Cosmos 13: Who Speaks for Earth?
December 21, 1980


Sagan reflects on the future of humanity and the question of "who speaks for Earth?" when meeting extraterrestrials. He discusses the very different meetings of the Tlingit people and explorer Jean-Francois de La Perouse with the destruction of the Aztecs by Spanish conquistadors, the looming threat of nuclear warfare, and the threats shown by destruction of the Library of Alexandria and the murder of Hypatia. The episode ends with an overview of the beginning of the universe, the evolution of life, and the accomplishments of humanity and makes a plea to mankind to cherish life and continue its journey in the cosmos. The Cosmos Update notes the preliminary reconnaissance of planets with spacecraft, the fall of the Berlin Wall and the end of apartheid in South Africa, and measures towards the reduction of nuclear weapons.

Narrator: "I call Heaven and Earth to witness against you this day that I have set before thee life and death the blessing and the curse. Therefore choose life, that thou mayest live thou and thy seed.


Nearly 200 years ago, in the Gulf of Alaska at a place called Lituya Bay, two cultures that had never met experienced a first encounter. The Tlingit people lived more or less as their ancestors had for thousands of years. They were nomads, moving often by canoe between numerous campsites where they caught the plentiful fish and sea otters, and traded with neighboring tribes.

The creator they worshiped was the raven god, whom they pictured as an enormous black bird with white wings. And one July day in 1786, the raven god appeared. The Tlingit were terrified. They knew that anyone looking directly at the god would be turned to stone.

From the other side of the planet had come an expedition led by the French explorer Lapérouse. It was, in fact, the most elaborately planned scientific voyage of the 18th century sent around the world to gather knowledge about the geography, natural history, and peoples of distant lands. But to the Tlingit, whose world was confined to the islands and inlets of southern Alaska, this great vessel could have come only from the gods.

There was one among them who dared to look more deeply. He was an old warrior, and nearly blind. He said that his life was almost over. For the common good he would approach the raven to learn whether the god really would turn his people to stone. He set out on his own voyage of discovery to confront the end of the world.

The old man made himself look hard at the raven and saw that it was not a great bird from the sky but the work of men like himself. This first encounter turned out to be peaceful. The men of the Lapérouse expedition were under orders to treat with respect any people they might discover. An exceptional policy for its time, and after. Lapérouse and the Tlingit exchanged goods and then the strange ship sailed away, never to return.

Not all encounters between nations had been so peaceful. Before 1519, the Aztecs of Mexico had never seen a gun. And they, too, believed at first that their strange visitors had come from the sky. The Spaniards, under Cortés, were not constrained by any injunctions against violence. Their true nature and intentions soon became clear. Unlike the Lapérouse expedition, the Conquistadors sought not knowledge, but gold. They used their superior weapons to loot and murder. In their madness, they obliterated a civilization. In the name of piety, in a mockery of their religion, the Spaniards utterly destroyed a society with an art, astronomy, and architecture; the equal of anything in Europe. We revile the Conquistadors for their cruelty and shortsightedness; for choosing death. We admire Lapérouse and the Tlingit for their courage and wisdom; for choosing life.

The choice is with us still. But the civilization now in jeopardy is all humanity. As the ancient mythmakers knew, we’re children equally of the Earth and the sky. In our tenure on this planet we’ve accumulated dangerous evolutionary baggage: propensities for aggression and ritual, submission to leaders, hostility to outsiders. All of which puts our survival in some doubt. But we’ve also acquired compassion for others, love for our children, a desire to learn from history and experience, and a great, soaring, passionate intelligence—the clear tools for our continued survival and prosperity. Which aspects of our nature will prevail is uncertain, particularly when our visions and prospects are bound to one small part of the small planet Earth. But up there, in the cosmos, an inescapable perspective awaits. National boundaries are not evident when we view the Earth from space. Fanatic ethnic or religious or national identifications are a little difficult to support when we see our planet as a fragile blue crescent, fading to become an inconspicuous point of light against the bastion and citadel of the stars. There are not yet obvious signs of extraterrestrial intelligence, and this makes us wonder whether civilizations like ours rush inevitably, headlong, to self-destruction. I dream about it. And sometimes they’re bad dreams.

In the vision of a dream, I once imagined myself searching for other civilizations in the cosmos. Among a hundred billion galaxies and a billion trillion stars, life and intelligence should have arisen on many worlds. Some worlds are barren and desolate. On them, life never began or may have been extinguished in some cosmic catastrophe. There may be worlds rich in life but not yet evolved to intelligence and high technology. There may be civilizations that achieve technology and then promptly use it to destroy themselves. And perhaps there are also beings who learned to live with their technology and themselves; beings who endure and become citizens of the cosmos.

