In a very real sense human beings are machines constructed by the nucleic acids to arrange for the efficient replication of more nucleic acids. In a sense our strongest urges, noblest enterprises, most compelling necessities, and apparent free wills are all an expression of the information coded in the genetic material: We are, in a way, temporary ambulatory repositories for our nucleic acids. This does not deny our humanity; it does not prevent us from pursuing the good, the true, and the beautiful. But it would be a great mistake to ignore where we have come from in our attempt to determine where we are going.

In our earliest history, so far as we can tell, individuals held an allegiance toward their immediate tribal group, which may have numbered no more than ten or twenty individuals, all of whom were related by consanguinity. As time went on, the need for cooperative behavior – in the hunting of large animals or large herds, in agriculture, and in the development of cities – forced human beings into larger and larger groups. The group that was identified with, the tribal unit, enlarged at each stage of this evolution. Today, a particular instant in the 4.5- billion-year history of Earth and in the several-million-year history of mankind, most human beings owe their primary allegiance to the nation-state (although some of the most dangerous political problems still arise from tribal conflicts involving smaller population units).

[...]

If we survive these perilous times, it is clear that even an identification with all of mankind is not the ultimate desirable identification. If we have a profound respect for other human beings as co-equal recipients of this precious patrimony of 4.5 billion years of evolution, why should the identification not apply also to all the other organisms on Earth, which are equally the product of 4.5 billion years of evolution? We care for a small fraction of the organisms on Earth – dogs, cats, and cows, for example – because they are useful or because they flatter us. But spiders and salamanders, salmon and sunflowers are equally our brothers and sisters.

I believe that the difficulty we all experience in extending our identification horizons in this way is itself genetic. Ants of one tribe will fight to the death intrusions by ants of another. Human history is filled with monstrous cases of small differences – in skin pigmentation, or abstruse theological speculation, or manner of dress and hair style – being the cause of harassment, enslavement, and murder.

[...]

The time has come for a respect, a reverence, not just for all human beings, but for all life forms – as we would have respect for a masterpiece of sculpture or an exquisitely tooled machine. This, of course, does not mean that we should abandon the imperatives for our own survival. Respect for the tetanus bacillus does not extend to volunteering our body as a culture medium. But at the same time we can recall that here is an organism with a biochemistry that tracks back deep into our planet's past. The tetanus bacillus is poisoned by molecular oxygen, which we breathe so freely. The tetanus bacillus, but not we, would be at home in the hydrogen-rich, oxygen-free atmosphere of primitive Earth.

[...]

There may be a time, as I describe in Part III of this book, when contact will be made with another intelligence on a planet of some far-distant star, beings with billions of years of quite independent evolution, beings with no prospect of looking very much like us – although they may think very much like us. It is important that we extend our identification horizons, not just down to the simplest and most humble forms of life on our own planet, but also up to the exotic and advanced forms of life that may inhabit, with us, our vast galaxy of stars.

What sexuality there is in the message also drew epistolary fire. The Los Angeles Times published a letter from an irate reader that went:

I must say I was shocked by the blatant display of both male and female sex organs on the front page of the Times. Surely this type of sexual exploitation is below the standards our community has come to expect from the Times. Isn't it enough that we must tolerate the bombardment of pornography through the media of film and smut magazines? Isn't it bad enough that our own space agency officials have found it necessary to spread this filth even beyond our own solar system?

This was followed several days later by another letter in the Times:

I certainly agree with those people who are protesting our sending those dirty pictures of naked people out into space. I think the way it should have been done would have been to visually bleep out the reproductive organs of the drawings of the man and the woman. Next to them should have been a picture of a stork carrying a little bundle from heaven. Then if we really want our celestial neighbors to know how far we have progressed intellectually, we should have included pictures of Santa Claus, the Easter Bunny, and the Tooth Fairy.

The New York Daily News headlined the story in typical fashion: "Nudes and Map tell about Earth to Other Worlds."

[...]

An article in Catholic Review criticizes the plaque because it "includes everything but God," and suggests that, rather than a pair of human beings, it would have been better to have borne a sketch of a pair of praying hands.

Another correspondent maintains that the perspective conventions are insuperably difficult, and urges us to send the complete cadavers of a man and a woman. They would be perfectly preserved in the cold of space, and could be examined by extraterrestrials in detail. We declined on grounds of excess weight.

The front page of the Berkeley, California, Barb, apparently intending to convey an impression that the man and woman on the message were too straight, reproduced them with the caption, "Hello. We're from Orange County."

This comment touches on an aspect of the representation of the man and woman that I personally feel much worse about, although it has received almost no other public notice. In the original sketches from which the engravings were made, we made a conscious attempt to have the man and woman panracial. The woman was given epicanthian folds and in other ways a partially Asian appearance. The man was given a broad nose, thick lips, and a short "Afro" haircut. Caucasian features were also present in both. We had hoped to represent at least three of the major races of mankind. The epicanthian folds, the lips, and the nose have survived into the final engraving. But because the woman's hair is drawn only in outline, it appears to many viewers as blond, thereby destroying the possibility of a significant contribution from an Asian gene pool. Also, somewhere in the transcription from the original sketch drawing to the final engraving the Afro was transmuted into a very non-African Mediterranean-curly haircut. Nevertheless, the man and woman on the plaque are, to a significant degree, representative of the sexes and races of mankind.

On the title page of this chapter is shown the message. It is etched on a 6-inch by 9-inch gold-anodized aluminum plate, attached to the antenna support struts of Pioneer 10. The expected erosion rate in interstellar space is sufficiently small that this message should remain intact for hundreds of millions of years, and probably for a much longer period of time. It is, thus, the artifact of mankind with the longest expected lifetime.

The message itself intends to communicate the locale, epoch, and something of the nature of the builders of the spacecraft. It is written in the only language we share with the recipients: Science. At top left is a schematic representation of the hyperfine transition between parallel and antiparallel proton and electron spins of the neutral hydrogen atom. Beneath this representation is the binary number 1. Such transitions of hydrogen are accompanied by the emission of a radiofrequency photon of wavelength about 21 centimeters and frequency of about 1,420 Megahertz. Thus, there is a characteristic distance and a characteristic time associated with the transition. Since hydrogen is the most abundant atom in the Galaxy, and physics is the same throughout the Galaxy, we think there will be no difficulty for an advanced civilization to understand this part of the message. But as a check, on the right margin is the binary number 8 (1---) between two tote marks, indicating the height of the Pioneer 10 spacecraft, schematically represented behind the man and the woman. A civilization that acquires the plaque will, of course, also acquire the spacecraft, and will be able to determine that the distance indicated is indeed close to 8 times 21 centimeters, thus confirming that the symbol at top left represents the hydrogen hyperfine transition.

