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What are these graceful visitors to our skies? We now know that they bring both life and death and teach us about our origins.
Comet begins with a breathtaking journey through space astride a comet. Pulitzer Prize-winning astronomer Carl Sagan, author of Cosmos and Contact, and writer Ann Druyan explore the origin, nature, and future of comets, and the exotic myths and portents attached to them. The authors show how comets have spurred some of the great discoveries in the history of science and raise intriguing questions about these brilliant visitors from the interstellar dark.
Were the fates of the dinosaurs and the origins of humans tied to the wanderings of a comet? Are comets the building blocks from which worlds are formed?
Lavishly illustrated with photographs and specially commissioned full-color paintings, Comet is an enthralling adventure, indispensable for anyone who has ever gazed up at the heavens and wondered why. Praise for Comet
"Simply the best." —The Times of London
"Fascinating, evocative, inspiring." —The Washington Post
"Comet humanizes science. A beautiful, interesting book." —United Press International
"Masterful . . . science, poetry, and imagination." —The Atlanta Journal & Constitution
Under the Cover
An excerpt from Comet
Astride the Comet
How vast is creation! I see the planets rise and the stars hurry by, carried along with their light! What, then, is this hand which propels them? The sky broadens the more I ascend. Worlds revolve around me. And I am the center of this restless creation.
Oh, how great is my spirit! I feel superior to that miserable world lost in the immeasurable distance beneath me; planets frolic about me—comets pass by casting forth their fiery tails, and centuries hence they will return, still running like horses on the field of space. How I am soothed by this immensity! Yes, this is indeed made for me; the infinite surrounds me on all sides. I am devouring it with ease.
—GUSTAVE FLAUBERT, SMARH, 1839, TRANSLATED BY JIM LEBLANC
These are the snows of yesteryear, the pristine remnants of the origin of the solar system, waiting frozen in the interstellar dark. Out here trillions of orbiting snowbanks and icebergs are stored, gently suspended about the Sun. They cruise no faster than a small propeller-driven aircraft would, buzzing through the blue skies of far-off Earth. The slowness of their motion just balances the gravity of the distant Sun, and, poised between feeble contending forces, they take millions of years to complete one orbit around that yellow point of light. Out here you are a third of the way to the nearest star. Or rather, to the next nearest star: In the depth and utter blackness of the dark sky around you, it is entirely clear that the Sun is one of the stars. It is not even the brightest star in the sky. Sirius is brighter, and Canopus. If there are planets circling the star called the Sun, there is no hint of them from this remote vantage point.
These trillions of floating icebergs fill an immense volume of space; the nearest one is three billion kilometers away from you, about the distance of the Earth from Uranus. There are many icebergs, but the space they fill, a thick shell surrounding the Sun, is incomprehensibly vast. Most of them have been out here since the solar system began, quarantined from whatever mischief may be going on down there, in that alien and hostile region bordering the Sun.
Beyond the occasional soft ping of a cosmic ray from some collapsed star at the other end of the Milky Way, hardly anything ever happens here. It is very peaceful. But something has happened, a gravitational intrusion, not by the Sun or its possible planets, but by another star. It was slow in coming, and at its closest it was never very near. You can see it over there, glowing faintly red, much dimmer than the Sun. This cloud of icebergs has been carried with the Sun on its motions through the Milky Way Galaxy. But other stars have their own characteristic motions, and sometimes by accident approach us. So on occasion, as now, there is a little gravitational rumbling, and the cloud trembles.
Since your iceberg is bound so weakly to the Sun, even a little push or tug is enough to throw it onto some new trajectory. The neighboring icebergs—much too small and distant for you to see directly—have been similarly affected, and are now hurrying off in many directions. Some have been shaken loose from the gravitational shackles that had bound them to the Sun, and are now liberated from their ancient servitude, embarking on odysseys into the vast spaces between the stars. But for your iceberg there is a different destiny working itself out: You have been tugged in such a way that you are now falling, slowly at first, but with gradually increasing speed—down, down, down toward the point of light about which this vast collection of little worlds slowly revolves.
Imagine that you are as patient and long-lived as the iceberg on which you are standing, that you have adequate life expectancy and life-support equipment for a journey of thousands to millions of years. You are falling toward the bright yellow star. Your worldlet and its brethren have been given a name. Comets, they are called. Your comet— perhaps it is one called Hale-Bopp—is an emissary from the kingdom of ice to the infernal realm near the Sun.
Out here a comet is only an iceberg. Later on, the iceberg will be just one part of the comet, called the nucleus. A typical cometary nucleus is a few kilometers across. Its surface area is the size of a small city. If you were standing on it you would see the smoothly curving contours of gracefully sculpted hillocks built of very dark, reddish-brown ice. There is no air on this small world, nothing liquid, and—apart from you yourself—nothing alive, at least so far as you can see. You can, over the following millions of years, explore every corner, every mountain, every crevice. With the skies perfectly clear, and with no particularly urgent tasks before you, you can also spend a little time studying the magnificent array of unwinking bright stars that surrounds you.
