Cosmos - Carl Sagan (1980)


Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.

—Albertus Magnus, thirteenth century

We may mount from this dull Earth, and viewing it from on high, consider whether Nature has laid out all her cost and finery upon this small speck of Dirt. So, like Travellers into other distant countries, we shall be better able to judge of what’s done at home, know how to make a true estimate of, and set its own value upon every thing. We shall be less apt to admire what this World calls great, shall nobly despise those Trifles the generality of Men set their Affections on, when we know that there are a multitude of such Earths inhabited and adorn’d as well as our own.

—Christiaan Huygens,
The Celestial Worlds Discovered, c. 1690

This is the time when humans have begun to sail the sea of space. The modern ships that ply the Keplerian trajectories to the planets are unmanned. They are beautifully constructed, semi-intelligent robots exploring unknown worlds. Voyages to the outer solar system are controlled from a single place on the planet Earth, the Jet Propulsion Laboratory (JPL) of the National Aeronautics and Space Administration in Pasadena, California.

On July 9, 1979, a spacecraft called Voyager 2 encountered the Jupiter system. It had been almost two years sailing through interplanetary space. The ship is made of millions of separate parts assembled redundantly, so that if some component fails, others will take over its responsibilities. The spacecraft weighs 0.9 tons and would fill a large living room. Its mission takes it so far from the sun that it cannot be powered by solar energy, as other spacecraft are. Instead, Voyager relies on a small nuclear power plant, drawing hundreds of watts from the radioactive decay of a pellet of plutonium. Its three integrated computers and most of its house-keeping functions—for example, its temperature-control system—are localized in its middle. It receives commands from Earth and radios its findings back to Earth through a large antenna, 3.7 meters in diameter. Most of its scientific instruments are on a scan platform, which tracks Jupiter or one of its moons as the spacecraft hurtles past. There are many scientific instruments—ultraviolet and infrared spectrometers, devices to measure charged particles and magnetic fields and the radio emission from Jupiter—but the most productive have been the two television cameras, designed to take tens of thousands of pictures of the planetary islands in the outer solar system.

Jupiter is surrounded by a shell of invisible but extremely dangerous high-energy charged particles. The spacecraft must pass through the outer edge of this radiation belt to examine Jupiter and its moons close up, and to continue its mission to Saturn and beyond. But the charged particles can damage the delicate instruments and fry the electronics. Jupiter is also surrounded by a ring of solid debris, discovered four months earlier by Voyager 1, which Voyager 2 had to traverse. A collision with a small boulder could have sent the spacecraft tumbling wildly out of control, its antenna unable to lock on the Earth, its data lost forever. Just before encounter, the mission controllers were restive. There were some alarms and emergencies, but the combined intelligence of the humans on Earth and the robot in space circumvented disaster.

Launched on August 20, 1977, it moved on an arcing trajectory past the orbit of Mars, through the asteroid belt, to approach the Jupiter system and thread its way past the planet and among its fourteen or so moons. Voyager’s passage by Jupiter accelerated it toward a close encounter with Saturn. Saturn’s gravity will propel it on to Uranus. After Uranus it will plunge on past Neptune, leaving the solar system, becoming an interstellar spacecraft, fated to roam forever the great ocean between the stars.

These voyages of exploration and discovery are the latest in a long series that have characterized and distinguished human history. In the fifteenth and sixteenth centuries you could travel from Spain to the Azores in a few days, the same time it takes us now to cross the channel from the Earth to the Moon. It took then a few months to traverse the Atlantic Ocean and reach what was called the New World, the Americas. Today it takes a few months to cross the ocean of the inner solar system and make planet-fall on Mars or Venus, which are truly and literally new worlds awaiting us. In the seventeenth and eighteenth centuries you could travel from Holland to China in a year or two, the time it has taken Voyager to travel from Earth to Jupiter.* The annual costs were, relatively, more then than now, but in both cases less than 1 percent of the appropriate Gross National Product. Our present spaceships, with their robot crews, are the harbingers, the vanguards of future human expeditions to the planets. We have traveled this way before.

The fifteenth through seventeenth centuries represent a major turning point in our history. It then became clear that we could venture to all parts of our planet. Plucky sailing vessels from half a dozen European nations dispersed to every ocean. There were many motivations for these journeys: ambition, greed, national pride, religious fanaticism, prison pardons, scientific curiosity, the thirst for adventure and the unavailability of suitable employment in Estremadura. These voyages worked much evil as well as much good. But the net result has been to bind the Earth together, to decrease provincialism, to unify the human species and to advance powerfully our knowledge of our planet and ourselves.

Emblematic of the epoch of sailing-ship exploration and discovery is the revolutionary Dutch Republic of the seventeenth century. Having recently declared its independence from the powerful Spanish Empire, it embraced more fully than any other nation of its time the European Enlightenment. It was a rational, orderly, creative society. But because Spanish ports and vessels were closed to Dutch shipping, the economic survival of the tiny republic depended on its ability to construct, man and deploy a great fleet of commercial sailing vessels.

The Dutch East India Company, a joint governmental and private enterprise, sent ships to the far corners of the world to acquire rare commodities and resell them at a profit in Europe. Such voyages were the life blood of the Republic. Navigational charts and maps were classified as state secrets. Ships often embarked with sealed orders. Suddenly the Dutch were present all over the planet. The Barents Sea in the Arctic Ocean and Tasmania in Australia are named after Dutch sea captains. These expeditions were not merely commercial exploitations, although there was plenty of that. There were powerful elements of scientific adventure and the zest for discovery of new lands, new plants and animals, new people; the pursuit of knowledge for its own sake.