Further binary numbers are shown in the radial pattern comprising the main part of the diagram at left center. These numbers, if written in decimal notation, would be ten digits long. They must represent either distances or times. If distances, they are of the order of several times 1011 centimeters, or a few dozen times the distance between the Earth and the Moon. It is highly unlikely that we would consider them useful to communicate. Because of the motion of objects within the Solar System, such distances vary in continuous and complex ways.

However, the corresponding times are on the order of 1/10 second to 1 second. These are the characteristic periods of the pulsars, natural and regular sources of cosmic radio emission; pulsars are rapidly rotating neutron stars produced in catastrophic stellar explosions (see Chapter 38). We believe that a scientifically sophisticated civilization will have no difficulty understanding the radial burst pattern as the positions and periods of 14 pulsars with respect to the Solar System of launch.

But pulsars are cosmic clocks that are running down at largely known rates. The recipients of the message must ask themselves not only where it was ever possible to see 14 pulsars arrayed in such a relative position, but also when it was possible to see them. The answers are: Only from a very small volume of the Milky Way Galaxy and in a single year in the history of the Galaxy. Within that small volume there are perhaps a thousand stars; only one is anticipated to have the array of planets with relative distances as indicated at the bottom of the diagram. The rough sizes of the planets and the rings of Saturn are also schematically shown. A schematic representation of the initial trajectory of the spacecraft launched from Earth and passing by Jupiter is also displayed. Thus, the message specifies one star in about 250 billion and one year (1970) in about 10 billion.

The content of the message to this point should be clear to an advanced extraterrestrial civilization, which will, of course, have the entire Pioneer 10 spacecraft to examine as well. The message is probably less clear to the man on the street, if the street is on the planet Earth. (However, scientific communities on Earth have had little difficulty decoding the message.) The opposite is the case with the representations of human beings to the right. Extraterrestrial beings, which are the product of 4.5 billion years or more of independent biological evolution, may not at all resemble humans, nor may the perspective and linedrawing conventions be the same there as here. The human beings are the most mysterious part of the message.

[...]

In the same way, the relative distances of the planets from the Sun, shown by binary notation at the bottom of the plaque, indicate that we use base-10 arithmetic. From the fact that we have 10 fingers and 10 toes – drawn with some care on the plaque – I hope any extraterrestrial recipients will be able to deduce that we use base-10 arithmetic and that some of us count on our fingers. From the stumpiness of our toes they may even be able to deduce that we evolved from arboreal ancestors.

The message aboard Pioneer 10 has been good fun. But it has been more than that. It is a kind of cosmic Rorschach test, in which many people see reflected their hopes and fears, their aspirations and defeats – the darkest and the most luminous aspects of the human spirit.

Our instincts and emotions are those of our hunter-gatherer ancestors of a million years ago. But our society is astonishingly different from that of a million years ago. In times of slow change, the insights and skills learned by one generation are useful, tried, and adaptive, and are gladly received when passed down to the next generation. But in times like today, when the society changes significantly in less than a human lifetime, the parental insights no longer have unquestioned validity for the young. The so-called generation gap is a consequence of the rate of social and technological change.

Even within a human lifetime, the change is so great that many people are alienated from their own society. Margaret Mead has described older people today as involuntary immigrants from the past to the present.

Old economic assumptions, old methods of determining political leaders, old methods of distributing resources, old methods of communicating information from the government to the people – and vice versa – all of these may once have been valid or useful or at least somewhat adaptive, but today may no longer have survival value at all. Old oppressive and chauvinistic attitudes among the races, between the sexes, and between economic groups are being justifiably challenged. The fabric of society throughout the world is ripping.

At the same time, there are vested interests opposed to change. These include individuals in power who have much to gain in the short run by maintaining the old ways, even if their children have much to lose in the long run. They are individuals who are unable in middle years to change the attitudes inculcated in their youth.

The situation is a very difficult one. The rate of change cannot continue indefinitely; as the example of the rate of communication indicates, limits must be reached. We cannot communicate faster than the velocity of light. We cannot have a population larger than Earth's resources and economic distribution facilities can maintain. Whatever the solutions to be achieved, hundreds of years from now the Earth is unlikely still to be experiencing great social stress and change. We will have reached some solution to our present problems. The question is, which solution?

In science a situation as complicated as this is difficult to treat theoretically. We do not understand all the factors that influence our society and, therefore, cannot make reliable predictions on what changes are desirable. There are too many complex interactions. Ecology has been called the subversive science because every time a serious effort to preserve a feature of the environment is made, it runs into enormous numbers of social or economic vested interests. The same is true every time we attempt to make a major change in anything that is wrong; the change runs through society as a whole. It is difficult to isolate small fragments of the society and change them without having profound influences on the rest of society.

When theory is not adequate in science, the only realistic approach is experimental. Experiment is the touchstone of science on which the theories are framed. It is the court of last resort. What is clearly needed are experimental societies!

[...]

Social mutations, it seems to me, are what we need. Perhaps because of a hoary science-fiction tradition that mutants are ugly and hateful, it might be better to use another term. But social mutation – a variation on a social system which breeds true, which, if it works, is the path to the future – seems to be precisely the right phrase. It would be useful to examine why some of us find the phrase objectionable.

We should be encouraging social, economic, and political experimentation on a massive scale in all countries. Instead, the opposite seems to be occurring. In countries such as the United States and the Soviet Union the official policy is to discourage significant experimentation, because it is, of course, unpopular with the majority. The practical consequence is vigorous popular disapproval of significant variation. Young urban idealists immersed in a drug culture, with dress styles considered bizarre by conventional standards, and with no prior knowledge of agriculture, are unlikely to succeed in establishing Utopian agricultural communities in the American Southwest – even without local harassment. Yet such experimental communities throughout the world have been subjected to hostility and violence by their more conventional neighbors. In some cases the vigilantes are enraged because they themselves have only within the previous generation been accepted into the conventional system.

We should not be surprised, then, if experimental communities fail. Only a small fraction of mutations succeed. But the advantage social mutations have over biological mutations is that individuals learn; the participants in unsuccessful communal experiments are able to assess the reasons for failure and can participate in later experiments that attempt to avoid the causes of initial failure.

