AMONG THE PLANETS, Jupiter has always been more saturnine than Saturn, a huge, paranoid presence that could have been a contender: at creation, it gathered up almost enough mass to give off its own light. All the planets, at the moment of their birth, were too hot not to cool down, but Jupiter is still cooling down, even now giving off more heat than it takes from the Sun. It has been called "a star that failed," its necklace of moons a sort of second-prize solar system. The "Jovian satellites" are the only ones anywhere to have their own atmospheres (if just barely), battered almost-planets abused by an unhappy almost-Sun. One of the closest, Io, astonished those running the space probe Voyager 1, whose 1979 flyby revealed "gigantic…volcanos spewing debris a hundred miles above [the] sulfur- and lava-strewn surface." Galileo, who first found Io with his brand-new telescope in 1610, had no real idea of its wretchedness. He concentrated on the way it and the three other moons might aid shipboard navigation, and on the rewards that would be coming his way for their discovery. The announcement of these "Medicean Stars" set off an orgy of dynastic self-congratulation, a production of masques and medals to reinforce the Florentine ruling family's self-proclaimed association with Jupiter, first the god and then the planet.
The Medici understood the sphere's vastness, but had little notion of how much actually went on there. Over the next 300 years, as Galileo's telescope was enlarged and refined, man learned to focus in on the planet's swirling bands and girdles, the colors and storms raking the surface of this great gasbag 1,300 times the size of Earth. Since the nineteenth century, astronomers have pondered Jupiter's Great Red Spot, wondering if it might be the dilating birth canal of yet another moon or just the solar system's most enduring hurricane. The White Ovals, 10,000 kilometers or so long, were not discovered until the 1930s, because they didn't exist before that: Jupiter is so changeable, so inanimatedly alive, that the usual question about whether it could support Life, of the organic kind, seems almost too dull to ask.
All our knowledge of the place was nothing compared to what we found out fifteen years ago, when Voyagers 1 and 2 passed it on a grand tour of the outer planets (then favorably aligned in a way that occurs only once every 175 years) and sent back to Earth a series of frighteningly gorgeous television pictures. The Voyagers were beautiful, swan-like sailing ships. This time Leda had gone to Zeus, putting on just Knowledge, not Power. Jupiter itself remained huge and malevolent, more fiercely Greek than judiciously Roman. It still boils with jealousy, and uses the asteroid belt as a first line of defense, something space probes have to get through before the next gantlets they must run, belts of rock and radiation that further dent and discourage them from going nearer the surface.
"Over millions of years," writes Bruce Murray, who from Pasadena's Jet Propulsion Laboratory saw the Pioneer and Voyager spacecraft survive brushes with the planet, "Jupiter's great gravity field domesticates once-pristine comets, reshaping the wild orbits initiated by the original chance fall toward the Sun. Little by little, the mature domesticated comets mimic the orbits of asteroids." But once in a while, sometime every eighty or one hundred or one thousand years (depending on which astronomer you talk to), a comet will find its way toward the gassy skull of Goliath. Within the last century Jupiter has captured at least three comets and made them orbit the planet itself, a particularly humiliating fate for a comet, like the demotion of a coltish ballerina, used to streaking across the whole heavenly stage. Two of these comets are believed to have escaped, but the third and most temperamental is now staging a suicidal rebellion. Two summers ago it came within 10,000 miles of the planet's surface, an act of presumption that provoked Jupiter to tear it into almost two dozen pieces. Since then the fragments have stuck together. Astronomers now routinely compare them to a string of pearls, but these pearls are closer to the bullets in an automatic clip; there is no stopping what they will do sometime during the third week of July, and that is strike the planet, one by one, at 130,000 mph. The same Jovian gravity that once put them in their place will now ensure that they crash with an impact all out of proportion to their size.
