Cosmos

by Carl Sagan

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.*

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.

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 world is my country,” he said, “science my religion.”

of Titan, the largest moon of Saturn and, as we now know, the largest moon in the solar system—a

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.

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.

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 passes 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

If a faithful account was rendered of Man’s ideas upon Divinity, he would be obliged to acknowledge, that for the most part the word “gods” has been used to express the concealed, remote, unknown causes of the effects he witnessed; that he applies this term when the spring of the natural, the source of known causes, ceases to be visible: as soon as he loses the thread of these causes, or as soon as his mind can no longer follow the chain, he solves the difficulty, terminates his research, by ascribing it to his gods … When, therefore, he ascribes to his gods the production of some phenomenon … does he, in fact, do any thing more than substitute for the darkness of his own mind, a sound to which he has been accustomed to listen with reverential awe? —Paul Heinrich Dietrich, Baron von Holbach, Système de la Nature, London, 1770

But the planets do not shine by their own light, as the Sun does. They merely reflect light from the Sun.

Metaphors like those about celestial campfires or galactic backbones were eventually replaced in most human cultures by another idea: The powerful beings in the sky were promoted to gods. They were given names and relatives, and special responsibilities for the cosmic services they were expected to perform. There was a god or goddess for every human concern. Gods ran Nature. Nothing could happen without their direct intervention. If they were happy, there was plenty of food, and humans were happy. But if something displeased the gods—and sometimes it took very little—the consequences were awesome: droughts, storms, wars, earthquakes, volcanoes, epidemics. The gods had to be propitiated, and a vast industry of priests and oracles arose to make the gods less angry. But because the gods were capricious, you could not be sure what they would do. Nature was a mystery. It was hard to understand the world.

Hera, who began her career as goddess of the sky. She was the patron deity of Samos, playing the same role there as Athena did in Athens. Much later she married Zeus, the chief of the Olympian gods. They honeymooned on Samos, the old stories tell us. The Greek religion explained that diffuse band of light in the night sky as the milk of Hera, squirted from her breast across the heavens, a legend that is the origin of the phrase Westerners still use—the Milky Way. Perhaps it originally represented the important insight that the sky nurtures the Earth; if so, that meaning seems to have been forgotten millennia ago.

Ionia was the place where science was born.

Thus he showed the world philosophers can easily be rich if they like, but that their ambition is of another sort.

In his book On Ancient Medicine, Hippocrates wrote: “Men think epilepsy divine, merely because they do not understand it. But if they called everything divine which they do not understand, why, there would be no end of divine things.”

For Democritus all of life was to be enjoyed and understood; understanding and enjoyment were the same thing.

Democritus invented the word atom, Greek for “unable to be cut.” Atoms were the ultimate particles, forever frustrating our attempts to break them into smaller pieces. Everything, he said, is a collection of atoms, intricately assembled. Even we. “Nothing exists,” he said, “but atoms and the void.”

somewhat unusual. Women, children and sex discomfited him, in part because they took time away from thinking. But he valued friendship, held cheerfulness to be the goal of life and devoted a major philosophical inquiry to the origin and nature of enthusiasm.

A portrait of Democritus is now on the Greek hundred-drachma bill.

“Irrational” originally meant only that a number could not be expressed as a ratio. But for the Pythagoreans it came to mean something threatening, a hint that their world view might not make sense, which is today the other meaning of “irrational.”

Kepler was both inspired in his search for the harmony of planetary motion and delayed for more than a decade by the attractions of Pythagorean doctrine.

Plutarch wrote: “It does not of necessity follow that, if the work delight you with its grace, the one who wrought it is worthy of esteem.”

Of the seventy-three books Democritus is said to have written, covering all of human knowledge, not a single work survives. All we know is from fragments, chiefly on ethics, and secondhand accounts. The same is true of almost all the other ancient Ionian scientists.

The resistance to Aristarchus and Copernicus, a kind of geocentrism in everyday life, remains with us: we still talk about the Sun “rising” and the Sun “setting.” It is 2,200 years since Aristarchus, and our language still pretends that the Earth does not turn.