Immersed in these thoughts, I found myself approaching a world that was clearly inhabited, a world I had visited before. I saw a planet encompassed by light and recognized the signature of intelligence. But suddenly: darkness, total and absolute. In my dream I could read the Book of Worlds, a vast encyclopedia of a billion planets within the Milky Way. What could the computer tell me about this—now darkened—world? They must have survived some earlier catastrophe. “Locally initiated contact”… maybe their television broadcasts. Their biology was different from ours. High technology. I wondered what those lights had been for. There must have been signs of trouble. Probability of survival in a century: less than 1%. Not very good odds. “Communications interrupted.” Their world society had failed. They had made the ultimate mistake. I felt a longing to return to Earth. The television transmissions of Earth rushed past me expanding away from our planet at the speed of light.

Then, suddenly, silence. Total and absolute. But the dream was not yet done. Had we destroyed our home? What had we done to the Earth? There had been many ways for life to perish at our hands. We had poisoned the air and water. We had ravaged the land. Perhaps we had changed the climate. Could it have been a plague or nuclear war?

I remembered the galactic computer. What would it say about the Earth? There was our region of the galaxy. There was our world. I had found the entry for Earth. Humanity, third from the sun. They had heard our television broadcasts and thought them an application for cosmic citizenship. Our technology had been growing enormously. They got that right. 200 nation states, about six global powers. The potential to become one planet. Probability of survival: over a century—here, also, less than l%.

So it was nuclear war. A full nuclear exchange. There would be no more big questions. No more answers. Never again a love or a child. No descendants to remember us and be proud. No more voyages to the stars. No more songs from the Earth.

I saw East Africa and thought: a few million years ago, we humans took our first steps there. Our brains grew and changed. The old parts began to be guided by the new parts. And this made us human with compassion and foresight and reason. But instead, we listened to that reptilian voice within us counseling fear, territoriality, aggression. We accepted the products of science. We rejected its methods. Maybe the reptiles will evolve intelligence once more. Perhaps, one day, there will be civilizations again on Earth. There will be life. There will be intelligence. But there will be no more humans. Not here, not on a billion worlds.

Every thinking person fears nuclear war and every technological nation plans for it. Everyone knows it’s madness and every country has an excuse. There’s a dreary chain of causality. The Germans were working on the bomb at the beginning of World War II, so the Americans had to make one first. If the Americans had one, the Russians had to have one. Then, the British, the French, the Chinese, the Indians, the Pakistanis. Many nations now collect nuclear weapons. They’re easy to make. You can steal fissionable material from nuclear reactors. Nuclear weapons have almost become a home handicraft industry.

The conventional bombs of World War II were called “blockbusters.” Filled with 20 tons of TNT, they could destroy a city block. All the bombs dropped on all the cities of World War II amounted to some two million tons of TNT—two megatons. Coventry and Rotterdam, Dresden and Tokyo, all the death that rained from the skies between 1939 and 1945: a hundred thousand blockbusters. Two megatons.

Today, two megatons is the equivalent of a single thermonuclear bomb. One bomb with the destructive force of the Second World War. But there are tens of thousands of nuclear weapons. The missile and bomber forces of the Soviet Union and United States have warheads aimed at over 15,000 designated targets. No place on the planet is safe.

The energy contained in these weapons—genies of death, patiently awaiting the rubbing of the lamps—totals far more than 10,000 megatons, but with the destruction concentrated efficiently not over six years, but over a few hours. A blockbuster for every family on the planet. A World War II every second for the length of a lazy afternoon.

The bomb dropped on Hiroshima killed 70,000 people. In a full nuclear exchange, in the paroxysm of global death, the equivalent of a million Hiroshima bombs would be dropped all over the world. But in such an exchange not everyone would be killed by the blast and the firestorm and the immediate radiation. There would be other agonies: the loss of loved ones, the legions of the burned and blinded and mutilated, the absence of medical care, disease, plague, long-lived radiation poisoning of the soil and the water, the threat of tumors and stillbirths and malformed children, and the hopeless sense of a civilization destroyed for nothing. The knowledge that we could have prevented it and did not.