The Amsterdam Town Hall reflects the confident and secular self-image of seventeenth-century Holland. It took shiploads of marble to build. Constantijn Huygens, a poet and diplomat of the time, remarked that the Town Hall dispelled “the Gothic squint and squalor.” In the Town Hall to this day, there is a statue of Atlas supporting the heavens, festooned with constellations. Beneath is Justice, brandishing a golden sword and scales, standing between Death and Punishment, and treading underfoot Avarice and Envy, the gods of the merchants. The Dutch, whose economy was based on private profit, nevertheless understood that the unrestrained pursuit of profit posed a threat to the nation’s soul.

A less allegorical symbol may be found under Atlas and Justice, on the floor of the Town Hall. It is a great inlaid map, dating from the late seventeenth or early eighteenth centuries, reaching from West Africa to the Pacific Ocean. The whole world was Holland’s arena. And on this map, with disarming modesty the Dutch omitted themselves, using only the old Latin name Belgium for their part of Europe.

In a typical year many ships set sail halfway around the world. Down the west coast of Africa, through what they called the Ethiopian Sea, around the south coast of Africa, within the Straits of Madagascar, and on past the southern tip of India they sailed, to one major focus of their interests, the Spice Islands, present-day Indonesia. Some expeditions journeyed from there to a land named New Holland, and today called Australia. A few ventured through the Straits of Malacca, past the Philippines, to China. We know from a mid-seventeenth-century account of an “Embassy from the East India Company of the United Provinces of the Netherlands, to the Grand Tartar, Cham, Emperor of China.” The Dutch burgers, ambassadors and sea captains stood wide-eyed in amazement, face to face with another civilization in the Imperial City of Peking.*

Never before or since has Holland been the world power it was then. A small country, forced to live by its wits, its foreign policy contained a strong pacifist element. Because of its tolerance for unorthodox opinions, it was a haven for intellectuals who were refugees from censorship and thought control elsewhere in Europe—much as the United States benefitted enormously in the 1930’s by the exodus of intellectuals from the Nazi-dominated Europe. So seventeenth-century Holland was the home of the great Jewish philosopher Spinoza, whom Einstein admired; of Descartes, a pivotal figure in the history of mathematics and philosophy; and of John Locke, a political scientist who influenced a group of philosophically inclined revolutionaries named Paine, Hamilton, Adams, Franklin and Jefferson. Never before or since has Holland been graced by such a galaxy of artists and scientists, philosophers and mathematicians. This was the time of the master painters Rembrandt and Vermeer and Frans Halls; of Leeuwenhoek, the inventor of the microscope; of Grotius, the founder of international law, of Willebrord Snellius, who discovered the law of the refraction of light.

In the Dutch tradition of encouraging freedom of thought, the University of Leiden offered a professorship to an Italian scientist named Galileo, who had been forced by the Catholic Church under threat of torture to recant his heretical view that the Earth moved about the Sun and not vice versa.* Galileo had close ties with Holland, and his first astronomical telescope was an improvement of a spyglass of Dutch design. With it he discovered sunspots, the phases of Venus, the craters of the Moon, and the four large moons of Jupiter now called, after him, the Galilean satellites. Galileo’s own description of his ecclesiastical travails is contained in a letter he wrote in the year 1615 to the Grand Duchess Christina:

Some years ago as Your Serene Highness well knows, I discovered in the heavens many things that had not been seen before our own age. The novelty of these things, as well as some consequences which followed from them in contradiction to the physical notions commonly held among academic philosophers, stirred up against me no small number of professors [many of them ecclesiastics]—as if I had placed these things in the sky with my own hands in order to upset Nature and overturn the sciences. They seemed to forget that the increase of known truths stimulates the investigation, establishment, and growth of the arts.

The connection between Holland as an exploratory power and Holland as an intellectual and cultural center was very strong. The improvement of sailing ships encouraged technology of all kinds. People enjoyed working with their hands. Inventions were prized. Technological advance required the freest possible pursuit of knowledge, so Holland became the leading publisher and bookseller in Europe, translating works written in other languages and permitting the publication of works proscribed elsewhere. Adventures into exotic lands and encounters with strange societies shook complacency, challenged thinkers to reconsider the prevailing wisdom and showed that ideas that had been accepted for thousands of years—for example, on geography—were fundamentally in error. In a time when kings and emperors ruled much of the world, the Dutch Republic was governed, more than any other nation, by the people. The openness of the society and its encouragement of the life of the mind, its material well-being and its commitment to the exploration and utilization of new worlds generated a joyful confidence in the human enterprise.*

In Italy, Galileo had announced other worlds, and Giordano Bruno had speculated on other lifeforms. For this they had been made to suffer brutally. But in Holland, the astronomer Christiaan Huygens, who believed in both, was showered with honors. His father was Constantijn Huygens, a master diplomat of the age, a litterateur, poet, composer, musician, close friend and translator of the English poet John Donne, and the head of an archetypical great family. Constantijn admired the painter Rubens, and “discovered” a young artist named Rembrandt van Rijn, in several of whose works he subsequently appears. After their first meeting, Descartes wrote of him: “I could not believe that a single mind could occupy itself with so many things, and equip itself so well in all of them.” The Huygens home was filled with goods from all over the world. Distinguished thinkers from other nations were frequent guests. Growing up in this environment, the young Christiaan Huygens became simultaneously adept in languages, drawing, law, science, engineering, mathematics and music. His interests and allegiances were broad. “The world is my country,” he said, “science my religion.”

Light was a motif of the age: the symbolic enlightenment of freedom of thought and religion, of geographical discovery; the light that permeated the paintings of the time, particularly the exquisite work of Vermeer; and light as an object of scientific inquiry, as in Snell’s study of refraction, Leeuwenhoek’s invention of the microscope and Huygens’ own wave theory of light.* These were all connected activities, and their practitioners mingled freely. Vermeer’s interiors are characteristically filled with nautical artifacts and wall maps. Microscopes were drawing-room curiosities. Leeuwenhoek was the executor of Vermeer’s estate and a frequent visitor at the Huygens home in Hofwijck.