There should be not only popular approval for such experiments, but also official governmental support for them. Volunteers for such experiments in Utopia – facing long odds for the benefit of society as a whole – will, I hope, be thought of as men and women of exemplary courage. They are the cutting edge of the future. One day there will arise an experimental community that works much more efficiently than the polyglot, rubbery, hand-patched society we are living in. A viable alternative will then be before us.

The virtue of thinking about life elsewhere is that it forces us to stretch our imaginations. Can we think of alternative solutions to biological problems already solved in one particular way on Earth? For example, the wheel is a comparatively recent invention on the planet Earth. It seems to have been invented in the ancient Near East less than ten thousand years ago. In fact, the high civilizations of Meso- America, the Aztecs and the Mayas, never employed the wheel, except for children's toys. Biology – the evolutionary process – has never invented the wheel, in spite of the fact that its selective advantages are manifest. Why are there no wheeled spiders or goats or elephants rolling along the highways? The answer is clearly that, until recently, there were no highways. Wheels are of use only when there are surfaces to roll on. Since the planet Earth is a heterogeneous, bumpy place with few long, smooth areas, there was no advantage to evolving the wheel. We can very well imagine another planet with enormous long stretches of smooth lava fields in which wheeled organisms are abundant. The late Dutch artist M. C. Escher designed a salamander-like organism that would do very well in such an environment.

Charles Darwin's insights into natural selection have shown that there are no evolutionary pathways leading unerringly from simple forms to Man; rather, evolution proceeds by fits and starts, and most life forms lead to evolutionary dead-ends. We are the products of a long series of biological accidents. In the cosmic perspective there is no reason to think that we are the first or the last or the best.

These realizations of the Copernican and Darwinian revolutions are profound – and, to some, disturbing. But they bring with them compensatory insights. We realize our deep connectedness with other life forms, both simple and complex. We know that the atoms that make us up were synthesized in the interiors of previous generations of dying stars. We are aware of our deep connection, both in form and in matter, with the rest of the universe. The cosmos revealed to us by the new advances in astronomy and biology is far grander and more awesome than the tidy world of our ancestors. And we are becoming a part of it, the cosmos as it is, not the cosmos of our desires.

The experience of space exploration gives no unique philosophy; to some extent, each group tends to see its own philosophical view reflected, and not always by the soundest logic: Nikita Khrushchev stressed that in the space flight of Yuri Gagarin no angels or other supernatural beings were detected; and, in almost perfect counterpoint, the Apollo 8 astronauts read from lunar orbit the Babylonian cosmogony enshrined in Genesis, Chapter 1, as if to reassure their American audience that the exploration of the Moon was not really in contradiction to anyone's religious beliefs. But it is striking how space exploration leads directly to religious and philosophical questions.

I believe that military control of manned space flight – in practice in the Soviet Union and a subject of current debate in the United States – is a step that supporters of peace should back. The military establishments of the United States and the Soviet Union are, I am afraid, establishments with vested interests in war. They are meticulously trained for war; in time of war, there are rapid promotions, increases in pay, and opportunities for valor that are absent in peacetime. Where eager readiness for warfare exists, the likelihood of intentional or accidental warfare becomes much greater. By virtue of their training and temperament, military men are often not interested in other sorts of gainful employment. There are few other ways of life with the perquisites of power of the military officer. If peace broke out, the officer corps, their services no longer as necessary, would be profoundly discomfited. Premier Khrushchev once attempted to cashier a large number of senior officers in the Red Army, putting them in charge of hydroelectric power stations and the like. This was not to their liking, and in something like a year most of them were back in their old jobs. In fact, the military establishments in the United States and the Soviet Union owe their jobs to each other, and there is a very real sense in which they form a natural alliance against the rest of us.

In all the history of mankind, there will be only one generation that will be first to explore the Solar System, one generation for which, in childhood, the planets are distant and indistinct discs moving through the night sky, and for which, in old age, the planets are places, diverse new worlds in the course of exploration.

There will be a time in our future history when the Solar System will be explored and inhabited. To them, and to all who come after us, the present moment will be a pivotal instant in the history of mankind. There are not many generations given an opportunity as historically significant as this one. The opportunity is ours, if we but grasp it. To paraphrase K. E. Tsiolkovsky, the founder of astronautics: The Earth is the cradle of mankind, but one cannot live in the cradle forever.

A human infant begins to achieve maturity by the experimental discovery that he is not the whole of the universe. The same is true of societies engaged in the exploration of their surroundings. The perspective carried by space exploration may hasten the maturation of mankind – a maturation that cannot come too soon.

Venus thus seems to be a place quite different from the Earth, and alarmingly unappealing: Broiling temperatures, crushing pressures, noxious and corrosive gases, sulfurous smells, and a landscape immersed in a ruddy gloom.

Curiously enough, there is a place astonishingly like this in the superstition, folklore and legends of men. We call it Hell. In the older belief – that of the Greeks, for example – it was the place where all human souls journeyed after death. In Christian times it has been thought of as the post-mortem destination only of one of two categories of moral persuasion. But there is little doubt that the average person's view of Hell – sizzling, choking, sulfurous, and red – is a dead ringer for the surface of Venus.

Because of their small sizes, Phobos and Deimos have very low gravitational accelerations. Their gravities do not pull very hard. The pull on Phobos is only about one one-thousandth of that on Earth. If you can perform a standing high jump of two or three feet on Earth, you could perform a standing high jump of half a mile on Phobos. It would not take many such jumps to circumnavigate Phobos. They would be graceful, slow, arcing leaps, taking many minutes to reach the high point of the self-propelled trajectory and then to return gently to the ground.

Even more interesting would be a game like baseball on Phobos. The velocity necessary to launch an object into orbit about Phobos is only about twenty miles per hour. An amateur baseball pitcher could easily launch a baseball into orbit around Phobos. The escape velocity from Phobos is only about thirty miles per hour, a speed easily reached by professional baseball pitchers. A baseball that had escaped from Phobos would still be in orbit about Mars – a man-launched moonlet. If Phobos were perfectly spherical, a lonely astronaut with an interest in baseball could invent a curious but somewhat sluggish version of this already rather sluggish game. First, as pitcher, he could throw the ball sidearm – at the horizon at between twenty and thirty miles per hour. He could then go home for lunch, because it will take about two hours for the baseball to circumnavigate Phobos. After lunch, he can pick up a bat, face the other direction and await his pitch of two hours earlier. Apart from the fact that good pitchers are seldom good hitters, hitting this pitch would be pretty easy: About fifteen seconds elapse from the appearance of the baseball at the horizon to its arrival in the vicinity of our astronaut. If he swings and misses – or, more likely, if the ball is wide of the plate – he can then go home for a two-hour nap, returning with his catcher's mitt to catch the ball. Alternatively, if he succeeds in hitting a fly ball at a velocity somewhere between twenty and thirty miles per hour, he can go home and take his nap, returning this time with a fielder's mitt, awaiting the return of the ball from the opposite horizon two hours later. Because Phobos is gravitationally lumpy, the game would be more difficult than I have indicated. Since daylight on Phobos lasts only about four hours, lights would have to be erected, or the game modified so that all pitching, hitting, and catching events happen on the day side.