The global balance of terror pioneered by the United States and the Soviet Union holds hostage all the citizens of the Earth. Each side persistently probes the limits of the other’s tolerance, like the Cuban missile crisis, the testing of anti-satellite weapons, the Vietnam and Afghanistan wars. The hostile military establishments are locked in some ghastly mutual embrace. Each needs the other. But the balance of terror is a delicate balance with very little margin for miscalculation.

And the world impoverishes itself by spending a trillion dollars a year on preparations for war, and by employing perhaps half the scientists and high technologists on the planet in military endeavors. How would we explain all this to a dispassionate extraterrestrial observer? What account would we give of our stewardship of the planet Earth? We have heard the rationales offered by the superpowers. We know who speaks for the nations. But who speaks for the human species? Who speaks for Earth?

From an extraterrestrial perspective, our global civilization is clearly on the edge of failure in the most important task it faces: preserving the lives and well-being of its citizens and the future habitability of the planet. But if we’re willing to live with the growing likelihood of nuclear war, shouldn’t we also be willing to explore vigorously every possible means to prevent nuclear war? Shouldn’t we consider, in every nation, major changes in the traditional ways of doing things? A fundamental restructuring of economic, political, social, and religious institutions? We’ve reached a point where there can be no more special interests or special cases. Nuclear arms threaten every person on the Earth.

Fundamental changes in society are sometimes labeled impractical or contrary to human nature, as if nuclear war were practical or as if there were only one human nature. But fundamental changes can clearly be made; we’re surrounded by them. In the last two centuries, abject slavery—which was with us for thousands of years—has almost entirely been eliminated in a stirring worldwide revolution. Women, systematically mistreated for millennia, are gradually gaining the political and economic power traditionally denied to them. And some wars of aggression have recently been stopped or curtailed because of a revulsion felt by the people in the aggressor nations. The old appeals to racial, sexual, and religious chauvinism, and to rabid nationalist fervor are beginning not to work. A new consciousness is developing which sees the Earth as a single organism, and recognizes that an organism at war with itself is doomed. We are one planet.

One of the great revelations of the age of space exploration is the image of the Earth, finite and lonely, somehow vulnerable, bearing the entire human species through the oceans of space and time. But this is an ancient perception. In the 3rd century B.C., our planet was mapped and accurately measured by a Greek scientist named Eratosthenes, who worked in Egypt. This was the world as he knew it. Eratosthenes was the director of the great Library of Alexandria the center of science and learning in the ancient world.

Aristotle had argued that humanity was divided into Greeks and everybody else, who he called "barbarians," and that the Greeks should keep themselves racially pure. He thought it was fitting for the Greeks to enslave other peoples. But Eratosthenes criticized Aristotle for his blind chauvinism. He believed there was good and bad in every nation. The Greek conquerors had invented a new god for the Egyptians but he looked remarkably Greek. Alexander was portrayed as pharaoh in a gesture to the Egyptians. But in practice, the Greeks were confident of their superiority. The casual protests of the librarian hardly constituted a serious challenge to prevailing prejudices. Their world was as imperfect as our own.

But the Ptolemies, the Greek kings of Egypt who followed Alexander, had at least this virtue: they supported the advancement of knowledge. Popular ideas about the nature of the cosmos were challenged and some of them discarded. New ideas were proposed and found to be in better accord with the facts. There were imaginative proposals, vigorous debates, brilliant syntheses, and the resulting treasure of knowledge was recorded and preserved for centuries on these shelves. Science came of age in this library.

The Ptolemies didn’t merely collect old knowledge. They supported scientific research and generated new knowledge. The results were amazing. Eratosthenes accurately calculated the size of the Earth. He mapped it and he argued that it could be circumnavigated. Hipparchus anticipated that stars come into being, slowly move during the course of centuries, and eventually perish. It was he who first catalogued the positions and magnitudes of the stars in order to determine whether there were such changes. Euclid produced a textbook on geometry which human beings learned from for 23 centuries. It’s still a great read, full of the most elegant proofs. Galen wrote basic works on healing and anatomy which dominated medicine until the Renaissance. These are just a few examples. There were dozens of great scholars here, and hundreds of fundamental discoveries.

Some of those discoveries have a distinctly modern ring. Apollonius of Perga studied the parabola and the ellipse, curves that we know today describe the paths of falling objects in a gravitational field and space vehicles traveling between the planets. Heron of Alexandria invented steam engines and gear trains. He was the author of the first book on robots. Imagine how different our world would be if those discoveries had been used for the benefit of everyone, if the humane perspective of Eratosthenes had been widely adopted and applied. But this was not to be.