Leeuwenhoek’s microscope evolved from the magnifying glasses employed by drapers to examine the quality of cloth. With it he discovered a universe in a drop of water: the microbes, which he described as “animalcules” and thought “cute.” Huygens had contributed to the design of the first microscopes and himself made many discoveries with them. Leeuwenhoek and Huygens were among the first people ever to see human sperm cells, a prerequisite for understanding human reproduction. To explain how microorganisms slowly develop in water previously sterilized by boiling, Huygens proposed that they were small enough to float through the air and reproduced on alighting in water. Thus he established an alternative to spontaneous generation—the notion that life could rise, in fermenting grape juice or rotting meat, entirely independent of preexisting life. It was not until the time of Louis Pasteur, two centuries later, that Huygens’ speculation was proved correct. The Viking search for life on Mars can be traced in more ways than one back to Leeuwenhoek and Huygens. They are also the grandfathers of the germ theory of disease, and therefore of much of modern medicine. But they had no practical motives in mind. They were merely tinkering in a technological society.

The microscope and telescope, both developed in early seventeenth-century Holland, represent an extension of human vision to the realms of the very small and the very large. Our observations of atoms and galaxies were launched in this time and place. Christiaan Huygens loved to grind and polish lenses for astronomical telescopes and constructed one five meters long. His discoveries with the telescope would by themselves have ensured his place in the history of human accomplishment. In the footsteps of Eratosthenes, he was the first person to measure the size of another planet. He was also the first to speculate that Venus is completely covered with clouds; the first to draw a surface feature on the planet Mars (a vast dark windswept slope called Syrtis Major); and by observing the appearance and disappearance of such features as the planet rotated, the first to determine that the Martian day was, like ours, roughly twenty-four hours long. He was the first to recognize that Saturn was surrounded by a system of rings which nowhere touches the planet.* And he was the discoverer of Titan, the largest moon of Saturn and, as we now know, the largest moon in the solar system—a world of extraordinary interest and promise. Most of these discoveries he made in his twenties. He also thought astrology was nonsense.

Huygens did much more. A key problem for marine navigation in this age was the determination of a longitude. Latitude could easily be determined by the stars—the farther south you were, the more southern constellations you could see. But longitude required precise timekeeping. An accurate shipboard clock would tell the time in your home port; the rising and setting of the Sun and stars would specify the local shipboard time; and the difference between the two would yield your longitude. Huygens invented the pendulum clock (its principle had been discovered earlier by Galileo), which was then employed, although not fully successfully, to calculate position in the midst of the great ocean. His efforts introduced an unprecedented accuracy in astronomical and other nautical clocks. He invented the spiral balance spring still used in some watches today; made fundamental contributions to mechanics—e.g., the calculation of centrifugal force—and, from a study of the game of dice, to the theory of probability. He improved the air pump, which was later to revolutionize the mining industry, and the “magic lantern,” the ancestor of the slide projector. He also invented something called the “gunpowder engine,” which influenced the development of another machine, the steam engine.

A detail from Christiaan Huygens’ Systema Saturnium, published in 1659. Shown is his (correct) explanation of the changing appearance of the rings of Saturn over the years as the relative geometry of Earth and Saturn changes. In position B the comparatively paper-thin rings disappear as they are seen edge-on. In position A they display their maximum extent visible from Earth, the configuration that caused Galileo, with a significantly inferior telescope, considerable consternation.

Huygens was delighted that the Copernican view of the Earth as a planet in motion around the Sun was widely accepted even by the ordinary people in Holland. Indeed, he said, Copernicus was acknowledged by all astronomers except those who “were a bit slow-witted or under the superstitions imposed by merely human authority.” In the Middle Ages, Christian philosophers were fond of arguing that, since the heavens circle the Earth once every day, they can hardly be infinite in extent; and therefore an infinite number of worlds, or even a large number of them (or even one other of them), is impossible. The discovery that the Earth is turning rather than the sky moving had important implications for the uniqueness of the Earth and the possiblity of life elsewhere. Copernicus held that not just the solar system but the entire universe was heliocentric, and Kepler denied that the stars have planetary systems. The first person to make explicit the idea of a large—indeed, an infinite—number of other worlds in orbit about other suns seems to have been Giordano Bruno. But others thought that the plurality of worlds followed immediately from the ideas of Copernicus and Kepler and found themselves aghast. In the early seventeenth century, Robert Merton contended that the heliocentric hypothesis implied a multitude of other planetary systems, and that this was an argument of the sort called reductio ad absurdum (Appendix 1), demonstrating the error of the initial assumption. He wrote, in an argument which may once have seemed withering,

For if the firmament be of such an incomparable bigness, as these Copernical giants will have it …, so vast and full of innumerable stars, as being infinite in extent … why may we not suppose … those infinite stars visible in the firmament to be so many suns, with particular fixed centers; to have likewise their subordinate planets, as the sun hath his dancing still around him?… And so, in consequence, there are infinite habitable worlds; what hinders?… these and suchlike insolent and bold attempts, prodigious paradoxes, inferences must needs follow, if it once be granted which … Kepler … and others maintain of the Earth’s motion.

But the Earth does move. Merton, if he lived today, would be obliged to deduce “infinite, habitable worlds.” Huygens did not shrink from this conclusion; he embraced it gladly: Across the sea of space the stars are other suns. By analogy with our solar system, Huygens reasoned that those stars should have their own planetary systems and that many of these planets might be inhabited: “Should we allow the planets nothing but vast deserts … and deprive them of all those creatures that more plainly bespeak their divine architect, we should sink them below the Earth in beauty and dignity, a thing very unreasonable.”*

These ideas were set forth in an extraordinary book bearing the triumphant title The Celestial Worlds Discover’d: Conjectures Concerning the Inhabitants, Plants and Productions of the Worlds in the Planets. Composed shortly before Huygens died in 1690, the work was admired by many, including Czar Peter the Great, who made it the first product of Western science to be published in Russia. The book is in large part about the nature or environments of the planets. Among the figures in the finely rendered first edition is one in which we see, to scale, the Sun and the giant planets Jupiter and Saturn. They are, comparatively, rather small. There is also an etching of Saturn next to the Earth: Our planet is a tiny circle.

