venus. And we can tell from
the absence of small impact craters that the atmosphere has been this thick,
driving as great a greenhouse effect, for as long as the present surface has
been in existence. (If it had been much thinner, intermediate-sized asteroids
would not have burned up on entry into the atmosphere, but would have survived
to excavate craters as they impact this planet's surface.) Lava flowing
downhill does make sinuous channels (sometimes under the ground, followed by
collapse of the roof of the channel). But even at the temperatures of Venus,
the lavas radiate heat, cool, slow, congeal, and stop. The magma freezes solid.
Lava channels cannot go even 10 percent of the length of the long Venus
channels before they solidify. Some planetary geologists think there must be a
special thin, watery, inviscid lava generated on Venus. But this is a
speculation supported by no other data, and a confession of our ignorance.
The thick atmosphere moves sluggishly;
because it's so dense, though, it's very good at lifting and moving fine
particles. There are wind streaks on Venus, largely emanating from impact
craters, in which the prevailing winds have scoured piles of sand and dust and
provided a sort of weather vane imprinted on the surface. Here and there we
seem to see fields of sand dunes, and provinces where wind erosion has sculpted
volcanic landforms. These aeolian processes take place in slow motion, as if at
the bottom of the sea. The winds are feeble at the surface of Venus. It may
take only a soft gust to raise a cloud of fine particles, but in that stifling
inferno a gust is hard to come by.
There are many impact craters on Venus,
but nothing like the number on the Moon or Mars. Craters smaller than a few
across are oddly missing. The reason is understood: Small asteroids and comets
are broken up on entry into the dense Venus atmosphere before they can hit the
surface. The observed cutoff in crater size corresponds very well to the
present density of the atmosphere of Venus. Certain irregular splotches seen on
the Magellan images are thought to be the remains of impactors that broke up in
the thick air before they could gouge out a crater.
Most of the impact craters are
remarkably pristine and well preserved; only a few percent of them have been
engulfed by subsequent lava flows. The surface of Venus as revealed by Magellan
is very young. There are so few impact craters that everything older than about
500 million years must have been eradicated-on a planet almost certainly 4. 5
billion years old. There is only one plausible erosive agent adequate for what
we see: vulcanism. All over the planet craters, mountains, and other geological
features have been inundated by seas of lava that once welled up from the
inside, flowed far, and froze.
After examining so young a surface
covered with congealed magma, you might wonder if there are any active volcanos
left. None has been found for certain, but there are a few-for example, one
called Maat Mons-that appear to be surrounded by fresh lava and which may
indeed still be churning and belching. There is some evidence that the
abundance of sulfur compounds in the high atmosphere varies with time, as if
volcanos at the surface were episodically injecting these materials into the
atmosphere. When the volcanos are quiescent, the sulfur compounds simply fall
out of the air. There's also disputed evidence of lightning playing around the
mountaintops of Venus, as sometimes happens on active volcanos on Earth. But we
do not know for certain whether there is ongoing vulcanism on Venus. That's a
matter for future missions.
Some scientists believe that until about
500 million years ago the Venus surface was almost entirely devoid of
landforms. Streams and oceans of molten rock were relentlessly pouring out of
the interior, filling in and covering over any relief that had managed to form.
Had you plummeted down through the clouds in that long-ago time, the surface
would have been nearly uniform and featureless. At night the landscape would
have been hellishly glowing from the red heat of molten lava. In this view, the
great internal heat engine of Venus, which supplied copious amounts of magma to
the surface until about 500 million years ago, has now turned off. The
planetary heat engine has finally run down.
In another provocative theoretical
model, this one by the geophysicist Donald Turcotte, Venus has plate tectonics
like the Earth's but it turns off and on. Right now, he proposes, the plate
tectonics are off; “continents” do not move along the surface, do not crash into
one another, do not thereby raise mountain ranges, and are not later subducted
into the deep interior. After hundreds of millions of years of quiescence,
though, plate tectonics always breaks out and surface features are flooded by
lava, destroyed by mountain building, subducted, and otherwise obliterated. The
last such breakout ended about 500 million years ago, Turcotte suggests, and
everything has been quiet since. However, the presence of coronae may
signify-on timescales that are geologically in the near future-that massive
changes on the surface of Venus are about to break out again.
EVEN MORE UNEXPECTED than the great
Martian volcanos or the magma-flooded surface of Venus is what awaited us when
the Voyager 1 spacecraft encountered lo, the innermost of the four large
Galilean moons of Jupiter, in March 1979. There we found a strange, small,
multihued world positively awash in volcanos. As we watched in astonishment,
eight active plumes poured gas and fine particles up into the sky. The largest,
now called Pele-after the Hawaiian volcano goddess-projected a fountain of
material 250 kilometers into space, higher above the surface of Io than some
astronauts have ventured above the Earth. By the time Voyager 2 arrived at Io,
four months later, Pele had turned itself off, although six of the other plumes
were still active, at least one new plume had been discovered, and another
caldera, named Surt, had changed its color dramatically.
The colors of Io, even though
exaggerated in NASA's color-enhanced images, are like none elsewhere in the
Solar System. The currently favored explanation is that the Ionian volcanos are
driven not by upwelling molten rock, as on the Earth, the Moon, Venus, and
Mars, but by upwelling sulfur dioxide and molten sulfur. The surface is covered
with volcanic mountains, volcanic calderas, vents, and lakes of molten sulfur.
Various forms and compounds of sulfur have been detected on the surface of Io
and in nearby space-the volcanos blow some of the sulfur off Io altogether.
These findings have suggested to some an underground sea of liquid sulfur that
issues to the surface at points of weakness, generates a shallow volcanic
mound, trickles downhill, and freezes, its final color determined by its
temperature on eruption.
On the Moon or Mars, you can find many
places that have changed little in a billion years. On Io, in a century, much
of the surface should be reflooded, filled in or washed away by new volcanic
flows. Maps of Io will then quickly become obsolete, and cartography of Io will
have become a growth industry.
All this seems to follow readily enough
from the Voyager observations. The rate at which the surface is covered over by
current volcanic flows implies massive changes in 50 or 100 years, a prediction
that luckily can be tested. The Voyager images of to can be compared with much
poorer images taken by ground-based telescopes 50 years earlier, and by the
Hubble Space Telescope 13 years later. The surprising conclusion seems to be
that the big surface markings on Io have hardly changed at all. Clearly, we're
missing something.
A VOLCANO in one sense represents the
insides of a planet gushing out, a wound that eventually heals itself by
cooling, only to be replaced by new stigmata. Different worlds have different
insides. The discovery of liquid-sulfur vulcanism on Io was a little like
finding that an old acquaintance, when cut, bleeds green. You had no idea such
differences were possible. He seemed so ordinary.
We are naturally eager to find
additional signs of vulcanism on other worlds. On Europa, the second of the
Galilean moons of Jupiter and Io's neighbor, there are no volcanic mountains at
all; but molten ice-liquid water-seems to have gushed to the surface through an
enormous number of crisscrossing dark markings before freezing. And further
out, among the moons of Saturn, there are signs that liquid water has gushed up
from the interior and wiped away impact craters. Still, we have never seen
anything that might plausibly be an ice volcano in either the Jupiter or Saturn
systems. On Triton, we may have observed nitrogen or methane vulcanism.
The volcanos of other worlds provide a
stirring spectacle. They enhance our sense of wonder, our joy in the beauty and
diversity of the Cosmos. But these exotic volcanos perform another service as
well: They help us to know the volcanos of our own world-and perhaps will help
one day even to predict their eruptions. If we cannot understand what's
happening in other circumstances, where the physical parameters are different,
how deep can our understanding be of the circumstance of most concern to us? A
general theory of vulcanism must cover all cases. When we stumble upon vast
volcanic eminences on a geologically quiet Mars; when we discover the surface
of Venus wiped clean only yesterday by floods of magma; when we find a world
melted not by the heat of radioactive decay, as on Earth, but by gravitational
tides exerted by nearby worlds; when we observe sulfur rather than silicate
vulcanism; and when we begin to wonder, in the moons of the outer planets,
whether we might be viewing water, ammonia, nitrogen, or methane vulcanism-then
we are learning what else is possible.
CHAPTER 13
The gift of Apollo
The gates of Heaven are open wide;
Off I ride...
—CH'U TZ'U /ATTRIBUTED TO CH'U YUAN),
“THE NINE SONGS,” SONG V, “THE GREAT
LORD OF LIVES”
(CHINA, CA. THIRD CENTURY B. C.)
It's a sultry night in July. You've
fallen asleep in the armchair. Abruptly, you startle awake, disoriented. The
television set is on, but not the sound. You strain to understand what you're
seeing. Two ghostly white figures in coveralls and helmets are softly dancing
under a pitch-black sky. They make strange little skipping motions, which
propel them upward amid barely perceptible clouds of dust. But something is
wrong. They take too long to come down. Encumbered as they are, they seem to be
flying a little. You rub your eyes, but the dreamlike tableau persists.
Of all the events surrounding Apollo
11's landing on the Moon on July 20, 1969, my most vivid recollection is its unreal
quality. Neil Armstrong and Buzz Aldrin shuffled along the gray, dusty lunar
surface, the Earth looming large in their sky, while Michael Collins, now the
Moon's own moon, orbited above them in lonely vigil. Yes, it was an astonishing
technological achievement and a triumph for the United States. Yes, the
astronauts displayed death-defying courage. Yes, as Armstrong said as he first
alighted, this was a historic step for the human species. But if you turned off
the by-play between Mission Control and the Sea of Tranquility, with its
deliberately mundane and routine chatter, and stared into that black-and-white
television monitor, you could glimpse that we humans had entered the realm of
myth and legend.
We knew the Moon from our earliest days.
It was there when our ancestors descended from the trees into the savannahs,
when we learned to walk upright, when we first devised stone tools, when we
domesticated fire, when we invented agriculture and built cities and set out to
subdue the Earth. Folklore and popular songs celebrate a mysterious connection
between the Moon and love. The word “month” and the second day of the week are
both named after the Moon. Its waxing and waning-from crescent to full to
crescent to new-was widely understood as a celestial metaphor of death and
rebirth. It was connected with the ovulation cycle of women, which has nearly
the same period-as the word “menstruation” (Latin mensis = month, from the word
“to measure”) reminds us. Those who sleep in moonlight go mad; the connection
is preserved in the English word “lunatic.” In the old Persian story, a vizier
renowned for his wisdom is asked which is more useful, the Sun or the Moon.
“The Moon,” he answers, “because the Sun shines in daytime when it's light out
anyway.” Especially when we lived out-of-doors, it was a major-if oddly
intangible-presence in our lives.
The Moon was a metaphor for the
unattainable: “You plight as well ask for the Moon,” they used to say. Or “You
can no more do that than fly to the Moon.” For most of our history, eve had no
idea what it was. A spirit? A god? A thing? It didn't look like something big
far away, but more like something small nearby-something the size of a plate,
maybe, hanging in the sky a little above our heads. Ancient Greek philosophers debated
the proposition “that the Moon is exactly as large as it looks” (betraying a
hopeless confusion between linear and angular size). Walking on the Moon would
have seemed a screwball idea; it made more sense to imagine somehow climbing up
into the sky on a ladder or on the back of a giant bird, grabbing the Moon, and
bringing it down to Earth. Nobody ever succeeded, although there were myths
aplenty about heroes who had tried.
Not until a few centuries ago did the
idea of the Moon as a place, a quarter-million miles away, gain wide currency.
And in that brief flicker of time, we've gone from the earliest steps in
understanding the Moon's nature to walking and joy-riding on its surface. We
calculated how objects move in space; liquefied oxygen from the air; invented
big rockets, telemetry, reliable electronics, inertial guidance, and much else.
Then we sailed out into the sky.
I was lucky enough to be involved in the
Apollo program, but I don't blame people who think the whole thing was faked in
a Hollywood movie studio. In the late Roman Empire, pagan philosophers had
attacked Christian doctrine on the ascension to Heaven of the body of Christ
and on the promised bodily resurrection of the dead-because the force of
gravity pulls down all “earthly bodies.” St. Augustine rejoined: “If human
skill can by some contrivance fabricate vessels that float, out of metals which
sink... how much more credible is it that God, by some hidden mode of
operation, should even more certainly effect that these earthly masses be
emancipated” from the chains that bind them to Earth? That humans should one
day discover such a “mode of operation” was beyond imagining. Fifteen hundred
years later, we emancipated ourselves.
The achievement elicited an amalgam of
awe and concern. Some remembered the story of the Tower of Babel. Some,
orthodox Moslems among them, felt setting foot on the Moon's surface to be
impudence and sacrilege. Many greeted it as a turning point in history.
The Moon is no longer unattainable. A
dozen humans, all Americans, have made those odd bounding motions they called
“moonwalks” on the crunchy, cratered, ancient gray lava-beginning on that July
day in 1969. But since 1972, no one from any nation has ventured back. Indeed,
none of us has gone anywhere since the glory days of Apollo except into low
Earth orbit-like a toddler who takes a few tentative steps outward and then,
breathless, retreats to the safety of his mother's skirts.
Once upon a time, we soared into the
Solar System. For a few years. Then we hurried back. Why? What happened? What
was Apollo really about?
The scope and audacity of John Kennedy's
May 25, 1961 message to a joint session of Congress on “Urgent National
Needs”-the speech that launched the Apollo program-dazzled me. We would use
rockets not yet designed and alloys not yet conceived, navigation and docking
schemes not yet devised, in order to send a man to an unknown world-a world not
yet explored, not even in a preliminary way, not even by robots-and we would
bring him safely back, and we would do it before the decade was over. This
confident pronouncement was made before any American had even achieved Earth
orbit.
As a newly minted Ph. D., I actually
thought all this had something centrally to do with science. But the President
did not talk about discovering the origin of the Moon, or even about bringing
samples of it back for study. All he seemed to be interested in was sending
someone there and bringing him home. It was a kind of gesture. Kennedy's
science advisor, Jerome Wiesner, later told me he had made a deal with the
President: If Kennedy would not claim that Apollo was about science, then he.
Wiesner, would support it. So if not science, what?
The Apollo program is really about
politics, others told me. This sounded more promising. Nonaligned nations would
be tempted to drift toward the Soviet Union if it was ahead in space
exploration, if the United States showed insufficient “national vigor.” I
didn't follow. Here was the United States, ahead of the Soviet Union in
virtually every area of technology-the world's economic, military, and, on
occasion, even moral leader-and Indonesia would go Communist because Yuri
Gagarin beat John Glenn to Earth orbit? What's so special about space
technology? Suddenly I understood.
Sending people to orbit the Earth or
robots to orbit the Sun requires rockets-big, reliable, powerful rockets. Those
same rockets can be used for nuclear war. The same technology that transports a
man to the Moon can carry nuclear warheads halfway around the world. The same technology
that puts an astronomer and a telescope in Earth orbit can also put up a laser
“battle station.” Even back then, there was fanciful talk in military circles,
East and West, about space as the new “high ground,” about the nation that
“controlled” space “controlling” the Earth. Of course strategic rockets were
already being tested on Earth. But heaving a ballistic missile with a dummy
warhead into a target zone in the middle of the Pacific Ocean doesn't buy much
glory. Sending people into space captures the attention and Imagination of the
world.
You wouldn't spend the money to launch
astronauts for this reason alone, but of all the ways of demonstrating rocket
potency, this one works best. It was a rite of national manhood; the shape of
the boosters made this point readily understood without anyone actually having
to explain it. The communication seemed to be transmitted from unconscious mind
to unconscious mind without the higher mental faculties catching a whiff of
what was going on.
My colleagues today-struggling for every
space science dollar-may have forgotten how easy it was to get money for
“space” in the glory days of Apollo and just before. Of many examples, consider
this exchange before the Defense Appropriations Subcommittee of the House of Representatives
in 1958, only a few months after Sputnik 1. Air Force Assistant Secretary
Richard E. Horner is testifying; his interlocutor is Rep. Daniel J. Flood
(Democrat of Pennsylvania):
HORNER: [W]hy is it desirable from the
military point of view to have a man on the moon? Partly, from the classic
point of view, because it is there. Partly because we might be afraid that the
U. S. S. R. might get one there first and realize advantages which we had not
anticipated existed there...
FLOOD: [I]f we gave you all the money
you said was necessary, regardless of how much it was, can you in the Air Force
hit the moon with something, anything, before Christmas?
HORNER: I feel sure we can. There is
always a certain amount of risk in this kind of undertaking, but we feel that
we can do that; yes, sir.
FLOOD: Have you asked anybody in the Air
Force or the Department of Defense to give you enough money, hardware, and
people, starting at midnight tonight, to chip a. piece out of that ball of
green cheese for a Christmas present to Uncle Sam? Have you asked for that?
HORNER: We have submitted such a program
to the Office of the Secretary of Defense. It is currently under consideration.
FLOOD: I am for giving it to them as of
this minute, Mr. Chairman, with our supplemental, without waiting for somebody
downtown to make up his mind to ask for it. If this man means what he says and
if he knows what he is talking about-and I think he does-then this committee
should not wait five minutes more today. We should give him all the money and
all the hardware and all the people he wants, regardless of what anybody else
says or wants, and tell him to go up on top of some hill and do it without any
question.
When President Kennedy formulated the
Apollo program, the Defense Department had a slew of space projects under
development-ways of carrying military personnel up into space, means of
conveying them around the Earth, robot weapons on orbiting platforms intended
to shoot down satellites and ballistic missiles of other nations. Apollo
supplanted these programs. They never reached operational status. A case can be
made then that Apollo served another purpose-to move the U. S. -Soviet space
competition from a military to a civilian arena. There are some who believe
that Kennedy intended Apollo as a substitute for an anus race in space. Maybe.
For me, the most ironic token of that
moment in history is the plaque signed by President Richard M. Nixon that
Apollo 11 took to the Moon. It reads: “We came in peace for all mankind.” As
the United States was dropping 71/2 megatons of conventional explosives on
small nations in Southeast Asia, we congratulated ourselves on our humanity: We
would harm no one on a lifeless rock. That plaque is there still, attached to
the base of the Apollo 11 Lunar Module, on the airless desolation of the Sea of
Tranquility. If no one disturbs it, it will still be readable a million years
from now.
Six more missions followed Apollo 11,
all but one of which successfully landed on the lunar surface. Apollo 17 was
the first to carry a scientist. As soon as he got there, the program was
canceled. The first scientist and the last human to land on the Moon were the
same person. The program had already served its purpose that July night in
1969. The half-dozen subsequent missions were just momentum.
Apollo was not mainly about science. It
was not even mainly about space. Apollo was about ideological confrontation and
nuclear war-often described by such euphemisms as world “leadership” and
national “prestige.” Nevertheless, good space science was done. We now know
much more about the composition, age, and history of the Moon and the origin of
the lunar landforms. We have made progress in understanding where the Moon came
from. Some of us have used lunar cratering statistics to better understand the
Earth at the time of the origin of life. But more important than any of this,
Apollo provided an aegis, an umbrella under which brilliantly engineered robot
spacecraft were dispatched throughout the Solar System, making that preliminary
reconnaissance of dozens of worlds. The offspring of Apollo have now reached
the planetary frontiers.
If not for Apollo-and, therefore, if not
for the political purpose it served-I doubt whether the historic American
expeditions of exploration and discovery throughout the Solar System would have
occurred, The Mariners, Vikings, Pioneers, Voyagers ,and Galileo are among the
gifts of Apollo. Magellan and Cassini are more distant descendants. Something
similar is true for the pioneering Soviet efforts in Solar System exploration,
including the first soft landings of robot spacecraft-Luna 9, Mars 3, Venera
8-on other worlds.
Apollo conveyed a confidence, energy,
and breadth of vision that did capture the imagination of the world. That too
was part of its purpose. It inspired an optimism about technology, an
enthusiasm for the future. If we could fly to the Moon, as so many have asked,
what else were we capable of? Even those who opposed the policies and actions
of the United States-even those who thought the worst of us-acknowledged the
genius and heroism of the Apollo program. With Apollo, the United States
touched greatness.
When you pack your bags for a big trip,
you never know what's in store for you. The Apollo astronauts on their way to
and from the Moon photographed their home planet. It was a natural thing to do,
but it had consequences that few foresaw. For the first time, the inhabitants
of Earth could see their world from above-the whole Earth, the Earth in color,
the Earth as an exquisite spinning white and blue ball set against the vast
darkness of space. Those images helped awaken our slumbering planetary
consciousness. They provide incontestable evidence that we all share the same
vulnerable planet. They remind us of what is important and what is not. They
were the harbingers of Voyager's pale blue dot.
We may have found that perspective just
in time, just as our technology threatens the habitability of our world.
Whatever the reason we first mustered the Apollo program, however mired it was
in Cold War nationalism and the instruments of death, the inescapable
recognition of the unity and fragility of the Earth is its clear and luminous
dividend, the unexpected final gift of Apollo. What began in deadly competition
has helped us to see that global cooperation is the essential precondition for
of survival.
Travel is broadening.
It's time to hit the road again.
CHAPTER 14
Exploring other worlds and protecting this one
The planets, in their various stages of
development, are subjected to the same formative forces that operate on our
earth, and have, therefore, the same geologic formation, and probably life, of
our own past, and perhaps future; but. further than this, these forces are
acting, in some cases, under totally different conditions from those under
which they operate on the earth, and hence must evolve forms different from
those ever known to man. The value of such material as this to the comparative
sciences is too obvious to need discussion.
—ROBERT H. GODDARD, NOTEBOOK (1907)
For the first time in my life, I saw the
horizon as a curved line. It was accentuated by a thin seam of dark blue
light-our atmosphere. Obviously, this was not the “ocean” of air I had been
told it was so many times in my life. I was terrified by its fragile appearance.
—ULF MERBOLD, GERMAN SPACE SHUTTLE
ASTRONAUT (1988)
When you look down at the Earth from
orbital altitudes, you see a lovely, fragile world embedded in black vacuum.
But peering at a piece of the Earth through a spacecraft porthole is nothing
like the joy of seeing it entire against the backdrop of black,
or-better-sweeping across your field of view as you float in space unencumbered
by a spacecraft. The first human to have this experience was Alexei Leonov, who
on March 18, 1965, left Voskhod 2 in the original space “walk”: “I looked down
at the Earth,” he recalls, “and the first thought that crossed my mind was `The
world is round, after all. ' In one glance I could see from Gibraltar to the
Caspian Sea...I felt like a bird-with wings, and able to fly.”
When you view the Earth from farther
away, as the Apollo astronauts did, it shrinks in apparent size, until nothing
but a little geography remains. You're struck by how self-contained it is. An
occasional hydrogen atom leaves; a pitter-patter of cometary dust arrives.
Sunlight, generated in the immense, silent thermonuclear engine deep in the
solar interior, pours out of the Sun in all directions, and the Earth
intercepts enough of it to provide a little illumination and enough heat for
our modest purposes. Apart from that, this small world is on its own.
From the surface of the Moon you can see
it, perhaps as a crescent, even its continents now indistinct. And from the
vantage point of the outermost planet it is a mere point of pale light.
From Earth orbit, you are struck by the
tender blue arc of the horizon-the Earth's thin atmosphere seen tangentially.
You can understand why there is no longer such a thing as a local environmental
problem. Molecules are stupid. Industrial poisons, greenhouse gases, and substances
that attack the protective ozone layer, because of their abysmal ignorance, do
not respect borders. They are oblivious of the notion of national sovereignty.
And so, due to the almost mythic powers of our technology (and the prevalence
of short-term thinking), we are beginning-on Continental and on planetary
scales-to pose a danger to ourselves. Plainly, if these problems are to be
solved, it will require many nations acting in concert over many years.
I'm struck again by the irony that
spaceflight-conceived in tile cauldron of nationalist rivalries and
hatreds-brings with it a stunning transnational vision. You spend even a little
time contemplating the Earth from orbit and the most deeply engrained
nationalisms begin to erode. They seem the squabbles of mites on a plum.
If we're stuck on one world, we're
limited to a single case; we don't know what else is possible. Then-like an art
fancier familiar only with Fayoum tomb paintings, a dentist who knows only
molars, a philosopher trained merely in NeoPlatonism, a linguist who has
studied only Chinese, or a physicist whose knowledge of gravity is restricted
to falling bodies on Earth-our perspective is foreshortened, our insights
narrow, our predictive abilities circumscribed. By contrast, when we explore
other worlds, what once seemed the only way a planet could be turns out to be
somewhere in the middle range of a vast spectrum of possibilities. When we look
at those other worlds, we begin to understand what happens when we have too
much of one thing or too little of another. We learn how a planet can go wrong.
We gain a new understanding, foreseen by the spaceflight pioneer Robert
Goddard, called comparative planetology.
The exploration of other worlds has
opened our eyes in the study of volcanos, earthquakes, and weather. It may one
day have profound implications for biology, because all life on Earth is built
on a common biochemical master plan. The discovery of a single extraterrestrial
organism-even something as humble as a bacterium-would revolutionize our
understanding of living things. But the connection between exploring other
worlds and protecting this one is most evident in the study of Earth's climate
and the burgeoning threat to that climate that our technology poses. Other
worlds provide vital insights about what dumb things not to do on Earth.
Three potential environmental
catastrophes-all operating on a global scale-have recently been uncovered:
ozone layer depletion, greenhouse warming, and nuclear winter. All three
discoveries, it turns out, have strong ties to the exploration of the planets.
(1) It was disturbing to find that an
inert material with all sorts of practical applications-it serves as the
working fluid in refrigerators and air conditioners, as aerosol propellant for
deodorants and other products, as lightweight foamy packaging for fast foods,
and as a cleaning agent in microelectronics, to name only a few-can pose a
danger to life on Earth. Who would have figured?
The molecules in question are called
chlorofluorocarbons (CFCs). Chemically, they're extremely inert, which means
they're invulnerable-until they find themselves up in the ozone layer, where
they're broken apart by ultraviolet light from the Sun. The chlorine atoms thus
liberated attack and break down the protective ozone, letting more ultraviolet
light reach the ground. This increased ultraviolet intensity ushers in a
ghastly procession of potential consequences involving not just skin cancer and
cataracts, but weakening of the human immune system and, most dangerous of all,
possible harm to agriculture and to photosynthetic organisms at the base of the
food chain on which most life on Earth depends.
Who discovered that CFCs posed a threat
to the ozone layer? Was it the principal manufacturer, the DuPont Corporation,
exercising corporate responsibility? Was it the Environmental Protection Agency
protecting us? Was it the Department of Defense defending us? No, it was two
Ivory-tower, white-coated university scientists working on something
else-Sherwood Rowland and Mario Molina of the University of California, Irvine.
Not even an Ivy League university. no one instructed them to look for dangers
to the environment. They were pursuing fundamental research. They were
scientists following their own interests. Their names should be known to every
schoolchild.
In their original calculations, Rowland
and Molina used rate constants of chemical reactions involving chlorine and
other halogens that had been measured in part with NASA support. Why NASA?
Because Venus has chlorine and fluorine molecules in its atmosphere, and
planetary aeronomers had wanted to understand what's happening there.
Confirming theoretical work on the role
of CFCs in ozone depletion was soon done by a group led by Michael McElroy at
Harvard How is it they had all these branching networks of halogen chemical
kinetics in their computer ready to go? Because they were working on the
chlorine and fluorine chemistry of the atmosphere of Venus. Venus helped make
and helped confirm the discovery that the Earth's ozone layer is in danger. An
entirely unexpected connection was found between the atmospheric
photochemistries of the two planets. A result of importance to everyone on
Earth emerged from what might well have seemed the most blue-sky, abstract,
impractical kind of work, understanding the chemistry of minor constituents in
the upper atmosphere of another world.
There's also a Mars connection. With
Viking we found the surface of Mars to be apparently lifeless and remarkably
deficient even in simple organic molecules. But simple organic molecules ought
to be there, because of the impact of organic-rich meteorites from the nearby
asteroid belt. This deficiency is widely attributed to the lack of ozone on
Mars. The Viking microbiology experiments found that organic matter carried
from Earth to Mars and sprinkled on Martian surface dust is quickly oxidized
arid destroyed. The materials in the dust that do the destruction are molecules
something like hydrogen peroxide-which we use as an antiseptic because it kills
microbes by oxidizing there, Ultraviolet light from the Sun strikes the surface
of Mars unimpeded by an ozone layer; if any organic matter were there, it would
be quickly destroyed by the ultraviolet light itself and its oxidation
products. Thus part of the reason the topmost layers of Martian soil are
antiseptic is that Mars has an ozone hole of planetary dimensions-by itself a
useful cautionary tale for us, who are busily thinning and puncturing our ozone
layer.
(2) Global warming is predicted to
follow from the increasing greenhouse effect caused largely by carbon dioxide
generated in the burning of fossil fuels-but also from the buildup of other
infrared-absorbing gases (oxides of nitrogen, methane, those same CFCs, and
other molecules).
Suppose that we have a three-dimensional
general circulation computer model of the Earth's climate. Its programmers
claim it's able to predict what the Earth will be like if there's more of one
atmospheric constituent or less of another. The model does very well at
“predicting” the present climate. But there is a nagging worry: The model has
been “tuned” so it will come out right-that is, certain adjustable parameters
are chosen, not from first principles of physics, but to get the right answer.
This is not exactly cheating, but if we apply the same computer model to rather
different climatic regimes-deep global warming, for instance-the tuning might
then be inappropriate. The model might be valid for today's climate, but not
extrapolatable to others.
One way to test this program is to apply
it to the very different climates of other planets. Can it predict the
structure of the atmosphere on Mars and the climate there? The weather? What
about Venus? If it were to fail these test cases, we would be right in
mistrusting it when it makes predictions for our own planet. In fact, climate
models now in use do very well in predicting from first principles of physics
the climates on Venus and Mars.
On Earth, huge upwellings of molten lava
are known and attributed to superplumes convecting up from the deep mantle and
generating vast plateaus of frozen basalt. A spectacular example occurred about
a hundred million years ago, and added perhaps ten times the present carbon
dioxide content to the atmosphere, inducing substantial global warming. These
plumes, it is thought, occur episodically throughout Earth's history. Similar
mantle upwelling seem to have occurred on Mars and Venus. There are sound
practical reasons for us to want to understand how a major change to the
Earth's surface and climate could suddenly arrive unannounced from hundreds of
kilometers beneath our feet.