Alexandria was the greatest city the Western world had ever seen. People from all nations came here to live, to trade, to learn. On a given day these harbors were thronged with merchants and scholars, tourists. It’s probably here that the word “cosmopolitan” realized its true meaning of a citizen—not just of a nation, but of the cosmos. To be a citizen of the cosmos! Here were clearly the seeds of our modern world. But why didn’t they take root and flourish? Why, instead, did the West slumber through a thousand years of darkness until Columbus and Copernicus and their contemporaries rediscovered the work done here? I cannot give you a simple answer but I do know this: there is no record in the entire history of the library that any of the illustrious scholars and scientists who worked here ever seriously challenged a single political or economic or religious assumption of the society in which they lived. The permanence of the stars was questioned. The justice of slavery was not.

Science and learning in general were the preserve of the privileged few. The vast population of this city had not the vaguest notion of the great discoveries being made within these walls. How could they? The new findings were not explained or popularized. The progress made here benefited them little. Science was not part of their lives. The discoveries in mechanics, say, or steam technology mainly were applied to the perfection of weapons, to the encouragement of superstition, to the amusement of kings. Scientists never seemed to grasp the enormous potential of machines to free people from arduous and repetitive labor. The intellectual achievements of antiquity had few practical applications. Science never captured the imagination of the multitude. There was no counterbalance to stagnation, to pessimism, to the most abject surrender to mysticism. So when, at long last, the mob came to burn the place down, there was nobody to stop them.

Let me tell you about the end. It’s a story about the last scientist to work in this place. A mathematician, astronomer, physicist, and head of the school of Neoplatonic philosophy in Alexandria. That’s an extraordinary range of accomplishments for any individual in any age. Her name was Hypatia. She was born in this city in the year 370 A.D.

This was a time when women had essentially no options. They were considered property. Nevertheless, Hypatia was able to move freely, unselfconsciously, through traditional male domains. By all accounts, she was a great beauty. And although she had many suitors, she had no interest in marriage. The Alexandria of Hypatia’s time—by then long under Roman rule—was a city in grave conflict. Slavery, the cancer of the ancient world, had sapped classical civilization of its vitality. The growing Christian Church was consolidating its power and attempting to eradicate pagan influence and culture. Hypatia stood at the focus, at the epicenter, of mighty social forces. Cyril, the Bishop of Alexandria, despised her, in part because of her close friendship with a Roman governor, but also because she was a symbol of learning and science, which were largely identified by the early Church with paganism. In great personal danger Hypatia continued to teach and to publish until, in the year 415 A.D., on her way to work, she was set upon by a fanatical mob of Cyril’s followers. They dragged her from her chariot, tore off her clothes, and flayed her flesh from her bones with abalone shells. Her remains were burned, her works obliterated, her name forgotten. Cyril was made a saint.

The glory you see around me is nothing but a memory. It does not exist. The last remains of the library were destroyed within a year of Hypatia’s death. It’s as if an entire civilization had undergone a sort of self-inflicted radical brain surgery, so that most of its memories, discoveries, ideas, and passions were irrevocably wiped out. The loss was incalculable. In some cases, we know only the tantalizing titles of books that had been destroyed. In most cases, we know neither the titles nor the authors. We do know that, in this library, there were 123 different plays by Sophocles, of which only seven have survived to our time. One of those seven is Oedipus Rex. Similar numbers apply to the lost works of Aeschylus, Euripides, Aristophanes. It’s a little as if the only surviving works of a man named William Shakespeare were Coriolanus and A Winter’s Tale, although we had heard he had written some other things which were highly prized in his time, plays called Hamlet, Macbeth, A Midsummer Night’s Dream, Julius Caesar, King Lear, Romeo and Juliet. History is full of people who—out of fear, or ignorance, or the lust for power—have destroyed treasures of immeasurable value which truly belong to all of us. We must not let it happen again.