These sports possibilities may, one day a century or two hence, provide a tourist industry for Phobos and Deimos. But baseball on Phobos is no more an argument for going there than, to take a random example, golf is for going to the Moon. The scientific interest in the moons of Mars – whether captured asteroids or debris from the formation of the planet – is, however, immense. Sooner or later, certainly on a time scale of centuries, there will be instruments – and then men – on the surface of Phobos looking up with awe at an immense red planet that fills the sky from zenith to horizon.

At the very beginning of the twentieth century competent scientific and lay opinion held that airplanes were impossible. The end of the century, barring the dark specter of nuclear or ecological catastrophes, will probably see joint Soviet and American manned space expeditions to the nearer planets.

This is the century in which some of the oldest dreams of Man have been realized, in which mankind has sprouted wings and realized the aspirations of Daedalus and da Vinci. Air-breathing, man-carrying machines now circumnavigate our planet in less than a day; other machines, skimming above the atmosphere, carry men around our globe in ninety minutes.

There is a generation of men and women for whom, in their youth, the planets were unimaginably distant points of light, and the Moon was the paradigm of the unattainable. Those same men and women, in middle life, have seen their fellows walk upon the surface of the Moon; in their old age, they will likely see men wandering along the dusty surface of Mars, their journeys illuminated by the battered face of Phobos. There is only one generation of humans in the tenmillion- year history of mankind that will live through such a transition. That generation is alive today

The Earth is overcrowded. Not yet in a literal sense: Our technology is adequate to maintain comfortably a population significantly larger than our present 3.6 billion. The Earth is overcrowded in a psychological sense. For that restless and ambition-driven fraction of mankind that has blazed new paths for our species, there are no new places to go. There are places inside of ourselves, but this is not the forte of such individuals. There are the ocean basins, but we are not yet committed to exploring them seriously; and when we do, they are likely to be exploited rapidly

At just this time in our history comes the possibility of exploring and colonizing our neighboring worlds in space. The opportunity has come to us not a moment too soon.

The Solar System is much vaster than the Earth, but the speeds of our spacecraft are, of course, much greater than the speeds of the sailing ships of the fifteenth and sixteenth centuries. The spacecraft trip from the Earth to the Moon is faster than was the galleon trip from Spain to the Canary Islands. The voyage from Earth to Mars will take as long as did the sailing time from England to North America; the journey from Earth to the moons of Jupiter will require about the same time as did the voyage from France to Siam in the eighteenth century. Moreover, the fraction of the gross national product of the United States or the Soviet Union that is being expended even in the more costly manned space programs is just comparable to the fraction of the gross national product spent by England and France in the sixteenth and seventeenth centuries on their exploratory ventures by sailing ships. In economic terms and in human terms, we have performed such voyages before!

Pioneer 10 is the first interstellar spacecraft launched by mankind. It was also the fastest spacecraft launched, to the date of its departure. But it will take eighty thousand years for Pioneer 10 to reach the distance of the nearest star. Because space is so empty, it will never enter another Solar System. The little golden message aboard Pioneer 10 will be read, but only if there are interstellar voyagers able to detect and intercept Pioneer 10.

I believe that such an interception may occur, but by interstellar voyagers from the planet Earth, overtaking and heaving to this ancient space derelict – as if the Nina, with its crew jabbering in Castilian about falling off the edge of the world, were to be intercepted, somewhere off Tristan da Cunha, by the aircraft carrier John F. Kennedy.

It is at this point that the ultimate significance of dolphins in the search for extraterrestrial intelligence emerges. It is not a question of whether we are emotionally prepared in the long run to confront a message from the stars. It is whether we can develop a sense that beings with quite different evolutionary histories, beings who may look far different from us, even "monstrous," may, nevertheless, be worthy of friendship and reverence, brotherhood and trust. We have far to go; while there is every sign that the human community is moving in this direction, the question is, are we moving fast enough? The most likely contact with extraterrestrial intelligence is with a society far more advanced than we (Chapter 31). But we will not at any time in the foreseeable future be in the position of the American Indians or the Vietnamese – colonial barbarity practiced on us by a technologically more advanced civilization – because of the great spaces between the stars and what I believe is the neutrality or benignness of any civilization that has survived long enough for us to make contact with it. Nor will the situation be the other way around, terrestrial predation on extraterrestrial civilizations – they are too far away from us and we are relatively powerless. Contact with another intelligent species on a planet of some other star – a species biologically far more different from us than dolphins or whales – may help us to cast off our baggage of accumulated jingoisms, from nationalism to human chauvinism. Though the search for extraterrestrial intelligence may take a very long time, we could not do better than to start with a program of rehumanization by making friends with the whales and the dolphins.

During the filming of 2001, Kubrick, who obviously has a grasp for detail, became concerned that extraterrestrial intelligence might be discovered before the $10.5 million film was released, rendering the plot line obsolete, if not erroneous. Lloyd's of London was approached to underwrite an insurance policy protecting against losses should extraterrestrial intelligence be discovered. Lloyd's of London, which insures against the most implausible contingencies, declined to write such a policy. In the mid-1960s there was no search being performed for extraterrestrial intelligence, and the chance of accidentally stumbling on extraterrestrial intelligence in a few years' period was extremely small. Lloyd's of London missed a good bet.

The fate of individual human beings may not now be connected in a deep way with the rest of the universe, but the matter out of which each of us is made is intimately tied to processes that occurred immense intervals of time and enormous distances in space away from us. Our Sun is a second- or third-generation star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff.