Some of the most important recent work
on global warming has been done by James Hansen and his colleagues at the
Goddard Institute for Space Sciences, a NASA facility in New York City. Hansen
developed one of the major computer climate models and employed it to predict
what will happen to our climate as the greenhouse gases continue to build up.
He has been in the forefront of testing these models against ancient climates of
the Earth. (During the last ice ages, it is of interest to note, more carbon
dioxide and methane are strikingly correlated with higher temperatures.) Hansen
collected a wide range of weather data from this century and last, to see what
actually happened to the global temperature, and then compared it to the
computer model's predictions of what should have happened. The two agree to
within the errors of measurement and calculation, respectively. He courageously
testified before Congress in the face of a politically generated order from the
White House Office of Management and Budget (this was in the Reagan years) to
exaggerate the uncertainties and minimize the dangers. His calculation on the
explosion of the Philippine volcano Mt. Pinatubo and his prediction of the
resulting temporary decline in the Earth's temperature (about half a degree
Celsius) were right on the money. He has been a force in convincing governments
worldwide that global warming is something to be taken seriously.
How did Hansen get interested in the
greenhouse effect in the first place? His doctoral thesis (at the University of
Iowa in 1967) was about Venus. He agreed that the high radio brightness of
Venus is due to a very hot surface, agreed that greenhouse gases keep the heat
in, but proposed that heat from the interior rather than sunlight was the
principal energy source. The Pioneer 12 mission to Venus in 1978 dropped entry
probes into the atmosphere; they showed directly that the ordinary greenhouse
effect-the surface heated by the Sun and the heat retained by the blanket of
air-was the operative cause. But it's Venus that got Hansen thinking about the
greenhouse effect.
Radio astronomers, you note, find Venus
to be an intense source of radio waves. Other explanations of the radio emission
fail. You conclude that the surface must be ridiculously hot. You try to
understand where the high temperatures come from and are led inexorably to one
or another kind of greenhouse effect. Decades later you find that this training
has prepared you to understand and help predict an unexpected threat to our
global civilization. I know many other instances where scientists who first
tried to puzzle out the atmospheres of other worlds are making important and
highly practical discoveries about this one. The other planets are a superb
training ground for students of the Earth. They require both breadth and depth
of knowledge, and they challenge the imagination.
Those who are skeptical about carbon
dioxide greenhouse warming might profitably note the massive greenhouse effect
on Venus. no one proposes that Venus's greenhouse effect derives froth
imprudent Venusians who burned too much coal, drove fuel-inefficient autos, and
cut down their forests. My point is different. The climatological history of
our planetary neighbor, an otherwise Earthlike planet on which the surface
became hot enough to melt tin or lead, is worth considering-especially by those
who say that the increasing greenhouse effect on Earth will be self-correcting,
that we don't really have to worry about it, or (you can see this in the
publications of some groups that call themselves conservative) that the
greenhouse effect itself is a “hoax.”
(3) Nuclear winter is the predicted
darkening and cooling of the Earth-mainly from fine smoke particles injected
into the atmosphere from the burning of cities and petroleum facilities-that is
predicted to follow a global thermonuclear war. A vigorous scientific debate
ensued on just how serious nuclear winter might be. The various opinions have
now converged. All three-dimensional general circulation computer models
predict that the global temperatures resulting from a worldwide thermonuclear
war would be colder than those in the Pleistocene ice ages. The implications
for our planetary civilization-especially through the collapse of
agriculture-are very dire. It is a consequence of nuclear war that was somehow
overlooked by the civil and military authorities of the United States, the
Soviet Union, Britain, France, and China when they decided to accumulate well
over 60,000 nuclear weapons. Although it's hard to be certain about such
things, a case can be made that nuclear Winter played a constructive role
(there were other causes, of course) in convincing the nuclear-armed nations,
especially the Soviet Union, of the futility of nuclear war.
Nuclear winter was first calculated and
named in 1982/83 by a group of five scientists, to which I'm proud to belong.
This team was given the acronym TTAPS (for Richard P. Turco, (even B. Toon,
Thomas Ackerman, James Pollack, and myself). Of the five TTAPS scientists, two
were planetary scientists, and the other three had published many papers in
planetary science, The earliest intimation of nuclear winter came during that
same Mariner 9 mission to Mars, when there was a global dust storm and we were
unable to see the surface of the planet; the infrared spectrometer on the
spacecraft found the high atmosphere to be warmer and the surface colder than
they ought to have been. Jim Pollack and I sat down and tried to calculate how
that could come about. Over the subsequent twelve years, this line of inquiry
led from dust storms on Mars to volcanic aerosols on Earth to the possible
extinction of the dinosaurs by impact dust to nuclear winter. You never know
where science will take you.
PLANETARY SCIENCE fosters a broad
interdisciplinary point of view that proves enormously helpful in discovering
and attempting to defuse these looming environmental catastrophes. When you cut
your teeth on other worlds, you gain a perspective about the fragility of
planetary environments and about what other, quite different, environments are
possible. There may well be potential global catastrophes still to be
uncovered. If there are, I bet planetary scientists will play a central role in
understanding them.
Of all the fields of mathematics,
technology, and science, the one with the greatest international cooperation
(as determined by how often the co-authors of research papers hail from two or
more countries) is the field called “Earth and space sciences.” Studying this
world and others, by its very nature, tends to be non-local, non-nationalist,
non-chauvinist. Very rarely do people go into these fields because they are
internationalists. Almost always, they enter for other reasons, and then
discover that splendid work, work that complements their own, is being done by
researchers in other nations; or that to solve a problem, you need data or a
perspective (access to the southern sky, for example) that is unavailable in
your country. And once you experience such cooperation-humans from different
parts of the planet working in a mutually intelligible scientific language as
partners on matters of common concern-it's hard not to imagine it happening on
other, nonscientific matters. I myself consider this aspect of Earth and space
sciences as a healing and unifying force in world politics; but, beneficial or
not, it is inescapable.
When I look at the evidence, it seems to
me that planetary exploration is of the most practical and urgent utility for
us here on Earth. Even if we were not roused by the prospect of exploring other
worlds, even if we didn't have a nanogram of adventuresome spirit in us, even
if we were only concerned for ourselves and in the narrowest sense, planetary
exploration would still constitute a superb investment.
CHAPTER 15
The gates of the wonder world open
The great floodgates of the wonder-world
swung open.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 1
(1851)
Sometime coming up, perhaps just around
the corner, there will be a nation-more likely, a consortium of nations-that
will work the next major step in the human venture into space. Perhaps it will
be brought about by circumventing bureaucracies and making efficient use of
present technologies. Perhaps it will require new technologies, transcending
the great blunderbuss chemical rockets. The crews of these ships will set foot
on new worlds. The first baby will be born somewhere up there. Early steps
toward living off the land will be made. We will be on our way. And the future
will remember.
TANTALIZING AND MAJESTIC, Mars is the
world next door, the nearest planet on which an astronaut or cosmonaut could
safely land. Although it is sometimes as warm as a New England October, Mars is
a chilly place, so cold that some of its thin carbon dioxide atmosphere freezes
out as dry ice at the winter pole.
It is the nearest planet whose surface
we can see with a small telescope. In all the Solar System, it is the planet
most like Earth. Apart from flybys, there have been only two fully successful
missions to Mars: Mariner 9 in 1971, and Vikings 1 and 2 in 1976. They revealed
a deep rift valley that would stretch from New York to San Francisco; immense
volcanic mountains, the largest of which towers 80,000 feet above the average
altitude of the Martian surface, almost three times the height of Mount
Everest; an intricate layered structure in and among the polar ices, resembling
a pile of discarded poker chips, and probably a record of past climatic change;
bright and dark streaks painted down on the surface by windblown dust,
providing high-speed wind maps of Mars over the past decades and centuries;
vast globe-girdling dust storms; and enigmatic surface features.
Hundreds of sinuous channels and valley
networks dating back several billion years can be found, mainly in the cratered
southern highlands. They suggest a previous epoch of more benign and Earthlike
conditions-very different from what we find beneath the tenuous and frigid
atmosphere of our time. Some ancient channels seem to have been carved by
rainfall, some by underground sapping and collapse, and some by great floods
that gushed up out of the ground. Rivers were pouring into and filling great
thousand-kilometer-diameter impact basins that today are dry as dust.
Waterfalls dwarfing any on Earth today cascaded into the lakes of ancient Mars.
Vast oceans, hundreds of meters, perhaps even a kilometer, deep may have gently
lapped shorelines barely discernible today. That would have been a world to
explore. We are four billion years late.
On Earth in just the same period, the
first microorganisms arose and evolved. Life on Earth is intimately connected,
for the most basic chemical reasons, with liquid water. We humans are ourselves
made of some three-quarters water. The same sorts of organic molecules that
fell out of the sky and were generated in the air and seas of ancient Earth,
should also have accumulated on ancient Mars. Is it plausible that life quickly
came to be in the waters of early Earth, but was somehow restrained and
inhibited in the waters of early Mars? Or might the Martian seas have been
filled with life-floating, spawning, evolving? What strange beasts once swum
there?
Whatever the dramas of those distant
times, it all started to go wrong around 3. 8 billion years ago. We can see
that the erosion of ancient craters dramatically began to slow about then. As
the atmosphere thinned, as the rivers flowed no more, as the oceans began to
dry, as the temperatures plummeted, life would have retreated to the few
remaining congenial habitats, perhaps huddling at the bottom of ice-covered
lakes, until it too vanished and the dead bodies and fossil remains of exotic
organisms-built, it might be, on principles very different from life on
Earth-were deep-frozen, awaiting the explorers who might in some distant future
arrive on Mars.
METEORITES ARE FRAGMENTS OF OTHER WORLDS
recovered on Earth. Most originate in collisions among the numerous asteroids
that orbit the Sun between the orbits of Mars and Jupiter. But a few are
generated when a large meteorite impacts a planet or asteroid at high speed,
gouges out a crater, and propels the excavated surface material into space. A
very small fraction of the ejected rocks, millions of years later, may
intercept another world.
In the wastelands of Antarctica, the ice
is here and there dotted with meteorites, preserved by the low temperatures and
until recently undisturbed by humans. A few of them, called SNC (pronounced
“snick”) meteorites§§§§§ have an aspect about them that at first seemed almost
unbelievable: Deep inside their mineral and glassy structures, locked away from
the contaminating influence of the Earth's atmosphere, a little gas is trapped.
When the gas is analyzed, it turns out to have exactly the same chemical
composition and isotopic ratios as the air on Mars. We know about Martian air
not just from spectroscopic inference but from direct measurement on the
Martian surface by the Viking landers. To the surprise of nearly everyone, the
SNC meteorites come from Mars.
Originally, they were rocks that had
melted and refrozen. Radioactive dating of all the SNC meteorites shows their
parent rocks condensed out of lava between 180 million and 1. 3 billion years
ago. Then they were driven off the planet by collisions from space. From how
long they've been exposed to cosmic rays on their interplanetary journeys
between Mars and Earth, we can tell how old they are-how long ago they were
ejected from Mars. In this sense, they are between 10 million and 700,000 years
old. They sample the most recent 0. l percent of Martian history.
Some of the minerals they contain show
clear evidence of having once been in water, warm liquid water. These
hydro-thermal minerals reveal that somehow, probably all over Mars, there was
recent liquid water. Perhaps it came about when the interior heat melted
underground ice. But however it happened, it's natural to wonder if life is not
entirely extinct, if somehow it's managed to hang on into our time in transient
underground lakes, or even in thin films of water wetting subsurface grains.
The geochemists Everett Gibson and Hal
Karlsson of NASA's Johnson Space Flight Center have extracted a single drop of
water from one of the SNC meteorites. The isotopic ratios of the oxygen and
hydrogen atoms that it contains are literally unearthly. I look on this water
from another world as an encouragement for future explorers and settlers.
Imagine what we might find if a large
number of samples, including never melted soil and rocks, were returned to
Earth from Martian locales selected for their scientific interest. We are very
close to being able to accomplish this with small roving robot vehicles.
The transportation of subsurface
material from world to world raises a tantalizing question: Four billion years
ago there were two neighboring planets, both warm, both wet. Impacts from
space, in the final stages of the accretion of these planets, were occurring at
a much higher rate than today. Samples from each world were being flung out
into space. We are sure there was life on at least one of them in this period.
We know that a fraction of the ejected debris stays cool throughout the
processes of impact, ejection, and interception by another world. So could some
of the early organisms on Earth have been safely transplanted to Mars four
billion years ago, initiating life on that planet' Or, even more speculative,
could life on Earth have arisen by such a transfer from Mars? Might the two
planets have regularly exchanged life-forms for hundreds of millions of years?
The notion might be testable. If we were to discover life on Mars and found it
very similar to life on Earth-and if, as well, e were sure it wasn't microbial
contamination that we ourselves had introduced in the course of our
explorations-the proposition that life was long ago transferred across
interplanetary space would have to be taken seriously.
IT WAS ONCE THOUGHT that life is
abundant on Mars. Even the dour and skeptical astronomer Simon Newcomb (in his
Astronomy for Everybody, which went through many editions in the early decades
of this century and was the astronomy text of my childhood) concluded, “There
appears to be life on the planet Mars. A few years ago this statement was
commonly regarded as fantastic. Now it is commonly accepted.” Not “intelligent
human life,” he was quick to add, but green plants. However, we have now been
to Mars and looked for plants-as well as animals, microbes, and intelligent
beings. Even if the other forms were absent, we might have imagined, as in
Earth's deserts today, and as on Earth for almost all its history, abundant
microbial life.
The “life detection” experiments on
Viking were designed to detect only a certain subset of conceivable biologies;
they were biased to find the kind of life about which we know. It would have
been foolish to send instruments that could not even detect life on Earth. They
were exquisitely sensitive, able to find microbes in the most unpromising, arid
deserts and wastelands on Earth.
One experiment measured the gases
exchanged between Martian soil and the Martian atmosphere in the presence of
organic matter from Earth. A second brought a wide variety organic foodstuffs
marked by a radioactive tracer to see if there were bugs in the Martian soil
who ate the food and oxidized it to radioactive carbon dioxide. A third
experiment introduced radioactive carbon dioxide (and carbon monoxide) to the
Martian soil to see if any of it was taken up by Martian microbes. To the
initial astonishment of, I think, all the scientists involved, each of the
three experiments gave what at first seemed to be positive results. Gases were
exchanged; organic matter was oxidized; carbon dioxide was incorporated into
the soil.
But there are reasons for caution. These
provocative results are not generally thought to be good evidence for life on
Mars: The putative metabolic processes of Martian microbes occurred under a
very wide range of conditions inside the Viking landers-wet (with liquid water
brought from Earth) and dry, light and dark, cold (only a little above
freezing) to hot (almost the normal boiling point of water). Many
microbiologists deem it unlikely that Martian microbes would be so capable
under such varied conditions. Another strong inducement to skepticism is that a
fourth experiment, to look for organic chemicals in the Martian soil, gave uniformly
negative results despite its sensitivity. We expect life on Mars, like life on
Earth, to be organized around carbon-based molecules. To find no such molecules
at all was daunting for optimists among the exobiologists.
The apparently positive results of the
life detection experiments is now generally attributed to chemicals that
oxidize the soil, deriving ultimately from ultraviolet sunlight (as discussed
in the previous chapter). There is still a handful of Viking scientists who
wonder if there might be extremely tough and competent organisms very thinly
spread over the Martian soil-so their organic chemistry could not be detected,
but their metabolic processes could. Such scientists do not deny that
ultraviolet-generated oxidants are present in the Martian soil, but stress that
no thorough explanation of the liking life detection results from oxidants
alone has been forthcoming. Tentative claims have been made of organic matter
in SNC meteorites, but they seem instead to be contaminants that have entered
the meteorite after its arrival on our world. So far, there are no claims of
Martian microbes in these rocks from the sky.
Perhaps because it seems to pander to
public interest, NASA and most Viking scientists have been very chary about
pursuing the biological hypothesis. Even now, much more could be done in going
over the old data, in looking with Viking-type instruments at Antarctic and
other soils that have few microbes in them, in laboratory simulation of the
role of oxidants in the Martian soil, and in designing experiments to elucidate
these matters-not excluding further searches for life-with future Mars landers.
If indeed no unambiguous signatures of
life were determined by a variety of sensitive experiments at two sites 5,000
kilometers apart on a planet marked by global wind transport of fine particles,
this is at least suggestive that Mars may be, today at least, a lifeless
planet. But if Mars is lifeless, we have two planets, of virtually identical
age and early conditions, evolving next door to one another in the same polar
system: Life evolves and proliferates on one, but not the other. Why?
Perhaps the chemical or fossil remains
of early Martian life can still be found-subsurface, safely protected from the
ultraviolet radiation and its oxidation products that today fry the surface.
Perhaps in a rock face exposed by a landslide, or in the banks of an ancient
river valley or dry lake bed, or in the polar, laminated terrain, key evidence
for life on another planet is waiting.
Despite its absence on the surface of
Mars, the planet's two moons, Phobos and Deimos, seem to be rich in complex
organic matter dating back to the early history of the Solar System. The Soviet
Phobos 2 spacecraft found evidence of water vapor being out-gassed from Phobos,
as if it has an icy interior heated by radioactivity. The moons of Mars may
have long ago been captured from somewhere in the outer Solar System;
conceivably, they are among the nearest available examples of unaltered stuff
from the earliest days of the Solar System. Phobos and Deimos are very small,
each roughly 10 kilometers across; the gravity they exert is nearly negligible.
So it's comparatively easy to rendezvous with them, land on them, examine them,
use them as a base of operations to study Mars, and then go home.
Mars calls, a storehouse of scientific
information-important in its own right but also for the light it casts on the
environment of our own planet. There are mysteries waiting to be resolved about
the interior of Mars and its mode of origin, the nature of volcanos on a world
without plate tectonics, the sculpting of landforms on a planet with sandstorms
undreamt of on Earth, glaciers and polar landforms, the escape of planetary
atmospheres, and the capture of moons-to mention a more or less random sampling
of scientific puzzles. If Mars once had abundant liquid water and a clement
climate, what went wrong? How did an Earthlike world become so parched, frigid,
and comparatively airless? Is there something here we should know about our own
planet?
We humans have been this way before. The
ancient explorers would have understood the call of Mars. But mere scientific
exploration does not require a human presence. We tan always send smart robots.
They are far cheaper, they don't talk back, you can send them to much more
dangerous locales, arid, with some chance of mission failure always before us,
no lives are put at risk.
“HAVE YOU SEEN ME?” the back of the milk
carton read. “Mars Observer, 6' x 4. 5' x 3', 2500 kg. Last heard from on
8/21/93, 627,000 km from Mars.”
“M. O. call home” was the plaintive
message on a banner hung outside the jet Propulsion Laboratory's Mission
Operations Facility in late August 1993. The failure of the United States' Mars
Observer spacecraft just before it was to insert itself into orbit around Mars
was a great disappointment. It was the first post-launch mission failure of an
American lunar or planetary spacecraft in 26 years. Many scientists and
engineers had devoted a decade of their professional lives to M. O. It was the
first U. S. mission to Mars in 17 years-since Viking's two orbiters and two
landers in 1976. It was also the first real post-Cold War spacecraft: Russian
scientists were on several of the investigator teams, and Mars Observer was to
act as an essential radio relay link for larders from what was then scheduled
to be the Russian Mars '94 mission, as well as for a daring rover and balloon
mission slated for Mars '96.
The scientific instruments aboard Mars
Observer would have napped the geochemistry of the planet and prepared the way
for future missions, guiding landing site decisions. It might have cast a new
light on the massive climate change that seems to have occurred in early
Martian history. It would have photographed some of the surface of Mars with
detail better than two meters across. Of course, we do not know what wonders
Mars Observer would have uncovered. But every time we examine a world with new
instruments and in vastly improved detail, a dazzling array of discoveries
emerges just as it did when Galileo turned the first telescope toward the
heavens and opened the era of modern astronomy.
According to the Commission of Inquiry,
the cause of the failure was probably a rupture of the fuel tank during
pressurization, gases and liquids sputtering out, and the wounded spacecraft
spinning wildly out of control. Perhaps it was avoidable. Perhaps it was an
unlucky accident. But to keep this matter in perspective, let's consider the
full range of missions to the Moon and the planets attempted by the United
States and the former Soviet Union:
In the beginning, our track records were
poor. Space vehicles blew up at launch, missed their targets, or failed to
function when they got there. As time went on, we humans got ;)otter at
interplanetary flight. There was a learning curve. The ,adjacent figures show
these curves (based on NASA data with NASA definitions of mission success). We
learned very well. Our present ability to fix spacecraft in flight is best
illustrated by the Voyager missions described earlier.
We see that it wasn't until about its
thirty-fifth launch to the Moon or the planets that the cumulative U. S.
mission success rate got as high as 50 percent. The Russians took about 50
launches to get there. Averaging the shaky start and the better recent
performance, we find that both the United States and Russia have a cumulative
launch success rate of about 80 percent. But the cumulative mission success
rate is still under 70 percent for the U. S. and under 60 percent for the U. S.
S. R. /Russia. Equivalently, lunar and planetary missions have failed on
average 30 or 40 percent of the time.
Missions to other worlds were from the
beginning at the cutting edge of technology. They continue to be so today. They
. ire designed with redundant subsystems, and operated by dedicated and
experienced engineers, but they are not perfect. The amazing thing is not that
we have done so poorly, but that we leave done so well.
We don't know whether the Mars Observer
failure was due to incompetence or just statistics. But we must expect a steady
background of mission failures when we explore other worlds. no human lives are
risked when a robot spacecraft is lost. Even if we were able to improve this
success rate significantly, it would be far too costly. It is much better to
take more risks and fly more spacecraft:.
Knowing about irreducible risks, why do
we these days fly only one spacecraft per mission? In 1962 Mariner 1, intended
for Venus, fell into the Atlantic; the nearly identical Mariner 2 became the
human species' first successful planetary mission. Mariner 3 failed, arid its
twin Mariner 4 became, in 1964, the first spacecraft to take close-up pictures
of Mars. Or consider the 1971 Mariner 8/Mariner 9 dual launch mission to Mars.
Mariner 8 Was to map the planet. Mariner 9 was to study the enigmatic seasonal
and secular changes of surface markings. The spacecraft were otherwise
identical. Mariner 8 fell into the ocean. Mariner 9 flew on to Mars arid became
the first spacecraft in human history to orbit another planet. It discovered
the volcanos, the laminated terrain in the polar caps, the ancient river
valleys, and the aeolian nature of the surface changes. It disproved the
“canals.” It mapped the planet pole to pole and revealed all the major
geological features of Mars known to us today. It provided the first close-up
observations of members of a whole class of small worlds (by targeting the
Martian moons, Phobos and Deimos). If we had launched only Mariner 8, the
endeavor would have been an unmitigated failure. With a dual launch it became a
brilliant and historic success.
There were also two Vikings, two
Voyagers, two Vegas, many pairs of Veneras. Why was only one Mars Observer
flown? The standard answer is cost. Part of the reason it was so costly,
though, is that it was planned to be launched by shuttle, which is an almost
absurdly expensive booster for planetary missions-in this case too expensive
for two M. O. launches. After many shuttle-connected delays and cost increases,
NASA changed its mind and decided to launch Mars Observer on a Titan booster.
This required an additional two-year delay and an adapter to mate the
spacecraft to the new launch vehicle. If NASA had not been so intent on
providing business for the increasingly uneconomic shuttle, we could have
launched a couple of years earlier and maybe with two spacecraft instead of
one.
But whether in single launches or in
pairs, the space-faring nations have clearly decided that the time is ripe to
return robot explorers to Mars. Mission designs change; new nations enter the
field; old nations find they no longer have the resources. Even already funded
programs cannot always be relied upon. But current plans do reveal something of
the intensity of effort and the depth of dedication.
As I write this book, there are
tentative plans by the United States, Russia, France, Germany, Japan, Austria,
Finland, Italy, Canada, the European Space Agency, and other entities for a
coordinated robotic exploration of Mars. In the seven years between 1996 and
2003, a flotilla of some twenty-five spacecraft-most of them comparatively
small and cheap-are to be sent from Earth to Mars. There will be no quick
flybys among them; these are all long-duration orbiter and lander missions. The
United States will re-fly all of the scientific instruments that were lost on
Mars Observer. The Russian spacecraft will contain particularly ambitious
experiments involving some twenty nations. Communications satellites will
permit experimental stations anywhere on Mars to relay their data back to
Earth. Penetrators screeching down from orbit will punch into the Martian soil,
transmitting data from underground. Instrumented balloons and roving
laboratories will wander over the sands of Mars. Some microrobots will weigh no
more than a few pounds. Landing sites are being planned and coordinated.
Instruments will be cross-calibrated. Data will be freely exchanged. There is
every reason to think that in the coming years Mars and its mysteries will
become increasingly familiar to the inhabitants of the planet Earth.
IN THE COMMAND CENTER on Earth, in a
special room, you are helmeted and gloved. You turn your head to the left, and
the cameras on the Mars robot rover turn to the left. You see, in very high
definition and in color, what the cameras see. You take a step forward, and the
rover walks forward. You reach out your arm to pick up something shiny in the
soil, and the robot arm does likewise. The sands of Mars trickle through your
fingers. The only difficulty with this remote reality technology is that all
this must occur in tedious slow motion: The round-trip travel time of the
up-link commands from Earth to Mars and the down-link data returned from Mars
to Earth might take half an hour or more. But this is something we can learn to
do. We can learn to contain our exploratory impatience if that's the price of
exploring Mars. The rover can be made smart enough to deal with routine
contingencies. Anything more challenging, and it makes a dead stop, puts itself
into a safeguard mode, and radios for a very patient human controller to take
over.
Conjure up roving, smart robots, each of
them a small scientific laboratory, landing in the safe but dull places and
wandering to view close-up some of that profusion of Martian Wonders. Perhaps
every day a robot would rove to its own horizon; each morning we would see
close-up what had yesterday been only a distant eminence. The lengthening
progress of a traverse route over the Martian landscape would appear oil news
programs and in schoolrooms. People would speculate on what will be found.
Nightly newscasts from another planet, with their revelations of new terrains
and new scientific findings would make everyone on Earth a party to the
adventure.
Then there's Martian virtual reality:
The data sent back from Mars, stored in a modern computer, are fed into your
helmet and gloves and boots. You are walking in an empty room on Earth, but to
you you are on Mars: pink skies, fields of boulders, sand dunes stretching to
the horizon where an immense volcano looms; you hear the sand crunching under
your boots, you turn rocks over, dig a hole, sample the thin air, turn a
corner, and come face to face with...whatever new discoveries we will make on
Mars-all exact copies of what's on Mars, and all experienced from the safety of
a virtual reality salon in your hometown. This is not why we explore Mars, but
clearly we will need robot explorers to return the real reality before it can
be reconfigured into virtual reality.
Especially with continuing investment in
robotics and machine intelligence, sending humans to Mars can't be justified by
science alone. And many more people can experience the virtual Mars than could
possibly be sent to the real one. We can do very well with robots. If we're
going to send people, we'll need a better reason than science and exploration.
In the 1980s, I thought I saw a coherent
justification for human missions to Mars. I imagined the United States and the
Soviet Union, the two Cold War rivals that had put our global civilization at
risk, joining together in a far-seeing, high-technology endeavor that would
give hope to people everywhere. I pictured a kind of Apollo program in reverse,
in which cooperation, not competition, was the driving force, in which tire two
leading space-faring nations would together lay the groundwork for a major
advance in human history-the eventual settlement of another planet.
The symbolism seemed so apt. The same
technology that can propel apocalyptic weapons from continent to continent
would enable the first human voyage to another planet. It was a choice of
fitting mythic power: to embrace the planet named after, rather the madness
ascribed to, the god of war.
We succeeded in interesting Soviet
scientists and engineers in such a joint endeavor. Roald Sagdeev, then director
of the Institute for Space Research of the Soviet Academy of Sciences in
Moscow, was already deeply engaged in international cooperation on Soviet
robotic missions to Venus, Mars, and Halley's Comet, long before the idea
became fashionable. Projected joint use of the Soviet Mir space station and the
Saturn V-class launch vehicle Energiya made cooperation attractive to the
Soviet organizations that manufactured these items of hardware; they were
otherwise having difficulty justifying their wares. Through a sequence of
arguments (helping to bring the Cold War to an end being chief among them),
then-Soviet leader Mikhail S. Gorbachev was convinced. During the December 1987
Washington summit, Mr. Gorbachev-asked what was the most important joint
activity through which the two countries might symbolize the change in their
relationship-unhesitatingly replied, “Let's go to Mars together.”
But the Reagan Administration was not
interested. Cooperating with the Soviets, acknowledging that certain Soviet
technologies were more advanced than their American counterparts, making some
American technology available to the Soviets, sharing credit, providing an
alternative for the arms manufacturers-these were not to the Administration's
liking. The offer was turned down. Mars would have to wait.
In only a few years, times have changed.
The Cold War is over. The Soviet Union is no more. The benefit deriving from
the two nations working together has lost some of its force. Other
nations-especially Japan and the constituent members of the European Space
Agency-have become interplanetary travelers. Many just and urgent demands are
levied on the discretionary budgets of the nations.
But the Energiya heavy-lift booster
still awaits a mission. The workhorse Proton rocket is available. The Mir space
station-with a crew on board almost continuously-still orbits the Earth every
hour and a half. Despite internal turmoil, the Russian space program continues
vigorously. Cooperation between Russia and America in space is accelerating. A
Russian cosmonaut, Sergei Krikalev, in 1994 flew on the shuttle Discovery (for
the usual one-week shuttle mission duration; Krikalev had already logged 464
days aboard the Mir space station). U. S. astronauts will visit Mir. American
instruments-including one to examine the oxidants thought to destroy organic
molecules in the Martian soil-are to be carried by Russian space vehicles to
Mars. Mars Observer was designed to serve as a relay station for landers in
Russian Mars missions. The Russians have offered to include a U. S. orbiter in
a forthcoming Proton-launched multipayload mission to Mars.
The American and Russian capabilities in
space science and technology mesh; they interdigitate. Each is strong where the
other is weak. This is a marriage made in heaven-but one that has been
surprisingly difficult to consummate.
On September 2, 1993, an agreement to
cooperate in depth was signed in Washington by Vice President Al Gore and Prime
Minister Viktor Chernomyrdin. The Clinton Administration has ordered NASA to
redesign the U. S. space station (called Freedom in the Reagan years) so it is
in the same orbit as Mir and can be mated to it: Japanese and European modules
will be attached, as will a Canadian robot arm. The designs have now evolved
into what is called Space Station Alpha, involving almost all the spacefaring
nations. (China is the most notable exception.)