We have considered the destruction of worlds and the end of civilizations. But there is another perspective by which to measure human endeavors. Let me tell you a story about the beginning. Some 15 billion years ago, our universe began with the mightiest explosion of all time. The universe expanded, cooled, and darkened. Energy condensed into matter—mostly hydrogen atoms—and these atoms accumulated into vast clouds, rushing away from each other, that would one day become the galaxies. Within these galaxies, the first generation of stars was born, kindling the energy hidden in matter, flooding the cosmos with light. Hydrogen atoms had made suns and starlight. There were, in those times, no planets to receive the light and no living creatures to admire the radiance of the heavens. But deep in the stellar furnaces, nuclear fusion was creating the heavier atoms: carbon and oxygen, silicon and iron. These elements, the ash left by hydrogen, were the raw materials from which planets and life would later arise. At first, the heavy elements were trapped in the hearts of the stars. But massive stars soon exhausted their fuel and, in their death throes, returned most of their substance back into space. The interstellar gas became enriched in heavy elements. In the Milky Way galaxy, the matter of the cosmos was recycled into new generations of stars, now rich in heavy atoms—a legacy from their stellar ancestors. And in the cold of interstellar space, great turbulent clouds were gathered by gravity and stirred by starlight. In their depths, the heavy atoms condensed into grains of rocky dust and ice and complex carbon-based molecules. In accordance with the laws of physics and chemistry, hydrogen atoms had brought forth the stuff of life.

In other clouds, more massive aggregates of gas and dust formed later generations of stars. As new stars were formed, tiny condensations of matter accreted near them—inconspicuous motes of rock and metal, ice and gas, that would become the planets. And on these worlds—as in interstellar clouds—organic molecules formed, made of atoms that had been cooked inside the stars. In the tide pools and oceans of many worlds, molecules were destroyed by sunlight and assembled by chemistry. One day, among these natural experiments, a molecule arose that, quite by accident, was able to make crude copies of itself. As time passed, self-replication became more accurate. Those molecules that copied better produced more copies. Natural selection was underway. Elaborate molecular machines had evolved. Slowly, imperceptibly, life had begun.

Collectives of organic molecules evolved into one-celled organisms. These produced multi-celled colonies. Their various parts became specialized organs. Some colonies attached themselves to the sea floor, others swam freely. Eyes evolved, and now the cosmos could see. Living things moved on to colonize the land. The reptiles held sway for a time, but they gave way to small warm-blooded creatures with bigger brains who developed dexterity and curiosity about their environment. They learned to use tools and fire and language. Star stuff, the ash of stellar alchemy, had emerged into consciousness.

We are a way for the cosmos to know itself. We are creatures of the cosmos and have always hungered to know our origins, to understand our connection with the universe. How did everything come to be? Every culture on the planet has devised its own response to the riddle posed by the universe. Every culture celebrates the cycles of life and nature. There are many different ways of being human. But an extraterrestrial visitor examining the differences among human societies would find those differences trivial compared to the similarities. We are one species. We are star stuff, harvesting starlight. Our lives, our past and our future, are tied to the sun, the moon, and the stars.

Our ancestors knew that their survival depended on understanding the heavens. They built observatories and computers to predict the changing of the seasons by the motions in the skies. We are, all of us, descended from astronomers. The discovery that there is order in the universe, that there are laws of nature, is the foundation on which science builds today. Our conception of the cosmos—all of modern science and technology—trace back to questions raised by the stars. Yet, even 400 years ago, we still had no idea of our place in the universe. The long journey to that understanding required both an unflinching respect for the facts and a delight in the natural world. Johannes Kepler wrote:

We do not ask for what useful purpose the birds do sing, for song is their pleasure, since they were created for singing. Similarly, we ought not to ask why the human mind troubles to fathom the secrets of the heavens. The diversity of the phenomena of nature is so great, and the treasures hidden in the heavens so rich, precisely in order that the human mind shall never be lacking in fresh nourishment.

It is the birthright of every child to encounter the cosmos anew in every culture and every age. When this happens to us, we experience a deep sense of wonder. The most fortunate among us are guided by teachers who channel this exhilaration. We are born to delight in the world. We are taught to distinguish our preconceptions from the truth. Then, new worlds are discovered as we decipher the mysteries of the cosmos. Science is a collective enterprise that embraces many cultures and spans the generations. In every age, and sometimes in the most unlikely places, there are those who wish with a great passion to understand the world. There is no way of knowing where the next discovery will come from, what dream of the mind’s eye will remake the world.