There are those who predict a dire catastrophe if we broadcast our presence to another star. The extraterrestrials will come and – eat us, or something equally unpleasant. (Actually, if we are especially tasty, they need only sample one of us, determine what sequence of our amino acids makes us appetizing, and then reconstruct the relevant proteins on their own planet. The high freightage makes us economically, if not gastronomically, unappetizing.) The message aboard Pioneer 10 was criticized by a few because it "gave away" our position in the Galaxy. I very much doubt if we pose any threat to anybody out there. We are the most backward possible civilization able to engage in communication, and the vast spaces between the stars are a kind of natural quarantine, preventing us at any time in the near future from messing around out there.

But, in any case, it is too late. We have already announced our presence. The initial radio broadcasts, starting with Marconi and reaching significant intensity in the 1920s, have leaked through the ionosphere and are expanding at the velocity of light in a spherical wavefront centered around the Earth. And in that wavefront, an advanced technical civilization can pick up the tinny transmissions of Enrico Caruso arias, the Scopes trial, the 1928 election returns, the big jazz bands. These are the harbingers of the cultures of Earth, our first emissaries to the stars.

If there are technical civilizations some fifty light-years out, they will just now be detecting these strange, primitive signals. Even if they are poised to respond instantly with the fastest spaceship possible, it will be at least another fifty years before we hear from them. Pioneer 10 will take a million years to cover the same distance.

It is too late to be shy and hesitant. We have announced our presence to the cosmos – in a backward and groping and unrepresentative manner, to be sure – but here we are!

Some individuals find the absence of a dialogue distressing – as if meaningful dialogues were commonplace on this planet. Philip Morrison, of the Massachusetts Institute of Technology, has pointed out that such cultural monologues are entirely common in the history of mankind; that, for example, the entire cultural patrimony of classical Greece, which has influenced our civilization in a profound way, has traveled in only one direction in time. We have not sent our wisdom to the Greeks. The Greeks have sent their wisdom to us – on paper and parchment, and not by radio waves, but the principle is the same.

But there may be more significant ways to characterize civilizations than by the energy they use for communications purposes. An important criterion of a civilization is the total amount of information that it stores. This information can be described in terms of bits, the number of yes-no statements concerning itself and the universe that such a civilization knows.

An example of this concept is the popular game of "Twenty Questions," as played on Earth. One player imagines an object or concept and makes an initial classification of it into animal, vegetable, mineral, or none of these three. To identify the object or concept, the other players then have a total of twenty questions, which can only be answered "Yes" or "No." How much information can be discriminated in this manner?

The initial characterization can be thought of as three yes-no questions: Conceptual or objective? Biological or nonbiological? Plant or animal? If we agree that a particular game of "Twenty Questions" is in pursuit of something alive, we have, in effect, answered three questions already by the time the game begins. The first question divided the universe into two (unequal) pieces. The second question divided one of those pieces into two more, and the third divided one of those pieces into yet two more. At this stage we have divided the universe crudely into 2×2×2=2³=8 pieces. When we have finished with our twenty questions, we have "divided the universe into 2²⁰ additional (probably unequal) pieces. Now, 2¹⁰ is 1,024. We can perform such calculations fairly quickly if we approximate 2¹⁰ by 1,000=10³; therefore, 2²⁰ equals (2¹⁰)², which approximately equals (10³)²=10⁶. The total number of effective questions, twenty-three, has divided the universe into about 2²³, or approximately 10⁷ pieces or bits. Thus, it is possible for skillful players to win at "Twenty Questions" only if they live in a civilization that has an information content of about 10⁷ bits.

But, as I discuss below, our civilization is characterized by perhaps 10¹⁴ bits. Therefore, skillful players should win at "Twenty Questions" only about 10⁷ out of 10¹⁴ times, or one in 10⁷, or one in ten million times. That the game is won more often in practice is because there is an additional rule – usually unstated but well understood: Namely, that the object or concept being named should be one in the general cultural heritage of all the players. But this must mean that 10⁷ bits can convey a great deal of information about a civilization, as indeed it can. Philip Morrison has estimated that the total written contribution to our present civilization from classical Greek civilization is only about 10⁹ bits. Thus, a one-way message, containing what, by the standards of modern radio astronomy, is a very small number of bits, can contain a very significant amount of new information and can have a powerful influence on a society in the long run.

It is possible to speculate on the very distant future of advanced civilizations. We can imagine such societies in excellent harmony with their environments, their biology, and the vagaries of their politics, so that they enjoy extraordinarily long lifetimes. Communications would long have been established with many other such civilizations. The diffusion of knowledge, techniques, and points of view would occur at the velocity of light. In time, the diverse cultures of the Galaxy, involving a large number of quite different-looking organisms, based on different biochemistries and different initial cultures, would become homogenized – just as the diverse cultures of Earth today are in the process of homogenization.

But such cultural homogenization of the Galaxy will take a long time. One round-trip communication by radio between us and the center of the Milky Way Galaxy requires sixty thousand years. Cultural homogenization of the Galaxy would require many such exchanges, even if each exchange involved very large amounts of information conveyed very efficiently. I find it difficult to believe that fewer than one hundred exchanges between the remotest parts of the Galaxy would be adequate for galactic cultural homogenization.

The minimum lifetime for the homogenization of the Galaxy would thus be many millions of years. The constituent societies must, of course, be stable for comparable periods of time. Such homogenization need not be desirable, but there are still strong and obvious pressures for it to occur, as is also the case on the Earth. If there exists a galactic community of civilizations that truly embraces much of the Milky Way, and if we are right that no information can be transmitted at a velocity faster than light, then most of the members – and all of the founding members – of such a community must be at least millions of years more advanced than we are. For this reason, I think it a great conceit, the idea of the present Earth establishing radio contact and becoming a member of a galactic federation – something like a bluejay or an armadillo applying to the United Nations for member-nation status.

We conclude that there cannot be a strongly cohesive network of communicating, unifying intelligences through the whole universe if (1) such galactic civilizations evolve upward from individual planetary societies and if (2) the velocity of light is indeed a fixed limit on the speed of information transmission, as special relativity requires (i.e., if we ignore such possibilities as using black holes for fast transport: See Chapter 39). Such a universal intelligence is a kind of god that cannot exist.

In a way, St. Augustine and many other thoughtful theologians have come to rather the same conclusion – God must not live from moment to moment, but during all times simultaneously. This is, in a way, the same as saying that special relativity does not apply to Him. But supercivilization gods, perhaps the only ones that this kind of scientific speculation admits, are fundamentally limited. There may be such gods of galaxies, but not of the universe as a whole.