In return for U. S. space cooperation
and an infusion of hard currency, Russia in effect agreed to halt its sale of
ballistic missile components to other nations, and generally to exercise tight
controls on its export of strategic weapons technology. In this Way, space
becomes once again, as it was at the height of the Cold War, an instrument of
national strategic policy.
This new trend has, though, made some of
the American aerospace industry and some key members of Congress profoundly
uneasy. Without international competition, can we motivate such ambitious
efforts? Does every Russian launch vehicle used cooperatively mean less support
for the American aerospace industry? Can Americans rely on stable support and
continuity of effort in joint projects with the Russians? (The Russians, of
course, ask similar questions about the Americans.) But cooperative programs in
the long term save money, draw upon the extraordinary scientific and
engineering talent distributed over our planet, and provide inspiration about
the global future. There may be fluctuations in national commitments. We are
likely to take backward as well as forward steps. But the overall trend seems
clear.
Despite growing pains, the space
programs of the two former adversaries are beginning to join. It is now
possible to foresee a world space station-not of any one nation but of the
planet Earth-being assembled at 51° inclination to the equator and a few
hundred miles up. A dramatic joint mission, called “Fire and Ice,” is being
discussed in which a fast flyby would be sent to Pluto, the last unexplored
planet; but to get there, a gravity assist from the Sun would be employed, in
the course of Which small probes would actually enter the Sun's atmosphere. And
we seem to be on the threshold of a World Consortium for the scientific
exploration of Mars. It very much looks as though such projects will be done
cooperatively or not at all.
WHETHER THERE ARE VALID, cost-effective,
broadly supportable reasons for people to venture to Mars is an open question.
Certainly there is no consensus. The matter is treated in the next chapter.
I would argue that if we are not
eventually going to send people to worlds as far away as Mars, we have lost the
chief reason for a space station-a permanently (or intermittently) occupied
human outpost in Earth orbit. A space station is far from an optimum platform
for doing science either looking down at the Earth, or looking out into space,
or for utilizing microgravity (the very presence of astronauts messes things
up). For military reconnaissance it is much inferior to robotic spacecraft.
There are no compelling economic or manufacturing applications. It is expensive
compared to robotic spacecraft. And of course it runs some risk of losing human
lives. Every shuttle launch to help build or supply a space station has an
estimated 1 or 2 percent chance of catastrophic failure. Previous civilian and
military space activities have littered low Earth orbit with fast-moving
debris-that sooner or later will collide with a space station (although, so
far, Mir has had no failures from this hazard). A space station is also
unnecessary for human exploration of the Moon. Apollo got there very well with
no space station at all. With Saturn V or Energiya class launchers, it also may
be possible to get to near-earth asteroids or even Mars without having to
assemble the interplanetary vehicle on an orbiting space station.
A space station could serve
inspirational and educational purposes, and it certainly can help to solidify
relations among the spacefaring nations-particularly the United States and
Russia. But the only substantive function of a space station, as far as I can
see, is for long-duration spaceflight. How do humans behave in micro gravity?
How can we counter progressive changes in blood chemistry and an estimated 6
percent bone loss per year in zero gravity? (For a three- or four-year mission
to Mars this adds up, if the travelers have to go at zero g.)
These are hardly questions in
fundamental biology such as DNA or the evolutionary process; instead they
address issues of applied human biology. It's important to know the answers,
but only if we intend to go somewhere in space that's far away and takes a long
time to get there. The only tangible and coherent goal of a space station is
eventual human missions to near-Earth asteroids, Mars, and beyond. Historically
NASA has been cautious about stating this fact clearly, probably for fear that
members of Congress will throw up their hands in disgust, denounce the space
station as the thin edge of an extremely expensive wedge, and declare the
country unready to commit to launching people to Mars. In effect, then, NASA
has kept quiet about what the space station is really for. And yet if we had
such a space station, nothing would require us to go straight to Mars. We could
use a space station to accumulate and refine the relevant knowledge, and take
as long as we like to do so-so that when the time does come, when we are ready
to go to the planets, we will have the background and experience to do so
safely.
The Mars Observer failure, and the
catastrophic loss of the space shuttle Challenger in 1986, remind us that there
will be a certain irreducible chance of disaster in future human flights to
Mars and elsewhere. The Apollo 13 mission, which was unable to land on the Moon
and barely returned safely to Earth, underscores how lucky we've been. We
cannot make perfectly safe autos or trains even though we've been at it for
more than a century. Hundreds of thousands of years after we first domesticated
fire, every city in the world has a service of firefighters biding their time
until there's a blaze that needs putting out. In Columbus' four voyages to the
New World, he lost ships left and right, including one third of the little
fleet that set out in 1492.
If we are to send people, it must be for
a very good reason-and with a realistic understanding that almost certainly we
will lose lives. Astronauts and cosmonauts have always understood this.
Nevertheless, there has been and will be no shortage of volunteers.
But why Mars? Why not return to the
Moon? It's nearby, and we've proved we know how to send people there. I'm
concerned that the Moon, close as it is, is a long detour, if not a dead end.
We've been there. We've even brought some of it back. People have seen the Moon
rocks, and, for reasons that I believe are fundamentally sound, they are bored
by the Moon. It's a static, airless, waterless, black-sky, dead world. Its most
interesting aspect perhaps is its cratered surface, a record of ancient
,catastrophic impacts, on the Earth as well as on the Moon.
Mars, by contrast, has weather, dust
storms, its own moons, volcanos, polar ice caps, peculiar landforms, ancient
river valleys, and evidence of massive climatic change on a once-Earthlike
world. It holds some prospect of past or maybe even present life, and is the
most congenial planet for future life-humans transplanted from Earth, living
off the land. None of this is true for the Moon. Mars also has its own legible
cratering history. It Mars, rather than the Moon, had been within easy reach,
we would not have backed off from manned space flight.
Nor is the Moon an especially desirable
test bed or way station for Mars. The Martian and lunar environments are very
different, and the Moon is as distant from Mars as is the Earth. The machinery
for Martian exploration can at least equally well be tested in Earth orbit, or
on near-Earth asteroids, or on the Earth itself-in Antarctica, for instance.
Japan has tended to be skeptical of the
commitment of the United States and other nations to plan and execute major
cooperative projects in space. This is at least one reason that Japan, more
than any other spacefaring nation, has tended to go it alone. The Lunar and
Planetary Society of Japan is an organization representing space enthusiasts in
the government, universities, and major industries. As I write, the Society is
proposing to construct and stock a lunar base entirely with robot labor. It is
said to take about 30 years and to cost about a billion U. S. dollars a year
(which would represent 7 percent of the present U. S. civilian space budget).
Humans would arrive only when the base is fully ready. The use of robot
construction crews under radio command from Earth is said to reduce the cost
tenfold. The only trouble with the scheme, according to reports, is that other
scientists in Japan keep asking, “What's it for?” That's a good question in
every nation.
The first human mission to Mars is now
probably too expensive for any one nation to pull off by itself. Nor is it
fitting that such a historic step be taken by representatives of only a small
fraction of the human species. But a cooperative venture among the United
States, Russia, Japan, the European Space Agency-and perhaps other nations,
such as China-might be feasible in the not too distant future. The
international space station will have tested our ability to work together on
great engineering projects in space.
The cost of sending a kilogram of
something no farther away than low Earth orbit is today about the same as the
cost of kilogram of gold. This is surely a major reason we have yet to stride
the ancient shorelines of Mars. Multistage chemical rockets are the means that
first took us into space, and that's what we've been using ever since. We've
tried to refine them, to make there safer, more reliable, simpler, cheaper. But
that hasn't happened, or at least not nearly as quickly as many had hoped.
So maybe there's a better way: maybe
single-stage rockets that can launch their payloads directly to orbit; maybe
many small payloads shot from guns or rocket-launched from airplanes; maybe
supersonic ramjets. Maybe there's something much better that we haven't thought
of yet. If we can manufacture propellants for the return trip from the air and
soil of our destination world, the difficulty of the voyage would be greatly
eased.
Once we're up there in space, venturing
to the planets, rocketry is not necessarily the best means to move large
payloads around, even with gravity assists. Today, we make a few early rocket
burns and later midcourse corrections, and coast the rest of the way. But there
are promising ion and nuclear/ electric propulsion systems by which a small and
steady acceleration is exerted. Or, as the Russian space pioneer Konstantin
Tsiolkovsky first envisioned, we could employ solar sails-vast but very thin
films that catch sunlight and the solar wind, a caravel kilometers wide plying
the void between the worlds. Especially for trips to Mars and beyond, such
methods are far better than rockets.
As with most technologies, when
something barely works, when it's the first of its kind, there's a natural
tendency to improve it, develop it, exploit it. Soon there's such an
institutional investment in the original technology, no matter how flawed, that
it's very hard to move on to something better. NASA has almost no resources to
pursue alternative propulsion technologies. That money would have to come out
of near-term missions, missions which could provide concrete results and
improve NASA's success record. Spending money on alternative technologies pays
off a decade or two in the future. We tend to be very little interested in a
decade or two in the future. This is one of the ways by which initial success
can sow the seeds of ultimate failure; and is very similar to what sometimes
happens in biological evolution. But sooner or later some nation-perhaps one
without a huge investment in marginally effective technology-will develop
effective alternatives.
Even before then, if we take a
cooperative path, there will come a time-perhaps in the first decades of the
new century and the new millennium-when an interplanetary spacecraft is
assembled in Earth orbit, the progress in full view on the evening news.
Astronauts and cosmonauts, hovering like gnats, guide and mate the
prefabricated parts. Eventually the ship, tested and ready, is boarded by its
international crew, and boosted to escape velocity. For the whole of the voyage
to Mars and back, the lives of the crew members depend on one another, a
microcosm of our actual circumstances down here on Earth. Perhaps the first
joint interplanetary mission with human crews will be only a flyby or orbit of
Mars. Earlier, robot vehicles, with aerobraking, parachutes, and retrorockets,
will have set gently down on the Martian surface to collect samples and return
them to Earth, and to emplace supplies for future explorers. But whether or not
we have compelling, coherent reasons, I am sure-unless we destroy ourselves
first-that the day will come when we humans set foot on Mars. It is only a
matter of when.
According to solemn treaty, signed in
Washington and Moscow on January 27, 1967, no nation may lay claim to part or all
of another planet. Nevertheless-for historical reasons that Columbus would have
understood well-some people are concerned about who first sets foot on Mars. If
this really worries us, we can arrange for the ankles of the crew members to be
tied together as they alight in the gentle Martian gravity.
The crews would acquire new and
previously sequestered samples, in part to search for life, in part to
understand the past and future of Mars and Earth. They would experiment, for
later expeditions, on extracting water, oxygen, and hydrogen from the rocks and
the air and from the underground permafrost-to drink, to breathe, to power
their machines and, as rocket fuel and oxidizer, to propel the return voyage.
They would test Martian materials for eventual fabrication of bases and
settlements on Mars.
And they would go exploring. When I
imagine the early human exploration of Mars, it's always a roving vehicle, a
little like a jeep, wandering down one of the valley networks, the crew with
geological hammers, cameras, and analytic instruments at the ready. They're
looking for rocks from ages past, signs of ancient cataclysms, clues to climate
change, strange chemistries, fossils, or-most exciting and most
unlikely-something alive. Their discoveries are televised back to Earth at the
speed of light. Snuggled up in bed with the kids, you explore the ancient
riverbeds of Mars.
CHAPTER 16
Scaling heaven
Who, my friend, can scale heaven?
—THE EPIC OF GILGAMESH (SUMER, THIRD
MILLENNIUM B. C.)
What?, I sometimes ask myself in
amazement: Our ancestors walked from East Africa to Novaya Zemlya and Ayers
Rock and Patagonia, hunted elephants with stone spearpoints, traversed the
polar seas in open boats 7,000 years ago, circumnavigated the Earth propelled
by nothing but wind, walked the Moon a decade after entering space-and we're
daunted by a voyage to Mars? But then I remind myself of the avoidable human
suffering on Earth, how a few dollars can save the life of a child dying of
dehydration, how many children we could save for the cost of a trip to Mars-and
for the moment I change my mind. Is it unworthy to stay home or unworthy to go?
Or have I posed a false dichotomy? Isn't it possible to make a better life for
everyone on Earth and to reach for the planets and the stars?
We had an expansive run in the '60s and
'70s. You might have thought, as I did then, that our species would be on Mars
before the century was over. But instead, we've pulled inward. Robots aside,
we've backed off from the planets and the stars. 1 keep asking myself Is it a
failure of nerve or a sign of maturity?
Maybe it's the most we could reasonably
have expected. In a way it's amazing that it was possible at all: We sent a
dozen humans on week-long excursions to the Moon. And we were given the
resources to make a preliminary reconnaissance of the whole Solar System, out
to Neptune anyway-missions that returned a wealth of data, but nothing of
short-term, everyday, bread-on-the-table practical value. They lifted the human
spirit, though. They enlightened us about our place in the Universe. It's easy
to imagine skeins of historical causality in which there were no race to the
Moon and no planetary program.
But it's also possible to imagine a much
more serious devotion to exploration, because of which we would today have
robot vehicles probing the atmospheres of all the Jovian planets and dozens of
moons, comets, and asteroids; a network of automatic scientific stations
emplaced on Mars would daily be reporting their findings; and samples from many
worlds would be under examination in the laboratories of Earth-revealing their
geology, chemistry, and perhaps even their biology. Human outposts might be
already established on the near-Earth asteroids, the Moon, and Mars.
There were many possible historical
paths. Our particular causality skein has brought us to a modest and
rudimentary, although in many respects heroic, series of explorations. But it
is far interior to what might have been-and what may one day be.
TO CARRY THE GREEN Promethean spark of
Life with us into the sterile void and ignite there a firestorm of animate
matter is the very destiny of our race,” reads the brochure of something called
the First Millennial Foundation. It promises, for $120 a year, “citizenship” in
“space colonies-when the time comes.” “Benefactors” who contribute more also
receive “the undying gratitude of a star-flung civilization, and their name
carved on the monolith to be erected on the Moon.” This represents one extreme
in the continuum of enthusiasm for a human presence in space. The other
extreme-better represented in Congress-questions why we should be in space at
all, especially people rather than robots. The Apollo program was a
“moondoggle,” the social critic Amitai Etzioni once called it; with the Cold
War over, there is no justification whatever, proponents of this orientation
hold, for a manned space program. Where in this spectrum of policy options
should we be?
Ever since the United States beat the
Soviet Union to the Moon, a coherent, widely understood justification for
humans in space seems to have vanished. Presidents and Congressional committees
puzzle over what to do with the manned space program. What is it for? Why do we
need it? But the exploits of the astronauts and the moon landings had
elicited-and for good reason-the admiration of the world. It would be a
rejection of that stunning American achievement, the political leaders tell
themselves, to back off from manned spaceflight. Which President, which
Congress wishes to be responsible for the end of the American space program?
And in the former Soviet Union a similar argument is heard: Shall we abandon,
they ask themselves, the one remaining high technology in which we are still
world leaders? Shall we be faithless heirs of Konstantin Tsiolkovsky, Sergei Korolev,
and Yuri Gagarin?
The first law of bureaucracy is to
guarantee its own continuance. Left to its own devices, without clear
instructions from above, NASA gradually devolved into a program that would
maintain profits, jobs, and perquisites. Pork-barrel politics, with Congress
playing a leading role, became an increasingly powerful force in the design and
execution of missions and long-term goals. The bureaucracy ossified. NASA lost
its way.
On July 20, 1989, the twentieth
anniversary of the Apollo 11 landing on the Moon, President George Bush
announced a long-term direction for the U. S. space program. Called the Space
Exploration Initiative (SEI), it proposed a sequence of goals including a U. S.
space station, a return of humans to the Moon, and the first landing of humans
on Mars. In a later statement, Mr. Bush set 2019 as the target date for the
first footfall on that planet.
And yet the Space Exploration
Initiative, despite clear direction from the top, foundered. Four years after
it was mandated, it did not even have a NASA office dedicated to it. Small and
inexpensive lunar robotic missions-that otherwise might well have been
approved-were canceled by Congress because of guilt by association with SEI.
What went wrong?
One problem was the timescale. SEI
extended five or so presidential terms of office into the future (taking the
average presidency as one and a half terms). That makes it easy for a president
to attempt to commit his successors, but leaves in considerable doubt how
reliable such a commitment might be. SEI contrasted dramatically with the
Apollo program-which, it might have been conjectured at the time it began,
could have triumphed when President Kennedy or his immediate political heir was
still in office.
Second, there was concern about whether
NASA, which had recently experienced great difficulty in safely lifting a few
astronauts 200 miles above the Earth, could send astronauts on an arcing
year-long trajectory to a destination 100 million miles away and bring them
back alive.
Third, the program was conceived
exclusively in nationalist terms. Cooperation with other nations was not
fundamental to either design or execution. Vice President Dan Quayle, who had
nominal responsibility for space, justified the space station as a demonstration
that the United States was “the world's only superpower.” But since the Soviet
Union had an operational space station that was a decade ahead of the United
States, Mr. Quayle's argument proved difficult to follow.
Finally, there was the question of
where, in terms of practical politics, the money was supposed to come from. The
costs of getting the first humans to Mars had been variously estimated, ranging
as high as $500 billion.
Of course, it's impossible to predict
costs before you have a mission design. And the mission design depends on such
matters as the size of the crew; the extent to which you take mitigating steps
against solar and cosmic radiation hazards, or zero gravity; and what other
risks you are willing to accept with the lives of the men and women on board.
If every crew member has one essential specialty, what happens if one of them
falls ill? The larger the crew, the more reliable the backups. You would almost
certainly not send a full-time oral surgeon, but what happens if you need root
canal work and you're a hundred million miles from the nearest dentist? Or
could it be done by an endodontist on Earth, using telepresence?
Wernher von Braun was the Nazi-American
engineer who, more than anyone else, actually took us into space. His 1952 book
Das Marsprojekt envisioned a first mission with 10 interplanetary spacecraft,
70 crew members, and 3 “landing boats.” Redundancy was uppermost in his mind.
The logistical requirements, he wrote, “are no greater than those for a minor
military operation extending over a limited theater of war.” He meant to
“explode once and for all the theory of the solitary space rocket and its
little band of bold interplanetary adventurers,” and appealed to Columbus'
three ships without which “history tends to prove that he might never have
returned to Spanish shores.” Modern Mars mission designs have ignored this
advice. They are much less ambitious than yon Braun's, typically calling for
one or two spacecraft crewed by three to eight astronauts, with another robotic
cargo ship or two. The solitary rocket and the little band of adventurers are
still with us.
Other uncertainties affecting mission
design and cost include whether you pre-emplace supplies from Earth and launch
humans to Mars only after the supplies are safely landed; whether you can use
Martian materials to generate oxygen to breathe, water to drink, and rocket
propellants to get home; whether you land using the thin Martian atmosphere for
aerobraking; the degree of redundancy in equipment thought prudent; the extent
to which you use closed ecological systems or just depend on the food, water,
and waste disposal facilities you've brought from Earth; the design of roving
vehicles for the crew to explore the Martian landscape; and how much equipment
you're willing to carry to test our ability to live off the land in later
voyages.
Until such questions are decided, it's
absurd to accept any figure for the cost of the program. On the other hand, it
was equally clear that SEI would be extremely expensive. For all these reasons,
the program was a nonstarter. It was stillborn.
There was no effective attempt by the
Bush Administration to spend political capital to get SEI going.
The lesson to me seems clear: There may
be no way to Send humans to Mars in the comparatively near future-despite the
fact that it is entirely within our technological capability. Governments do
not spend these vast sums just for science, or merely to explore. They need
another purpose, and it must make real political sense.
It may be impossible to go just yet, but
when it is possible, the mission, I think, must be international from the
start, with costs and responsibilities equitably shared and the expertise of
many nations tapped; the price must be reasonable; the time from approval to
launch must fit within practical political timescales; and the space agencies
concerned must demonstrate their ability to muster pioneering exploratory
missions with human crews safely, on time, and on budget. If it were possible
to imagine such a mission for less than $100 billion, and for a time from
approval to launch less than 15 years, maybe it would be feasible. (In terms of
cost, this would represent only a fraction of the annual civilian space budgets
of the present spacefaring nations.) With aerobraking and manufacturing fuel
and oxygen for the return trip out of Martian air, it's now beginning to look
as if such a budget and such a timescale might actually be realistic.
The cheaper and quicker the mission is,
necessarily the more risk we must be willing to take with the lives of the
astronauts and cosmonauts aboard. But as is illustrated, among countless
examples, by the samurai of medieval Japan, there are always competent
volunteers for highly dangerous missions in what is perceived as a great cause.
no budget, no timeline can be really reliable when we attempt to do something
on so grand a scale, something that has never been done before. The more leeway
we ask, the greater is the cost and the longer it takes to get there. Finding
the right compromise between political feasibility and mission success may be
tricky.
IT'S NOT ENOUGH to go to Mars because
some of us have dreamt of doing so since childhood, or because it seems to us
the obvious long-term exploratory goal for the human species. If we're talking about
spending this much money, we must justify the expense.
There are now other matters-clear,
crying national needs-that cannot be addressed without major expenditures; at
the same time, the discretionary federal budget has become painfully
constrained. Disposal of chemical and radioactive poisons, energy efficiency,
alternatives to fossil fuels, declining rates of technological innovation, the
collapsing urban infrastructure, the AIDS epidemic, a witches' brew of cancers,
homelessness, malnutrition, infant mortality, education, jobs, health
care-there is a painfully long list. Ignoring them will endanger the well-being
of the nation. A similar dilemma faces all the spacefaring nations.
Nearly every one of these matters could
cost hundreds of billions of dollars or more to address. Fixing infrastructure
will cost several trillion dollars. Alternatives to the fossil-fuel economy
clearly represent a multitrillion-dollar investment worldwide, if we can do it.
These projects, we are sometimes told, are beyond our ability to pay. How then
can we afford to go to Mars?
If there were 20 percent more
discretionary funds in the U. S. federal budget (or the budgets of the other
spacefaring nations), I probably would not feel so conflicted about advocating
sending humans to Mars. If there were 20 percent less, I don't think the most
diehard space enthusiast would be urging such a mission. Surely there is some
point at which the national economy is in such dire straits that sending people
to Mats is unconscionable. The question is where we draw the line. plainly such
a line exists, and every participant in these debates should stipulate where
that line should be drawn, what fraction of the gross national product for
space is too much. I'd like the same thing done for “defense.”
Public opinion polls show that many
Americans think the NASA budget is about equal to the defense budget. In fact,
the entire NASA budget, including human and robotic missions and aeronautics,
is about 5 percent of the U. S. defense budget. How much spending for defense
actually weakens the country? And even if NASA were cancelled altogether, would
we free up what is needed to solve our national problems?
HUMAN SPACEFLIGHT In general-to say
nothing of expeditions to Mars-would be much more readily supportable if, as in
the fifteenth-century arguments of Columbus and Henry the Navigator, there were
a profit lure. Some arguments have been
advanced. The high vacuum or low gravity or intense radiation environment of
near-Earth space might be utilized, it is said, for commercial benefit. All
such proposals must be challenged by this question: Could comparable or better
products be manufactured down here on Earth if the development money made
available were comparable to what is being poured into the space program?
Judging by how little money corporations have been willing to invest in such
technology-apart from the entities building the rockets and spacecraft
themselves-the prospects, at least at present, seem to be not very high.
The notion that rare materials might be
available elsewhere is tempered by the fact that freightage is high. There may,
for all we know, be oceans of petroleum on Titan, but transporting it to Earth
will be expensive. Platinum-group metals may be abundant in certain asteroids.
If we could move these asteroids into orbit around the Earth, perhaps we could
conveniently mine them. But at least for the foreseeable future this seems
dangerously imprudent, as I describe later in this book.
In his classic science fiction novel The
Man Who Sold the Moon, Robert Heinlein imagined the profit motive as the key to
space travel. He hadn't foreseen that the Cold War would sell the Moon. But he
did recognize that an honest profit argument would be difficult to come by.
Heinlein envisioned, therefore, a scam in which the lunar surface was salted
with diamonds so later explorers could breathlessly discover them and initiate
a diamond rush. We've since returned samples from the Moon, though, and there
is not a hint of commercially interesting diamonds there.
However, Kiyoshi Kuramoto and Takafumi
Matsui of the University of Tokyo have studied how the central iron cores of
Earth, Venus, and Mars formed, and find that the Martian mantle (between crust
and core) should be rich in carbon-richer than that of the Moon or Venus or
Earth. Deeper than 300 kilometers, the pressures should transform carbon into
diamond. We know that Mars has been geologically active over its history.
Material from great depth will occasionally be extruded up to the surface, and
not just in the great volcanos. So there does seem to be a case for diamonds on
other worlds-on Mars, lied not the Moon. In what quantities, of what quality
and size, and in which locales we do not yet know.
The return to Earth of a spacecraft
stuffed with gorgeous multicarat diamonds would doubtless depress prices (as
well as the shareholders of the de Beers and General Electric corporations).
But because of the ornamental and industrial applications of diamonds, perhaps
there is a lower limit below which prices will not go. Conceivably, the
affected industries might find cause to promote the early exploration of Mars.
The idea that Martian diamonds will pay
for exploring Mars is at best a very long shot, but it's an example of how rare
and valuable substances may be discoverable on other worlds. It would be
foolish, though, to count on such contingencies. If we seek to justify missions
to other worlds, we'll have to find other reasons.
BEYOND DISCUSSIONS OF PROFITS and costs,
even reduced costs, we must also describe benefits, if they exist. Advocates of
human missions to Mars must address whether, in the long term, missions up
there are likely to mitigate any of the problems down here. Consider now the
standard set of justifications and see if you find them valid, invalid, or
indeterminate:
Human missions to Mars would
spectacularly improve our knowledge of the planet, including the search for
present and past life. The program is likely to clarify our understanding of
the environment of our own planet, as robotic missions have already begun to
do. The history of our civilization shows that the pursuit of basic knowledge
is the way the most significant practical advances come about. Opinion polls
suggest that the most popular reason for “exploring space” is “increased
knowledge.” But are humans in space essential to achieve this goal? Robotic
missions, given high national priority and equipped with improved machine
intelligence, seem to me entirely capable of answering, as well as astronauts
can, all the questions we need to ask-and at Maybe 10 percent the cost.
It is alleged that “spinoff” will
transpire-huge technological benefits that would otherwise fail to come
about-thereby improving our international competitiveness and the domestic
economy. But this is an old argument: Spend $80 billion (in contemporary money)
to send Apollo astronauts to the Moon, and we'll throw in a free stickless
frying pan. Plainly, if we're after frying pans, we can invest the money
directly and save almost all of that $80 billion.
The argument is specious for other
reasons as well, one of which is that DuPont's Teflon technology long antedated
Apollo. The same is true of cardiac pacemakers, ballpoint pens, Velcro, and
other purported spinoffs of the Apollo program. (I once had the opportunity to
talk with the inventor of the cardiac pacemaker, who himself nearly had a
coronary accident describing the injustice of what he perceived as NASA taking
credit for his device.) If there are technologies we urgently need, then spend
the money and develop them. Why go to Mars to do it?
Of course it would be impossible for so
much new technology as NASA requires to be developed and not have some
spillover into the general economy, some inventions useful down here. For
example, the powdered orange juice substitute Tang was a product of the manned
space program, and spinoffs have occurred in cordless tools, implanted cardiac
defibrillators, liquid-cooled garments, and digital imaging-to name a few. But
they hardly justify human voyages to Mars or the existence of NASA.
We could see the old spinoff engine
wheezing and puffing in the waning days of the Reagan-era Star Wars office.
Hydrogen bomb-driven X-ray lasers on orbiting battle stations will help perfect
laser surgery, they told us. But if we need laser surgery, if it's a high
national priority, by all means let's allocate the funds to develop it. just
leave Star Wars out of it. Spinoff justifications constitute an admission that
the program can't stand on its own two feet, cannot be justified by the purpose
for which it was originally sold.
Once upon a time it was thought, on the
basis of econometric models, that for every dollar invested in NASA many
dollars were pumped into the U. S. economy. If this multiplier effect applied
more to NASA than to most government agencies, it would provide a potent fiscal
and social justification for the space program. NASA supporters were not shy
about appealing to this argument. But a 1994 Congressional Budget Office study
found it to be a delusion. While NASA spending benefits some production
segments of the U. S. economy-especially the aerospace industry-there is no
preferential multiplier effect. Likewise, while NASA spending certainly creates
or maintains jobs and profits, it does so no more efficiently than many other
government agencies.
Then there's education, an argument that
has proved from time to time very attractive in the White House. Doctorates in
science peaked somewhere around the time of Apollo 11, maybe even with the
proper phase lag after the start of the Apollo program. The cause-and-effect
relationship is perhaps undemonstrated, although not implausible. But so what?
If we're interested in improving education, is going to Mars the best route?
Think of what we could do with $100 billion for teacher training and salaries,
school laboratories and libraries, scholarships for disadvantaged students,
research facilities, and graduate fellowships. Is it really true that the best
way to promote science education is to go to Mars?
Another argument is that human missions
to Mars will occupy the military-industrial complex, diffusing the temptation
to use its considerable political muscle to exaggerate external threats and
pump up defense funding. The other side of this particular coin is that by
going to Mars we maintain a standby technological capacity that might be
important for future military contingencies. Of course, we might simply ask
those guys to do something directly useful for the civilian economy. But as we
saw in the 1970s with Grumman buses and Boeing/Vertol commuter trains, the
aerospace industry experiences real difficulty in producing competitively for
the civilian economy. Certainly a tank may travel 1,000 miles a year and a bus
1,000 miles a week, so the basic designs must be different. But on matters of
reliability at least, the Defense Department seems to be much less demanding.
Cooperation in space, as I've already
mentioned, is becoming an instrument of international cooperation-for example,
in slowing the proliferation of strategic weapons to new nations. Rockets
decommissioned because of the end of the Cold War might be gainfully employed
in missions to Earth orbit, the Moon, the planets, asteroids, and comets. But
all this can be accomplished without human missions to Mars.