THE PEARLS WERE DISCOVERED one night in March 1993 by two astronomers, one professional (Carolyn Shoemaker) and one amateur (David Levy), whose nightly search for comets and asteroids at Palomar Observatory was then being hampered, predictably, by Jupiter. As they recall it: "David had difficulty finding the star on which he would guide the telescope for the eight-minute exposure. Jupiter was close by, and its glare swamped the field of the guiding eyepiece." They finally managed to make the two films of a single field necessary for spectroscopic scanning. Looking at them two days later, on a cold, unproductive afternoon, Shoemaker suddenly saw something peculiar, a "squashed" comet. She and Levy called for help to a friend at the Spacewatch telescope at Kitt Peak, Arizona, and that night he called back with a confirmation, describing the comet's "appearance, the multiple nuclei, the tails, and the wings, just as the fourth movement of Beethoven's first symphony was wafting through the [Palomar] dome."
Soon afterward, extrapolation of the fragments' path yielded the conclusion that the comet, now named Shoemaker-Levy 9, had only sixteen months of celestial life left. Between the 16th and 22nd of this month the pearls will, as it is now routinely put, cause "the largest planetary impact ever witnessed by humanity." Never have so many telescopes, from the lofty Hubble (finally seeing through its corrective lenses) to patio tripods purchased at Sears, regarded a single astronomical event as will be focusing upon what happens in the southwestern sky this July.
Anticipation rose last fall, when the New York Times's John Noble Wilford wrote that "a rain of cometary debris plowing into Jupiter could blow holes the size of Texas in the thick atmosphere, send out titanic shock waves, kick up showers of dust and radiate flashes of light out among the Jovian moons." The excitement of amateurs has diminished a bit over the past several months, as the pearls began to look smaller and less destructive than they first did. Still, big-ticket telescope time has been allotted across the globe, and the Planetary Society, an organization of over 100,000 buffs, ex-space jocks, and serious scientists, has not lost faith in the spectacular possibilities: they are giving away "Jupiter Watch" T-shirts, setting up a special phone number (1-900-88-IMPACT) with updates, and planning a big viewing party at the U.S. Naval Observatory in Washington.
Since all of the crashes will take place on the planet's far side, revelers will have to wait for Jupiter to come around in its once-every-ten-hours orbit (amazingly fast for its size) in order to assess the damage. The impacts themselves will be left to a variety of space vehicles to detect and photograph: the Galileo probe -- the last of the great missions to the outer planets, launched in 1989 and still heading for its own rendezvous with Jupiter -- will send down its pictures, like a tape-delayed Olympic event. Voyager 2's observations will be a nostalgic effort, a chance for the spacecraft, by now billions of miles beyond the giant planet, to remember its visit to Jupiter fifteen summers before. Its cameras were turned off in 1989, after passing Neptune, but it can still make radio measurements of Shoemaker's impact. Little Pioneers 10 and 11, more than twenty years on the road, decades past their own encounters with Jupiter and traveling past the reaches of the solar system, may still be able to squint back hard enough and notice something.
JUST WHAT THERE WILL BE to see has been, all spring, a matter for conjecture and debate. If the pearls are no longer thought to be the 9-mile-wide monsters first assumed, disagreement about what the smaller pieces will do has, if anything, enhanced professional astronomers' interest beyond what it was. A drive down to the Jet Propulsion Laboratory from Los Angeles (whose springtime yellow smog isn't too different in appearance from what ammonia and methane create above Jupiter's clouds) brought one to a group of scientists trying to predict July's impact even as part of their own cafeteria was closed for earthquake-related repairs. (Comets were considered to be portents of quakes as early as the fifteenth century, and on an April morning, Dr. Ray Newburn, a 38-year JPL veteran cheerfully wearing his comet-decorated tie, pointed to the high pile of scientific journals that had tumbled to the floor of his office at dawn on January 17.)