In fact, our own universe is very likely itself a vast black hole. We have no knowledge of what lies outside our universe. This is true by definition, but also because of the properties of black holes. Objects that reside in them cannot ordinarily leave them. In a strange sense, our universe may be filled with objects that are not here. They are not separate universes. They do not have the mass of our universe. But in their separateness and their isolation they are autonomous universes.

Once upon a time, about ten or fifteen billion years ago, the universe was without form. There were no galaxies. There were no stars. There were no planets. And there was no life. Darkness was upon the face of the deep. The universe was hydrogen and helium. The explosion of the Big Bang had passed, and the fires of that titanic event – either the creation of the universe or the ashes of a previous incarnation of the universe – were rumbling feebly down the corridors of space.

But the gas of hydrogen and helium was not smoothly distributed. Here and there in the great dark, by accident, somewhat more than the ordinary amount of gas was collected. Such clumps grew imperceptibly at the expense of their surroundings, gravitationally attracting larger and larger amounts of neighboring gas. As such clumps grew in mass, their denser parts, governed by the inexorable laws of gravitation and conservation of angular momentum, contracted and compacted, spinning faster and faster. Within these great rotating balls and pinwheels of gas, smaller fragments of greater density condensed out; these shattered into billions of smaller shrinking gas balls.

Compaction led to violent collisions of the atoms at the centers of the gas balls. The temperatures became so great that electrons were stripped from protons in the constituent hydrogen atoms. Because protons have like positive charges, they ordinarily electrically repel one another. But after a while the temperatures at the centers of the gas balls became so great that the protons collided with extraordinary energy – an energy so great that the barrier of electrical repulsion that surrounds the proton was penetrated. Once penetration occurred, nuclear forces – the forces that hold the nuclei of atoms together – came into play. From the simple hydrogen gas the next atom in complexity, helium, was formed. In the synthesis of one helium atom from four hydrogen atoms there is a small amount of excess energy left over. This energy, trickling out through the gas ball, reached the surface and was radiated into space. The gas ball had turned on. The first star was formed. There was light on the face of the heavens.

The stars evolved over billions of years, slowly turning hydrogen into helium in their deep interiors, converting the slight mass difference into energy, and flooding the skies with light. There were in these times no planets to receive the light, and no life forms to admire the radiance of the heavens.

The conversion of hydrogen into helium could not continue indefinitely. Eventually, in the hot interiors of the stars, where the temperatures were high enough to overcome the forces of electrical repulsion, all the hydrogen was consumed. The fires of the stars were stoked. The pressures in the interiors could no longer support the immense weight of the overlying layers of star. The stars then continued their process of collapse, which had been interrupted by the nuclear fires of a billion years before.

In contracting further, higher temperatures were reached, temperatures so high that helium atoms – the ash of the previous epoch of nuclear reaction – became usable as stellar fuel. More complex nuclear reactions occurred in the insides of the stars – now swollen, distended red giant stars. Helium was converted to carbon, carbon to oxygen and magnesium, oxygen to neon, magnesium to silicon, silicon to sulfur, and upward through the litany of the periodic table of the elements – a massive stellar alchemy. Vast and intricate mazes of nuclear reactions built up some nuclei. Others coalesced to form much more complex nuclei. Still others fragmented or combined with protons to build only slightly more complex nuclei.

But the gravity on the surfaces of red giants is low, because the surfaces have expanded outward from the interiors. The outer layers of red giants are slowly dissipated into interstellar space, enriching the space between the stars in carbon and oxygen and magnesium and iron and all the elements heavier than hydrogen and helium. In some cases, the outer layers of the star were slowly stripped off, like the successive skins of an onion. In other cases, a colossal nuclear explosion rocked the star, propelling at immense velocity into interstellar space most of the outside of the star. Either by leakage or explosion, by dissipation slow or dissipation fast, star-stuff was spewed back to the dark, thin gas from which the stars had come.

But here, later generations of stars were aborning. Again the condensations of gas spun their slow gravitational pirouettes, slowly transmogrifying gas cloud into star. But these new second- and third-generation stars were enriched in heavy elements, the patrimony of their stellar antecedents. Now, as stars were formed, smaller condensations formed near them, condensations far too small to produce nuclear fires and become stars. They were little dense, cold clots of matter, slowly forming out of the rotating cloud, later to be illuminated by the nuclear fires that they themselves could not generate. These unprepossessing clots became the planets: Some giant and gaseous, composed mostly of hydrogen and helium, cold and far from their parent star; others, smaller and warmer, losing the bulk of their hydrogen and helium by a slow trickling away to space, formed a different sort of planet – rocky, metallic, hard-surfaced.

These smaller cosmic debris, congealing and warming, released small quantities of hydrogen-rich gases, trapped in their interiors during the processes of formation. Some gases condensed on the surface, forming the first oceans; other gases remained above the surface, forming the first atmospheres – atmospheres different from the present atmosphere of Earth, atmospheres composed of methane, ammonia, hydrogen sulfide, water, and hydrogen – an unpleasant and unbreathable atmosphere for humans. But this is not yet a story about humans.

Starlight fell on this atmosphere. Storms were driven by the Sun, producing thunder and lightning. Volcanoes erupted, hot lava heating the atmosphere near the surface. These processes broke apart molecules of the primitive atmosphere. But the fragments reassorted into more and more complex molecules, falling into the early oceans, there interacting with each other, falling by chance upon clays, a dizzying process of breakdown, resynthesis, transformation – slowly moving toward molecules of greater and greater complexity, driven by the laws of physics and chemistry. After a time, the oceans achieved the constituency of a warm dilute broth.

Among the innumerable species of complex organic molecules forming and dissipating in this broth there one day arose a molecule able crudely to make copies of itself – a molecule which weakly guided the chemical processes in its vicinity to produce molecules like itself – a template molecule, a blueprint molecule, a self-replicating molecule. This molecule was not very efficient. Its copies were inexact. But soon it gained a significant advantage over the other molecules in the early waters. The molecules that could not copy themselves did not. Those that could, did. The number of copying molecules greatly increased.

As time passed, the copying process became more exact. Other molecules in the waters were reprocessed to form the jigsaw puzzle pieces to fit the copying molecules. A minute and imperceptible statistical advantage of the molecules that could copy themselves was soon transformed by the arithmetic of geometrical progression into the dominant process in the oceans.