Other justifications are offered. It is
argued that the ultimate solution to world energy problems is to strip-mine the
Moon, return the solar-wind-implanted helium-3 back to Earth, and use it in
fusion reactors. What fusion reactors? Even if this were possible, even if it
were cost-effective, it is a technology 50 or 100 years away. Our energy
problems need to be solved at a less leisurely pace.
Even stranger is the argument that we
have to send human beings into space in order to solve the world population
crisis. But some 250,000 more people are born than die every day— which means
cans that we would have to launch 250,000 people per day into space to maintain
world population at its present levels. This appears to be beyond our present
capability.
I RUN THROUGH such a list and try to add
up the pros and cons, bearing in mind the other urgent claims on the federal
budget. To me, the argument so far comes down to this question: Can the sum of
a large number of individually inadequate Justifications add up to an adequate
justification?
I don't think any of the items on my
list of purported justifications is demonstrably worth $500 billion or even
$100 billion, certainly not in the short term. On the other hand, most of them
are worth something, and if I have five items each worth $20 billion, maybe it
adds up to $100 billion. If we can be clever about reducing costs and making
true international partnerships, the justifications become more compelling.
Until a national debate on this topic
has transpired, until we have a better idea of the rationale and the
cost/benefit ratio of human missions to Mars, what should we do? My suggestion
is that we pursue research and development projects that can be justified on
their own merits or by their relevance to other goals, but that can also
contribute to human missions to Mars should we later decide to go. Such an
agenda would include:
•U. S. astronauts on the Russian space
station Mir for joint flights of gradually increasing duration, aiming at one
to two years, the Mars flight time.
•Configuration of the international
space station so its principal function is to study the long-term effects of
the space environment on humans.
• Early implementation of a rotating or
tethered “artificial gravity” module on the international space station, for
other animals and then for humans.
• Enhanced studies of the Sun, including
a distributed set of robot probes in orbit about the Sun, to monitor solar
activity and give the earliest possible warning to astronauts of hazardous
“solar flares”-mass ejections of electrons and protons from the Sun's corona.
• U. S. /Russian and multilateral
development of Energiya and Proton rocket technology for the U. S. and
international space programs. Although the United States is unlikely to depend
primarily on a Soviet booster, Energiya has roughly the lift of the Saturn V
that sent the Apollo astronauts to the Moon. The United States let the Saturn V
assembly line die, and it cannot readily be resuscitated. Proton is the most
reliable large booster now in service. Russia is eager to sell this technology
for hard currency.
• Joint projects with NASDA (the
Japanese space agency) and Tokyo University, the
European Space Agency, and the Russian
Space Agency, along with Canada and other nations. In most cases these should
be equal partnerships, not the United States insisting on calling the shots.
For the robotic exploration of Mars, such programs are already under way. For
human flight, the chief such activity is clearly the international space
station. Eventually, we might muster joint simulated planetary missions in low
Earth orbit. One of the principal objectives of these programs should be to
build a tradition of cooperative technical excellence.
• Technological development-using
state-of-the-art robotics and artificial intelligence— of rovers, balloons, and
aircraft for the exploration of Mars, and implementation of the first
international return sample mission. Robotic spacecraft that can return samples
from Mars can be tested on near-Earth asteroids and the Moon. Samples returned
from carefully selected regions of the Moon can have their ages determined and
contribute in a fundamental way to our understanding of the early history of
the Earth.
•Further development of technologies to
manufacture fuel and oxidizer out of Martian materials. In one estimate, based
on a prototype instrument designed by Robert Zubrin and colleagues at the
Martin Marietta Corporation, several kilograms of Martian soil can be
automatically returned to Earth using a modest and reliable Delta launch
vehicle, all for no more than a song (comparatively speaking).
• Simulations on Earth of long-duration
trips to Mars, concentrating on potential social and psychological problems.
•Vigorous pursuit of new technologies
such as constant-thrust propulsion to get us to
Mars quickly; this may be essential if
the radiation or microgravity hazards make one—year (or longer) flight times
too risky.
•Intensive study of near-Earth
asteroids, which may provide superior intermediate— timescale objectives for human
exploration than does the Moon.
•A greater emphasis on science-including
the fundamental sciences behind space exploration, and the thorough analysis of
data already obtained-by NASA and other space agencies.
These recommendations add up to a
fraction of the full cost of a human mission to Mars and-spread out over a
decade or so and done jointly with other nations-a fraction of current space
budgets. But, if implemented, they would help us to make accurate cost
estimates and better assessment of the dangers and benefits. They would permit
us to maintain vigorous progress toward human expeditions to Mars without
premature commitment to any specific mission hardware. Most, perhaps all, of
these recommendations have other justifications, even if We were sure wed be
unable to send humans to any other world in the next few decades. And a steady
drumbeat of accomplishments increasing the feasibility of human voyages to Mars
would-in the minds of many at least-combat widespread pessimism about the
future.
THERE'S SOMETHING MORE. There's a set of
less tangible arguments, many of which, I freely admit, I find attractive and
resonant. Spaceflight speaks to something deep inside us-many of us, if not
all. An emerging cosmic perspective, an improved understanding of our place in
the Universe, a highly visible program affecting our view of ourselves might
clarify the fragility of our planetary environment and the common peril and
responsibility of all the nations and peoples of Earth. And human missions to
Mars would provide hopeful prospects, rich in adventure, for the wanderers
among us, especially the young. Even vicarious exploration has social utility.
I repeatedly find that when I give talks
on the future of the space program-to universities, business and military
groups, professional organizations-the audiences are much less patient with
practical, real-world political and economic obstacles than 1. They long to
sweep away the impediments, to recapture the glory days of Vostok and Apollo,
to get on with it and once more tread other worlds. We did it before; we can do
it again, they say. But, I caution myself, those who attend such talks are
self-selected space enthusiasts.
In 1969, less than half the American
people thought the Apollo program was worth the cost. But on the twenty-fifth
anniversary of the Moon landing, the number had risen to two thirds. Despite
its problems, NASA was rated as doing a good-to-excellent job by 63 percent of
Americans. With no reference to cost, 75 percent of Americans (according to a CBS
News poll) favored “the United States sending astronauts to explore Mars. ' For
young adults, the figure was 68 percent. I think “explore” is the operative
word.
It is no accident that, whatever their
human flaws, and how ever moribund the human space program has become (a trend
that the Hubble Space Telescope repair mission may have helped to reverse),
astronauts and cosmonauts are still widely regarded as heroes of our species. A
scientific colleague tells me about a recent trip to the New Guinea highlands
where she visited a stone age culture hardly contacted by Western civilization.
They were ignorant of wristwatches, soft drinks, and frozen food. But they knew
about Apollo 11. They knew that humans had walked on the Moon. They knew the
names of Armstrong and Aldrin and Collins. They wanted to know who was visiting
the Moon these days.
Projects that are future-oriented, that,
despite their political difficulties, can be completed only in some distant
decade are continuing reminders that there will be a future. Winning a foothold
on other worlds whispers in our ears that we're more than Picts or Serbs or
Tongans: We're humans.
Exploratory spaceflight puts scientific
ideas, scientific thinking, and scientific vocabulary in the public eye. It
elevates the general level of intellectual inquiry. The idea that we've now
understood something never grasped by anyone who ever lived before-that
exhilaration, especially intense for the scientists involved, but perceptible
to nearly everyone-propagates through the society, bounces off walls, and comes
back at us. It encourages us to address problems in other fields that have also
never before been solved. It increases the general sense of optimism in the
society. It gives currency to critical thinking of the sort urgently needed if
we are to solve hitherto intractable social issues. It helps stimulate a new
generation of scientists. The more science in the media-especially if methods
are described, as well as conclusions and implications-the healthier, I
believe, the society is. People everywhere hunger to understand.
WHEN I WAS A CHILD, my most exultant
dreams were about flying-not in some machine, but all by myself. I would be
skipping or hopping, and slowly I could pull my trajectory higher. It would
take longer to fall back to the ground. Soon I would be on such a high arc that
I wouldn't come down at all. I would alight like a gargoyle in a niche near the
pinnacle of a skyscraper, or gently settle down on a cloud. In the dream-which
I must have had in its many variations at least a hundred times-achieving
flight required a certain cast of mind. It's impossible to describe it in
words, but I can remember what it was like to this day. You did something
inside your head and at the pit of your stomach, and then you could lift
yourself up by an effort of will alone, your limbs hanging limply. Off you'd
soar.
I know many people have had similar
dreams. Maybe most people. Maybe everyone. Perhaps it goes back 10 million
years or more, when our ancestors were gracefully flinging themselves from
branch to branch in the primeval forest. A wish to soar like the birds
motivated many of the pioneers of flight, including Leonardo da Vinci and the
Wright brothers. Maybe that's part of the appeal of spaceflight, too.
In orbit about any world, or in
interplanetary flight, you are literally weightless. You can propel yourself to
the spacecraft ceiling with a slight push off the floor. You can go tumbling
through the air down the long axis of the spacecraft. Humans experience
weightlessness as joy; this has been reported by almost every astronaut and
cosmonaut. But because spacecraft are still so small, and because space “walks”
have been done with extreme caution, no human has yet enjoyed this wonder and
glory: propelling yourself by an almost imperceptible push, with no machinery
driving you, untethered, high up into the sky, into the blackness of
interplanetary space. You become a living satellite of the Earth, or a human
planet of the Sun.
Planetary exploration satisfies our
inclination for great enterprises and wanderings and quests that has been with
us since our days as hunters and gatherers on the East African savannahs a
million years ago. By chance-it is possible, I say, to imagine many skeins of
historical causality in which this would not have transpired-in our age we are
able to begin again.
Exploring other worlds employs precisely
the same qualities of daring, planning, cooperative enterprise, and valor that
mark the finest in the military tradition. Never mind the night launch of an
Apollo spacecraft bound for another world. That makes the conclusion foregone.
Witness mere F-14s taking off from adjacent flight decks, gracefully canting
left and right, afterburners flaming, and there's something that sweeps you
away-or at least it does me. And no amount of knowledge of the potential abuses
of carrier task forces can affect the depth of that feeling. It simply speaks
to another part of me. It doesn't want recriminations or politics. It just
wants to fly.
“I...had ambition not only to go farther
than anyone had done before,” wrote Captain James Cook, the eighteenth-century
explorer of the Pacific, “but as far as it was possible for man to go.” Two
centuries later, Yuri Romanenko, on returning to Earth after what was then the
longest space flight in history, said “The Cosmos is a magnet...Once you've
been there, all you can think of is how to get back.”
Even Jean-Jacques Rousseau, no
enthusiast of technology, felt it:
The stars are far above us; we need
preliminary instruction, instruments and machines, which are like so many
immense ladders enabling us to approach them and bring them within our grasp.
“The future possibilities of
space-travel,” wrote the philosopher Bertrand Russell in 1959which are now left
mainly to unfounded fantasy, could be more soberly treated without ceasing to
be interesting and could show to even the most adventurous of the young that a
world without war need not be a world without adventurous and hazardous glory.
To this kind of contest there is no limit. Each victory is only a prelude to
another, and no boundaries can be set to rational hope.
In the long run, these-more than any of
the “practical” justifications considered earlier-may be the reasons we will go
to Mars and other worlds. In the meantime, the most important step we can take
toward Mars is to make significant progress on Earth. Even modest improvements
in the social, economic, and political problems that our global civilization
now faces could release enormous resources, both material and human, for other
goals.
There's plenty of housework to be done
here on Earth, and our commitment to it must be steadfast. But we're the kind
of species that needs a frontier-for fundamental biological reasons.
Every time humanity stretches itself and
turns a new corner, it receives a jolt of productive vitality that can carry it
for centuries.
There's a new world next door. And we
know how to get there.
CHAPTER 17
Routine interplanetary violence
It is a law of nature that Earth and all
other bodies should remain in their proper places and be moved from them only
by violence.
—ARISTOTLE (384-322 B. C.), PHYSICS
There was something funny about Saturn.
When, in 1610, Galileo used the world's first astronomical telescope to view
the planet-then the most distant world known-he found two appendages, one on
either side. He likened them to “handles.” Other astronomers called them
“ears.” The Cosmos holds many wonders, but a planet with jug ears is dismaying.
Galileo went to his grave with this bizarre matter unresolved.
As the years passed, observers found the
ears...Well, waxing and waning. Eventually, it became clear that what Galileo
had discovered was an extremely thin ring that surrounds Saturn at its equator
but touches it nowhere. In some years, because of the changing orbital
positions of Earth and Saturn, the ring had been seen edge-on and, because of
its thinness, it seemed to disappear. In other years, it had been viewed more
face-oil, and the “ears” grew bigger. But what does it mean that there's a ring
around Saturn? A thin, flat, solid plate with a hole cut out for the planet to
fit into? Where does that come from?
This line of inquiry will shortly take
us to world-shattering collisions, to two quite different perils for our
species, and to a reason-beyond those already described-that we must, for our
very survival, be out there among the planets.
We now know that the rings (emphatically
plural) of Saturn are a vast horde of tiny ice worlds, each on its separate
orbit, each bound to Saturn by the giant planet's gravity. In size, these
worldlets range from particles of fine dust to houses. None is big enough to
photograph even from close flybys. Spaced out in an exquisite set of fine
concentric circles, something like the grooves on a phonograph record (which in
reality make, of course, a spiral), the rings were first revealed in their true
majesty by the two Voyager spacecraft in their 1980/81 flybys. In our century,
the Art Deco rings of Saturn have become an icon of the future.
At a scientific meeting in the late
1960s, I was asked to summarize the outstanding problems in planetary science.
One, I suggested, was the question of why, of all the planets, only Saturn had
rings. This, Voyager discovered, is a nonquestion. All four giant planets in
our Solar System- Jupiter, Saturn, Uranus, and Neptune-in fact have rings. But
no one knew it then.
Each ring system has distinctive
features. Jupiter's is tenuous and made mainly of dark, very small particles.
The bright rings of Saturn are composed mainly of frozen water; there are
thousands of separate rings here, some twisted, with strange, dusky, spoke-like
markings forming and dissipating. The dark rings of Uranus seem to be composed
of elemental carbon and organic molecules-something like charcoal or chimney
soot; Uranus has nine main rings, a few of which sometime seem to “breathe,”
expanding and contracting. Neptune's rings are the most tenuous of all, varying
so much in thickness that, when detected from Earth, they appear only as arcs
and incomplete circles. A number of rings seem to be maintained by the
gravitational tugs of two shepherd moons, one a little nearer and the other a
little farther from the planet than the ring. Each ring system displays its
own, appropriately unearthly, beauty.
How do rings form? One possibility is
tides: If an errant world passes close to a planet, the interloper's near side
is gravitationally pulled toward the planet more than its far side; if it comes
close enough, if its internal cohesion is low enough, it can be literally torn to
pieces. Occasionally we see this happening to comets as they pass too close to
Jupiter, or the Sun. Another possibility, emerging from the Voyager
reconnaissance of the outer Solar System, is this: Rings are made when worlds
collide and moons are smashed to smithereens. Both mechanisms may have played a
role.
The space between the planets is
traversed by an odd collection of rogue worldlets, each in orbit about the Sun.
A few are as big as a county or even a state; many more have surface areas like
those of a village or a town. More little ones are found than big ones, and
they range in size down to particles of dust. Some of them travel on long,
stretched-out elliptical paths, which make them periodically cross the orbit of
one or more planets.
Occasionally, unluckily, there's a world
in the way. The collision can shatter and pulverize both the interloper and the
moon that's hit (or at least the region around ground zero). The resulting
debris-ejected from the moon but not so fast-moving as to escape from the
planet's gravity-may form, for a time, a new ring. It's made of whatever the
colliding bodies were made of, but usually more of the target moon than the
rogue impactor. If the colliding worlds are icy, the net result will be rings
of ice particles; if they're made of organic molecules, the result will be
rings of organic particles (which will slowly be processed by radiation into
carbon). All the mass in the rings of Saturn is no more than 'would result from
the complete impact pulverization of a single icy moon. The disintegration of
small moons can likewise account for the ring systems of the three other giant
planets.
Unless it's very close to its planet, a
shattered moon gradually reaccumulates (or at least a fair fraction of it
does). The pieces, big and small, still in approximately the same orbit as the
moon was before the impact, fall together helter-skelter. What used to be a
piece of the core is now at the surface, and vice versa. The resulting
hodgepodge surfaces might seem very odd. Miranda, one of the moons of Uranus,
looks disconcertingly jumbled and may have had such an origin.
The American planetary geologist Eugene
Shoemaker proposes that many moons in the outer Solar System have been
annihilated and reformed-not just once but several times each over the 4. 5
billion years since the Sun and the planets condensed out of interstellar gas
and dust. The picture emerging from the Voyager reconnaissance of the outer
Solar System is of worlds whose placid and lonely vigils are spasmodically
troubled by interlopers from space; of world-shattering collisions; and of
moons re-forming from debris, reconstituting themselves like phoenixes from
their own ashes.
But a moon that lives very close to a
planet cannot re-form if it is pulverized-the gravitational tides of the nearby
planet prevent it. The resulting debris, once spread out into a ring system,
might be very long-lived-at least by the standard of a human lifetime. Perhaps
many of the small, inconspicuous moons now orbiting the giant planets will one
day blossom forth into vast and lovely rings.
These ideas are supported by the
appearance of a number of satellites in the Solar System. Phobos, the inner
moon of Mars, has a large crater named Stickney; Mimas, an inner moon of
Saturn, has a big one named Herschel. These craters-like those on our own Moon
and, indeed, throughout the Solar System-are produced by collisions. An
interloper smashes into a bigger world and makes an immense explosion at the
point of impact, A bowl-shaped crater is excavated, and the smaller impacting
object is destroyed. If the interlopers that dug out the Stickney and Herschel
craters had been only a little larger, they would have had enough energy to
blow Phobos and Mimas to bits. These moons barely escaped the cosmic wrecking
ball. Many others did not.
Every time a world is smashed into,
there's one less interloper-something like a demolition derby on the scale of
the Solar System, a war of attrition. The very fact that many such collisions
have occurred means that the rogue worldlets have been largely used up. Those
on circular trajectories around the Sun, those that don't intersect the orbits
of other worlds, will be unlikely to smash into a planet. Those on highly
elliptical trajectories, those that cross the orbits of other planets, Will
sooner or later collide or, by a near miss, be gravitationally ejected from the
Solar System.
The planets almost certainly accumulated
from worldlets which in turn had condensed out of a great flat cloud of gas and
dust Surrounding the Sun-the sort of cloud that can now be seen around young
nearby stars. So, in the early history of the solar System before collisions
cleaned things up, there should have been many more worldlets than we see
today.
Indeed, there is clear evidence for this
in our own backyard: If we count up the interloper worldlets in our
neighborhood in space, we can estimate how often they'll hit the Moon. Let us
make the very modest assumption that the population of interlopers has never
been smaller than it is today. We can then calculate how many craters there
should be on the Moon. The number we figure turns out to be much less than the
number we see on the Moon's ravaged highlands. The unexpected profusion of
craters on the Moon speaks to us of an earlier epoch when the Solar System was
in wild turmoil, churning with worlds on collision trajectories. This makes
good sense, because they formed from the aggregation of much smaller
worldlets-which themselves had grown out of interstellar dust. Four billion
years ago, the lunar impacts were hundreds of times more frequent than they are
today; and 4. 5 billion years ago, when the planets were still incomplete,
collisions happened perhaps a billion times more often than in our becalmed
epoch.
The chaos may have been relieved by much
more flamboyant ring systems than grace the planets today. If they had small
moons in that time, the Earth, Mars, and the other small planets may also have
been adorned with rings.
The most satisfactory explanation of the
origin of our own Moon, based on its chemistry (as revealed by samples returned
from the Apollo missions), is that it was formed almost 4. 5 billion years ago,
when a world the size of Mars struck the Earth. Much of our planet's rocky
mantle was reduced to dust and hot gas and blasted into space. Some of the
debris, in o: -bit around the Earth, then gradually reaccumulated-atom by atom,
boulder by boulder. If that unknown impacting world had been only a little
larger, the result would have been the obliteration of the Earth. Perhaps there
once were other worlds in our Solar System-perhaps even worlds on which life
was stirring-hit by some demon worldlet, utterly demolished, and of which today
we have not even an intimation.
The emerging picture of the early Solar
System does not resemble a stately progression of events designed to form the
Earth. Instead, it looks as if our planet was made, and survived, by mere lucky
chance, amid unbelievable violence. Our world does not seem to have been
sculpted by a master craftsman. Here too, there is no hint of a Universe made
for us.
THE DWINDLING SUPPLY of worldlets is
today variously labeled: asteroids, comets, small moons. But these are
arbitrary categories-real worldlets are able to breach these human-made
partitions. Some asteroids (the word means “starlike,” which they certainly are
not) are rocky, others metallic, still others rich in organic matter. None is
bigger than 1,000 kilometers across. They are found mainly in a belt between
the orbits of Mars and Jupiter. Astronomers once thought the “main-belt”
asteroids were the remains of a demolished world, but, as I've been describing,
another idea is now more fashionable: The Solar System was once filled with
asteroid-like worlds, some of which went into building the planets. Only in the
asteroid belt, near Jupiter, did the gravitational tides of this most massive
planet prevent the nearby debris from coalescing into a new world. The
asteroids, instead of representing a world that once was, seem to be the
building blocks of a world destined never to be.
Down to kilometer size, there may be
several million asteroids, but, in the enormous volume of interplanetary space,
even that's still far too few to cause any serious hazard to spacecraft on
their way to the outer Solar System. The first main-belt asteroids, Gaspra and
Ida, were photographed, in 1991 and 1993 respectively, by the Galileo
spacecraft on its tortuous journey to Jupiter.
Main-belt asteroids mostly stay at home.
To investigate them. we must go and visit them, as Galileo did. Comets, on the
other hand, sometimes come and visit us, as Halley's Comet did most recently in
1910 and 1986. Comets are made mainly of ice, plus smaller amounts of rocky and
organic material. When heated, the ice vaporizes, forming the long and lovely
tails blown outward by the solar wind and the pressure of sunlight. After many
passages by the Sun, the ice is all evaporated, sometimes leaving a dead rocky
and organic world. Sometimes the remaining particles, the ice that held them
together now gone, spread out in the comet's orbit, generating a debris trail
around the Sun.
Every time a bit of cometary fluff the
size of a grain of sand enters the Earth's atmosphere at high speed, it burns
up, producing a momentary trail of light that Earthbound observers call a
sporadic meteor or “shooting star.” Some disintegrating comets have orbits that
cross the Earth's. So every year, the Earth, on its steady circumnavigation of
the Sun, also plunges through belts of orbiting cometary debris. We may then
witness a meteor shower, or even a meteor storm-the skies ablaze with the body
parts of a comet. For example, the Perseid meteors, seen on or about August 12
of each year, originate in a dying comet called Swift-Tuttle. But the beauty of
a meteor shower should not deceive us: There is a continuum that connects these
shimmering visitors to our night skies with the destruction of worlds.
A few asteroids now and then give off
little puffs of gas or even form a temporary tail, suggesting that they are in
transition between cometdom and asteroidhood. Some small moons going around the
planets are probably captured asteroids or comets; the moons of Mars and the
outer satellites of Jupiter may be in this category.
Gravity smooths down everything that
sticks out too far. But only in large bodies is the gravity enough to make
mountains and other projections collapse of their own weight, rounding the
world. And, indeed, when we observe their shapes, almost always we find that
small worldlets are lumpy, irregular, potato-shaped.
THERE ARE ASTRONOMERS whose idea of a
good time is to stay up till dawn on a cold, moonless night taking pictures of
the sky-the same sky they photographed the year before...and the year before
that. If they got it right last time, you might well ask, why are they doing it
again? The answer is: The sky changes. In any given year there might be
worldlets wholly unknown, never seen before, that approach the Earth and are
spied by these dedicated observers.
On March 25, 1993, a group of asteroid
and comet hunters, looking at the photographic harvest from an intermittently
cloudy night at Mount Palomar in California, discovered a faint elongated
smudge on their films. It was near a very bright object in the sky, the planet
Jupiter. Carolyn and Eugene Shoemaker and David Levy then asked other observers
to take a look. The smudge turned out to be something astonishing: some twenty
small, bright objects orbiting Jupiter, one behind the other, like pearls on a
string. Collectively they are called Comet Shoemaker-Levy 9 (this is the ninth
time that these collaborators have together discovered a periodic comet).
But calling these objects a comet is
confusing. There was a horde of them, probably the fragmented remains of a
single, hitherto undiscovered, comet. It silently orbited the Sun for 4 billion
years before passing too close to Jupiter and being captured, a few decades
ago, by the gravity of the Solar System's largest planet. On July 7, 1992, it
was torn apart by Jupiter's gravitational tides.
You can recognize that the inner part of
such a comet would be pulled toward Jupiter a little more strongly than the
outer part, because the inner part is closer to Jupiter than the outer part.
The difference in pull is certainly small. Our feet are a little closer to the
center of the Earth than our heads, but we are not in consequence torn to
pieces by the Earth's gravity. For such tidal disruption to have occurred, the
original comet must have been held together very weakly. Before fragmentation,
it was, we think, a loosely consolidated mass of ice, rock, and organic matter,
maybe 10 kilometers (about 6 miles) across.
The orbit of this disrupted comet was
then determined to high precision. Between July 16 and 22, 1994, all the
cometary fragments, one after another, collided with Jupiter. The biggest
pieces seem to have been a few kilometers across. Their impacts with Jupiter
were spectacular.
No one knew beforehand what these
multiple impacts into the atmosphere and clouds of Jupiter would do. Perhaps
the cometary fragments, surrounded by halos of dust, were much smaller than
they seemed. Or perhaps they were not coherent bodies at all, but loosely
consolidated-something like a heap of gravel with all the particles traveling
through space together, in nearly identical orbits. If either of these
possibilities were true Jupiter might swallow the comets without a trace. Other
astronomers thought there would at least be bright fireballs and giant plumes
as the cometary fragments plunged into the atmosphere. Still others suggested
that the dense cloud of fine particles accompanying the fragments of Comet
Shoemaker-Levy 9 into Jupiter would disrupt the magnetosphere of Jupiter or
form a new ring.
A comet this size should impact Jupiter,
it is calculated, only once every thousand years. It's the astronomical event
not of one lifetime, but of a dozen. Nothing on this scale has occurred since
the invention of the telescope. So in mid July 1994, in a beautifully
coordinated international scientific effort, telescopes all over the Earth and
in space turned towards Jupiter.
Astronomers had over a year to prepare.
The trajectories of the fragments in their orbits around Jupiter were
estimated. It was discovered that they would all hit Jupiter. Predictions of
the timing were refined. Disappointingly, the calculations revealed that all
impacts would occur on the night side of Jupiter, the side invisible from the
Earth (although accessible to the Galileo and Voyager spacecraft in the outer
Solar System). But, happily, all impacts would occur only a few minutes before
the Jovian dawn, before the impact site would be carried by Jupiter's rotation
into the line of sight from Earth.
The appointed moment for the impact of
the first piece, Fragment A, came and went. There were no reports from
ground-based telescopes. Planetary scientists stared with increasing gloom at a
television monitor displaying the data transmitted to the Space Telescope
Science Institute in Baltimore from the Hubble Space Telescope. There was
nothing anomalous Shuttle astronauts took time off from the reproduction of
fruit flies, fish, and newts to look at Jupiter through binoculars. They
reported seeing nothing. The impact of the millennium was beginning to look
very much like a fizzle.
Then there was a report from a
ground-based optical telescope in La Palma in the Canary Islands, followed by
announcements from a radiotelescope in Japan; from the European Southern
Observatory in Chile; and from a University of Chicago instrument in the frigid
wastelands of the South Pole. In Baltimore the young scientists crowding around
the TV monitor-themselves monitored by the cameras of CNN-began to see
something, and in exactly the right place on Jupiter. You could witness
consternation turn into puzzlement, and then exultation. They cheered; they
screamed; they jumped up and down. Smiles filled the room. They broke out the
champagne. Here was a group of young American scientists-about a third of them,
including the team leader, Heidi Hammel, women-and you could imagine youngsters
all over the world thinking that it might be fun to be a scientist, that this
might be a good daytime job, or even a means to spiritual fulfillment.
For many of the fragments, observers
somewhere on Earth noticed the fireball rise so quickly and so high that it
could be seen even though the impact site below it was still in Jovian
darkness. Plumes ascended and then flattened into pancake-like forms. Spreading
out from the point of impact we could see sound and gravity waves, and a patch
of discoloration that for the largest fragments became as big as the Earth.
Slamming into Jupiter at 60 kilometers a
second (130,000 miles an hour), the large fragments converted their kinetic
energy partly into shock waves, partly into heat. The temperature in the
fireball was estimated at thousands of degrees. Some of the fireballs and
plumes were far brighter than all the rest of Jupiter put together.
What is the cause of the dark stains
left after the impact? It might be stuff from the deep clouds of Jupiter-from
the region to which ground-based observers cannot ordinarily see-that welled up
and spread out. However, the fragments do not seem to have penetrated to such
depths. Or the molecules responsible for the stains might have been in the
cometary fragments in the first place. We know from the Vega 1 and 2 Soviet
missions and the Giotto mission of the European Space Agency-both to Halley's
Comet-that comets may be as much as a quarter composed of complex organic
molecules. They are the reason that the nucleus of Halley's Comet is pitch
black. If some of the cometary organics survived the impact events, they may
have been responsible for the stain. Or, finally, the stain may be due to
organic matter not delivered by the impacting cometary fragments, but
synthesized by their shock waves from the atmosphere of Jupiter.
Impact of the fragments of Comet
Shoemaker-Levy 9 with Jupiter was witnessed on seven continents. Even amateur
astronomers with small telescopes could see the plumes and the subsequent
discoloration of the Jovian clouds. Just as sporting events are covered at all
angles by television cameras on the field and from a dirigible high overhead,
six NASA spacecraft deployed throughout the Solar System, with different
observational specialties, recorded this new wonder-the Hubble Space Telescope,
the International Ultraviolet Explorer, and the Extreme Ultraviolet Explorer
all in Earth orbit; Ulysses, taking time out from it investigation of the South
Pole of the Sun; Galileo, on the way to its own rendezvous with Jupiter; and
Voyager 2, far beyond Neptune on its way to the stars. As the data are
accumulated and analyzed, our knowledge of comets, of Jupiter, and of the
violent collisions of worlds should all be substantially improved.