Will the whole thing disappoint, like the overhyped reappearance of Halley in '86, or the almost-forgotten but then-eagerly-awaited Kohoutek, which turned out, in 1973, to be an extremely damp firework? "It may be," writes astronomer Stephen J. Edberg, "that Jupiter is so, gigantic…it will simply swallow the fragments of the comet's nucleus without so much as a burp." Newburn and his colleague, Dr. Zdenek Sekanina, whose models and forecasts have been changing by the month, say that's unlikely, but Glenn Orton, a Jupiter specialist, does sometimes wonder if the tiger they think they have by the tail won't turn out to be a kitten. He doesn't expect the explosions to create any vast new vortices -- certainly nothing like the Great Red Spot -- but he thinks some of the storms and swirls they engender may last for months. Even if the insulted Jupiter quickly sets itself to rights, repairing its wounds like one of the self-healing targets used by archers, the impacts will still have been extraordinarily forceful. Orton is glad that the event is taking place -- and glad it's happening five astronomical units (the mean distance between the sun and Earth) away from here.
TINY AS THE PEARLS ARE, they would almost certainly wipe out human life, if they were headed this way, and what happens in July will set catastrophists to worrying over what a similar shelling could do to us. The destruction of hundreds of thousands of Siberian acres in 1908 is thought to have been accomplished by a single meteor or asteroid no more than 50 meters wide that never even made land: it exploded five miles up in the atmosphere. Something just a bit larger -- like the asteroid that landed in the Yucatan 65 million years ago and wiped out the dinosaurs -- would surely kick up enough dust to block out sunlight and set off a version of nuclear winter. The only thing that could keep man from being killed by such a cosmic stray bullet is a nuclear weapon of our own, which, properly launched, might deflect or pulverize the invader. You won't find NASA talking openly about this, but as we emerge from the age of Mutually Assured Destruction, it may be levelheaded to keep a bomb or two around, like one of those remaining vials of smallpox virus.
J.L. Lagrange's eighteenth-century theory that comets were actually created by Jupiter -- belched like baseballs from an automatic pitching-machine -- is only one of many that had to give way before we arrived at our current idea of their ingredients: rock and ice and other leftover creational crud that gathered at the far reaches of the solar system, beyond Uranus. Aristotle made a wrong but enduring guess that they were sublunary fireballs and always bad news. (At least for someone: when the weavers of Bayeux put the still-unnamed Halley's comet into the tapestry they made in 1066, they presumably took it as a bad omen for the English, not the French.) Only when Edmond Halley forecast its reappearance in 1758, were comets proved to be governed by physical law. As John C. Brandt and Robert D. Chapman write: "Their orbits could be calculated and their return predicted years in advance…Any rational fear of [them] as signs of disaster or evil should have vanished [then]."
Handfuls of witchy doomsayers -- those expecting not collisions, only imminent bad luck -- remain among us. (One of them in England insists that Shoemaker-Levy is actually Halley, and that its coming destruction is a very bad sign.) But comets have undergone a celestial seachange in their public image. They now delight instead of alarm. Astronomical buffs regard these vulnerable but can-do rovers as heavenly jalopies, capable of astonishing mileage before they fall apart. Ask Dr. Sekanina what attracted him to contemplate them years ago, when he was still a boy in Czechoslovakia, and the first thing he mentions is their size. They are small enough to wrap the mind around, almost human in scale, like the asteroid Ida, just 35 miles long, discovered only last summer to be in possession of its own moon, a single mile in diameter. The tininess of such objects, their moving and enduring within the void, is what enchants, a version of the desert-island or magic-carpet fantasy. In the case of the comets, the tails are, to the beholder, more important than the nuclei emitting them. It is the fiery tresses that let us picture comets doing what they do, and that is move. On the sky's clock-face they sweep like second-hands, while all the other bodies, Jupiter included, proceed too slowly to watch. We take the planets' movements, like the hour-hand's, on faith, but the travels of comets are something we can -- or at least think we can -- see. Comets plot time against space, and their flaming tails remind humans of something they themselves used to do long ago: climb aboard rockets, and set out for new worlds.
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