By and large Huygens imagined the environments and inhabitants of other planets to be rather like those of seventeenth-century Earth. He conceived of “planetarians” whose “whole Bodies, and every part of them, may be quite distinct and different from ours … ’tis a very ridiculous opinion … that it is impossible a rational Soul should dwell in any other shape than ours.” You could be smart, he was saying, even if you looked peculiar. But he then went on to argue that they would not look very peculiar—that they must have hands and feet and walk upright, that they would have writing and geometry, and that Jupiter has its four Galilean satellites to provide a navigational aid for the sailors in the Jovian oceans. Huygens was, of course, a citizen of his time. Who of us is not? He claimed science as his religion and then argued that the planets must be inhabited because otherwise God had made worlds for nothing. Because he lived before Darwin, his speculations about extraterrestrial life are innocent of the evolutionary perspective. But he was able to develop on observational grounds something akin to the modern cosmic perspective:

What a wonderful and Amazing scheme have we here of the magnificant vastness of the universe … So many Suns, so many Earths … and every one of them stock’d with so many Herbs, Trees, and Animals, adorn’d with so many Seas and Mountains!… And how must our Wonder and Admiration be increased when we consider the prodigious Distance and Multitude of the Stars.

The Voyager spacecraft are the lineal descendants of those sailing-ship voyages of exploration, and of the scientific and speculative tradition of Christiaan Huygens. The Voyagers are caravels bound for the stars, and on the way exploring those worlds that Huygens knew and loved so well.

One of the main commodities returned on those voyages of centuries ago were travelers’ tales,* stories of alien lands and exotic creatures that evoked our sense of wonder and stimulated future exploration. There had been accounts of mountains that reached the sky; of dragons and sea monsters; of everyday eating utensils made of gold; of a beast with an arm for a nose; of people who thought the doctrinal disputes among Protestants, Catholics, Jews and Muslims to be silly; of a black stone that burned; of headless humans with mouths in their chests; of sheep that grew on trees. Some of these stories were true, some were lies. Others had a kernel of truth, misunderstood or exaggerated by the explorers or their informants. In the hands of Voltaire, say, or Jonathan Swift, these accounts stimulated a new perspective on European society, forcing a reconsideration of that insular world.

Modern Voyagers also return travelers’ tales, tales of a world shattered like a crystal sphere; a globe where the ground is covered, pole to pole, with what looks like a network of cobwebs; tiny moons shaped like potatoes; a world with an underground ocean; a land that smells of rotten eggs and looks like a pizza pie, with lakes of molten sulfur and volcanic eruptions ejecting smoke directly into space; a planet called Jupiter that dwarfs our own—so large that 1,000 Earths would fit within it.

The Galilean satellites of Jupiter are each almost as big as the planet Mercury. We can measure their sizes and masses and so calculate their density, which tells us something about the composition of their interiors. We find that the inner two, Io and Europa, have a density as high as rock. The outer two, Ganymede and Callisto, have a much lower density, halfway between rock and ice. But the mixture of ice and rocks within these outer moons must contain, as do rocks on Earth, traces of radioactive minerals, which heat their surroundings. There is no effective way for this heat, accumulated over billions of years, to reach the surface and be lost to space, and the radioactivity inside Ganymede and Callisto must therefore melt their icy interiors. We anticipate underground oceans of slush and water in these moons, a hint, before we have ever seen the surfaces of the Galilean satellites close up, that they may be very different one from another. When we do look closely, through the eyes of Voyager, this prediction is confirmed. They do not resemble each other. They are different from any worlds we have ever seen before.

The Voyager 2 spacecraft will never return to Earth. But its scientific findings, its epic discoveries, its travelers’ tales, do return. Take July 9, 1979, for instance. At 8:04 Pacific Standard Time on this morning, the first pictures of a new world, called Europa after an old one, were received on Earth.

How does a picture from the outer solar system get to us? Sunlight shines on Europa in its orbit around Jupiter and is reflected back to space, where some of it strikes the phosphors of the Voyager television cameras, generating an image. The image is read by the Voyager computers, radioed back across the immense intervening distance of half a billion kilometers to a radio telescope, a ground station on the Earth. There is one in Spain, one in the Mojave Desert of Southern California and one in Australia. (On that July morning in 1979 it was the one in Australia that was pointed toward Jupiter and Europa.) It then passses the information via a communications satellite in Earth orbit to Southern California, where it is transmitted by a set of microwave relay towers to a computer at the Jet Propulsion Laboratory, where it is processed. The picture is fundamentally like a newspaper wirephoto, made of perhaps a million individual dots, each a different shade of gray, so fine and close together that at a distance the constituent dots are invisible. We see only their cumulative effect. The information from the spacecraft specifies how bright or dark each dot is to be. After processing, the dots are then stored on a magnetic disc, something like a phonograph record. There are some eighteen thousand photographs taken in the Jupiter system by Voyager 1 that are stored on such magnetic discs, and an equivalent number for Voyager 1. Finally the end product of this remarkable set of links and relays is a thin piece of glossy paper, in this case showing the wonders of Europa, recorded, processed and examined for the first time in human history on July 9, 1979.