For many scientists-but especially for
Carolyn and Eugene Shoemaker and David Levy-there was something poignant about
the cometary fragments, one after the other, making their death plunges into
Jupiter. They had lived with this comet, in a manner of speaking, for 16
months, watched it split, the pieces, enshrouded by clouds of dust, playing
hide-and-seek and spreading out in their orbits. In a limited way, each
fragment had its own personality. Now they're all gone, ablated into molecules
and atoms in the upper atmosphere of the Solar System's largest planet. In a
way, we almost mourn them. But we're learning from their fiery deaths. It is
perhaps some reassurance to know that there are a hundred trillion more of them
in the Sun's vast treasure-house of worlds.
THERE ARE ABOUT 200 known asteroids
whose paths take them near the Earth. They are called, appropriately enough,
“near-Earth” asteroids. Their detailed appearance (like that of their main-belt
cousins) immediately implies that they are the products of a violent
collisional history. Many of them may be the shards and remnants of once-larger
worldlets.
With a few exceptions, the near-Earth
asteroids are only a few kilometers across or smaller, and take one to a few
years to make a circuit around the Sun. About 20 percent of them, sooner or
later, are bound to hit the Earth-with devastating consequences. (But in
astronomy, “sooner or later” can encompass billions of years.) Cicero's
assurance that “nothing of chance or hazard” is to be found in an absolutely
ordered and regular heaven is a profound misperception. Even today, as Comet
Shoemaker-Levy 9's encounter with Jupiter reminds us, there is routine
interplanetary violence, although not on the scale that marked the early
history of the Solar System.
Like main-belt asteroids, many
near-Earth asteroids are rocky. A few are mainly metal, and it has been
suggested that enormous rewards might attend moving such an asteroid into orbit
around the Earth, and then systematically mining it-a mountain of high-grade
ore a few hundred miles overhead. The value of platinum-group metals alone in a
single such world has been estimated as many trillions of dollars-although the
unit price would plummet spectacularly if such materials became widely
available. Methods of extracting metals and minerals from appropriate asteroids
are being studied, for example by John Lewis, a planetary scientist at the
University of Arizona.
Some near-Earth asteroids are rich in
organic matter, apparently preserved from the very earliest Solar System. Some
have been found, by Steven Ostro of the Jet Propulsion Laboratory, to be
double, two bodies in contact. Perhaps a larger world has broken in two as it
passed through the strong gravitational tides of a planet like Jupiter; more
interesting is the possibility that two worlds on similar orbits made a gentle
overtaking collision and stuck. This process may have been key to the building
of planets and the Earth. At least one asteroid (Ida, as viewed by Galileo) has
its own small moon. We might guess that two asteroids in contact and two
asteroids orbiting one another have related origins.
Sometimes, we hear about an asteroid
making a “near miss.” (Why do we call it a “near miss”? A “near hit” is what we
really mean.) But then we read a little more carefully, and it turns out that
its closest approach to the Earth was several hundreds of thousands or millions
of kilometers. That doesn't count-that's too far away, farther even than the
Moon. If we had an inventory of all the near-Earth asteroids, including those
considerably smaller than a kilometer across, we could project their orbits
into the future and predict which ones are potentially dangerous. There are an
estimated 2,000 of them bigger than a kilometer across, of which we have
actually observed only a few percent. There are maybe 200,000 bigger than 100
meters in diameter.
The near-Earth asteroids have evocative
mythological names: Orpheus, Hathor, Icarus, Adonis, Apollo, Cerberus, Khufu,
Amor, Tantalus, Aten, Midas, Ra-Shalom, Phaethon, Toutatis, Quetzalcoatl. There
are a few of special exploratory potential-for example, Nereus. In general,
it's much easier to get onto and off of near-Earth asteroids than the Moon.
Nereus, a tiny world about a kilometer across, is one of the easiest. It would
be real exploration of a truly new world.
Some humans (all from the former Soviet
Union) have already been in space for periods longer than the entire roundtrip
time to Nereus. The rocket technology to get there already exists. It's a much
smaller step than going to Mars or even, in several respects, than returning to
the Moon. If something went wrong, though, we would be unable to run home to
safety in only a few days. In this respect, its level of difficulty lies
somewhere between a voyage to Mars and one to the Moon.
Of many possible future missions to
Nereus, there's one that takes 10 months to get there from Earth, spends 30
days there, and then requires only 3 weeks to return to home. We could visit
Nereus with robots, or-if we're up to it-with humans. We could examine this
little world's shape, constitution, interior, past history, organic chemistry,
cosmic evolution, and possible tie to comets. We could bring samples back for
examination at leisure in Earthbound laboratories. We could investigate whether
there really are commercially valuable resources-metals or minerals-there. If
we are ever going to send humans to Mars, near-Earth asteroids provide a
convenient and appropriate intermediate goal-to test out the equipment and
exploratory protocols while studying an almost wholly unknown little world.
Here's a way to get our feet wet again when we're ready to reenter the cosmic
ocean.
CHAPTER 18
The marsh of camarina
[I]t's too late to make any improvements
now. The universe is finished; the copestone is on, and the chips were carted
off a million years ago.
—HERMAN MELVILLE, MOBY DICK, CHAPTER 2
(1851)
Camarina was a city in southern Sicily,
founded by colonists from Syracuse in 598 B. C. A generation or two later, it
was threatened by a pestilence-festering, some said, in the adjacent marsh.
(While the germ theory of disease was certainly not widely accepted in the
ancient world, there were hints-for example, Marcus Varro in the first century
B. C. advised explicitly against building cities near swamps “because there are
bred certain minute creatures which cannot be seen by the eyes, which float in
the air and enter the body through the mouth and nose and there cause serious
disease.”) The danger to Camarina was great. Plans were drawn to drain the
marsh. When the oracle was consulted, though, it forbade such a course of
action, counseling patience instead. But lives were at stake, the oracle was
ignored, and the marsh was drained. The pestilence was promptly halted. Too
late, it was recognized that the marsh had protected the city from its
enemies-among whom there had now to be counted their cousins the Syracusans. As
in America 2,300 years later, the colonists had quarreled with the mother
country. In 552 B. C., a Syracusan force crossed over the dry land where the
marsh had been, slaughtered every man, woman, and child, and razed the city.
The marsh of Camarina became proverbial for eliminating a danger in such a way
as to usher in another, much worse.
THE CRETACEOUS-TERTIARY COLLISION (or
collisions-there may have been more than one) illuminates the peril from
asteroids and comets. In sequence, a world-immolating fire burned vegetation to
a crisp all over the planet; a stratospheric dust cloud so darkened the sky
that surviving plants had trouble making a living from photosynthesis; there
were worldwide freezing temperatures, torrential rains of caustic acids,
massive depletion of the ozone layer, and, to top it off, after the Earth
healed itself from these assaults, a prolonged greenhouse warming (because the
main impact seems to have volatilized a deep layer of sedimentary carbonates,
pouring huge amounts of carbon dioxide into the air). It was not a single
catastrophe, but a parade of them, a concatenation of terrors. Organisms
weakened by one disaster were finished off by the next. It is quite uncertain
whether our civilization would survive even a considerably less energetic
collision.
Since there are many more small
asteroids than large ones, run-of-the-mill collisions with the Earth will be
made by the little guys. But the longer you're prepared to wait, the more devastating
the impact you can expect. On average, once every few hundred years the Earth
is hit by an object about 70 meters in diameter; the resulting energy released
is equivalent to the largest nuclear weapons explosion ever detonated. Every
10,000 years, we're hit by a 200-meter object that might induce serious
regional climatic effects. Every million years, an impact by a body over 2
kilometers in diameter occurs, equivalent to nearly a million megatons of
TNT-an explosion that would work a global catastrophe, killing (unless
unprecedented precautions were taken) a significant fraction of the human
species. A million megatons of TNT is 100 times the explosive yield of all the
nuclear weapons on the planet, if simultaneously blown up. Dwarfing even this,
in a hundred million years or so, you can bet on something like the
Cretaceous-Tertiary event, the impact of a world 10 kilometers across or
bigger. The destructive energy latent in a large near-Earth asteroid dwarfs
anything else the human species can get its hands on.
As first shown by the American planetary
scientist Christopher Chyba and his colleagues, little asteroids or comets, a
few tens of meters across, break and burn up on entering our atmosphere. They
arrive comparatively often but do no significant harm. Some idea of how
frequently they enter the Earth's atmosphere has been revealed by declassified
Department of Defense data obtained from special satellites monitoring the
Earth for clandestine nuclear explosions. There seem to have been hundreds of
small worldlets (and at least one larger body) impacting in the last 20 years.
They did no harm. But, we need to be very sure we can distinguish a small
colliding comet or asteroid from an atmospheric nuclear explosion.
Civilization-threatening impacts require
bodies several hundred meters across, or more. (A meter is about a yard; 100
meters is roughly the length of a football field.) They arrive something like
once every 200,000 years. Our civilization is only about 10,000 years old, so
we should have no institutional memory of the last such impact. Nor do we.
Comet Shoemaker-Levy 9, in its
succession of fiery explosions on Jupiter in July 1994, reminds us that such
impacts really do occur in our time-and that the impact of a body a few
kilometers across can spread debris over an area as big as the Earth. It was a
kind of portent.
In the very week of the Shoemaker-Levy
impact, the Science and Space Committee of the U. S. House of Representatives
drafted legislation that requires NASA “in coordination with the Department of
Defense and the space agencies of other countries” to identify and determine
the orbital characteristics of all Earth-approaching “comets and asteroids that
are greater than 1 kilometer in diameter.” The work is to be completed by the
year 2005. Such a search program had been advocated by many planetary
scientists. But it took the death throes of a comet to move it toward practical
implementation.
Spread out over the waiting time, the
dangers of asteroid collision do not seem very worrisome. But if a big impact
happens, it would be an unprecedented human catastrophe. There's something like
one chance in two thousand that such a collision will happen in the lifetime of
a newborn baby. Most of us would not fly in an airplane if the chance of
crashing were one in two thousand. (In fact for commercial flights the chance
is one in two million. Even so, many people consider this large enough to worry
about, or even to take out insurance for.) When our lives are at stake, we
often change our behavior to arrange more favorable odds. Those who don't tend
to be no longer with us.
Perhaps we should practice getting to
these worldlets and diverting their orbits, should the hour of need ever arise.
Melville notwithstanding, some of the chips of creation are still left, and
improvements evidently need to be made. Along parallel and only weakly
interacting tracks, the planetary science community and the U. S. and Russian
nuclear weapons laboratories, aware of the foregoing scenarios, have been
pursuing these questions: how to monitor all sizable near-Earth interplanetary
objects, how to characterize their physical and chemical nature, how to predict
which ones may be on a future collision trajectory with Earth, and, finally,
how to prevent a collision from happening.
The Russian spaceflight pioneer
Konstantin Tsiolkovsky argued a century ago that there must be bodies
intermediate ill size between the observed large asteroids and those asteroidal
fragments, the meteorites, that occasionally fall to Earth. He wrote about
living on small asteroids in interplanetary space. He did not have military
applications in mind. In the early 1980s, though, some in the U. S. weapons
establishment argued that the Soviets might use near-Earth asteroids as
first-strike weapons; the alleged plan was called “Ivan's Hammer.”
Countermeasures were needed. But, at the same time, it was suggested, maybe it
wasn't a bad idea for the United States to learn how to use small worlds as
weapons of its own. The Defense Department's Ballistic Missile Defense
Organization, the successor to the Star Wars office of the 1980s, launched an
innovative spacecraft called Clementine to orbit the Moon and fly by the
near-Earth asteroid Geographos. (After completing a remarkable reconnaissance
of the Moon in May 1994, the spacecraft failed before it could reach
Geographos.)
In principle, you could use big rocket
engines, or projectile impact, or equip tile asteroid with giant reflective
panels and shove it with sunlight or powerful Earth-based lasers. But with
technology that exists right now, there are only two ways. First, one or more
high-yield nuclear weapons might blast the asteroid or comet into fragments
that would disintegrate and atomize on entering the Earth's atmosphere. If the
offending worldlet is only weakly held together, perhaps only hundreds of
megatons would suffice. Since there is no theoretical upper limit to the
explosive yield of a thermonuclear weapon, there seem to be those in the
weapons laboratories who consider making bigger bombs not only as a stirring
challenge, but also as a way to mute pesky environmentalists by securing a seat
for nuclear weapons on the save-the-Earth bandwagon.
Another approach under more serious
discussion is less dramatic but still an effective way of maintaining the
weapons establishment-a plan to alter the orbit of any errant worldlet by
exploding nuclear weapons nearby. The explosions (generally near the asteroid's
closest point to the Sun) are arranged to deflect it away from the Earth. A flurry of low-yield nuclear weapons, each
giving a little push in the desired direction, is enough to deflect a
medium-sized asteroid with only a few weeks' warning. The method also offers,
it is hoped, a way to deal with a suddenly detected long-period comet on imminent
collision trajectory with the Earth: The comet would be intercepted with a
small asteroid. (Needless to say, this game of celestial billiards is even more
difficult and uncertain-and therefore even less practical in the near
future-than the herding of an asteroid on a known, well-behaved orbit with
months or years at our disposal.)
We don't know what a standoff nuclear
explosion would do to an asteroid. The answer may vary from asteroid to
asteroid. Some small worlds might be strongly held together; others might be
little more than self-gravitating gravel heaps. If an explosion breaks, let's
say, a 10kilometer asteroid up into hundreds of 1-kilometer fragments, the
likelihood that at least one of them impacts the Earth is probably increased,
and the apocalyptic character of the consequences may not be much reduced. On
the other hand, if the explosion disrupts the asteroid into a swarm of objects
a hundred meters in diameter or smaller, all of them might ablate away like
giant meteors on entering the Earth's atmosphere. In this case little impact
damage would be caused. Even if the asteroid were wholly pulverized into fine
powder, though, the resulting high-altitude dust layer might be so opaque as to
block the sunlight and change the climate. We do not yet know.
A vision of dozens or hundreds of
nuclear-armed missiles on ready standby to deal with threatening asteroids or
comets has been offered. However premature in this particular application, it
seems very familiar; only the enemy has been changed. It also seems very
dangerous.
The problem, Steven Ostro of JPL and I
have suggested, is that if you can reliably deflect a threatening worldlet so
it does not collide with the Earth, you can also reliably deflect a harmless
worldlet so it does collide with the Earth. Suppose you had a full inventory,
with orbits, of the estimated 300,000 near-Earth asteroids larger than 100
meters-each of them large enough, on impacting the Earth, to have serious
consequences. Then, it turns out, you also have a list of huge numbers of
inoffensive asteroids whose orbits could be altered with nuclear warheads so
they quickly collide with the Earth.
Suppose we restrict our attention to the
2,000 or so near-Earth asteroids that are a kilometer across or bigger-that is,
the ones most likely to cause a global catastrophe. Today, with only about 100
of these objects catalogued, it would take about a century to catch one when
it's easily deflectable to Earth and alter its orbit. We think we've found one,
an as-yet-unnamed asteroid so far denoted only as 1991OA. In 2070, this world,
about 1 kilometer in diameter, will come within 4. 5 million kilometers of the
Earth's orbit-only fifteen times the distance to the Moon. To deflect 1991OA so
it hits the Earth, only about 60 megatons of TNT equivalent needs to be
exploded in the right way-the equivalent of a small number of currently
available nuclear warheads.
Now imagine a time, a few decades hence,
when all such near-Earth asteroids are inventoried and their orbits compiled.
Then, as Alan Harris of JPL, Greg Canavan of the Los Alamos National
Laboratory, Ostro, and I have shown, it might take only a year to select a
suitable object, alter its orbit, and send it crashing into the Earth with
cataclysmic effect.
The technology required-large optical
telescopes, sensitive detectors, rocket propulsion systems able to lift a few
tons of payload and make precise rendezvous in nearby space, and thermonuclear
weapons-all exist today. Improvements in all but perhaps the last can be
confidently expected. If we're not careful, many nations may have these
capabilities in the next few decades. What kind of world will we then have
made?
We have a tendency to minimize the
dangers of new technologies. A year before the Chernobyl disaster, a Soviet
nuclear power industry deputy minister was asked about the safety of Soviet
reactors, and chose Chernobyl as a particularly safe site. The average waiting
time to disaster, he confidently estimated, was a hundred thousand years. Less
than a year later...devastation. Similar reassurances were provided by NASA
contractors the year before the Challenger disaster: You would have to wait ten
thousand years, they estimated, for a catastrophic failure of the shuttle. One
year later...heartbreak.
Chlorofluorocarbons (CFCs) were
developed specifically as a completely safe refrigerant-to replace ammonia and
other refrigerants that, on leaking out, had caused illness and some deaths.
Chemically inert, nontoxic (in ordinary concentrations), odorless, tasteless,
non-allergenic, nonflammable, CFCs represent a brilliant technical solution to
a well-defined practical problem. They found uses in many other industries
besides refrigeration and air conditioning. But, as I described above, the
chemists who developed CFCs overlooked one essential fact-that the molecules'
very inertness guarantees that they are circulated to stratospheric altitudes
and there cracked open by sunlight, releasing chlorine atoms which then attack
the protective ozone layer. Due to the work of a few scientists, the dangers
may have been recognized and averted in time. We humans have now almost stopped
producing CFCs. We won't actually know if we've avoided real harm for about a
century; that's how long it takes for all the CFC damage to be completed. Like
the ancient Camarinans, we make mistakes. Not only do we often ignore the
warnings of the Oracles; characteristically we do not even consult them.
The notion of moving asteroids into
Earth orbit has proved attractive to some space scientists and long-range
planners. They foresee mining the minerals and precious metals of these worlds
or providing resources for the construction of space infrastructure without
having to fight the Earth's gravity to get them up there. Articles have been
published on how to accomplish this end and what the benefits will be. In
modern discussions, the asteroid is inserted into orbit around the Earth by
first making it pass through and be braked by the Earth's atmosphere, a
maneuver with very little margin for error. For the near future we can, I
think, recognize this whole endeavor as unusually dangerous and foolhardy,
especially for metal worldlets larger than tens of meters across. This is the
one activity where errors in navigation or propulsion or mission design can
have the most sweeping and catastrophic consequences.
The foregoing are examples of
inadvertence. But there's another kind of peril: We are sometimes told that
this or that invention would of course not be misused. no sane person would be
so reckless. This is the “only a madman” argument. Whenever I hear it (and it's
often trotted out in such debates), I remind myself that madmen really exist.
Sometimes they achieve the highest levels of political power in modern
industrial nations. This is the century of Hitler and Stalin, tyrants who posed
the gravest dangers not just to the rest of the human family, but to their own
people as well. In the winter and spring of 1945, Hitler ordered Germany to be
destroyed-even “what the people need for elementary survival”-because the
surviving Germans had “betrayed” him, and at any rate were “inferior” to those
who had already died. If Hitler had had nuclear weapons, the threat of a
counterstrike by Allied nuclear weapons, had there been any, is unlikely to
have dissuaded him. It might have encouraged him.
Can we humans be trusted with
civilization-threatening technologies? If the chance is almost one in a
thousand that much of the human population will be killed by an impact in the
next century, isn't it more likely that asteroid deflection technology will get
into the wrong hands in another century-some misanthropic sociopath like a
Hitler or a Stalin eager to kill everybody, a megalomaniac lusting after
“greatness” and “glory,” a victim of ethnic violence bent on revenge, someone
in the grip of unusually severe testosterone poisoning, some religious fanatic
hastening the Day of judgment, or just technicians incompetent or
insufficiently vigilant in handling the controls and safeguards? Such people
exist. The risks seem far worse than the benefits, the cure worse than the
disease. The cloud of near-Earth asteroids through which the Earth plows may
constitute a modern Camarine marsh.
It's easy to think that all of this must
be very unlikely, mere anxious fantasy. Surely sober heads would prevail. Think
of how many people would be involved in preparing and launching warheads, in
space navigation, in detonating warheads, in checking what orbital perturbation
each nuclear explosion has made, in herding the asteroid so it is on an impact
trajectory with Earth, and so on. Isn't it noteworthy that although Hitler gave
orders for the retreating Nazi troops to burn Paris and to lay waste to Germany
itself, his orders were not carried out? Surely someone essential to the
success of the deflection mission will recognize the danger. Even assurances
that the project is designed to destroy some vile enemy nation would probably
be disbelieved, because the effects of collision are planet-wide (and anyway
it's very hard to make sure your asteroid excavates its monster crater in a
particularly deserving nation).
But now imagine a totalitarian state,
not overrun by enemy troops, but one thriving and self-confident. Imagine a
tradition in which orders are obeyed without question. Imagine that those
involved in the operation are supplied a cover story: The asteroid is about to
impact the Earth, and it is their job to deflect it-but in order not to worry
people needlessly, the operation must be performed in secret. In a military
setting with a command hierarchy firmly in place, compartmentalization of
knowledge, general secrecy, and a cover story, can we be confident that even
apocalyptic orders would be disobeyed? Are we really sure that in the next
decades and centuries and millennia, nothing like this might happen? How sure
are we?
It's no use saying that all technologies
can be used for good or for ill. That is certainly true, but when the “ill”
achieves a sufficiently apocalyptic scale, we may have to set limits on which
technologies may be developed. (In a way we do this all the time, because we
can't afford to develop all technologies. Some are favored and some are not.)
Or constraints may have to be levied by the community of nations on madmen and
autarchs and fanaticism.
Tracking asteroids and comets is
prudent, it's good science, and it doesn't cost much. But, knowing our
weaknesses, why would we even consider now developing the technology to deflect
small worlds? For safety, shall we imagine this technology in the hands of many
nations, each providing checks and balances against misuse by another? This is
nothing like the old nuclear balance of terror. It hardly inhibits some madman
intent on global catastrophe to know that if he does not hurry, a rival may
beat him to it. How confident can we be that the community of nations will be
able to detect a cleverly designed, clandestine asteroid deflection in time to
do something about it? If such a technology were developed, can any
international safeguards be envisioned that have a reliability commensurate
with the risk?
Even if we restrict ourselves merely to
surveillance, there's a risk. Imagine that in a generation we characterize the
orbits of 30,000 objects of 100-meter diameter or more, and that this
information is publicized, as of course it should be. Maps will be published
showing near-Earth space black with the orbits of asteroids and comets, 30,000
swords of Damocles hanging over our heads-ten times more than the number of
stars visible to the naked eye under conditions of optimum atmospheric clarity.
Public anxiety might be much greater in such a time of knowledge than in our
current age of ignorance. There might be irresistible public pressure to
develop means to mitigate even nonexistent threats, which would then feed the
danger that deflection technology would be misused. For this reason, asteroid
discovery and surveillance may not be a mere neutral tool of future policy, but
rather a kind of booby trap. To me, the only foreseeable solution is a
combination of accurate orbit estimation, realistic threat assessment, and
effective public education-so that in democracies at least, the citizens can
make their own, informed decisions. This is a job for NASA.
Near-Earth asteroids, and means of
altering their orbits, are being looked at seriously. There is some sign that
officials in the Department of Defense and the weapons laboratories are
beginning to understand that there may be real dangers in planning to push
asteroids around. Civilian and military scientists have met to discuss the
subject. On first hearing about the asteroid hazard, many people think of it as
a kind of Chicken Little fable; Goosey-Lucy, newly arrived and in great
excitement, is communicating the urgent news that the sky is falling. The
tendency to dismiss the prospect of any catastrophe that we have not personally
witnessed is in the long run very foolish. But in this case it may be an ally
of prudence.
MEANWHILE WE MUST STILL FACE the
deflection dilemma. If we develop and deploy this technology, it may do us in.
If we don't, some asteroid or comet may do us in. The resolution of the dilemma
hinges, I think, on the fact that the likely timescales of the two dangers are
very different-short for the former, long for the latter.
I like to think that our future
involvement with near-Earth asteroids will go something like this: From
ground-based observatories, we discover all the big ones, plot and monitor
their orbits, determine rotation rates and compositions. Scientists are
diligent in explaining the dangers-neither exaggerating nor muting the prospects.
We send robotic spacecraft to fly by a few selected bodies, orbit them, land on
them, and return surface samples to laboratories on Earth. Eventually we send
humans. (Because of the low gravities, they will be able to make standing broad
jumps of ten kilometers or more into the sky, and lob a baseball into orbit
around the asteroid.) Fully aware of the dangers, we make no attempts to alter
trajectories until the potential for misuse of world-altering technologies is
much less. That might take a while.
If we're too quick in developing the
technology to move worlds around, we may destroy ourselves; if we're too slow,
we will surely destroy ourselves. The reliability of world political
organizations and the confidence they inspire will have to make significant
strides before they can be trusted to deal with a problem of this seriousness.
At the same time, there seems to be no acceptable national solution. Who would
feel comfortable with the means of world destruction in the hands of some
dedicated (or even potential) enemy nation, whether or not our nation had
comparable powers? The existence of interplanetary collision hazards, when
widely understood, works to bring our species together. When facing a common
danger, we humans have sometimes reached heights widely thought impossible; we
have set aside our differences-at least until the danger passed.
But this danger never passes. The
asteroids, gravitationally churning, are slowly altering their orbits; without
warning, new comets come careening toward us from the transplutonian darkness.
There will always be a need to deal with them in a way that does not endanger
us. By posing two different classes of peril-one natural, the other
human-made-the small near-Earth worlds provide a new and potent motivation to create
effective transnational institutions and to unify the human species. It's hard
to see any satisfactory alternative.
In our usual jittery,
two-steps-forward-one-step-back mode, We are moving toward unification anyway.
There are powerful influences deriving from transportation and communications
technologies, the interdependent world economy, and the global environmental
crisis. The impact hazard merely hastens the pace.
Eventually, cautiously, scrupulously
careful to attempt nothing with asteroids that could inadvertently cause a
catastrophe on Earth, I imagine we will begin to learn how to change the orbits
of little nonmetallic worlds, smaller than 100 meters across. We begin with
smaller explosions and slowly work our way up. We gain experience in changing
the orbits of various asteroids and comets of different compositions and
strengths. We try to determine which ones can be pushed around and which
cannot. By the twenty-second century, perhaps, we move small worlds around the
Solar System, using (see next chapter) not nuclear explosions but nuclear
fusion engines or their equivalents. We insert small asteroids made of precious
and industrial metals into Earth orbit. Gradually develop a defensive
technology to deflect a large asteroid or comet that might in the foreseeable
future hit the Earth, while, with meticulous care, we build layers of
safeguards against misuse.
Since the danger of misusing deflection
technology seems so much greater than the danger of an imminent impact, we can
afford to wait, take precautions, rebuild political institutions-for decades
certainly, probably centuries. If we play our cards right and are not unlucky,
we can pace what we do up there by what progress we're making down here. The
two are in any case deeply connected.
The asteroid hazard forces our hand.
Eventually, we must establish a formidable human presence throughout the inner
Solar System. On an issue of this importance I do not think we will be content
with purely robotic means of mitigation. To do so safely we must make changes
in our political and international systems. While much about our future is
cloudy, this conclusion seems a little more robust, and independent of the
vagaries of human institutions.
In the long term, even if we were not
the descendants of professional wanderers, even if we were not inspired by
exploratory passions, some of us would still have to leave the Earth-simply to
ensure the survival of all of us. And once we're out there, we'll need bases,
infrastructures. It would not be very long before some of us were living in
artificial habitats and on other worlds. This is the first of two mussing
arguments, omitted in our discussion of missions to Mars, for a permanent human
presence in space.
OTHER PLANETARY SYSTEMS must face their
own impact hazards-because small primordial worlds, of which asteroids and
comets are remnants, are the stuff out of which planets form there as well.
After the planets are made, many of these planetesimals are left over. The
average time between civilization-threatening impacts on Earth is perhaps
200,000 years, twenty times the age of our civilization. Very different waiting
times may pertain to extraterrestrial civilizations, if they exist, depending
on such factors as the physical and chemical characteristics of the planet and
its biosphere, the biological and social nature of the civilization, and of
course the collision rate itself. Planets with higher atmospheric pressures
will be protected against somewhat larger 1mpactors, although the pressure
cannot be much greater before greenhouse warming and other consequences make
life improbable. If the gravity is much less than on Earth, impactors will make
less energetic collisions and the hazard will be reduced-although it cannot be
reduced very much before the atmosphere escapes to space.
The impact rate in other planetary
systems is uncertain. Our system contains two major populations of small bodies
that feed potential impactors into Earth-crossing orbits. Both the existence of
the source populations and the mechanisms that maintain the collision rate
depend on how worlds are distributed. For example, our Oort Cloud seems to have
been populated by gravitational ejections of icy worldlets from the vicinity of
Uranus and Neptune. If there are no planets that play the role of Uranus and
Neptune in systems otherwise like our own, their Oort Clouds may be much more
thinly populated. Stars in open and globular stellar clusters, stars in double
or multiple systems, stars closer to the center of the Galaxy, stars
experiencing more frequent encounters with Giant Molecular Clouds in
interstellar space, may all experience higher impact fluxes at their
terrestrial planets. The cometary flux might be hundreds or thousands of times
more at the Earth had the planet Jupiter never formed-according to a
calculation by George Wetherill of the Carnegie Institution of Washington. In
systems without Jupiter-like planets, the gravitational shield against comets
is down, and civilization-threatening impacts much more frequent.
To a certain extent, increased fluxes of
interplanetary objects might increase the rate of evolution, as the mammals
that flourished and diversified after the Cretaceous-Tertiary collision wiped
out the dinosaurs. But there must be a point of diminishing returns: Clearly, some
flux is too high for the continuance of any civilization.
One consequence of this train of
argument is that, even if civilizations commonly arise on planets throughout
the Galaxy, few of them will be both long-lived and non-technological. Since
hazards from asteroids and comets must apply to inhabited planets all over the
Galaxy, if there are such, intelligent beings everywhere will have to unify
their home worlds politically, leave their planets, and move small nearby
worlds around. Their eventual choice, as ours, is spaceflight or extinction.
CHAPTER 19
Remaking the planets
Who could deny that man could somehow
also make the heavens, could he only obtain the instruments and the heavenly
material?
—MARSILIO FICINO, “THE SOUL OF MAN” (CA.
1474)
In the midst of the Second World War, a
young American writer named Jack Williamson envisioned a populated Solar
System. In the twenty-second century, he imagined, Venus would be settled by
China, Japan, and Indonesia; Mars by Germany; and the moons of Jupiter by
Russia. Those who spoke English, the language in which Williamson was writing,
were confined to the asteroids-and of course the Earth.