Your footprints are deep, because the snow beneath your feet is weak. In a few places there are patches of ground so fragile that were you to walk upon them imprudently you would fall through—as in the legendary quicksand of Earth—onto deep shadowed ice, perhaps meters below. Your fall would be slow, though, almost languorous, because the acceleration caused by gravity, the downward force you feel here on the comet, is only a few thousandths of a percent of the familiar 1 g of Earth.
On more solid ground, the low gravity might tempt you into unprecedented athletic feats. But you must be careful. If you so much as stride purposefully, you walk off the comet altogether. With only a little effort, from a standing position you leap thirty kilometers into space, taking almost a week to reach the peak of your trajectory. There, gently tumbling, you have a comprehensive view of the comet slowly rotating beneath you, its axis by accident almost pointing toward the Sun. You can make out its lumpy shape; the comet is far from a perfect sphere. Perhaps you worry that you have jumped too high, that you will not fall back to the comet, that you will drift alone through space forever. But no, you see that your outbound velocity is gradually diminishing, and eventually, ten or twelve days after making this modest exertion, you tumble lightly back onto the somber snows. On this world you are dangerously strong.
Since it is hard to take a step without launching yourself on a small parabolic trajectory, team sports would be played in agonizing slow motion, the cluster of players rising and spinning in the space surrounding the comet like a swarm of gnats sizing up a grapefruit. A game of baseball would take years to complete—which is just as well, since you have a million years or so to idle away. But the ground rules would be unorthodox.
You pack the snow into an odd dark snowball and easily fling it off the comet, never to return again. With a flick of the wrist, without even engaging your arm in the throwing motion, you have launched a new comet on its own long, falling trajectory into the inner solar system. High above the equator of your comet, you can lightly pitch a snowball so that it hovers forever above the same point on the surface. You can make arrays of objects stationary in space, vast, apparently motionless three-dimensional assemblages, poised above the cometary surface.
As the millennia pass, you cannot help but notice that the yellow star is gradually growing more intense, until it has become by far the brightest star in the sky. The early phase of your voyage has been tedious, even if you are endowed with heroic patience, and in several million years hardly anything has happened. But you can at least see your surroundings more clearly now. The icy ground beneath you has hardly changed at all. The journey has been so long that you have been able to detect variations in the positions and, you think, even the brightness of many nearby stars. Your world is moving faster now, but otherwise everything is still, silent, cold, dark, changeless.
The comet eventually begins crossing the orbits of other kinds of objects, much larger bodies that are also bound in gravitational thrall to that beckoning point of light. As you pass close by them, you careen perceptibly. Their gravities are so large that they retain massive atmospheres. Your comet, by contrast, is so insubstantial that any puff of gas released escapes almost instantly to space. Accompanying the giant gas planets with their multicolored clouds is a retinue of smaller, airless worlds, some of them made of ice—much more kin to the comets than the huge ball of hydrogen that fills your sky.
You can feel the warmth of the Sun increasing. The comet feels it too. Little patches of snow are becoming agitated, frothy, unstable. Grains of dust are being levitated over the patches. Considering the feeble gravity, it is no surprise that even gentle puffs of gas send grains of ice and dust swirling skyward. A powerful jet gushes up from the ground and a fountain of fine particles is launched far above you. The ice crystals sparkle prettily in the sunlight. After a while the ground becomes covered with a light snow. As you plunge onward, closer to the Sun, its disk now easily visible, such blowoffs become more frequent. While on one of your excursions aloft, you chance to see an active jet, a geyser pouring out of the ground. You give it wide berth. But it reminds you of the instability—the literal volatility—of this tiny world.
Carl Sagan served as the David Duncan Professor of Astronomy and Space Sciences and Director of the Laboratory for Planetary Studies at Cornell University. He played a leading role in the Mariner, Viking, Voyager, and Galileo spacecraft expeditions, for which he received the NASA Medals for Exceptional Scientific Achievement and (twice) for Distinguished Public Service.
His Emmy- and Peabody–winning television series, Cosmos, became the most widely watched series in the history of American public television. The accompanying book, also called Cosmos, is one of the bestselling science books ever published in the English language. Dr. Sagan received the Pulitzer Prize, the Oersted Medal, and many other awards—including twenty honorary degrees from American colleges and universities—for his contributions to science, literature, education, and the preservation of the environment. In their posthumous award to Dr. Sagan of their highest honor, the National Science Foundation declared that his “research transformed planetary science . . . his gifts to mankind were infinite.” Dr. Sagan died on December 20, 1996.