What we saw on such pictures was absolutely astonishing. Voyager 1 obtained excellent imagery of the other three Galilean satellites of Jupiter. But not Europa. It was left for Voyager 2 to acquire the first close-up pictures of Europa, where we see things that are only a few kilometers across. At first glance, the place looks like nothing so much as the canal network that Percival Lowell imagined to adorn Mars, and that, we now know from space vehicle exploration, does not exist at all. We see on Europa an amazing, intricate network of intersecting straight and curved lines. Are they ridges—that is, raised? Are they troughs—that is, depressed? How are they made? Are they part of a global tectonic system, produced perhaps by fracturing of an expanding or contracting planet? Are they connected with plate tectonics on the Earth? What light do they shed on the other satellites of the Jovian system? At the moment of discovery, the vaunted technology has produced something astonishing. But it remains for another device, the human brain, to figure it out. Europa turns out to be as smooth as a billiard ball despite the network of lineations. The absence of impact craters may be due to the heating and flow of surface ice upon impact. The lines are grooves or cracks, their origin still being debated long after the mission.

If the Voyager missions were manned, the captain would keep a ship’s log, and the log, a combination of the events of Voyagers 1 and 2, might read something like this:

Day 1 After much concern about provisions and instruments, which seemed to be malfunctioning, we successfully lifted off from Cape Canaveral on our long journey to the planets and the stars.

Day 2 A problem in the deployment of the boom that supports the science scan platform. If the problem is not solved, we will lose most of our pictures and other scientific data.

Day 13 We have looked back and taken the first photograph ever obtained of the Earth and Moon as worlds together in space. A pretty pair.

Day 150 Engines fired nominally for a mid-course trajectory correction.

Day 170 Routine housekeeping functions. An uneventful few months.

Day 185 Successful calibration images taken of Jupiter.

Day 207 Boom problem solved, but failure of main radio transmitter. We have moved to back-up transmitter. If it fails, no one on Earth will ever hear from us again.

Day 215 We cross the orbit of Mars. That planet itself is on the other side of the Sun.

Day 295 We enter the asteroid belt. There are many large, tumbling boulders here, the shoals and reefs of space. Most of them are uncharted. Lookouts posted. We hope to avoid a collision.

Day 475 We safely emerge from the main asteroid belt, happy to have survived.

Day 570 Jupiter is becoming prominent in the sky. We can now make out finer detail on it than the largest telescopes on Earth have ever obtained.

Day 615 The colossal weather systems and changing clouds of Jupiter, spinning in space before us, have us hypnotized. The planet is immense. It is more than twice as massive as all the other planets put together. There are no mountains, valleys, volcanoes, rivers; no boundaries between land and air; just a vast ocean of dense gas and floating clouds—a world without a surface. Everything we can see on Jupiter is floating in its sky.

Day 630 The weather on Jupiter continues to be spectacular. This ponderous world spins on its axis in less than ten hours. Its atmospheric motions are driven by the rapid rotation, by sunlight and by the heat bubbling and welling up from its interior.

Day 640 The cloud patterns are distinctive and gorgeous. They remind us a little of Van Gogh’s Starry Night, or works by William Blake or Edvard Munch. But only a little. No artist ever painted like this because none of them ever left our planet. No painter trapped on Earth ever imagined a world so strange and lovely.

We observe the multicolored belts and bands of Jupiter close up. The white bands are thought to be high clouds, probably ammonia crystals; the brownish-colored belts, deeper and hotter places where the atmosphere is sinking. The blue places are apparently deep holes in the overlying clouds through which we see clear sky.

We do not know the reason for the reddish-brown color of Jupiter. Perhaps it is due to the chemistry of phosphorus or sulfur. Perhaps it is due to complex brightly colored organic molecules produced when ultraviolet light from the Sun breaks down the methane, ammonia, and water in the Jovian atmosphere and the molecular fragments recombine. In that case, the colors of Jupiter speak to us of chemical events that four billion years ago back on Earth led to the origin of life.

Day 647 The Great Red Spot. A great column of gas reaching high above the adjacent clouds, so large that it could hold half a dozen Earths. Perhaps it is red because it is carrying up to view the complex molecules produced or concentrated at greater depth. It may be a great storm system a million years old.

Day 650 Encounter. A day of wonders. We successfully negotiate the treacherous radiation belts of Jupiter with only one instrument, the photopolarimeter, damaged. We accomplish the ring plane crossing and suffer no collisions with the particles and boulders of the newly discovered rings of Jupiter. And wonderful images of Amalthea, a tiny, red, oblong world that lives in the heart of the radiation belt; of multicolored Io; of the linear markings on Europa; the cobwebby features of Ganymede; the great multi-ringed basin on Callisto. We round Callisto and pass the orbit of Jupiter 13, the outermost of the planet’s known moons. We are outward bound.

Day 662 Our particle and field detectors indicate that we have left the Jovian radiation belts. The planet’s gravity has boosted our speed. We are free of Jupiter at last and sail again the sea of space.

Day 874 A loss of the ship’s lock on the star Canopus—in the lore of constellations the rudder of a sailing vessel. It is our rudder too, essential for the ship’s orientation in the dark of space, to find our way through this unexplored part of the cosmic ocean. Canopus lock reacquired. The optical sensors seem to have mistaken Alpha and Beta Centauri for Canopus. Next port of call, two years hence: the Saturn system.

Of all the travelers’ tales returned by Voyager, my favorites concern the discoveries made on the innermost Galilean satellite, Io.* Before Voyager, we were aware of something strange about Io. We could resolve few features on its surface, but we knew it was red—extremely red, redder than Mars, perhaps the reddest object in the solar system. Over a period of years something seemed to be changing on it, in infrared light and perhaps in its radar reflection properties. We also know that partially surrounding Jupiter in the orbital position of Io was a great doughnut-shaped tube of atoms, sulfur and sodium and potassium, material somehow lost from Io.

When Voyager approached this giant moon we found a strange multicolored surface unlike any other in the solar system. Io is near the asteroid belt. It must have been thoroughly pummeled throughout its history by falling boulders. Impact craters must have been made. Yet there were none to be seen. Accordingly, there had to be some process on Io that was extremely efficient in rubbing craters out or filling them in. The process could not be atmospheric, since Io’s atmosphere has mostly escaped to space because of its low gravity. It could not be running water; Io’s surface is far too cold. There were a few places that resembled the summits of volcanoes. But it was hard to be sure.