The story, published in Astounding
Science Fiction in July 1942, was called “Collision Orbit” and written under
the pseudonym Will Stewart. Its plot hinged on the imminent collision of an
uninhabited asteroid with a colonized one, and the search for a means of
altering the trajectories of small worlds. Although no one on Earth was
endangered, this may have been the first appearance, apart from newspaper comic
strips, of asteroid collisions as a threat to humans. (Comets impacting the
Earth had been a staple peril.)
The environments of Mars and Venus were
poorly understood in the early 1940s; it was conceivable that humans could live
there without elaborate life-support systems. But the asteroids were another
matter. It was well known, even then, that asteroids were small, dry, airless
worlds. If they were to be inhabited, especially by large numbers of people,
these little worlds would somehow have to be fixed.
In “Collision Orbit,” Williamson
portrays a group of “spatial engineers,” able to render such barren outposts
clement. Coining a word, Williamson called the process of metamorphosis into an
Earth-like world “terraforming.” He knew that the low gravity on an asteroid
means that any atmosphere generated or transported there would quickly escape
to space. So his key terraforming technology was “paragravity,” an artificial
gravity that would hold a dense atmosphere.
As nearly as we can tell today,
paragravity is a physical impossibility. But we can imagine domed, transparent
habitats on the surfaces of asteroids, as suggested by Konstantin Tsiolkovsky,
or communities established in the insides of asteroids, as outlined in the
1920s by the British scientist J. D. Bernal. Because asteroids are small and
their gravities low, even massive subsurface construction might be
comparatively easy. If a tunnel were dug clean through, you could jump in at
one end and emerge some 45 minutes later at the other, oscillating up and down
along the toll diameter of this world indefinitely. Inside the right kind of
asteroid, a carbonaceous one, you can find materials for manufacturing stone,
metal, and plastic construction and plentiful water-all you might need to build
a subsurface closed ecological system, an underground garden. Implementation
would require a significant step beyond what we have today, but-unlike
“paragravity”-nothing in such a scheme seems impossible. All the elements can be
found in contemporary technology. If there were sufficient reason, a fair
number of us could be living on (or in asteroids by the twenty-second century.
They would of course need a source of
power, not just to sustain themselves, but, as Bernal suggested, to move their
asteroidal homes around. (It does not seem so big a step from explosive
alteration of asteroid orbits to a more gentle means of propulsion a century or
two later.) If an oxygen atmosphere were generated from chemically bound water,
then organics could be burned to generate power, just as fossil fuels are
burned on the Earth today. Solar power could be considered, although for the
main-belt asteroids the intensity of sunlight is only about 10 percent what it
is on Earth. Still, we could imagine vast fields of solar panels covering the
surfaces of inhabited asteroids and converting sunlight into electricity.
Photovoltaic technology is routinely used in Earth-orbiting spacecraft, and is
in increasing use on the surface of the Earth today. But while that might be
enough to warm and light the homes of these descendants, it does not seem
adequate to change asteroid orbits.
For that, Williamson proposed using
anti-matter. Antimatter is just like ordinary matter, with one significant
difference. Consider hydrogen: An ordinary hydrogen atom consists of a
positively charged proton on the inside and a negatively charged electron on
the outside. An atom of anti-hydrogen consists of a negatively charged proton
on the inside and a positively charged electron (also called a positron) on the
outside. The protons, whatever the sign of their charges, have the same mass;
and the electrons, whatever the sign of their charges, have the same mass.
Particles with opposite charges attract. A hydrogen atom and an anti-hydrogen
atom are both stable, because in both cases the positive and negative
electrical charges precisely balance.
Anti-matter is not some hypothetical
construct from the perfervid musings of science fiction writers or theoretical
physicists. Anti-matter exists. Physicists make it in nuclear accelerators; it
can be found in high-energy cosmic rays. So why don't we hear more about it?
Why has no one held up a lump of antimatter for our inspection? Because matter
and anti-matter, when brought into contact, violently annihilate each other,
disappearing in an intense burst of gamma rays. We cannot tell whether
something is made of matter or anti-matter just by looking at it. The
spectroscopic properties of, for example, hydrogen and anti-hydrogen are
identical.
Albert Einstein's answer to the question
of why we see only matter and not anti-matter was, “Matter won”-by which he
meant that in our sector of the Universe at least, after almost all the matter
and anti-matter interacted and annihilated each other long ago, there was some
of what we call ordinary matter left over.
As far as we can tell today, from gamma ray astronomy and other means,
the Universe is made almost entirely of matter. The reason for this engages the
deepest cosmological issues, which need not detain us here. But if there was
only a one-particle-in-a-billion difference in the preponderance of matter over
anti-matter at the beginning, even this would be enough to explain the Universe
we see today.
Williamson imagined that humans in the
twenty-second century would move asteroids around by the controlled mutual
annihilation of matter and anti-matter. All the resulting gamma rays, if
collimated, would make a potent rocket exhaust. The anti-matter would be
available in the main asteroid belt (between the orbits of Mars and Jupiter),
because this was his explanation for the existence of the asteroid belt. In the
remote past, he proposed, an intruder anti-matter worldlet arrived in the Solar
System from the depths of space, impacted, and annihilated what was then an
Earthlike planet, fifth from the Sun. The fragments of this mighty collision
are the asteroids, and some of them are still made of anti-matter. Harness an
anti-asteroid-Williamson recognized that this might be tricky-and you can move
worlds around at will.
At the time, Williamson's ideas were
futuristic, but far from foolish. Some of “Collision Orbit” can be considered
visionary. Today, however, we have good reason to believe that there are no
significant amounts of anti-matter in the Solar System, and that the asteroid
belt, far from being a fragmented terrestrial planet, is an enormous array of
small bodies prevented (by the gravitational tides of Jupiter) from forming an
Earthlike world.
However, we do generate (very) small
amounts of antimatter in nuclear accelerators today, and we will probably be
able to manufacture much larger amounts by the twenty-second century. Because
it is so efficient-converting all of the matter into energy, E = MC2, with 100
percent efficiency-perhaps anti-matter engines will be a practical technology
by then, vindicating Williamson Failing that, what energy sources can we
realistically expect to be available, to reconfigure asteroids, to light them
warm them, and move them around?
The Sun shines by jamming protons
together and turning them into helium nuclei. Energy is released in the
process, although with less than 1 percent the efficiency of the annihilation
of matter and anti-matter. But even proton-proton reactions are far beyond
anything we can realistically imagine for ourselves in the near future. The
required temperatures are much too high. Instead) of jamming protons together,
though, we might use heavier kinds of hydrogen. We already do so in
thermonuclear weapons. Deuterium is a proton bound by nuclear forces to a
neutron; tritium is a proton bound by nuclear forces to two neutrons. It seems
likely that in another century we will have practical power schemes that
involve the controlled fusion of deuterium and tritium, and of deuterium and
helium. Deuterium and tritium are present as minor constituents in water (on
Earth and other worlds). The kind of helium needed for fusion, 3He (two protons
and a neutron make up its nucleus), has been implanted over billions of years
by the solar wind in the surfaces of the asteroids. These processes are not
nearly as efficient as the proton-proton reactions in the Sun, but they could
provide enough power to run a small city for a year from a lode of ice only a
few meters in size.
Fusion reactors seem to be coming along
too slowly to play a major role in solving, or even significantly mitigating,
global warming. But by the twenty-second century, they ought to be widely
available. With fusion rocket engines, it will be possible to more asteroids
and comets around the inner Solar System taking a main-belt asteroid, for
example, and inserting it into orbit around the Earth. A world 10 kilometers
across could be transported from Saturn, say, to Mars through nuclear burning
of the hydrogen in an icy comet a kilometer across. (Again, I'm assuming a time
of much greater political stability and safety.)
PUT ASIDE FOR THE MOMENT any qualms you
might have about the ethics of rearranging worlds, or our ability to do so
without catastrophic consequences. Digging out the insides of worldlets,
reconfiguring them for human habitation, and moving them from one place in the
Solar System to another seems to be within our grasp in another century or two.
Perhaps by then we will have adequate international safeguards as well. But
what about transforming the surface environments not of asteroids or comets,
but of planets? Could we live on Mars?
If we wanted to set up housekeeping on
Mars, it's easy to see that, in principle at least, we could do it: There's
abundant sunlight. There's plentiful water in the rocks and in underground and
polar ice. The atmosphere is mostly carbon dioxide. It seems likely that in
self-contained habitats-perhaps domed enclosures-we could grow crops,
manufacture oxygen from water, recycle wastes.
At first we'd be dependent on
commodities resupplied from Earth, but in time we'd manufacture more and more
of them ourselves. We'd become increasingly self-sufficient. The domed
enclosures, even if made of ordinary glass, would let in the visible sunlight
and screen out the Sun's ultraviolet rays. With oxygen masks and protective
garments-but nothing as bulky and cumbersome as a spacesuit-we could leave
these enclosures to go exploring, or to build another domed village and farms.
It seems very evocative of the American
pioneering experience, but with at least one major difference: In the early
stages, large subsidies are essential. The technology required is too expensive
for some poor family, like my grandparents a century ago, to pay their own
passage to Mars. The early Martian pioneers will be sent by governments and
will have highly specialized skills. But in a generation or two, when children
and grandchildren are born there- and especially when self-sufficiency is
within reach-that will begin to change. Youngsters born on Mars will be given
specialized training in the technology essential for survival in this new
environment. The settlers will become less heroic and less exceptional. The
full range of human strengths and deficiencies will begin to assert themselves.
Gradually, precisely because of the difficulty of getting from Earth to Mars, a
unique Martian culture will begin to emerge-distinct aspirations and fears tied
to the environment they live in, distinct technologies, distinct social
problems, distinct solutions-and, as has occurred in every similar circumstance
throughout human history, a gradual sense of cultural and political
estrangement from the mother world.
Great ships will arrive carrying
essential technology from Earth, new families of settlers, scarce resources. It
is hard to know, on the basis of our limited knowledge of Mars, whether they
will go home empty-or whether they will carry with them something found only on
Mars, something considered very valuable on Earth. Initially much of the
scientific investigation of samples of the Martian surface will be done on
Earth. But in time the scientific study of Mars (and its moons Phobos and
Deimos) will be done from Mars.
Eventually-as has happened with
virtually every other form of human transportation-interplanetary travel will
become accessible to people of ordinary means: to scientists pursuing their own
research projects, to settlers fed up with Earth, even to venturesome tourists.
And of course there will be explorers.
If the time ever came when it was possible
to make the Martian environment much more Earth-like-so protective garments,
oxygen masks, and domed farmlands and cities could be dispensed with-the
attraction and accessibility of Mars would be increased many-fold. The same, of
course, would be true for any other world which could be engineered so that
humans could live there without elaborate contrivances to keep the planetary
environment out. We would feel much more comfortable in our adopted home if an
intact dome or spacesuit weren't all that stood between us and death. (But
perhaps I exaggerate the dangers. People who live in the Netherlands seem at
least as well adjusted and carefree as other inhabitants of Northern Europe;
vet their dikes are all that stand between them and the sea.
Recognizing the speculative nature of
the question and the limitations in our knowledge, is it nevertheless possible
to envision terraforming the planets?
We need look no further than our own
world to see that humans are now able to alter planetary environments in a profound
way. Depletion of the ozone layer, global warming from an increased greenhouse
effect, and global cooling from nuclear war are all ways in which present
technology can significantly alter the environment of our world-and in each
case as an inadvertent consequence of doing something else. If we had intended
to alter our planetary environment, we would be fully able to generate still
greater change. As our technology becomes more powerful, we will be able to
work still more profound changes.
But just as (in parallel parking) it's
easier to get out of a parking place than into one, it's easier to destroy a
planetary environment than to move it into a narrowly prescribed range of
temperatures, pressures, compositions, and so on. We already know of a multitude
of desolate and uninhabitable worlds, and-with very narrow margins-only one
green and clement one. This is a major conclusion from early in the era of
spacecraft exploration of the Solar System. In altering the Earth, or any world
with an atmosphere, we must be very careful about positive feedbacks, where we
nudge an environment a little bit and it takes of on its own-a little cooling
leading to runaway glaciation, as may have happened on Mars, or a little
warming to a runaway greenhouse effect, as happened on Venus. It is not at all
clear that our knowledge is sufficient to this purpose.
As far as I know, the first suggestion
in the scientific literature about terraforming the planets was made in a 1961
article I wrote about Venus. I was pretty sure then that Venus had a surface
temperature well above the normal boiling point of water, produced by a carbon
dioxide/water vapor greenhouse effect. I imagined seeding its high clouds with
genetically engineered microorganisms that would take CO2, N2, and H2O out of
the atmosphere and convert them into organic molecules. The more CO2 removed,
the smaller the greenhouse effect and the cooler the surface. The microbes
would be carried down through the atmosphere toward the ground, where they
would be fried, so water vapor would be returned to the atmosphere; but the
carbon from the CO2 would be converted irreversibly by the high temperatures
into graphite or some other involatile form of carbon. Eventually, the
temperatures would fall below the boiling point and the surface of Venus would
become habitable, dotted with pools and lakes of warm water.
The idea was soon taken up by a number
of science fiction authors in the continuing dance between science and science
fiction-in which the science stimulates the fiction, and the fiction stimulates
a new generation of scientists, a process benefiting both genres. But as the
next step in the dance, it is now clear that seeding Venus with special
photosynthetic microorganisms will not work. Since 1961 we've discovered that
the clouds of Venus are a concentrated solution of sulfuric acid, which makes
the genetic engineering rather more challenging. But that in itself is not a
fatal flaw. (There are microorganisms that live out their lives in concentrated
solutions of sulfuric acid.) Here's the fatal flaw: In 1961 I thought the
atmospheric pressure at the surface of Venus N\-as a few “bars,” a few times
the surface pressure on Earth. We now know it to be 90 bars, so that if the
scheme worked, the result would be a surface buried in hundreds of meters of
fine graphite, and an atmosphere made of 65 bars of almost pure molecular
oxygen. Whether we would first implode under the atmospheric pressure or
spontaneously burst into flames in all that oxygen is an open question. However,
long before so much oxygen could build up, the graphite would spontaneously
burn back into CO2, short-circuiting the process. At best, such a scheme can
carry the terraforming of Venus only partway.
Let's assume that by the early
twenty-second century we have comparatively inexpensive heavy-lift vehicles, so
we can carry large payloads to other worlds; abundant and powerful fusion
reactors; and well-developed genetic engineering. All three assumptions are
likely, given current trends. Could we terraform the planets? James Pollack of
NASA's Ames Research Center and I surveyed this problem. Here's a summary of
what we found:
VENUS: Clearly the problem with Venus is
its massive greenhouse effect. If we could reduce the greenhouse effect almost
to zero, the climate might be balmy. But a 90-bar CO2 atmosphere is
oppressively thick. Over every postage stamp-sized square inch of surface, the
air weighs as much as six professional football players, piled one on top of
another. Making all that go away will take some doing.
Imagine bombarding Venus with asteroids
and comets. Each impact would blow away some of the atmosphere. To blow away
almost all of it, though, would require using up more big asteroids and comets
than there are-at least in the planetary part of the Solar System. Even if that
many potential impactors existed, even if we could make them all collide with
Venus (this is the overkill approach to the impact hazard problem), think what
we would have lost. Who knows what wonders, what practical knowledge they
contain? We would also obliterate much of Venus' gorgeous surface geology-which
we've just begun to understand, and which may teach us much about the Earth.
This is an example of bruteforce terraforming. I suggest we want to steer
entirely clear of such methods, even if someday we'll be able to afford them
(which I very much doubt). We want something more elegant, more subtle, more
respectful of the environments of other worlds. A microbial approach has some
of those virtues, but does not do the trick, as we've just seen.
We can imagine pulverizing a dark
asteroid and spreading the powder through the upper atmosphere of Venus, or
carrying such dust up from the surface. This would be the physical equivalent
of nuclear winter or the Cretaceous-Tertiary post-impact climate. If the
sunlight reaching the ground is sufficiently attenuated, the surface
temperature must fall. But by its very nature, this option plunges Venus into
deep gloom, with daytime light levels perhaps only as bright as on a moonlit
night on Earth. The oppressive, crushing 90-bar atmosphere would remain
untouched. Since the emplaced dust would sediment out every few years, the
layer would have to be replenished in the same period of time. Perhaps such an
approach would be acceptable for short exploratory missions, but the
environment generated seems very stark for a self-sustaining human community on
Venus.
We could use a giant artificial sunshade
in orbit around Venus to cool the surface; but it would be enormously
expensive, as well as having many of the deficiencies of the dust layer.
However, if the temperatures could be lowered sufficiently, the CO2 in the
atmosphere would rain out. There would be a transitional time of CO2 oceans on
Venus. If those oceans could be covered over to prevent re-evaporation-for
example, with water oceans made by melting a large, icy moon transported from
the outer Solar System-then the CO2 might conceivably be sequestered away, and
Venus converted into a water (or low-fizz seltzer) planet. Ways have also been
suggested to convert the CO2 into carbonate rock.
Thus all proposals for terraforming
Venus are still brute-force, inelegant, and absurdly expensive. The desired
planetary metamorphosis may be beyond our reach for a very long time, even if
we thought it was desirable and responsible. The Asian colonization of Venus
that Jack Williamson imagined may have to be redirected somewhere else.
MARS: For Mars we have just the opposite
problem. There's not enough greenhouse effect. The planet is a frozen desert.
But the fact that Mars seems to have had abundant rivers, lakes, and perhaps
even oceans 4 billion years ago-at a time when the Sun was less bright than it
is today-makes you wonder if there's solve natural instability in the Martian
climate, something on hair trigger that once released would all by itself
return the planet to its ancient clement state. (Let's note from the start that
doing so would destroy Martian landforms that hold key data on the
past-especially the laminated polar terrain.)
As we know very well from Earth and
Venus, carbon dioxide is a greenhouse gas. There are carbonate minerals found
on Mars, and dry ice in one of the polar caps. They could be converted into CO2
gas. But to make enough of a greenhouse effect to generate comfortable temperatures
on Mars would require the entire surface of the planet to be plowed up and
processed to a depth of kilometers. Apart from the daunting obstacles in
practical engineering that this represents-fusion power or no fusion power-and
the inconvenience to whatever self-contained, closed ecological systems humans
had already established on the planet it would also constitute the
irresponsible destruction of a unique scientific resource and database, the
Martian surface.
What about other greenhouse gases?
Alternatively, we might take chlorofluorocarbons (CFCs or HCFCs) to Mars after
manufacturing them on Earth. These are artificial substances that, so far as we
know, are found nowhere else in the Solar System. We can certainly imagine
manufacturing enough CFCs on Earth to warm Mars, because by accident in a few
decades with present technology on Earth we've managed to synthesize enough to
contribute to global warming on our planet. Transportation to Mars would be
expensive, though: Even using Saturn V- or Energiya-class boosters, it would
require at least a launch a day for a century. But perhaps they could be
manufactured from fluorine-containing minerals on Mars.
There is, in addition, a serious
drawback: On Mars as on Earth, abundant CFCs would prevent formation of an
ozone layer. CFCs might bring Martian temperatures into a clement range, but
guarantee that the solar ultraviolet hazard would remain extremely serious.
Perhaps the solar ultraviolet light could be absorbed by an atmospheric layer
of pulverized asteroidal or surface debris injected in carefully titrated
amounts above the CFCs. But now we're in the troubling circumstance of having
to deal with propagating side effects, each of which requires its own
large-scale technological solution.
A third possible greenhouse gas for
warming Mars is ammonia (NH3). Only a little ammonia would be enough to warm
the Martian surface to above the freezing point of water. In principle, this
might be done by specially engineered microorganisms that would convert Martian
atmospheric N2 to NH3 as some microbes do on Earth, but do it under Martian
conditions. Or the same conversion might be done in special factories.
Alternatively, the nitrogen required could be carried to Mars from elsewhere in
the Solar System. (N2 is the principal constituent in the atmospheres of both
Earth and Titan.) Ultraviolet light would convert ammonia back into N2 in about
30 years, so there would have to be a continuous resupply of NH3.
A judicious combination of CO2, CFC, and
NH3 greenhouse effects on Mars looks as if it might be able to bring surface
temperatures close enough to the freezing point of water for the second phase
of Martian terraforming to begin-temperatures rising due to the pressure of
substantial water vapor in the air, widespread production of O2 by genetically
engineered plants, and fine-tuning the surface environment. Microbes and larger
plants and animals could be established on Mars before the overall environment
was suitable for unprotected human settlers.
Terraforming Mars is plainly much easier
than terraforming Venus. But it is still very expensive by present standards,
and environmentally destructive. If there were sufficient justification,
though, perhaps the terraforming of Mars could be under way by the
twenty-second century.
THE MOONS OF JUPITER AND SATURN:
Terraforming the satellites of the Jovian planets presents varying degrees of
difficulty. Perhaps the easiest to contemplate is Titan. It already has an
atmosphere, made mainly of N2 like the Earth's, and is much closer to
terrestrial atmospheric pressures than either Venus or Mars. Moreover,
important greenhouse gases, such as NH3 and H20, are almost certainly frozen
out on its surface. Manufacture of initial greenhouse gases that do not freeze
out at present Titan temperatures plus direct warming of the surface by nuclear
fusion could, it seems, be the key early steps to one day terraform Titan.
IF THERE WERE A COMPELLING REASON for
terraforming other worlds, this greatest of engineering projects might be
feasible on the timescale we've been describing-certainly for asteroids,
possibly for Mars, Titan, and other moons of the outer planets, and probably
not for Venus. Pollack and I recognized that there are those who feel a
powerful attraction to the idea of rendering other worlds in the Solar System
suitable for human habitation-in establishing observatories, exploratory bases,
communities, and homesteads there. Because of its pioneering history, this may
be a particularly natural and attractive idea in the United States.
In any case, massive alteration of the
environments of other worlds can be done competently and responsibly only when
we have a much better understanding of those worlds than is available today.
Advocates of terraforming must first become advocates of the long-term and
thorough scientific exploration of other worlds.
Perhaps when we really understand the
difficulties of terraforming, the costs or the environmental penalties will
prove too steep, and we will lower our sights to domed or subsurface cities or
other local, closed ecological systems, greatly improved versions of Biosphere
II, on other worlds. Perhaps we will abandon the dream of converting the
surfaces of other worlds to something approaching the Earth's. Or perhaps there
are much more elegant, cost-effective, and environmentally responsible ways of
terraforming that we have not yet imagined.
But if we are seriously to pursue the
matter, certain questions ought to be asked: Given that any terraforming scheme
entails a balance of benefits against costs, how certain must we be that key
scientific information will not thereby be destroyed before proceeding? How
much understanding of the world in question do we need before planetary
engineering can be relied upon to produce the desired end state? Can we
guarantee a long-term human commitment to maintain and replenish an engineered
world, when human political institutions are so short lived? If a world is even
conceivably inhabited-perhaps only by microorganisms-do humans have a right to
alter it? What is our responsibility to preserve the worlds of the Solar System
in their present wilderness states for future generations-who may contemplate
uses that today we are too ignorant to foresee? These questions may perhaps be
encapsulated into a final question: Can we, who have made such a mess of this
world, be trusted with others?
It is just conceivable that some of the
techniques that might eventually terraform other worlds might be applied to
ameliorate the damage we have done to this one. Considering the relative
urgencies, a useful indication of when the human species is ready to consider
terraforming seriously is when we have put our own world right. We can consider
it a test of the depth of our understanding and our commitment. The first step
in engineering the Solar System is to guarantee the habitability of the Earth.
Then we'll be ready to spread out to
asteroids, comets, Mars, the moons of the outer Solar System, and beyond. Jack
Williamson 's prediction that this will begin to come about by the twenty-second
century may not be far off the mark.
THE NOTION OF OUR DESCENDANTS living and
working on other worlds, and even moving some of them around for their
convenience, seems the most extravagant science fiction. Be realistic, a voice
inside my head counsels. But this is realistic. We're on the cusp of the
technology, near the midpoint between impossible and routine. It's easy to be
conflicted about it. If we don't do something awful to ourselves in the
interim, in another century terraforming may seem no more impossible than a
human-tended space station does today.
I think the experience of living on
other worlds is bound to change us. Our descendants, born and raised elsewhere,
will naturally begin to owe primary loyalty to the worlds of their birth,
whatever affection they retain for the Earth. Their physical needs, their
methods of supplying those needs, their technologies, and their social
structures will all have to be different.
A blade of grass is a commonplace on
Earth; it would be a miracle on Mars. Our descendants on Mars will know the
value of a patch of green. And if a blade of grass is priceless, what is the
value of a human being? The American revolutionary Tom Paine, in describing his
contemporaries, had thoughts along these lines:
The wants which necessarily accompany
the cultivation of a wilderness produced among them a state of society which
countries long harassed by the quarrels and intrigues of governments had
neglected to cherish. In such a situation man becomes what he ought to be. He
sees his species...as kindred.
Having seen at first hand a procession
of barren and desolate worlds, it will be natural for our spacefaring
descendants to cherish life. Having learned something from the tenure of our
species on Earth, they may wish to apply those lessons to other worlds-to spare
generations to come the avoidable suffering that their ancestors were obliged
to endure, and to draw upon our experience and our mistakes as we begin our
open-ended evolution into space.
CHAPTER 20
Darkness
Far away, hidden from the eyes of
daylight, there are watchers in the skies.
—EURIPIDES, THE BACCHAE /CA. 406 B. C.)
As children, we fear the dark. Anything
might be out. here. The unknown troubles us. Ironically, it is our fate to live
in the dark. This unexpected finding of science is only about three centuries
old. Head out from the Earth in any direction you choose, and-after an initial
flash of blue and a longer wait while the Sun fades-you are surrounded by
blackness, punctuated only here and there by the faint and distant stars.
Even after we are grown, the darkness
retains its power to frighten us. And so there are those who say we should not
inquire too closely into who else might be living in that darkness. Better not
to know, they say.
There are 400 billion stars in the Milky
Way Galaxy. Of this immense multitude, could it be that our humdrum Sun is the
only one with an inhabited planet? Maybe. Maybe the origin of life or
intelligence is exceedingly improbable. Or maybe civilizations arise all the time,
but wipe themselves out as soon as they are able.
Or, here and there, peppered across
space, orbiting other suns, maybe there are worlds something like our own, on
which other beings gaze up and wonder as we do about who else lives in the
dark. Could the Milky Way be rippling with life and intelligence-worlds calling
out to worlds-while we on Earth are alive at the critical moment when we first
decide to listen?
Our species has discovered a way to
communicate through the dark, to transcend immense distances. no means of
communication is faster or cheaper or reaches out farther. It's called radio.
After billions of years of biological
evolution-on their planet and ours-an alien civilization cannot be in
technological lockstep with us. There have been humans for more than twenty
thousand centuries, but we've had radio only for about one century. If alien
civilizations are behind us, they're likely to be too far behind to have radio.
And if they're ahead of us, they're likely to be far ahead of us. Think of the
technical advances on our world over just the last few centuries. What is for
us technologically difficult or impossible, what might seem to us like magic,
might for them be trivially easy. They might use other, very advanced means to
communicate with their peers, but they would know about radio as an approach to
newly emerging civilizations. Even with no more than our level of technology at
the transmitting and receiving ends, we could communicate today across much of
the Galaxy. They should be able to do much better.
If they exist.
But our fear of the dark rebels. The
idea of alien beings troubles us. We conjure up objections:
“It's too expensive.” But, in its
fullest modern technological expression, it costs less than one attack
helicopter a year.
“We'll never understand what they're
saying.” But, because the message -is transmitted by radio, we and they must
have radio physics, radio astronomy, and radio technology in common. The laws
of Nature are the same everywhere; so science itself provides a means and
language of communication even between very different kinds of beings-provided
they both have science. Figuring out the message, if we're fortunate enough to
receive one, may be much easier than acquiring it.
“It would be demoralizing to learn that
our science is primitive.” But by the standards of the next few centuries, at
least some of our present science will be considered primitive,
extraterrestrials or no extraterrestrials. (So will some of our present
politics, ethics, economics, and religion.) To go beyond present science is one
of. the chief goals of science. Serious students are not commonly plunged into
fits of despair on turning the pages of a textbook and discovering that some
further topic is known to the author but not yet to the student. Usually the
students struggle a little, acquire the new knowledge, and, following an
ancient human tradition, continue to turn the pages.
“All through history advanced
civilizations have ruined civilizations just slightly more backward.”
Certainly. But malevolent aliens, should they exist, will not discover our
existence from the fact that we listen. The search programs only receive; they
do not send.
THE DEBATE IS, for the moment, moot. We
are now, on an unprecedented scale, listening for radio signals from possible
other civilizations in the depths of space. Alive today is the first generation
of scientists to interrogate the darkness. Conceivably it might also be the
last generation before contact is made-and this the last moment before we
discover that someone in the darkness is calling out to us.
This quest is called the Search for
Extraterrestrial Intelligence (SETI). Let me describe how far we've come.
The first SETI program was carried out
by Frank Drake at the National Radio Astronomy Observatory in Greenbank, West
Virginia, in 1960. He listened to two nearby Sun-like stars for two weeks at
one particular frequency. (“Nearby” is a relative term: The nearest was 12
light-years-70 trillion miles-away.)
Almost at the moment Drake pointed the
radio telescope and turned the system on, he picked up a very strong signal.
Was it a message from alien beings? Then it went away. If the signal
disappears, you can't scrutinize it. You can't see if, because of the Earth's
rotation, it moves with the sky. If it's not repeatable, you've learned almost
nothing from it-it might be terrestrial radio interference, or a failure of
your amplifier or detector...or an alien signal. Unrepeatable data, no matter
how illustrious the scientist reporting them, are not worth much.
Weeks later, the signal was detected
again. It turned out to be a military aircraft broadcasting on an unauthorized
frequency. Drake reported negative results. But in science a negative result is
not at all the same thing as a failure. His great achievement was to show that
modern technology is fully able to listen for signals from hypothetical
civilizations on the planets of other stars.
Since then there've been a number of
attempts, often on time borrowed from other radio telescope observing programs,
and almost never for longer than a few months. There've been some more false
alarms, at Ohio State, in Arecibo, Puerto Rico, in France, Russia, and
elsewhere, but nothing that could pass muster with the world scientific
community.