It is now very clear that we live some 30,000 light-years from the galactic core, on the fringes of a spiral arm, where the local density of stars is relatively sparse. There may be those who live on a planet that orbits a central star in one of Shapley’s globular clusters, or one located in the core. Such beings may pity us for our handful of naked-eye stars, because their skies will be ablaze with them. Near the center of the Milky Way, millions of brilliant stars would be visible to the naked eye, compared to our paltry few thousand. Our Sun or suns might set, but the night would never come.

A handful of sand contains about 10,000 grains, more than the number of stars we can see with the naked eye on a clear night.

All places on Earth are, to high precision, the same distance from any star. This is why the star patterns in a given constellation do not change as we go from, say, Soviet Central Asia to the American Midwest. Astronomically, the U.S.S.R. and the United States are the same place. The stars in any constellation are all so far away that we cannot recognize them as a three-dimensional configuration as long as we are tied to Earth.

The appearance of the constellations changes not only in space but also in time; not only if we alter our position but also if we merely wait sufficiently long. Sometimes stars move together in a group or cluster; other times a single star may move very rapidly with respect to its fellows.

At some positions in its orbit, Proxima is the closest known star to the Sun—hence its name. Most stars in the sky are members of double or multiple star systems. Our solitary Sun is something of an anomaly.

If you are looking at a friend three meters (ten feet) away, at the other end of the room, you are not seeing her as she is “now”; but rather as she “was” a hundred millionth of a second ago. [(3 m) / (3 × 108 m/sec) = 1/(108 / sec) = 10–8 sec, or a hundredth of a microsecond. In this calculation we have merely divided the distance by the speed to get the travel time.] But the difference between your friend “now” and now minus a hundred-millionth of a second is too small to notice.

Light (reflected or emitted) from an object travels at the same velocity whether the object is moving or stationary: Thou shalt not add thy speed to the speed of light. Also, no material object may move faster than light: Thou shalt not travel at or beyond the speed of light.

a universe filled with stars rushing helter-skelter in all directions, there was no place that was “at rest,” no framework from which to view the universe that was superior to any other framework. This is what the word relativity means. The idea is very simple, despite its magical trappings: in viewing the universe, every place is as good as every other place. The laws of Nature must be identical no matter who is describing them.

The problems of simultaneity do not apply to sound as they do to light because sound is propagated through some material medium, usually air. The sound wave that reaches you when a friend is talking is the motion of molecules in the air. Light, however, travels in a vacuum. There are restrictions on how molecules of air can move which do not apply to a vacuum. Light from the Sun reaches us across the intervening empty space, but no matter how carefully we listen, we do not hear the crackle of sunspots or the thunder of the solar flares.

The electrical impulses in modern computers do, however, travel nearly at the speed of light.

Traveling close to the speed of light is a kind of elixir of life. Because time slows down close to the speed of light, special relativity provides us with a means of going to the stars.

Orion was designed to utilize explosions of hydrogen bombs, nuclear weapons, against an inertial plate, each explosion providing a kind of “putt-putt,” a vast nuclear motorboat in space. Orion seems entirely practical from an engineering point of view. By its very nature it would have produced vast quantities of radioactive debris, but for conscientious mission profiles only in the emptiness of interplanetary or interstellar space. Orion was under serious development in the United States until the signing of the international treaty that forbids the detonation of nuclear weapons in space. This seems to me a great pity. The Orion starship is the best use of nuclear weapons I can think of.

The Earth gravitationally attracts us with a certain force, which if we are falling we experience as an acceleration. Were we to fall out of a tree—and many of our proto-human ancestors must have done so—we would plummet faster and faster, increasing our fall speed by ten meters (or thirty-two feet) per second, every second.

This acceleration, which characterizes the force of gravity holding us to the Earth’s surface, is called 1 g, g for Earth gravity. We are comfortable with accelerations of 1 g; we have grown up with 1 g. If we lived in an interstellar spacecraft that could accelerate at 1 g, we would find ourselves in a perfectly natural environment. In fact, the equivalence between gravitational forces and the forces we would feel in an accelerating spaceship is a major feature of Einstein’s later general theory of relativity. With a continuous 1 g acceleration, after one year in space we would be traveling very close to the speed of light [(0.01 km/sec2) × (3 × 107 sec) = 3 × 105 km/sec].

But if we do not destroy ourselves, I believe that we will one day venture to the stars. When our solar system is all explored, the planets of other stars will beckon.