Linda Morabito, a member of the Voyager Navigation Team responsible for keeping Voyager precisely on its trajectory, was routinely ordering a computer to enhance an image of the edge of Io, to bring out the stars behind it. To her astonishment, she saw a bright plume standing off in the darkness from the satellite’s surface and soon determined that the plume was in exactly the position of one of the suspected volcanoes. Voyager had discovered the first active volcano beyond the Earth. We know now of nine large volcanoes, spewing out gas and debris, and hundreds—perhaps thousands—of extinct volcanoes on Io. The debris, rolling and flowing down the sides of the volcanic mountains, arching in great jets over the polychrome landscape, is more than enough to cover the impact craters. We are looking at a fresh planetary landscape, a surface newly hatched. How Galileo and Huygens would have marveled.

The volcanoes of Io were predicted, before they were discovered, by Stanton Peale and his co-workers, who calculated the tides that would be raised in the solid interior of Io by the combined pulls of the nearby moon Europa and the giant planet Jupiter. They found that the rocks inside Io should have been melted, not by radioactivity but by tides; that much of the interior of Io should be liquid. It now seems likely that the volcanoes of Io are tapping an underground ocean of liquid sulfur, melted and concentrated near the surface. When solid sulfur is heated a little past the normal boiling point of water, to about 115°C, it melts and changes color. The higher the temperature, the deeper the color. If the molten sulfur is quickly cooled, it retains its color. The pattern of colors that we see on Io resembles closely what we would expect if rivers and torrents and sheets of molten sulfur were pouring out of the mouths of the volcanoes: black sulfur the hottest, near the top of the volcano; red and orange, including the rivers, nearby; and great plains covered by yellow sulfur at a greater remove. The surface of Io is changing on a time scale of months. Maps will have to be issued regularly, like weather reports on Earth. Those future explorers on Io will have to keep their wits about them.

The very thin and tenuous atmosphere of Io was found by Voyager to be composed mainly of sulfur dioxide. But this thin atmosphere can serve a useful purpose, because it may be just thick enough to protect the surface from the intense charged particles in the Jupiter radiation belt in which Io is embedded. At night the temperature drops so low that the sulfur dioxide should condense out as a kind of white frost; the charged particles would then immolate the surface, and it would probably be wise to spend the nights just slightly underground.

The great volcanic plumes of Io reach so high that they are close to injecting their atoms directly into the space around Jupiter. The volcanoes are the probable source of the great doughnut-shaped ring of atoms that surrounds Jupiter in the position of Io’s orbit. These atoms, gradually spiraling in toward Jupiter, should coat the inner moon Amalthea and may be responsible for its reddish coloration. It is even possible that the material outgassed from Io contributes, after many collisions and condensations, to the ring system of Jupiter.

A substantial human presence on Jupiter itself is much more difficult to imagine—although I suppose great balloon cities permanently floating in its atmosphere are a technological possibility for the remote future. As seen from the near sides of Io or Europa, that immense and variable world fills much of the sky, hanging aloft, never to rise or set, because almost every satellite in the solar system keeps a constant face to its planet, as the Moon does to the Earth. Jupiter will be a source of continuing provocation and excitement for the future human explorers of the Jovian moons.

As the solar system condensed out of instellar gas and dust, Jupiter acquired most of the matter that was not ejected into interstellar space and did not fall inward to form the Sun. Had Jupiter been several dozen times more massive, the matter in its interior would have undergone thermonuclear reactions, and Jupiter would have begun to shine by its own light. The largest planet is a star that failed. Even so, its interior temperatures are sufficiently high that it gives off about twice as much energy as it receives from the Sun. In the infrared part of the spectrum, it might even be correct to consider Jupiter a star. Had it become a star in visible light, we would today inhabit a binary or double-star system, with two suns in our sky, and the nights would come more rarely—a commonplace, I believe, in countless solar systems throughout the Milky Way Galaxy. We would doubtless think the circumstances natural and lovely.

Deep below the clouds of Jupiter the weight of the overlying layers of atmosphere produces pressures much higher than any found on Earth, pressures so great that electrons are squeezed off hydrogen atoms, producing a remarkable substance, liquid metallic hydrogen—a physical state that has never been achieved on Earth. (There is some hope that metallic hydrogen is a superconductor at moderate temperatures. If it could be manufactured on Earth, it would work a revolution in electronics.) In the interior of Jupiter, where the pressures are about three million times the atmospheric pressure at the surface of the Earth, there is almost nothing but a great dark sloshing ocean of metallic hydrogen. But at the very core of Jupiter there may be a lump of rock and iron, an Earth-like world in a pressure vise, hidden forever at the center of the largest planet.

The electrical currents in the liquid metal interior of Jupiter may be the source of the planet’s enormous magnetic field, the largest in the solar system, and of its associated belt of trapped electrons and protons. These charged particles are ejected from the Sun in the solar wind and captured and accelerated by Jupiter’s magnetic field. Vast numbers of them are trapped far above the clouds and are condemned to bounce from pole to pole until by chance they encounter some high-altitude atmospheric molecule and are removed from the radiation belt. Io moves in an orbit so close to Jupiter that it plows through the midst of this intense radiation, creating cascades of charged particles, which in turn generate violent bursts of radio energy. (They may also influence eruptive processes on the surface of Io.) It is possible to predict radio bursts from Jupiter with better reliability than weather forecasts on Earth, by computing the position of Io.