Meanwhile, the technology for detection
has been getting cheaper; the sensitivity keeps improving; the scientific
respectability of SETI has continued to grow; and even NASA and Congress have
become a little less afraid to support it. Diverse, complementary search
strategies are possible and necessary. It was clear years ago that if the trend
continued, the technology for a comprehensive SETI effort would eventually fall
within the reach even of private organizations (or wealthy individuals); and
sooner or later, the government would be willing to support a major program.
After 30 years of work, for some of us it's been later rather than sooner. But
at last the time has come.
THE PLANETARY SOCIETY-a nonprofit
membership organization that Bruce Murray, then the Director of JPL, and I
founded in 1980-is devoted to planetary exploration and the search for
extraterrestrial life. Paul Horowitz, a physicist at Harvard University, had
made a number of important innovations for SETI and was eager to try them out.
If we could find the money to get him started, we thought we could continue to
support the program by donations from our members.
In 1983 Ann Druyan and I suggested to
the filmmaker Steven Spielberg that this was an ideal project for him to
support. Breaking with Hollywood tradition, he had in two wildly successful
movies conveyed the idea that extraterrestrial beings might not be hostile and
dangerous. Spielberg agreed. With his initial support through The Planetary
Society, Project META began.
META is an acronym for “Megachannel ExtraTerrestrial
Assay.” The single frequency of Drake's first system grew to 8. 4 million. But
each channel, each “station,” we tune to has an exceptionally narrow frequency
range. There are no known processes out among the stars and galaxies that can generate
such sharp radio “lines.” If we pick up anything falling into so narrow a
channel, it must, we think, be a token of intelligence and technology.
What's more, the Earth turns-which means
that any distant radio source will have a sizable apparent motion, like the
rising and setting of the stars. Just as the steady tone of a car's horn dips
as it drives by, so any authentic extraterrestrial radio source will exhibit a
steady drift in frequency due to the Earth's rotation. In contrast, any source
of radio interference at the Earth's surface will be rotating at the same speed
as the META receiver. META's listening frequencies are continuously changed to
compensate for the Earth's rotation, so that any narrow-band Signals from the
sky will always appear in a single channel. But any radio interference down
here on Earth will give itself away by racing through adjacent channels.
The META radio telescope at Harvard,
Massachusetts, is 26 meters (84 feet) in diameter. Each day, as the Earth
rotates the telescope beneath the sky, a swath of stars narrower than the full
moon is swept out and examined. Next day, it's an adjacent swath. Over a year,
all of the northern sky and part of the southern is observed. An identical
system, also sponsored by The Planetary Society, is in operation just outside
Buenos Aires, Argentina, to examine the southern sky. So together the two META
systems have been exploring the entire sky.
The radio telescope, gravitationally
glued to the spinning Earth, looks at any given star for about two minutes.
Then it's on to the next. 8. 4 million channels sounds like a lot, but
remember, each channel is very narrow. All of them together constitute only a
few parts in 100,000 of the available radio spectrum. So we have to park our 8.
4 million channels somewhere in the radio spectrum for each year of
observation, near some frequency that an alien civilization, knowing nothing
about us, might nevertheless conclude we're listening to.
Hydrogen is by far the most abundant
kind of atom in the Universe. It's distributed in clouds and as diffuse gas
throughout interstellar space. When it acquires energy, it releases some of it
by giving off radio waves at a precise frequency of 1420. 405751768 megahertz.
(One hertz means the crest and trough of a wave arriving at your detection
instrument each second. So 1420 megahertz means 1. 420 billion waves entering
your detector every second. Since the wavelength of light is just the speed of
light divided by the frequency of the wave, 1420 megahertz corresponds to a wavelength
of 21 centimeters.) Radio astronomers anywhere in the Galaxy will be studying
the Universe at 1420 megahertz and can anticipate that other radio astronomers,
no matter how different they may look, will do the same.
It's as if someone told you that there's
only one station on your home radio set's frequency band, but that no one knows
its frequency. Oh yes, one other thing: Your set's frequency dial, kith its
thin marker you adjust by turning a knob, happens to reach from the Earth to
the Moon. To search systematically through this vast radio spectrum, patiently
turning the knob, is going to be very time-consuming. Your problem is to set
the dial correctly from the beginning, to choose the right frequency. If you
can correctly guess what frequencies that extraterrestrials are broadcasting to
us on-the “magic” frequencies-then you can save yourself much time and trouble.
These are the sorts of reasons that we first listened, as Drake did, at
frequencies near 1420 megahertz, the hydrogen “magic” frequency.
Horowitz and I have published detailed
results from five years of full-time searching with Project META and two years
of follow-up. We can't report that we found a signal from alien beings. But we
did find something puzzling, something that for me in quiet moments, every now
and then, raises goose bumps:
Of course, there's a background level of
radio noise from Earth-radio and television stations, aircraft, portable
telephones, nearby and more distant spacecraft. Also, as with all radio
receivers, the longer you wait, the more likely it is that there'll be some
random fluctuation in the electronics so strong that it generates a spurious
signal. So we ignore anything that isn't much louder than the background.
Any strong narrow-band signal that
remains in a single channel we take very seriously. As it logs in the data,
META automatically tells the human operators to pay attention to certain
signals. Over five years we made some 60 trillion observations at various
frequencies, while examining the entire accessible sky. A few dozen signals
survive the culling. These are subjected to further scrutiny, and almost all of
them are rejected-for example, because an error has been found by
fault-detection microprocessors that examine the signal-detection microprocessors.
What's left-the strongest candidate
signals after three surveys of the sky-are 11 “events.” They satisfy all but
one of our criteria for a genuine alien signal. But the one failed criterion is
supremely important: Verifiability. We've never been able to find any of them
again. We look back at that part of the sky three minutes later and there's
nothing there. We look again the following day: nothing. Examine it a year
later, or seven years later, and still there's nothing.
It seems unlikely that every signal we
get from alien civilizations would turn itself off a couple of minutes after we
begin listening, and never repeat. (How would they know we're paying
attention?) But, just possibly, this is the effect of twinkling. Stars twinkle
because parcels of turbulent air are moving across the line of sight between
the star and us. Sometimes these air parcels act as a lens and cause the light
rays from a given star to converge a little, making it momentarily brighter.
Similarly, astronomical radio sources may also twinkle-owing to clouds of
electrically charged (or “ionized”) gas in the great near-vacuum between the
stars. We observe this routinely with pulsars.
Imagine a radio signal that's a little
below the strength that we could otherwise detect on Earth. Occasionally the
signal will by chance be temporarily focused, amplified, and brought within the
detectability range of our radio telescopes. The interesting thing is that the
lifetimes of such brightening, predicted from the physics of the interstellar gas,
are a few minutes-and the chance of reacquiring the signal is small. We should
really be pointing steadily at these coordinates in the sky, watching them for
months.
Despite the fact that none of these
signals repeats, there's an additional fact about them that, every time I think
about it, sends a chill down my spine: 8 of the 11 best candidate signals lie
in or near the plane of the Milky Way Galaxy. The five strongest are in the
constellations Cassiopeia, Monoceros, Hydra, and two in Sagittarius-in the
approximate direction of the center of the Galaxy. The Milky Way is a flat,
wheel-like collection of gas and dust and stars. Its flatness is why we see it
as a band of diffuse light across the night sky. That's where almost all the
stars in our galaxy are. If our candidate signals really were radio
interference from Earth or some undetected glitch in the detection electronics,
we shouldn't see them preferentially when we're pointing at the Milky Way.
But maybe we had an especially unlucky
and misleading run of statistics. The probability that this correlation with
the galactic plane is due merely to chance is less than half a percent. Imagine
a wall-size map of the sky, ranging from the North Star at the top to the
fainter stars toward which the Earth's south pole points at the bottom. Snaking
across this wall map are the irregular boundaries of the Milky Way. Now suppose
that you were blindfolded and asked to throw five darts at random at the map
(with much of the southern sky, inaccessible from Massachusetts, declared off
limits). You'd have to throw the set of five darts more than 200 times before,
by accident, you got them to fall as closely within the precincts of the Milky
Way as the five strongest META signals did. Without repeatable signals, though,
there's no way we can conclude that we've actually found extraterrestrial
intelligence.
Or maybe the events we've found are
caused by some new kind of astrophysical phenomenon, something that nobody has
thought of yet, by which not civilizations, but stars or gas clouds (or
something) that do lie in the plane of the Milky Way emit strong signals in
bafflingly narrow frequency bands.
Let's permit ourselves, though, a moment
of extravagant speculation. Let's imagine that all our surviving events are in
fact due to radio beacons of other civilizations. Then we can estimate-from how
little time we've spent watching each piece of sky-how many such transmitters
there are in the entire Milky Way. The answer is something approaching a
million. If randomly strewn through space, the nearest of them would be a few
hundred light years away, too far for them to have picked up our own TV or
radar signals yet. They would not know for another few centuries that a
technical civilization has emerged on Earth. The Galaxy would be pulsing with
life and intelligence, but-unless they're busily exploring huge numbers of
obscure star systems-wholly oblivious of what has been happening down here
lately. A few centuries from now, after they do hear from us, things might get
very interesting. Fortunately, we'd have many generations to prepare.
If, on the other hand, none of our
candidate signals is an authentic alien radio beacon, then we're forced to the
conclusion that very few civilizations are broadcasting, maybe none, at least
at our magic frequencies and strongly enough for us to hear:
Consider a civilization like our own,
but which dedicated all its available power (about 10 trillion watts) to
broadcasting a beacon signal at one of our magic frequencies and to all
directions in space. The META results would then imply that there are no such
civilizations out to 25 light-years-a volume that encompasses perhaps a dozen
Sun-like stars. This is not a very stringent limit. If, in contrast, that
civilization were broadcasting directly at our position in space, using an
antenna no more advanced than the Arecibo Observatory, then if META has found
nothing, it follows that there are no such civilizations anywhere in the Milky
Way Galaxy-out of 400 billion stars, not one. But even assuming they would want
to, how would they know to transmit in our direction?
Now consider, at the opposite
technological extreme, a very advanced civilization omnidirectionally and
extravagantly broadcasting at a power level 10 trillion times greater (1026
watts, the entire energy output of a star like the Sun). Then, if the META
results are negative, we can conclude not only that there are no such
civilizations in the Milky Way, but none out to 70 million light-years-none in
M31, the nearest galaxy like our own, none in M33, or the Fornax system, or
M81, or the Whirlpool Nebula, or Centaurus A, or the Virgo cluster of galaxies,
or the nearest Seyfert galaxies; none among any of the hundred trillion stars
in thousands of nearby galaxies. Stake through its heart or not, the geocentric
conceit stirs again.
Of course, it might be a token not of
intelligence but of stupidity to pour so much energy into interstellar (and
intergalactic) communication. Perhaps they have good reasons not to hail all
comers. Or perhaps they don't care about civilizations as backward as we are.
But still-not one civilization in a hundred trillion stars broadcasting with
such power on such a frequency? If the META results are negative, we have set
an instructive limit-but whether on the abundance of very advanced
civilizations or their communications strategy we have no way of knowing. Even
if META has found nothing, a broad middle range remains open-of abundant
civilizations, more advanced than we and broadcasting omnidirectionally at
magic frequencies. We would not have heard from them yet.
ON OCTOBER 12, 1992-auspiciously or
otherwise the 500th anniversary of the “discovery” of America by Christopher
Columbus-NASA turned on its new SETI program. At a radio telescope in the
Mojave Desert, a search was initiated intended to cover the entire sky
systematically-like META, making no guesses about which stars are more likely,
but greatly expanding the frequency coverage. At the Arecibo Observatory, an
even more sensitive NASA study began that concentrated on promising nearby star
systems. When fully operational, the NASA searches would have been able to
detect much fainter signals than META, and look for kinds of signals that META
could not.
The META experience reveals a thicket of
background static and radio interference. Quick reobservation and confirmation
of the signal-specially at other, independent radio telescopes-is the key to
being sure. Horowitz and I gave NASA scientists the coordinates of our fleeting
and enigmatic events. Perhaps they would be able to confirm and clarify our
results. The NASA program was also developing new technology, stimulating
ideas, and exciting schoolchildren. In the eyes of many it was well worth the
$10 million a year being spent on it. But almost exactly a year after
authorizing it, Congress pulled the plug on NASA's SETI program. It cost too
much, they said. The post-Cold War U. S. defense budget is some 30,000 times
larger.
The chief argument of the principal
opponent of the NASA SETI program-Senator Richard Bryan of Nevada-was this
[from the Congressional Record for September 22, 1993]:
So far, the NASA SETI Program has found
nothing. In fact, all the decades of SETI research have found no confirmable
signs of extraterrestrial life.
Even with the current NASA version of
SETI, I do not think many of its scientists would be willing to guarantee that
we are likely to see any tangible results in the [foreseeable] future...
Scientific research rarely, if ever,
offers guarantees of success-and I understand that-and the full benefits of
such research are often unknown until very late in the process. And I accept
that, as well.
In the case of SETI, however, the
chances of success are so remote, and the likely benefits of the program are so
limited, that there is little justification for 12 million taxpayer dollars to
be expended for this program.
But how, before we have found
extraterrestrial intelligence, can we “guarantee” that we will find it? How, on
the other hand, can we know that the chances of success are “remote”? And if we
find extraterrestrial intelligence, are the benefits really likely to be “so
limited”? As in all great exploratory ventures, we do not know what we will
find and we don't know the probability of finding it. If we did, we would not
have to look.
SETI is one of those search programs
irritating to those who want well-defined cost/benefit ratios. Whether ETI can
be found; how long it would take to find it; and what it would cost to do so
are all unknown. The benefits might be enormous, but we can't really be sure of
that either. It would of course be foolish to spend a major fraction of the
national treasure on such ventures, but I wonder if civilizations cannot be
calibrated by whether they pay some attention to trying to solve the great
problems.
Despite these setbacks, a dedicated band
of scientists and engineers, centered at the SETI Institute in Palo Alto,
California, has decided to go ahead, government or no government. NASA has
given them permission to use the equipment already paid for; captains of the
electronics industry have donated a few million dollars; at least one
appropriate radio telescope is available; and the initial stages of this
grandest of all SETI programs is on track. If it can demonstrate that a useful
sky survey is possible without being swamped by background noise-and especially
if, as is very likely from the META experience, there are unexplained candidate
signals-perhaps Congress will change its mind once more and fund the project.
Meanwhile, Paul Horowitz has come up
with a new program-different from META, different from what NASA was
doing-called BETA. BETA stands for “Billion-channel ExtraTerrestrial Assay.” It
combines narrow-band sensitivity, wide frequency coverage, and a clever way to
verify signals as they're detected. If The Planetary Society can find the
additional support, this system-much cheaper than the former NASA
program-should be on the air soon.
WOULD I LIKE TO BELIEVE that with META
we've detected transmissions from other civilizations out there in the dark, sprinkled
through the vast Milky Way Galaxy? You bet. After decades of wondering and
studying this problem, of course I would. To me, such a discovery would be
thrilling. It would change everything. We would be hearing from other beings,
independently evolved over billions of years, viewing the Universe perhaps very
differently, probably much smarter, certainly not human. How much do they know
that we don't?
For me, no signals, no one calling out
to us is a depressing prospect. “Complete silence,” said Jean-Jacques Rousseau
in a different context, “induces melancholy; it is an image of death.” But I'm
with Henry David Thoreau: “Why should I feel lonely? Is not our planet in the
Milky Way?”
The realization that such beings exist
and that, as the evolutionary process requires, they must be very different
from us, would have a striking implication: Whatever differences divide us down
here on Earth are trivial compared to the differences between any of us and any
of them. Maybe it's a long shot, but the discovery of extraterrestrial
intelligence might play a role in unifying our squabbling and divided planet.
It would be the last of the Great Demotions, a rite of passage for our species
and a transforming event in the ancient quest to discover our place in the Universe.
In our fascination with SETI, we might
be tempted, even without good evidence, to succumb to belief but this would be
self-indulgent and foolish. We must surrender our skepticism only in the face
of rock-solid evidence. Science demands a tolerance for ambiguity. Where we are
ignorant, we withhold belief. Whatever annoyance the uncertainty engenders
serves a higher purpose: It drives us to accumulate better data. This attitude
is the difference between science and so much else. Science offers little in
the way of cheap thrills. The standards of evidence are strict. But when
followed they allow us to see far, illuminating even a great darkness.
CHAPTER 21
To the sky!
The stairs of the sky are let down for
him that he may ascend thereon to heaven. O gods, put your arms under the king:
raise him, lift him to the sky. To the
sky! To the sky!
—HYMN FOR A DEAD PHARAOH (EGYPT, CA.
2600 B. C.)
When my grandparents were children, the
electric light, the automobile, the airplane, and the radio were Stupefying
technological advances, the wonders of the age. You might hear wild stories
about them, but you could not find a single exemplar in that little village in
Austria-Hungary, near the banks of the river Bug. But in that same time, around
the turn of the last century, there were two men who foresaw other, far more
ambitious, inventions-Konstantin Tsiolkovsky, the theoretician, a nearly deaf
schoolteacher in the obscure Russian town of Kaluga, and Robert Goddard, the
engineer, a professor at an equally obscure American college in Massachusetts.
They dreamt of using rockets to journey to the planets and the stars. Step by
step, they worked out the fundamental physics and many of the details.
Gradually, their machines took shape. Ultimately, their dream proved infectious.
In their time, the very idea was
considered disreputable, or even a symptom of some obscure derangement. Goddard
found that merely mentioning a voyage to other worlds subjected him to
ridicule, and he dared not publish or even discuss in public his long-term
vision of flights to the stars. As teenagers, both had epiphanal visions of
spaceflight that never left them. “I still have dreams in which I fly up to the
stars in my machine,” Tsiolkovsky wrote in middle age. “It is difficult to work
all on your own for many years, in adverse conditions without a gleam of hope,
without any help.” Many of his contemporaries thought he was truly mad. Those
who knew physics better than Tsiolkovsky and Goddard-including The New York
Times in a dismissive editorial not retracted until the eve of Apollo
11-insisted that rockets could not work in a vacuum, that the Moon and the
planets were forever beyond human reach.
A generation later, inspired by
Tsiolkovsky and Goddard, Wernher von Braun was constructing the first rocket
capable of reaching the edge of space, the V-2. But in one of those ironies
with which the twentieth century is replete, von Braun was building it for the
Nazis-as an instrument of indiscriminate slaughter of civilians, as a
“vengeance weapon” for Hitler, the rocket factories staffed with slave labor,
untold human suffering exacted in the construction of every booster, and von
Braun himself made an officer in the SS. He was aiming at the Moon, he joked
unselfconsciously, but hit London instead.
Another generation later, building on
the work of Tsiolkovsky and Goddard, extending von Braun's technological
genius, we were up there in space, silently circumnavigating the Earth,
treading the ancient and desolate lunar surface. Our machines-increasingly competent
and autonomous-were spreading through the Solar System, discovering new worlds,
examining them closely, searching for life, comparing them with Earth.
This is one reason that in the long
astronomical perspective there is something truly epochal about “now”-which we
can define as the few centuries centered on the year you're reading this book.
And there's a second reason: This is the first moment in the history of our
planet when any species, by its own voluntary actions, has become a danger to
itself-as well as to vast numbers of others. Let me recount the ways:
•We've been burning fossil fuels for
hundreds of thousands of years. By the 1960s, there were so many of us burning
wood, coal, oil, and natural gas on so large a scale, that scientists began to
worry about the increasing greenhouse effect; the dangers of global warming
began slowly slipping into public consciousness.
•CFCs were invented in the 1920s and
1930s; in 1974 they were discovered to attack the protective ozone layer.
Fifteen years later a worldwide ban on their production was going into effect.
• Nuclear weapons were invented in 1945.
It took until 1983 before the global consequences of thermonuclear war were
understood. By 1992, large numbers of warheads were being dismantled.
•The first asteroid was discovered in
1801. More or less serious proposals to move them around were floated beginning
in the 1980s. Recognition of the potential dangers of asteroid deflection
technology followed shortly after.
•Biological warfare has been with us for
centuries, but its deadly mating with molecular biology has occurred only
lately.
•We humans have already precipitated
extinctions of species on a scale unprecedented since the end of the Cretaceous
Period. But only in the last decade has the magnitude of these extinctions
become clear, and the possibility raised that in our ignorance of the
interrelations of life on Earth we may be endangering our own future.
Look at the dates on this list and
consider the range of new technologies currently under development. Is it not
likely that other dangers of our own making are yet to be discovered, some
perhaps even more serious?
In the littered field of discredited
self-congratulatory chauvinisms, there is only one that seems to hold up, one
sense in which we are special: Due to our own actions or inactions, and the
misuse of our technology, we live at an extraordinary moment, for the Earth at
least-the first time that a species has become able to wipe itself out. But
this is also, we may note, the first time that a species has become able to
journey to the planets and the stars. The two times, brought about by the same
technology, coincide-a few centuries in the history of a 4. 5-billion-year-old
planet. If you were somehow dropped down on the Earth randomly at any moment in
the past (or future), the chance of arriving at this critical moment would be
less than 1 in 10 million. Our leverage on the future is high just now.
It might be a familiar progression,
transpiring on many worlds-a planet, newly formed, placidly revolves around its
star; life slowly forms; a kaleidoscopic procession of creatures evolves;
intelligence emerges which, at least up to a point, confers enormous survival
value; and then technology is invented. It dawns on them that there are such things
as laws of Nature, that these laws can be revealed by experiment, and that
knowledge of these laws can be made both to save and to take lives, both on
unprecedented scales. Science, they recognize, grants immense powers. In a
flash, they create world-altering contrivances. Some planetary civilizations
see their way through, place limits on what may and what must not be done, and
safely pass through the time of perils. Others are not so lucky or so prudent,
perish.
Since, in the long run, every planetary
society will be endangered by impacts from space, every surviving civilization
is obliged to become spacefaring—not because of exploratory or romantic zeal,
but for the most practical reason imaginable: staying alive. And once you're
out there in space for centuries and millennia, moving little worlds around and
engineering planets, your species has been pried loose from its cradle. If they
exist, many other civilizations will eventually venture far from home.
A MEANS HAS BEEN OFFERED of estimating
how precarious our circumstances are-remarkably, without in any way addressing
the nature of the hazards. J. Richard Gott III is an astrophysicist at
Princeton University. He asks us to adopt a generalized Copernican principle,
something I've described elsewhere as the Principle of Mediocrity. Chances are
that we do not live in a truly extraordinary time. Hardly anyone ever did. The
probability is high that we're born, live out our days, and die somewhere in
the broad middle range of the lifetime of our species (or civilization, or
nation). Almost certainly, Gott says, we do not live in first or last times. So
if your species is very young, it follows that it's unlikely to last
long-because if it were to last long, you (and the rest of us alive today)
would be extraordinary in living, proportionally speaking, so near the
beginning.
What then is the projected longevity of
our species? Gott concludes, at the 97. 5 percent confidence level, that there
will be humans for no more than 8 million years. That's his upper limit, about
the same as the average lifetime of many mammalian species. In that case, our
technology neither harms nor helps. But Gott's lower limit, with the same
claimed reliability, is only 12 years. He will not give you 40-to-1 odds that
humans will still be around by the time babies now alive become teenagers. In
everyday life we try very hard not to take risks so large, not to board
airplanes, say, with 1 chance in 40 of crashing. We will agree to surgery in
which 95 percent of patients survive only if our disease has a greater than 5
percent chance of killing us. Mere 40-to-1 odds on our species surviving
another 12 years Would be, if valid, a cause for supreme concern. If Gott is
right, not only may we never be out among the stars; there's a fair chance we
may not be around long enough even to make the first footfall on another
planet.
To me, this argument has a strange,
vaporish quality. Knowing nothing about our species except how old it is, we
make numerical estimates, claimed to be highly reliable, about its future
prospects. How? We go with the winners. Those who have been around are likely
to stay around. Newcomers tend to disappear. The only assumption is the quite
plausible one that there is nothing special about the moment at which we inquire
into the matter. So why is the argument unsatisfying? Is it just that we are
appalled by its implications?
Something like the Principle of
Mediocrity must have very broad applicability. But we are not so ignorant as to
imagine that everything is mediocre. There is something special about our
time-not just the temporal chauvinism that those who reside in any epoch
doubtless feel, but something, as outlined above, clearly unique and strictly
relevant to our species' future chances: This is the first time that (a) our
exponentiating technology has reached the precipice of self-destruction, but
also the first time that (b) we can postpone or avoid destruction by going
somewhere else, somewhere off the Earth.
These two clusters of capabilities, (a)
and (b), make our time extraordinary in directly contradictory ways-which both
(a) strengthen and (b) weaken Gott's argument. I don't know how to predict
whether the new destructive technologies will hasten, more than the new
spaceflight technologies will delay, human extinction. But since never before
have we contrived the means of annihilating ourselves, and never before have w
e developed the technology for settling other worlds, I think a compelling case
can be made that our time is extraordinary precisely in the context of Gott's
argument. If this is true, it significantly increases the margin of error in
such estimates of future longevity. The worst is worse, and the best better:
Our short-term prospects are even bleaker and-if we can survive the
short-term-our long-term chances even brighter than Gott calculates.
But the former is no more cause for
despair than the latter is for complacency. Nothing forces us to be passive
observers, clucking in dismay as our destiny inexorably works itself out. If we
cannot quite seize fate by the neck, perhaps we can misdirect it, or mollify
it, or escape it.
Of course we must keep our planet
habitable-not on a leisurely timescale of centuries or millennia, but urgently,
on a timescale of decades or even years. This will involve changes in
government, in industry, in ethics, in economics, and in religion. We've never
done such a thing before, certainly not on a global scale. It may be too
difficult for us. Dangerous technologies may be too widespread. Corruption may
be too pervasive. Too many leaders may be focused on the short term rather than
the long. There may be too many quarreling ethnic groups, nation-states, and
ideologies for the right kind of global change to be instituted. We may be too
foolish to perceive even what the real dangers are, or that much of what we
hear about them is determined by those with a vested interest in minimizing
fundamental change.
However, we humans also have a history
of making long-lasting social change that nearly everyone thought impossible. Since
our earliest days, we've worked not just for our own advantage but for our
children and our grandchildren. My grandparents and parents did so for me. We
have often, despite our diversity, despite endemic hatreds, pulled together to
face a common enemy. We seem, these days, much more willing to recognize the
dangers before us than we were even a decade ago. The newly recognized dangers
threaten all of us equally. no one can say how it will turn out down here.
THE MOON WAS WHERE the tree of
immortality grew in ancient Chinese myth. The tree of longevity if not of
immortality, it seems, indeed grows on other worlds. If we were up there among
the planets, if there were self-sufficient human communities oil many worlds,
our species would be insulated from catastrophe. The depletion of the
ultraviolet-absorbing shield on one world would, if anything, be a warning to
take special care of the shield on another. A cataclysmic impact on one world
would likely leave all the others untouched. The more of us beyond the Earth,
the greater the diversity of worlds we inhabit, the more varied the planetary
engineering, the greater the range of societal standards and values-then the
safer the human species will be.
If you grow up living underground in a
world with a hundredth of an Earth gravity and black skies through the portals,
you have a very different set of perceptions, interests, prejudices, and
predispositions than someone who lives on the surface of the home planet.
Likewise if you live on the surface of Mars in the throes of terraforming, or
Venus, or Titan. This strategy-breaking up into many smaller self-propagating
groups, each with somewhat different strengths and concerns, but all marked by
local pride-has been widely employed in the evolution of life on Earth, and by
our own ancestors in particular. It may, in fact, be key to understanding why
we humans are the way we are. This is
the second of the missing justifications for a permanent human presence in
space: to improve our chances of surviving, not just the catastrophes we can
foresee, but also the ones we cannot. Gott also argues that establishing human
communities on other worlds may offer us our best chance of beating the odds.
To take out this insurance policy is not
very expensive, not on the scale on which we do things on Earth. It would not
even require doubling the space budgets of the present spacefaring nations
(which, in all cases, are only a small fraction of the military budgets and
many other voluntary expenditures that might be considered marginal or even
frivolous). We could soon be setting humans down on near-Earth asteroids and
establishing bases on Mars. We know how to do it, even with present technology,
in less than a human lifetime. And the technologies will quickly improve. We
will get better at going into space.
A serious effort to send humans to other
worlds is relatively so inexpensive on a per annum basis that it cannot
seriously compete with urgent social agendas on Earth. If we take this path,
streams of images from other worlds will be pouring down on Earth at the speed
of light. Virtual reality will make the adventure accessible to millions of
stay-on-Earths. Vicarious participation will be much more real than at any
earlier age of exploration and discovery. And the more cultures and people it
inspires and excites, the more likely it will happen.
But by what right, we might ask
ourselves, do we inhabit, alter, and conquer other worlds? If anyone else were
living in the Solar System, this would be an important question. If, though,
there's no one else in this system but us, don't we have a right to settle it?
Of course, our exploration and
homesteading should be enlightened by a respect for planetary environments and
the scientific knowledge they hold. This is simple prudence. Of course,
exploration and settlement ought to be done equitably and transnationally, by
representatives of the entire human species. Our past colonial history is not
encouraging in these regards; but this time we are not motivated by gold or
spices or slaves or a zeal to convert the heathen to the One True Faith, as
were the European explorers of the fifteenth and sixteenth centuries. Indeed,
this is one of the chief reasons we're experiencing such intermittent progress,
so many fits and starts in the manned space programs of all nations.
Despite all the provincialisms I
complained about early in this book, here I find myself an unapologetic human
chauvinist. If there were other life in this solar system, it would be in
imminent danger because the humans are coming. In such a case, I might even be
persuaded that safeguarding our species by settling certain other worlds is
offset, in part at least, by the danger we would pose to everybody else. But as
nearly as we can tell, so far at least, there is no other life in this system,
not one microbe. There's only Earthlife.
In that case, on behalf of Earthlife, I
urge that, with full knowledge of our limitations, we vastly increase our
knowledge of the Solar System and then begin to settle other worlds.
These are the missing practical
arguments: safeguarding 'the Earth from otherwise inevitable catastrophic
impacts and hedging our bets on the many other threats, known and unknown, to
the environment that sustains us. Without these arguments, a compelling case
for sending humans to Mars and elsewhere might be lacking. But with them-and
the buttressing arguments involving science, education, perspective, and hope-I
think a strong case can be made. If our long-term survival is at stake, we have
a basic responsibility to our species venture to other worlds.