More and more elaborate reproductive systems arose. Those systems that copied better produced more copies. Those that copied poorly produced fewer copies. Soon most of the molecules were organized into molecular collectives, into self-replicating systems. It was not that any molecules had the glimmering of an idea or the ghostly passage of a need or want or aspiration; merely, those molecules that copied did, and soon the face of the planet became transformed by the copying process. In time, the seas became full of these molecular collectives, forming, metabolizing, replicating … forming, metabolizing, replicating … forming, metabolizing, mutating, replicating… Elaborate systems arose, molecular collectives exhibiting behavior, moving to where the replication building blocks were more abundant, avoiding molecular collectives that incorporated their neighbors. Natural selection became a molecular sieve, selecting out those combinations of molecules best suited by chance to further replication.

All the while the building blocks, the foodstuffs, the parts for later copies, were being produced, mainly by sunlight and lightning and thunder – all driven by the nearby star. The nuclear processes in the insides of the stars drove the planetary processes, which led to and sustained life.

As the supply of foodstuffs gradually was exhausted, a new kind of molecular collective arose, one able to produce molecular building blocks internally out of air and water and sunlight. The first animals were joined by the first plants. The animals became parasites upon the plants, as they had been earlier on the stellar manna falling from the skies. The plants slowly changed the composition of the atmosphere; hydrogen was lost to space, ammonia transformed to nitrogen, methane to carbon dioxide. For the first time, oxygen was produced in significant quantities in the atmosphere – oxygen, a deadly poisonous gas able to convert all the self-replicating organic molecules back into simple gases like carbon dioxide and water.

But life met this supreme challenge: In some cases by burrowing into environments where oxygen was absent, but – in the most successful variants – by evolving not only to survive the oxygen but to use it in the more efficient metabolism of foodstuffs.

[CONTINUED]

Sex and death evolved – processes that vastly increased the rate of natural selection. Some organisms evolved hard parts, climbed onto, and survived on the land. The pace of production of more complex forms accelerated. Flight evolved. Enormous four-legged beasts thundered across the steaming jungles. Small beasts emerged, born live, instead of in hard-shelled containers filled with replicas of the early oceans. They survived through swiftness and cunning – and increasingly long periods in which their knowledge was not so much preprogrammed in selfreplicating molecules as learned from parents and experiences.

All the while, the climate was variable. Slight variations in the output of sunlight, the orbital motion of the planet, clouds, oceans, and polar icecaps produced climatic changes – wiping out whole groups of organisms and causing the exuberant proliferation of other, once insignificant, groups.

And then … the Earth grew somewhat cold. The forests retreated. Small arboreal animals climbed down from the trees to seek a livelihood on the savannas. They became upright and tool-using. They communicated by producing compressional waves in the air with their eating and breathing organs. They discovered that organic material would, at a high enough temperature, combine with atmospheric oxygen to produce the stable hot plasma called fire. Postpartum learning was greatly accelerated by social interaction. Communal hunting developed, writing was invented, political structures evolved, superstition and science, religion and technology.

And then one day there came to be a creature whose genetic material was in no major way different from the self-replicating molecular collectives of any of the other organisms on his planet, which he called Earth. But he was able to ponder the mystery of his origins, the strange and tortuous path by which he had emerged from star-stuff. He was the matter of the cosmos, contemplating itself. He considered the problematical and enigmatic question of his future. He called himself Man. He was one of the starfolk. And he longed to return to the stars.

Eventually, many billions of years ago, a molecule was formed that had a remarkable capability. It was able to produce, out of the molecular building blocks of the surrounding waters, a fairly accurate copy of itself. In such a molecular system there is a set of instructions, a molecular code, containing the sequence of building blocks from which the larger molecule is constructed. When, by accident, there is a change in the sequence, the copy is likewise changed. Such a molecular system – capable of replication, mutation, and replication of its mutations – can be called "alive." It is a collection of molecules that can evolve by natural selection. Those molecules able to replicate faster, or to reprocess building blocks from their surroundings into a more useful variety, reproduced more efficiently than their competitors – and eventually dominated.

But conditions gradually changed. Hydrogen escaped to space. Production of the molecular building blocks declined. The foodstuffs formerly available in great abundance dwindled. Life was expelled from the molecular Garden of Eden. Only those simple collections of molecules able to transform their surroundings, able to produce efficient molecular machines for the conversion of simple into complex molecules, were able to survive. By isolating themselves from their surroundings, by maintaining the earlier idyllic conditions, those molecules that surrounded themselves by membranes had an advantage. The first cells arose.

With molecular building blocks no longer available for free, organisms had to work hard to make such building blocks. Plants are the result. Plants start with air and water, minerals and sunlight, and produce molecular building blocks of high complexity. Animals, such as human beings, are parasites on the plants.

Changing climate and competition among what was now a wide diversity of organisms produced greater and greater specialization, a sophistication of function, and an elaboration of form. A rich array of plants and animals began to cover the Earth. Out of the initial oceans in which life arose, new environments, such as the land and the air, were colonized. Organisms now live from the top of Mount Everest to the deepest portions of the abyssal depths. Organisms live in hot, concentrated solutions of sulfuric acid and in dry Antarctic valleys. Organisms live on the water adsorbed on a single crystal of salt.

Life forms developed that were finely attuned to their specific environments, exquisitely adapted to the conditions. But the conditions changed. The organisms were too specialized. They died. Other organisms were less well adapted, but they were more generalized. The conditions changed, the climate varied, but the organisms were able to continue. Many more species of organisms have died during the history of the Earth than are alive today. The secret of evolution is time and death.

Among the adaptations that seem to be useful is one that we call intelligence. Intelligence is an extension of an evolutionary tendency apparent in the simplest organisms – the tendency toward control of the environment. The standby biological method of control has been the hereditary material: Information passed on by nucleic acids from generation to generation – information on how to build a nest; information on the fear of falling, or of snakes, or of the dark; information on how to fly south for the winter. But intelligence requires information of an adaptive quality developed during the lifetime of a single individual. A variety of organisms on the Earth today have this quality we call intelligence: The dolphins have it, and so do the great apes. But it is most evident in the organism called Man.

In Man, not only is adaptive information acquired in the lifetime of a single individual, but it is passed on extra-genetically through learning, through books, through education. It is this, more than anything else, that has raised Man to his present pre-eminent status on the planet Earth.

We are the product of 4.5 billion years of fortuitous, slow, biological evolution. There is no reason to think that the evolutionary process has stopped. Man is a transitional animal. He is not the climax of creation.

There is a place with four suns in the sky – red, white, blue, and yellow; two of them are so close together that they touch, and star-stuff flows between them.

I know of a world with a million moons.