That Jupiter is a source of radio emission was discovered accidentally in the 1950’s, the early days of radio astronomy. Two young Americans, Bernard Burke and Kenneth Franklin, were examining the sky with a newly constructed and for that time very sensitive radio telescope. They were searching the cosmic radio background—that is, radio sources far beyond our solar system. To their surprise, they found an intense and previously unreported source that seemed to correspond to no prominent star, nebula or galaxy. What is more, it gradually moved, with respect to the distant stars, much faster than any remote object could.* After finding no likely explanation of all this in their charts of the distant Cosmos, they one day stepped outside the observatory and looked up at the sky with the naked eye to see if anything interesting happened to be there. Bemusedly they noted an exceptionally bright object in the right place, which they soon identified as the planet Jupiter. This accidental discovery is, incidentally, entirely typical of the history of science.

Every evening before Voyager l’s encounter with Jupiter, I could see that giant planet twinkling in the sky, a sight our ancestors have enjoyed and wondered at for a million years. And on the evening of Encounter, on my way to study the Voyager data arriving at JPL, I thought that Jupiter would never be the same, never again just a point of light in the night sky, but would forever after be a place to be explored and known. Jupiter and its moons are a kind of miniature solar system and exquisite worlds with much to teach us.

In composition and in many other respects Saturn is similar to Jupiter, although smaller. Rotating once every ten hours, it exhibits colorful equatorial banding, which is, however, not so prominent as Jupiter’s. It has a weaker magnetic field and radiation belt than Jupiter and a more spectacular set of circumplanetary rings. And it also is surrounded by a dozen or more satellites.

The most interesting of the moons of Saturn seems to be Titan, the largest moon in the solar system and the only one with a substantial atmosphere. Prior to the encounter of Voyager 1 with Titan in November 1980, our information about Titan was scanty and tantalizing. The only gas known unambiguously to be present was methane, CH4, discovered by G. P. Kuiper. Ultraviolet light from the sun converts methane to more complex hydrocarbon molecules and hydrogen gas. The hydrocarbons should remain on Titan, covering the surface with a brownish tarry organic sludge, something like that produced in experiments on the origin of life on Earth. The lightweight hydrogen gas should, because of Titan’s low gravity, rapidly escape to space by a violent process known as “blowoff,” which should carry the methane and other atmospheric constituents with it. But Titan has an atmospheric pressure at least as great as that of the planet Mars. Blowoff does not seem to be happening. Perhaps there is some major and as yet undiscovered atmospheric constituent—nitrogen, for example—which keeps the average molecular weight of the atmosphere high and prevents blowoff. Or perhaps blowoff is happening, but the gases lost to space are being replenished by others released from the satellite’s interior. The bulk density of Titan is so low that there must be a vast supply of water and other ices, probably including methane, which are at unknown rates being released to the surface by internal heating.

When we examine Titan through the telescope we see a barely perceptible reddish disc. Some observers have reported variable white clouds above that disc—most likely, clouds of methane crystals. But what is responsible for the reddish coloration? Most students of Titan agree that complex organic molecules are the most likely explanation. The surface temperature and atmospheric thickness are still under debate. There have been some hints of an enhanced surface temperature due to an atmospheric greenhouse effect. With abundant organic molecules on its surface and in its atmosphere, Titan is a remarkable and unique denizen of the solar system. The history of our past voyages of discovery suggests that Voyager and other spacecraft reconnaissance missions will revolutionize our knowledge of this place.

Through a break in the clouds of Titan, you might glimpse Saturn and its rings, their pale yellow color diffused by the intervening atmosphere. Because the Saturn system is ten times farther from the sun than is the Earth, the sunshine on Titan is only 1 percent as intense as we are accustomed to, and the temperatures should be far below the freezing point of water even with a sizable atmospheric greenhouse effect. But with abundant organic matter, sunlight and perhaps volcanic hot spots, the possibility of life on Titan* cannot be readily dismissed. In that very different environment, it would, of course, have to be very different from life on Earth. There is no strong evidence either for or against life on Titan. It is merely possible. We are unlikely to determine the answer to this question without landing instrumented space vehicles on the Titanian surface.

To examine the individual particles composing the rings of Saturn we must approach them closely, for the particles are small—snowballs and ice chips and tiny tumbling bonsai glaciers, a meter or so across. We know they are composed of water ice, because the spectral properties of sunlight reflected off the rings match those of ice in the laboratory measurements. To approach the particles in a space vehicle, we must slow down, so that we move along with them as they circle Saturn at some 45,000 miles per hour; that is, we must be in orbit around Saturn ourselves, moving at the same speed as the particles. Only then will we be able to see them individually and not as smears or streaks.

Why is there not a single large satellite instead of a ring system around Saturn? The closer a ring particle is to Saturn, the faster its orbital speed (the faster it is “falling” around the planet—Kepler’s third law); the inner particles are streaming past the outer ones (the “passing lane” as we see it is always to the left). Although the whole assemblage is tearing around the planet itself at some 20 kilometers per second, the relative speed of two adjacent particles is very low, only some few centimeters per minute. Because of this relative motion, the particles can never stick together by their mutual gravity. As soon as they try, their slightly different orbital speeds pull them apart. If the ring were not so close to Saturn, this effect would not be so strong, and the particles could accrete, making small snowballs and eventually growing into satellites. So it is probably no coincidence that outside the rings of Saturn there is a system of satellites varying in size from a few hundred kilometers across to Titan, a giant moon nearly as large as the planet Mars. The matter in all the satellites and the planets themselves may have been originally distributed in the form of rings, which condensed and accumulated to form the present moons and planets.

For Saturn as for Jupiter, the magnetic field captures and accelerates the charged particles of the solar wind. When a charged particle bounces from one magnetic pole to the other, it must cross the equatorial plane of Saturn. If there is a ring particle in the way, the proton or electron is absorbed by this small snowball. As a result, for both planets, the rings clear out the radiation belts, which exist only interior and exterior to the particle rings. A close moon of Jupiter or Saturn will likewise gobble up radiation belt particles, and in fact one of the new moons of Saturn was discovered in just this way: Pioneer 11 found an unexpected gap in the radiation belts, caused by the sweeping up of charged particles by a previously unknown moon.