Sailors on a becalmed sea, we sense the
stirring of a breeze.
CHAPTER 22
Tiptoeing through the Milky Way
I swear by the shelters of the stars (a
mighty oath, if you but knew it)...
—THE QUR'AN, SURA 56 (7TH CENTURY)
Of course, it is strange to inhabit the
earth no longer, To give up customs one barely had time to learn...
—RAINER MARIA RILKE, “THE FIRST ELEGY”
(1923)
The prospect of scaling heaven, of
ascending to the sky, of altering other worlds to suit our purposes-no matter
how well intentioned we may be-sets the warning flags flying: We remember the
human inclination toward overweening pride; we recall our fallibility and
misjudgments when presented with powerful new technologies. We recollect the
story of the Tower of Babel, a building “whose top may reach unto heaven,” and
God's fear . shout our species, that now “nothing will be restrained from them
which they have imagined to do.”
We come upon Psalm 15, which stakes a
divine claim to other worlds: “[T]he heavens are the Lord's, but the Earth lath
he given to the children of men.” Or Plato's retelling of the Greek analogue of
Babel-the tale of Otys and Ephialtes. They were mortals who “dared to scale
heaven.” The gods were faced with a choice. Should they kill the upstart humans
“and annihilate [their] race with thunderbolts”? On the one Band, “this would
be the end of the sacrifices and worship which mien offered” the gods and which
gods craved. “But, on the other hand, the gods could not suffer [such]
insolence to be unrestrained.''
If, in the long term, though, we have no
alternative, if our choice really is many worlds or none, we are in need of
other sorts of myths of encouragement. They exist. Many religions, from
Hinduism to Gnostic Christianity to Mormon doctrine, teach that-as impious as
it may sound-it is the goal of humans to become gods. Or consider a story in
the Jewish Talmud left out of the Book of Genesis. (It is in doubtful accord
with the account of the apple, the Tree of Knowledge, the Fall, and the
expulsion from Eden.) In the Garden, God tells Eve and Adam that He has
intentionally left the Universe unfinished. It is the responsibility of humans,
over countless generations, to participate with God in a “glorious”
experiment-” completing the Creation.”
The burden of such a responsibility is
heavy, especially on so weak and imperfect a species as ours, one with so
unhappy a history. Nothing remotely like “completion” can be attempted without
vastly more knowledge than we have today. But perhaps, if our very existence is
at stake, we will find ourselves Able to rise to this supreme challenge.
ALTHOUGH HE DID NOT quite use any of the
arguments of the preceding chapter, it was Robert Goddard's intuition that “the
navigation of interplanetary space must be effected to ensure the continuance
of the race.” Konstantin Tsiolkovsky made a similar judgment:
There are countless planets, like many
island Earths...Man occupies one of them. But why could he not avail himself of
others, and of the might of numberless suns?...When the Sun has exhausted its
energy, it would be logical to leave it and look for another, newly kindled,
star still in its prime.
This might be done earlier, he
suggested, long before the Sun dies, “by adventurous souls seeking fresh worlds
to conquer.”
But as I rethink this whole argument,
I'm troubled. Is it too much Buck Rogers? Does it demand an absurd confidence
in future technology? Does it ignore my own admonitions about human
fallibility? Surely in the short term it's biased against technologically
less-developed nations. Are there no practical alternatives that avoid these
pitfalls?
All our self-inflicted environmental
problems, all our weapons of mass destruction are products of science and
technology. So, you might say, let's just back off from science and technology.
Let's admit that these tools are simply too hot to handle. Let's create a
simpler society, in which no matter how careless or short-sighted we are, we're
incapable of altering the environment on a global or even on a regional scale.
Let's throttle back to a minimal, agriculturally intensive technology, with
stringent controls on new knowledge. An authoritarian theocracy is a
tried-and-true way to enforce the controls.
Such a world culture is unstable,
though, in the long run if not the short-because of the speed of technological
advance. Human propensities for self-betterment, envy, and competition will
always be throbbing subsurface; opportunities for short-term, local advantage
will sooner or later be seized. Unless there are severe constraints on thought
and action, in a flash we'll be back to where we are today. So controlled a
society must grant great powers to the elite that does the controlling,
inviting flagrant abuse and eventual rebellion. It's very hard-once we've seen
the riches, conveniences, and lifesaving medicines that technology offers-to
squelch human inventiveness and acquisitiveness. And while such a devolution of
the global civilization, were it possible, might conceivably address the
problem of self-inflicted technological catastrophe, it would also leave us
defenseless against eventual asteroidal and cometary impacts.
Or you might imagine throttling back
much further, back to hunter-gatherer society, where we live off the natural
products of the land and abandon even agriculture. Javelin, digging stick, bow,
arrow, and fire would then be technology enough. But the Earth could support at
the very most a few tens of millions of hunter-gatherers. How could we get down
to such low population levels without instigating the very catastrophes we are
trying to avoid? Besides, we hardly know how to live the hunter-gatherer life
anymore: We've forgotten their cultures, their skills, their tool-kits. We've
killed off almost all of them, and we've destroyed much of the environment that
sustained them. Except for a tiny remnant of us, we might not be able, even if
we gave it high priority, to go back. And again, even if we could return, we
would be helpless before the impact catastrophe that inexorably will come.
The alternatives seem worse than cruel:
They are ineffective. Many of the dangers we face indeed arise from science and
technology-but, more fundamentally, because we leave become powerful without
becoming commensurately wise. The world-altering powers that technology has
delivered into our hands now require a degree of consideration and foresight
that has never before been asked of us.
Science cuts two ways, of course; its
products can be used for both good and evil. But there's no turning back from
science. The early warnings about technological dangers also come from science.
The solutions may well require more of us than just a technological fix. Many
will have to become scientifically literate. We may have to change institutions
and behavior. But our problems, whatever their origin, cannot be solved apart
from science. The technologies that threaten us and the circumvention of those
threats both issue from the same font. They are racing neck and neck.
In contrast, with human societies on
several worlds, our prospects would be far more favorable. Our portfolio would
be diversified. Our eggs would be, almost literally, in many baskets. Each
society would tend to be proud of the virtues of its world, its planetary
engineering, its social conventions, its hereditary predispositions.
Necessarily, cultural differences would be cherished and exaggerated. This
diversity would serve as a tool of survival.
When the off Earth settlements are
better able to fend for themselves, they will have every reason to encourage
technological advance, openness of spirit, and adventure-even if those left on
Earth are obliged to prize caution, fear new knowledge, and institute Draconian
social controls. After the first few self-sustaining communities are
established on other worlds, the Earthlings might also be able to relax their
strictures and lighten up. The humans in space would provide those on Earth
with real protection against rare but catastrophic collisions by asteroids or
comets on rogue trajectories. Of course, for this very reason, humans in space
would hold the upper hand in any serious dispute with those on Earth.
The prospects of such a time contrast
provocatively with forecasts that the progress of science and technology is now
near some asymptotic limit; that art, literature, and music are never to
approach, much less exceed, the heights our species has, on occasion, already
touched; and that political life on Earth is about to settle into some
rock-stable liberal democratic world government, identified, after Hegel, as “the
end of history.” Such an expansion into space also contrasts with a different
but likewise discernible trend in recent times-toward authoritarianism,
censorship, ethnic hatred, and a deep suspicion of curiosity and learning.
Instead, I think that, after some debugging, the settlement of the Solar System
presages an open-ended era of dazzling advances in science and technology;
cultural flowering; and wide-ranging experiments, up there in the sky, in
government and social organization. In more than one respect, exploring the
Solar System and homesteading other worlds constitutes the beginning, much more
than the end, of history.
IT'S IMPOSSIBLE, for us humans at least,
to look into our future, certainly not centuries ahead. no one has ever done so
with any consistency and detail. I certainly do not imagine that I can. I have,
with some trepidation, gone as far as I have to this point in the book, because
we are just recognizing the truly unprecedented challenges brought on by our
technology. These challenges have, I think, occasional straightforward
implications, some of which I've tried briefly to lay out. There are also less
straightforward, much longer-term implications about which I'm even less
confident. Nevertheless, I'd like to present them too for your consideration:
Even when our descendants are
established on near-Earth asteroids and Mars and the moons of the outer Solar
System and the Kuiper Comet Belt, it still won't be entirely safe. In the long
run, the Sun may generate stupendous X-ray and ultraviolet outbursts; the Solar
System will enter one of the vast interstellar clouds lurking nearby and the
planets will darken and cool; a shower of deadly comets will come roaring out
of the Oort Cloud threatening civilizations on many adjacent worlds; we will
recognize that a nearby star is about to become a supernova. In the really long
run, the Sun-on its way to becoming a red giant star-will get bigger and
brighter, the Earth will begin to lose its air and water to space, the soil
will char, the oceans will evaporate and boil, the rocks will vaporize, and our
planet may even be swallowed up into the interior of the Sun.
Far from being made for us, eventually
the Solar System will become too dangerous for us. In the long run, putting all
our eggs in a single stellar basket, no matter how reliable the Solar System
has been lately, may be too risky. In the long run, as Tsiolkovsky and Goddard
long ago recognized, we need to leave the Solar System.
If that's true for us, you might very
well ask, why isn't it true for others? And if it is true for others, why
aren't they here? There are many possible answers, including the contention
that they have come here-although the evidence for that is pitifully slim. Or
there may be no one else out there, because they destroy themselves, with
almost no exceptions, before they achieve interstellar flight; or because in a
galaxy of 400 billion suns ours is the first technical civilization.
A more likely explanation, I think,
issues from the simple fact that space is vast and the stars are far apart.
Even if there were civilizations much older and more advanced than we-expanding
out from their home worlds, reworking new worlds, and then continuing onward to
other stars-they would be unlikely, according to calculations performed by
William I. Newman of UCLA and me, to be here. Yet. And because the speed of
light is finite, the TV and radar news that a technical civilization has arisen
on some planet of the Sun has not reached them. Yet.
Should optimistic estimates prevail and
one in every million stars shelters a nearby technological civilization, and if
as well they're randomly strewn through the Milky Way-were these provisos to
hold-then the nearest one, we recall, would be a few hundred light-years
distant: at the closest, maybe 100 lightyears, more likely a thousand
light-years-and, of course, perhaps nowhere, no matter how far. Suppose the
nearest civilization on a planet of another star is, say, 200 light-years away.
Then, some 150 years from now they'll begin to receive our feeble post-World
War II television and radar emission. What will they make of it? With each
passing year the signal will get louder, more interesting, perhaps more
alarming. Eventually, they may respond: by returning a radio message, or by
visiting. In either case, the response will likely be limited by the finite
value of the speed of light. With these wildly uncertain numbers, the answer to
our unintentional mid-century call into the depths of space will not arrive
until around the year 2350. If they're farther away, of course, it will take
longer; and if much farther away, much longer. The interesting possibility
arises that our first receipt of a message from an alien civilization, a
message intended for us (not just an all-points bulletin), will occur in a time
when we are well situated on many worlds in our solar system and preparing to
move on.
With or without such a message, though,
we will have reason to continue outward, seeking other solar systems. Or-even
safer in this unpredictable and violent sector of the Galaxy-to sequester some
of us in self-sufficient habitations in interstellar space, far from the
dangers constituted by the stars. Such a future would, I think, naturally
evolve, by slow increments, even without any grand goal of interstellar travel:
For safety, some communities may wish to
sever their ties with the rest of humanity-uninfluenced by other societies,
other ethical codes, other technological imperatives. In a time when comets and
asteroids are being routinely repositioned, we will be able to populate a small
world and then cut it loose. In successive generations, as this world sped
outward, the Earth would fade from bright star to pale dot to invisibility; the
Sun would appear dimmer, until it was no more than a vaguely yellow point of
light, lost among thousands of others. The travelers would approach
interstellar night. Some such communities may be content with occasional radio
and laser traffic with the old home worlds. Others, confident of the
superiority of their own survival chances and wary of contamination, may try to
disappear. Perhaps all contact with them will ultimately be lost, their very
existence forgotten.
Even the resources of a sizable asteroid
or comet are finite, though, and eventually more resources must be sought
elsewhere-especially water, needed for drink, for a breathable oxygen
atmosphere, and for hydrogen to power fusion reactors. So in the long run these
communities must migrate from world to world, with no lasting loyalty to any.
We might call it “pioneering,” or “homesteading.” A less sympathetic observer
might describe it as sucking dry the resources of little world after little
world. But there are a trillion little worlds in the Oort Comet Cloud.
Living in small numbers on a modest
stepmother world far from the Sun, we will know that every scrap of food and
every drop of water is dependent on the smooth operation of a farsighted
technology; but these conditions are not radically unlike those to which we are
already accustomed. Digging resources out of the ground and stalking passing
resources seem oddly familiar, like a forgotten memory of childhood: It is,
with a few significant changes, the strategy of our hunter-gatherer ancestors.
For 99. 9 percent of the tenure of humans on Earth, we lived such a life.
Judging from some of the last surviving hunter-gatherers just before they were
engulfed by the present global civilization, we may have been relatively happy.
It's the kind of life that forged us. So after a brief, only partially
successful sedentary experiment, we may become wanderers again-more
technological than last time, but even then our technology, stone tools and
fire, was our only hedge against extinction.
If safety lies in isolation and
remoteness, then some of our descendants will eventually emigrate to the outer
comets of the Oort Cloud. With a trillion cometary nuclei, each separated from
the next by about as much as Mars is from Earth, there will be a great deal to
do out there.
The outer edge of the Sun's Oort Cloud
is perhaps halfway to the nearest star. Not every other star has an Oort Cloud,
but many probably do. As the Sun passes nearby stars, our Oort Cloud will
encounter, and partially pass through, other comet clouds, like two swarms of
gnats interpenetrating but not colliding. To occupy a comet of another star
will then be not much more difficult than to occupy one of our own. From the
frontiers of some other solar system the children of the blue dot may peer
longingly at the moving points of light denoting substantial (and well-lit)
planets. Some communities-feeling the ancient human love for oceans and
sunlight stirring within them-may begin the long journey down to the bright,
warm, and clement planets of a new sun.
Other communities may consider this last
strategy a weakness. Planets are associated with natural catastrophes. Planets
may have pre-existing life and intelligence. Planets are easy for other beings
to find. Better to remain in the darkness. Better to spread ourselves among
many “small and obscure worlds. Better to stay hidden.
ONCE WE CAN SEND our machines and
ourselves far from home, far from the planets-once we really enter the theater
of the Universe-we are bound to come upon phenomena unlike anything we've ever
encountered. Here are three possible examples:
First: Starting some 550 astronomical
units (AU) out-about ten times farther from the Sun than Jupiter, and therefore
much more accessible than the Oort Cloud-there's something extraordinary. Just
as an ordinary lens focuses far-off images, so does gravity. (Gravitational
lensing by distant stars and galaxies is now being detected.) Five hundred
fifty AU from the Sun-only a year away if we could travel at 1 percent the
speed of light-is where the focus begins (although when effects of the solar
corona, the halo of ionized gas surrounding the Sun, are taken into account,
the focus may be considerably farther out). There, distant radio signals are
enormously enhanced, amplifying whispers. The magnification of distant images
would allow us (with a modest radio telescope) to resolve a continent at the
distance of the nearest star and the inner Solar System at the distance of the
nearest spiral galaxy. If you are free to roam an imaginary spherical shell at
the appropriate focal distance and centered on the Sun, you are free to explore
the Universe in stupendous magnification, to peer at it with unprecedented
clarity, to eavesdrop on the radio signals of distant civilizations, if any,
and to glimpse the earliest events in the history of the Universe.
Alternatively, the lens could be used the other way, to amplify a very modest
signal of ours so it could be heard over immense distances. There are reasons
that draw us to hundreds and thousands of AU. Other civilizations will have
their own regions of gravitational focusing, depending on the mass and radius
of their star, some a little closer, some a little farther away than ours.
Gravitational lensing may serve as a common inducement for civilizations to
explore the regions just beyond the planetary parts of their solar systems.
Second: Spend a moment thinking about
brown dwarfs, hypothetical very low temperature stars, considerably more
massive than Jupiter, but considerably less massive than the Sun. Nobody knows
if brown dwarfs exist. Some experts, using nearer stars as gravitational lenses
to detect the presence of more distant ones, claim to have found evidence of
brown dwarfs. From the tiny fraction of the whole sky that has so far been
observed by this technique, an enormous number of brown dwarfs is inferred.
Others disagree. In the 1950s, it was suggested by the astronomer Harlow
Shapley of Harvard that brown dwarfs-he called them “Lilliputian stars”-were
inhabited. He pictured their surfaces as warm as a June day in Cambridge, with
lots of area. They would be stars that humans could survive on and explore.
Third: The physicists B. J. Carr and
Stephen Hawking of Cambridge University have shown that fluctuations in the
density of matter in the earliest stages of the Universe could have generated a
wide variety of small black holes. Primordial black holes-if they exist-must
decay by emitting radiation to space, a consequence of the laws of quantum
mechanics. The less massive the black hole, the faster it dissipates. Any
primordial black hole in the final stages of decay today would have to weigh
about as much as a mountain. All the smaller ones are gone. Since the
abundance-to say nothing of the existence-of primordial black holes depends on
what happened in the earliest moments after the Big Bang, no one can be sure
that there are any to be found; we certainly can't be sure that any lie nearby.
Not very restrictive upper limits on their abundance have been set by the
failure so far to find short gamma ray pulses, a component of the Hawking
radiation.
In a separate study, G. E. Brown of
Caltech and the pioneering nuclear physicist Hans Bethe of Cornell suggest that
about a billion non-primordial black holes are strewn through the Galaxy,
generated in the evolution of stars. If so, the nearest may be only 10 or 20
lightyears away.
If there are black holes within
reach-whether they're as massive as mountains or as stars-we will have amazing
physics to study firsthand, as well as a formidable new source of energy. By no
means do I claim that brown dwarfs or primordial black holes are likely within
a few light-years, or anywhere. But as we enter interstellar space, it is
inevitable that we will stumble upon whole new categories of wonders and
delights, some with transforming practical applications.
I do not know where my train of argument
ends. As more time passes, attractive new denizens of the cosmic zoo will draw
us farther outward, and increasingly improbable and deadly catastrophes must
come to pass. The probabilities are cumulative. But, as time goes on,
technological species will also accrue greater and greater powers, far
surpassing any we can imagine today. Perhaps, if we are very skillful (lucky, I
think, won't be enough), we will ultimately spread far from home, sailing
through the starry archipelagos of the vast Milky Way Galaxy. If we come upon
anyone else-or, more likely, if they come upon us-we will harmoniously
interact. Since other spacefaring civilizations are likely to be much more
advanced than we, quarrelsome humans in interstellar space are unlikely to last
long.
Eventually, our future may be as
Voltaire, of all people, imagined:
Sometimes by the help of a sunbeam, and
sometimes by the convenience of a comet, [they] glided from sphere to sphere,
as a bird hops from bough to bough. In a very little time [they] posted through
the Milky Way...
We are, even now, discovering vast
numbers of gas and dust disks around young stars-the very structures out of
which, in our solar system four and a half billion years ago, the Earth and the
other planets formed. We're beginning to understand how fine dust grains slowly
grow into worlds; how big Earthlike planets accrete and then quickly capture
hydrogen and helium to become the hidden cores of gas giants; and how small
terrestrial planets remain comparatively bare of atmosphere. We are
reconstructing the histories of worlds-how mainly ices and organics collected
together in the chilly outskirts of the early Solar System, and mainly rock and
metal in the inner regions warmed by the young Sun. We have begun to recognize
the dominant role of early collisions in knocking worlds over, gouging huge
craters and basins in their surfaces and interiors, spinning them up, making
and obliterating moons, creating rings, carrying, it may be, whole oceans down
from the skies, and then depositing a veneer of organic matter as the neat
finishing touch in the creation of worlds. We are beginning to apply this
knowledge to other systems.
In the next few decades we have a real
chance of examining the layout and something of the composition of many other
mature planetary systems around nearby stars. We will begin to know which
aspects of our system are the rule and which the exception. What is more
common-planets like Jupiter, planets like Neptune, or planets like Earth? Or do
all other systems have Jupiters and Neptunes and Earths? What other categories
of worlds are there, currently unknown to us? Are all solar systems embedded in
a vast spherical cloud of comets? Most stars in the sky are not solitary suns
like our own, but double or multiple systems in which the stars are in mutual
orbit. Are there planets in such systems? If so, what are they like? If, as we
now think, planetary systems are a routine consequence of the origin of suns,
have they followed very different evolutionary paths elsewhere? What do elderly
planetary systems, billions of years more evolved than ours, look like? In the
next few centuries our knowledge of other systems will become increasingly
comprehensive. We will begin to know which to visit, which to seed, and which
to settle.
Imagine we could accelerate continuously
at 1 g-what we're comfortable with on good old terra firma-to the midpoint of
our voyage, and decelerate continuously at 1 g until we arrive at our
destination. It would then take a day to get to Mars, a week and a half to
Pluto, a year to the Oort Cloud, and a few years to the nearest stars.
Even a modest extrapolation of our
recent advances in transportation suggests that in only a few centuries we will
be able to travel close to the speed of light. Perhaps this is hopelessly
optimistic. Perhaps it will really take millennia or more. But unless we
destroy ourselves first we will be inventing new technologies as strange to us
as Voyager might be to our hunter-gatherer ancestors. Even today we can think
of ways-clumsy, ruinously expensive, inefficient to be sure-of constructing a
starship that approaches light speed. In time, the designs will become more
elegant, more affordable, more efficient. The day will come when we overcome
the necessity of jumping from comet to comet. We will begin to soar through the
light-years and, as St. Augustine said of the gods of the ancient Greeks and
Romans, colonize the sky.
Such descendants may be tens or hundreds
of generations removed from anyone who ever lived on the surface of a planet.
Their cultures will be different, their technologies far advanced, their
languages changed, their association with machine intelligence much more
intimate, perhaps their very appearance markedly altered from that of their
nearly mythical ancestors who first tentatively set forth in the late twentieth
century into the sea of space. But they will be human, at least in large part;
they will be practitioners of high technology; they will have historical
records. Despite Augustine's judgment on Lot's wife, that “no one who is being
saved should long for what he is leaving,” they will not wholly forget the
Earth.
But we're not nearly ready, you may be
thinking. As Voltaire put it in his Memnon, “our little terraqueous globe is
the madhouse of those hundred thousand millions of worlds.” We, who cannot even
put our own planetary home in order, riven with rivalries and hatreds,
despoiling our environment, murdering one another through irritation and
inattention as well as on deadly purpose, and moreover a species that until
only recently was convinced that the Universe was made for its sole benefit-are
we to venture out into space, move worlds, reengineer planets, spread to
neighboring star systems?
I do not imagine that it is precisely
we, with our present customs and social conventions, who will be out there. If
we continue to accumulate only power and not wisdom, we will surely destroy
ourselves. Our very existence in that distant time requires that we will have
changed our institutions and ourselves. How can I dare to guess about humans in
the far future? It is, I think, only a matter of natural selection. If we
become even slightly more violent, shortsighted, ignorant, and selfish than we
are now, almost certainly we will have no future.
If you're young, it's just possible that
we will be taking our first steps on near-Earth asteroids and Mars during your
lifetime. To spread out to the moons of the Jovian planets and the Kuiper Comet
Belt will take many generations more. The Oort Cloud will require much longer
still. By the time we're ready to settle even the nearest other planetary
systems, we will have changed. The simple passage of so many generations will
have changed us. The different circumstances we will be living under will have
changed us. Prostheses and genetic engineering will have changed us. Necessity
will have changed us. We're an adaptable species.
It will not be we who reach Alpha
Centauri and the other nearby stars. It will be a species very like us, but
with more of our strengths and fewer of our weaknesses, a species returned to
circumstances more like those for which it was originally evolved, more
confident, farseeing, capable, and prudent-the sorts of beings we would want to
represent us in a Universe that, for all we know, is filled with species much
older, much more powerful, and very different.
The vast distances that separate the
stars are providential. Beings and worlds are quarantined from one another. The
quarantine is lifted only for those with sufficient self-knowledge and judgment
to have safely traveled from star to star.
ON IMMENSE TIMESCALES, in hundreds of
millions to billions of years, the centers of galaxies explode. We see,
scattered across deep space, galaxies with “active nuclei,” quasars, galaxies
distorted by collisions, their spiral arms disrupted, star systems blasted with
radiation or gobbled up by black holes-and we gather that on such timescales
even interstellar space, even galaxies may not be safe.
There is a halo of dark matter
surrounding the Milky Way, extending perhaps halfway to the distance of the
next spiral galaxy (M31 in the constellation Andromeda, which also contains
hundreds of billions of stars). We do not know what this dark matter is, or how
it is arranged-but some of it may be in worlds untethered to individual stars.
If so, our descendants of the remote future will have an opportunity, over
unimaginable intervals of time, to become established in intergalactic space,
and to tiptoe to other galaxies.
But on the timescale for populating our
galaxy, if not long before, we must ask: How immutable is this longing for
safety that drives us outward? Will we one day feel content with the time our
species has had and our successes, and willingly exit the cosmic stage?
Millions of years from now-probably much sooner-we will have made ourselves
into something else. Even if we do nothing intentionally, the natural process
of mutation and selection will have worked our extinction or evolved us into
some other species on just such a timescale (if we may judge by other mammals).
Over the typical lifetime of a mammalian species, even if we were able to
travel close to the speed of light and were dedicated to nothing else, we could
not, I think, explore even a representative fraction of the Milky Way Galaxy.
There's just too much of it. And beyond are a hundred billion galaxies more.
Will our present motivations remain unchanged over geological, much less
cosmological, timescales-when we ourselves have been transfigured? In such
remote epochs, we may discover outlets for our ambitions far grander and more
worthy than merely populating an unlimited number of worlds.
Perhaps, some scientists have imagined,
we will one day create new forms of life, link minds, colonize stars,
reconfigure galaxies, or prevent, in a nearby volume of space, the expansion of
the Universe. In a 1993 article in the journal Nuclear Physics, the physicist
Andrei Linde-conceivably, in a playful mood-suggests that laboratory
experiments (it would have to be quite a laboratory) to create separate, closed-off,
expanding universes might ultimately be possible. “However,” he writes to me,
“I myself do not know whether [this suggestion] is simply a joke or something
else.” In such a list of projects for the far future, we will have no
difficulty in recognizing a continuing human ambition to arrogate powers once
considered godlike-or, in that other more encouraging metaphor, to complete the
Creation.
FOR MANY PAGES NOW, we have left the
realm of plausible conjecture for the heady intoxication of nearly unconstrained
speculation. It is time to return to our own age.
My grandfather, born before radio waves
were even a laboratory curiosity, almost lived to see the first artificial
satellite beeping down at us from space. There are people who were born before
there was such a thing as an airplane, and who in old age saw four ships
launched to the stars. For all our failings, despite our limitations and
fallibilities, we humans are capable of greatness. This is true of our science
and some areas of our technology, of our art, music, literature, altruism, and
compassion, and even, on rare occasion, of our statecraft. What new wonders
undreamt of in our time will we have wrought in another generation? And
another? How far will our nomadic species have wandered by the end of the next
century? And the next millennium?
Two billion years ago our ancestors were
microbes; a halfbillion years ago, fish; a hundred million years ago, something
like mice; ten million years ago, arboreal apes; and a million years ago,
proto-humans puzzling out the taming of fire. Our evolutionary lineage is
marked by mastery of change. In our time, the pace is quickening.
When we first venture to a near-Earth
asteroid, we will have entered a habitat that may engage our species forever.
The first voyage of men and women to Mars is the key step in transforming us
into a multiplanet species. These events are as momentous as the colonization
of the land by our amphibian ancestors and the descent from the trees by our
primate ancestors.
Fish with rudimentary lungs and fins
slightly adapted for walking must have died in great numbers before
establishing a permanent foothold on the land. As the forests slowly receded,
our upright apelike forebears often scurried back into the trees, fleeing the
predators that stalked the savannahs. The transitions were painful, took
millions of years, and were imperceptible to those involved. In our case the
transition occupies only a few generations, and with only a handful of lives
lost. The pace is so swift that we are still barely able to grasp what is
happening.
Once the first children are born off
Earth; once we have bases and homesteads on asteroids, comets, moons, and
planets; once we're living off the land and bringing up new generations on
other worlds, something will have changed forever in human history. But
inhabiting other worlds does not imply abandoning this one, any more than the
evolution of amphibians meant the end of fish. For a very long time only a
small fraction of us will be out there.
“In modern Western society,” writes the
scholar Charles Lindholm, the erosion of tradition and the collapse of accepted
religious belief leaves us without a telos [an end to which we strive], a
sanctified notion of humanity's potential. Bereft of a sacred project, we have only
a demystified image of a frail and fallible humanity no longer capable of
becoming god-like.
I believe it is healthy-indeed,
essential-to keep our frailty and fallibility firmly in mind. I worry about
people who aspire to be “god-like.” But as for a long-term goal and a sacred
project, there is one before us. On it the very survival of our species
depends. If we have been locked and bolted into a prison of the self, here is
an escape hatch-something worthy, something vastly larger than ourselves, a crucial
act on behalf of humanity. Peopling other worlds unifies nations and ethnic
groups, binds the generations, and requires us to be both smart and wise. It
liberates our nature and, in part, returns us to our beginnings. Even now, this
new telos is within our grasp.
The pioneering psychologist William
James called religion a “feeling of being at home in the Universe.” Our
tendency has been, as I described in the early chapters of this book, to
pretend that the Universe is how we wish our home would be, rather than to
revise our notion of what's homey so it embraces the Universe. If, in
considering James' definition, we mean the real Universe, then we have no true
religion yet. That is for another time, when the sting of the Great Demotions
is well behind us, when we are acclimatized to other worlds and they to us,
when we are spreading outward to the stars.
The Cosmos extends, for all practical
purposes, forever. After a brief sedentary hiatus, we are resuming our ancient
nomadic way of life. Our remote descendants, safely arrayed on many worlds
through the Solar System and beyond, will be unified by their common heritage,
by their regard for their home planet, and by the knowledge that, whatever
other life may be, the only humans in all the Universe come from Earth.
They will gaze up and strain to find the
blue dot in their skies. They will love it no less for its obscurity and
fragility. They will marvel at how vulnerable the repository of all our
potential once was, how perilous our infancy, how humble our beginnings, how
many rivers we had to cross before we found our way.