I know of a sun the size of the Earth – and made of diamond.

There are atomic nuclei a mile across that rotate thirty times a second.

There are tiny grains between the stars, with the size and atomic composition of bacteria.

There are stars leaving the Milky Way. There are immense gas clouds falling into the Milky Way.

There are turbulent plasmas writhing with X- and gamma-rays and mighty stellar explosions.

There are, perhaps, places outside our universe.

The universe is vast and awesome, and for the first time we are becoming a part of it.

The mathematician Freeman Dyson, of the Institute for Advanced Study, offers a scheme in which the planet Jupiter is broken down piece by piece, transported to the distance of the Earth from the Sun, and reconstructed into a spherical shell – a swarm of individual fragments revolving about the Sun. The advantage of Dyson's proposal is that all of the sunlight now wasted by not falling upon an inhabited planet could then be gainfully employed; and a population greatly in excess of that which now inhabits the Earth could be maintained. Whether such a vast population is desirable is an important and unsolved question. But what seems clear is that at the present rate of technological progress it will be possible to construct such a Dyson sphere in perhaps some thousands of years. In that case, other civilizations older than we may have already constructed such spherical swarms.

A Dyson sphere absorbs visible light from the Sun. But it does not continue indefinitely to absorb this light without re-radiating; otherwise, the temperature would become impossibly high. The exterior of the Dyson sphere radiates infrared radiation into space. Because of the large dimensions of the sphere, the infrared flux from a Dyson sphere should be detectable over quite sizable distances – with present infrared technology, over distances of hundreds to thousands of light-years. Remarkably enough, large infrared objects of roughly Solar System dimensions and of temperatures less than 1,000 degrees Fahrenheit have been detected in recent years. These, of course, are not necessarily Dyson civilizations. They may be vast dust clouds surrounding stars in the process of formation. But we are beginning to detect objects that are not dissimilar to the artifacts of advanced civilizations.

I was swimming in a large indoor pool with Peter. When I threw the pool's rubber ball to Peter (as was natural for me to have done), he dove under the ball as it hit the water and batted it with his snout accurately into my hands. After a few throws and precision returns, Peter's returns became increasingly inaccurate – forcing me to swim first to one side of the pool and then to the other in order to retrieve the ball. Eventually, it became clear that Peter chose not to place the ball within ten feet of me. He had changed the rules of the game.

Peter was performing a psychological experiment on me – to learn to what extreme lengths I would go to continue this pointless game of catch. It was the same kind of psychological testing that Elvar had conducted in our first meeting. Such testing is one clue to the bond that draws dolphins to humans: We are one of the few species that have pretensions of psychological knowledge; therefore, we are one of the few that would permit, however inadvertently, dolphins to perform psychological experiments on us.

There is another approach to the extraterrestrial hypothesis of UFO origins. This assessment depends on a large number of factors about which we know little, and a few about which we know literally nothing. I want to make some crude numerical estimate of the probability that we are frequently visited by extraterrestrial beings.

Now, there is a range of hypotheses that can be examined in such a way. Let me give a simple example: Consider the Santa Claus hypothesis, which maintains that, in a period of eight hours or so on December 24-25 of each year, an out-sized elf visits one hundred million homes in the United States. This is an interesting and widely discussed hypothesis. Some strong emotions ride on it, and it is argued that at least it does no harm.

We can do some calculations. Suppose that the elf in question spends one second per house. This isn't quite the usual picture – "Ho, Ho, Ho," and so on – but imagine that he is terribly efficient and very speedy; that would explain why nobody ever sees him very much – only one second per house, after all. With a hundred million houses he has to spend three years just filling stockings. I have assumed he spends no time at all in going from house to house. Even with relativistic reindeer, the time spent in a hundred million houses is three years and not eight hours. This is an example of hypothesis-testing independent of reindeer propulsion mechanisms or debates on the origins of elves. We examine the hypothesis itself, making very straightforward assumptions, and derive a result inconsistent with the hypothesis by many orders of magnitude. We would then suggest that the hypothesis is untenable.

For me, some of the most moving responses to the message are the works of art and poetry that it evoked. Mr. 'Aim Morhardt is a painter of water colors of the desert and sierras who lives in Bishop, California, where, perhaps not coincidentally, the giant Goldstone tracking station, which commands Pioneer 10, is located. Mr. Morhardt's poem follows:

Pioneer 10: The Golden Messenger.
The dragon prows that cruised the northern seas,
Questing adventure with the fighting clan;
The gallant mermaid bows blown down the breeze
On barquentine and slim-hulled merchantman;
All the discoverers of unknown lands
Gone in this winged age where naught remains
Of new strange treasure on some foreign strand,
So well-known earth, such charted routes and lanes.
Now the new figurehead of man appears,
Facing the vast immeasurable unknown,
Naked, star-sped, beyond the call of years,
Hand in hand, outward bound, and so alone.
Go, tiny messenger of our your race,
Touch, if you can, harbor in some far place.

Mr. Arvid F. Sponberg, of Belfast, Northern Ireland, writes: "The voyage of Pioneer 10 – and the voyages of those like her – will have an effect that poets, painters and musicians will not long ignore. The existence of the idea of Pioneer 10 is proof of this. The scientific mission of course is of incalculable value and interest, but the idea of the journey is of even greater imaginative value. Pioneer 10 brings closer the day when artists must confront man's new voyage as experience and not fantasy."

Mr. Sponberg composed for us a poem in sonnet form:

New Odyssey
Away, afar, beyond, bereft of kin,
Wayward, wandering, far ranging vagabonds,
Yearning, stardrawn, the Pioneers sweep on,
Outward bound, adrift on the solar wind.
A man, a woman, orphans of warm earth
Or splendid voyageurs with golden sails,
Or gypsies roaming ancient stellar trails,
A caravan in quest of celestial berth.
If, deep within cold interstellar space,
Some fearful eye spies life on this raft,
Will it perceive the heart within our craft,
A pulsar pounding out the rhythms of peace?
A spirit's starburst pierces new frontiers;
An Odyssey is our home; let us praise Pioneers!

There is, of course, the possibility that the message on Pioneer 10 – invented by human beings but directed at creatures of a very different kind – may prove ultimately mysterious to them. We think not. We think we have written the message – except for the man and woman – in a universal language. The extraterrestrials cannot possibly understand English or Russian or Chinese or Esperanto, but they must share with us common mathematics and physics and astronomy. I believe that they will understand, with no very great effort, this message written in the galactic language: "Scientific."