The solar wind trickles into the outer solar system far beyond the orbit of Saturn. When Voyager reaches Uranus and the orbits of Neptune and Pluto, if the instruments are still functioning, they will almost certainly sense its presence, the wind between the worlds, the top of the sun’s atmosphere blown outward toward the realm of the stars. Some two or three times farther from the Sun than Pluto is, the pressure of the interstellar protons and electrons becomes greater than the minuscule pressure there exerted by the solar wind. That place, called the heliopause, is one definition of the outer boundary of the Empire of the Sun. But the Voyager spacecraft will plunge on, penetrating the heliopause sometime in the middle of the twenty-first century, skimming through the ocean of space, never to enter another solar system, destined to wander through eternity far from the stellar islands and to complete its first circumnavigation of the massive center of the Milky Way a few hundred million years from now. We have embarked on epic voyages.

*Or, to make a different comparison, a fertilized egg takes as long to wander from the fallopian tubes and implant itself in the uterus as Apollo 11 took to journey to the Moon; and as long to develop into a full-term infant as Viking took on its trip to Mars. The normal human lifetime is longer than Voyager will take to venture beyond the orbit of Pluto.

*We even know what gifts they brought the Court. The Empress was presented with “six little chests of divers pictures.” And the Emperor received “two fardels of cinnamon.”

*In 1979 Pope John Paul II cautiously proposed reversing the condemnation of Galileo done 346 years earlier by the “Holy Inquisition.”

The courage of Galileo (and Kepler) in promoting the heliocentric hypothesis was not evident in the actions of others, even those residing in less fanatically doctrinal parts of Europe. For example, in a letter dated April 1634, René Descartes, then living in Holland, wrote:

Doubtless you know that Galileo was recently censured by the Inquisitors of the Faith, and mat his views about the movement of the Earth were condemned as heretical. I must tell you that all the things I explained in my treatise, which included the doctrine of the movement of the Earth, were so interdependent that it is enough to discover that one of them is false to know that all the arguments I was using are unsound. Though I thought they were based on very certain and evident proofs, I would not wish, for anything in the world, to maintain them against the authority of the Church.… I desire to live in peace and to continue the life I have begun under the motto to live well you must live unseen.

*This exploratory tradition may account for the fact that Holland has, to this day, produced far more than its per capita share of distinguished astronomers, among them Gerard Peter Kuiper, who in the 1940’s and 1950’s was the world’s only full-time planetary astrophysicist. The subject was men considered by most professional astronomers to be at least slightly disreputable, tainted with Lowellian excesses. I am grateful to have been Kuiper’s student.

*Isaac Newton admired Christiaan Huygens and thought him “the most elegant mathematician” of their time, and the truest follower of the mathematical tradition of the ancient Greeks—then, as now, a great compliment. Newton believed, in part because shadows had sharp edges, that light behaved as if it were a stream of tiny particles. He thought that red light was composed of the largest particles and violet the smallest. Huygens argued that instead light behaved as if it were a wave propagating in a vacuum, as an ocean wave does in the sea—which is why we talk about the wavelength and frequency of light. Many properties of light, including diffraction, are naturally explained by the wave theory, and in subsequent years Huygens’ view carried the day. But in 1905, Einstein showed that the particle theory of light could explain the photoelectric effect, the ejection of electrons from a metal upon exposure to a beam of light. Modern quantum mechanics combines both ideas, and it is customary today to think of light as behaving in some circumstances as a beam of particles and in others as a wave. This wave-particle dualism may not correspond readily to our common-sense notions, but it is in excellent accord with what experiments have shown light really does. There is something mysterious and stirring in this marriage of opposites, and it is fitting that Newton and Huygens, bachelors both, were the parents of our modern understanding of the nature of light.

*Galileo discovered the rings, but had no idea what to make of them. Through his early astronomical telescope, they seemed to be two projections symmetrically attached to Saturn, resembling, he said in some bafflement, ears.

*A few others had held similar opinions. In his Harmonice Mundi Kepler remarked “it was Tycho Brahe’s opinion concerning that bare wilderness of globes that it does not exist fruitlessly but is filled with inhabitants.”

*Such tales are an ancient human tradition; many of them have had, from the beginning of exploration, a cosmic motif. For example, the fifteenth-century explorations of Indonesia, Sri Lanka, India, Arabia and Africa by the Ming Dynasty Chinese were described by Fei Hsin, one of the participants, in a picture book prepared for the Emperor, as “The Triumphant Visions of the Starry Raft.” Unfortunately, the pictures—although not the text—have been lost.

*Frequently pronounced “eye-oh” by Americans, because this is the preferred enunciation in the Oxford English Dictionary. But the British have no special wisdom here. The word is of Eastern Mediterranean origin and is pronounced throughout the rest of Europe, correctly, as “ee-oh.”

*Because the speed of light is finite (see Chapter 8).

*The view of Huygens, who discovered Titan in 1655, was: “Now can any one look upon, and compare these Systems [of Jupiter and Saturn] together, without being amazed at the vast Magnitude and noble Attendants of these two Planets, in respect of this little pitiful Earth of ours? Or can they force themselves to think, that the wise Creator has disposed of all his Animals and Plants here, has furnished and adorn’d this Spot only, and has left all those Worlds bare and destitute of Inhabitants, who might adore and worship Him; or that all those prodigious Bodies were made only to twinkle to, and be studied by some few perhaps of us poor Fellows?” Since Saturn moves around the sun once every thirty years, the length of the seasons on Saturn and its moons is much longer than on Earth. Of the presumed inhabitants of the moons of Saturn, Huygens therefore wrote: “It is impossible but that their way of living must be very different from ours, having such tedious Winters.”