These dreams begin as impossibilities. Once, even to see a planet through a telescope was an astonishment. But we studied these worlds, we figured out how they moved in their orbits, and soon we were planning voyages of discovery beyond the Earth and sending robot explorers to the planets and the stars. We humans long to be connected with our origins, so we create rituals. Science is another way to express this longing. It also connects us with our origins. And it, too, has its rituals and its commandments. Its only sacred truth is that there are no sacred truths. All assumptions must be critically examined. Arguments from authority are worthless. Whatever is inconsistent with the facts, no matter how fond of it we are, must be discarded or revised

Science is not perfect. It’s often misused. It’s only a tool. But it’s the best tool we have self-correcting, ever-changing applicable to everything. With this tool, we vanquish the impossible. With the methods of science we have begun to explore the cosmos. For the first time, scientific discoveries are widely accessible. Our machines—the products of our science—are now beyond the orbit of Saturn. A preliminary spacecraft reconnaissance has been made of twenty new worlds. We have learned to value careful observations, to respect the facts—even when they are disquieting, when they seem to contradict conventional wisdom. The Canterbury monks faithfully recorded an impact on the moon, and the Anasazi people an explosion of a distant star. They saw for us as we see for them. We see further than they only because we stand on their shoulders. We build on what they knew. We depend on free inquiry and free access to knowledge.

We humans have seen the atoms which constitute all of matter, and the forces that sculpt this world and others. We know the molecules of life are easily formed under conditions common throughout the cosmos. We have mapped the molecular machines at the heart of life. We have discovered a microcosm in a drop of water. We have peered into the bloodstream, and down on our stormy planet to see the Earth as a single organism. We have found volcanoes on other worlds and explosions on the sun, studied comets from the depths of space and traced their origins and destinies, listened to pulsars and searched for other civilizations. We humans have set foot on another world in a place called the Sea of Tranquility, an astonishing achievement for creatures such as we, whose earliest footsteps—3.5 million years old—are preserved in the volcanic ash of East Africa. We have walked far.

These are some of the things that hydrogen atoms do, given 15 billion years of cosmic evolution. It has the sound of epic myth. But it’s simply a description of the evolution of the cosmos as revealed by science in our time. And we—we who embody the local eyes and ears and thoughts and feelings of the cosmos—we’ve begun, at last, to wonder about our origins. Star stuff contemplating the stars—organized collections of 10 billion-billion-billion atoms contemplating the evolution of matter, tracing that long path by which it arrived at consciousness here on the planet Earth, and perhaps, throughout the cosmos. Our loyalties are to the species and the planet. We speak for Earth. Our obligation to survive and flourish is owed not just to ourselves, but also to that cosmos—ancient and vast—from which we spring.

Cosmos Update

The greatest thrill for me in reliving this adventure has been not just that we’ve completed the preliminary reconnaissance with spacecraft of the entire solar system, and not just that we’ve discovered astonishing structures in the realm of the galaxies, but especially that some of Cosmos’s boldest dreams about this world are coming closer to reality. Since this series’ maiden voyage, the impossible has come to pass. Mighty walls that maintained insuperable ideological differences have come tumbling down. Deadly enemies have embraced and begun to work together. The imperative to cherish the Earth and to protect the global environment that sustains all of us has become widely accepted. And we’ve begun, finally, the process of reducing the obscene number of weapons of mass destruction. Perhaps we have, after all, decided to choose life.

But we still have light-years to go to ensure that choice, even after the summits and the ceremonies and the treaties. There are still some 50,000 nuclear weapons in the world. And it would require the detonation of only a tiny fraction of them to produce a nuclear winter—the predicted global climatic catastrophe that would result from the smoke and the dust lifted into the atmosphere by burning cities and petroleum facilities.

The world’s scientific community has begun to sound the alarm about the grave dangers posed by depleting the protective ozone shield, and by greenhouse warming. And again, we’re taking some mitigating steps. But again, those steps are too small and too slow. The discovery that such a thing as nuclear winter was really possible evolved out of studies of Martian dust storms. The surface of Mars, fried by ultraviolet light, is also a reminder of why it’s important to keep our ozone layer intact. The runaway greenhouse effect on Venus is a valuable reminder that we must take the increasing greenhouse effect on Earth seriously. Important lessons about our environment have come from spacecraft missions to the planets. By exploring other worlds we safeguard this one. By itself, I think this fact more than justifies the money our species has spent in sending ships to other worlds.

It is our fate to live during one of the most perilous and, at the same time, one of the most hopeful chapters in human history. Our science and our technology have posed us a profound question: will we learn to use these tools with wisdom and foresight before it’s too late? Will we see our species safely through this difficult passage so that our children and grandchildren will continue the great journey of discovery still deeper into the mysteries of the cosmos? That same rocket and nuclear and computer technology that sends our ships past the farthest known planet can also be used to destroy our global civilization. Exactly the same technology can be used for good and for evil. It is as if there were a god who said to us: “I set before you two ways. You can use your technology to destroy yourselves, or to carry you to the planets and the stars. It’s up to you.”



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