Good Show, By Jove
By all rights, it was the comet that should have come out ahead. But in a celestial fusillade whose like had never before been seen by the eyes of earthlings, the rock-hard balls of frozen gas and cosmic dust were vaporized by a close encounter with a veil of gas less substantial than a wisp of water vapor. As each of the 21 big fragments of Comet Shoemaker-Levy 9 barreled into the diaphanous atmosphere of the planet Jupiter at 134,000 miles an hour last week, friction with the sparse molecules of the Jovian clouds stopped them dead. Each fragment's huge energy of motion was converted immediately into heat so intense that it speckled the stratosphere with patches of fried debris. The fatal plunge of a single fragment -- christened G -- triggered fireballs as powerful as a 6 million-megaton explosion (the largest H-bomb ever detonated weighed in at a piddling 58 megatons). Plumes of fire shot up hundreds of miles, high enough to peek over Jupiter's horizon from the backside, where they hit, and become visible to telescopes on Earth and in space. Midway through the six-day barrage, the impact of fragment H created a bubble of superheated gas that blazed with 50 times the infrared luminosity of the entire planet, briefly blinding some telescopes. "Jupiter," said amateur observer David Levy, one of the comet's codiscoverers, "is getting the stuffing knocked out of it."
Having downplayed expectations in advance, astronomers were delighted and amazed at the show Shoemaker-Levy put on. The fragments did smack into Jupiter's far side, as predicted, but the rapidly rotating planet quickly spun the resulting fireballs into view. The pyrotechnics were so bright that even little telescopes dug out of dusty attics for the occasion caught a glimpse of them. Amateur sky watchers could even spy the prominent black scars the impacts left above Jupiter's clouds -- fried gobs of particles left over after a fragment got incinerated. "This is not just the astronomers' comet," said Levy. "This is everybody's."
There were almost too many everybodies. An electronic bulletin board that astronomer Lucy McFadden set up at the University of Maryland to collect reports of sightings from Hawaii to Texas to Australia, was jammed all week; Internet services carrying news and images of the bombardment were flooded with more than 60,000 users in one day. In New York, 2,000 people flocked to Central Park to view the impacts through telescopes set up by avid amateur observers. "The interest has been tremendous," said Wade Sylvester of the Boston Museum of Science, which had six times the usual number of visitors at its planetarium shows.
Shoemaker-Levy 9 had meandered, intact and unnamed, through the outskirts of the solar system for as long as the sun and planets have existed -- 4.5 billion years. Then, by chance, the sun nudged it off course enough so that, on July 7, 1992, it hurtled deep within the clutches of Jupiter's gravitational field. That close encounter ripped apart the icy dust ball (or dusty ice ball: astronomers are divided on how best to describe these chunks of frozen gas and dust), leaving 21 big fragments, the largest 2.5 miles across, and uncounted thousands of smaller ones. The detritus orbited Jupiter along an elongated elliptical path (diagram) for the next two years -- a glowing string of pearls coming perilously and inexorably close to its fatal rendezvous. The fragments' next encounter with the largest planet in the solar system was their last.
Last week's collisions marked the first time humans had ever observed the impact of one member of the solar system on another. All astronomers had seen before were accident reports -- eerily straight lines of craters on the Jovian moons Callisto and Ganymede, which scientists now suspect were created by bombardments like last week's, and the chaotic patterns of craters on the moon, evidence that the solar system was once a place where it was wise to duck. For this was indeed how the solar system was shaped, not just by whimpers of coalescing gas clouds but by bangs stupendous in their power. Although last week was more spectacle than science, once the data are analyzed astronomers hope they will reveal exactly what role comets played in forming the planets and the life on them. Some theories hold that comets brought to Earth the water that formed the life-giving seas. The icy wanderers may also have sprinkled Earth with organic molecules that, after eons of evolution, came together into the first organism. By analyzing spectra of Shoemaker-Levy's ingredients, astronomers hope to pin down whether comets indeed could have carried the seeds of life to Earth.
The bombardment also promises data that will fill in the gaps in astronomers' knowledge about Jupiter. "The best way to learn about something is to poke it and see what happens," says astronomer Roger Yelle of the University of Arizona. "We just gave the atmosphere of Jupiter a giant poke." The Voyager and Pioneer space probes had already confirmed that Jupiter is basically an unignited sun: largely liquid hydrogen and helium, it is sheathed in clouds of ammonia crystals. Even its "surface" is gas, with perhaps a metallic interior and a rocky core. The impacts shot gobs of matter into the upper stratosphere: an analysis of these black spots could reveal more precisely what Jupiter is made of.
Even before those answers come, the violent sky show has had a practical effect. Just about the time Jupiter was getting its black eyes, a House of Representatives subcommittee voted to require NASA to track the thousands of comets and asteroids that travel the solar system in orbits that periodically intersect Earth's. There is a one in 10,000 chance, every human lifetime, of a killer asteroid hitting Earth. Last week it was only a comet that died, albeit spectacularly; if something like Shoemaker-Levy hit Earth, the planet might become no more than ashes, and the music of the spheres an eternal dirge.
For six days last week, 21 fragments of a fractured comet smacked into the far side of Jupiter at 134,000 miles an hour. The force was greater than Earth's entire cold-war nuclear arsenal. One impact triggered a fireball whose infrared luminosity outshone the planet below. Others produced plumes of burning gas that rose so high they peeked over the horizon and were visible to delighted astronomers on Earth. In the months ahead astronomers expect that the discs upon discs of data collected last week will tell them what Jupiter's interior is made of -- and perhaps whether comets could have seeded life on Earth.
DIAGRAM: Comet Trajectory: Shoemaker-Levy 9 was spotted in March 1992 but has probably traveled a highly elliptical orbit around Jupiter for at least 20 years. After getting too close to Jupiter in July 1992, it broke into dozens of fragments fated to orbit the planet once more before their fatal rendezvous last week.
DIAGRAM: Shoemaker-Levy's Destruction: Each impact ignited a fireball that expanded so fast the flaming gases of the explosion blasted out of the entry tunnel carved by the cometary fragment. Although the fireball dissipated within minutes, some of the hot spots in the atmosphere were still seething days after the barrage ended.
DIAGRAM: Glowing Embers: Jupiter rotates on its axis once every 9 hours and 56 minutes. Its radius is 11 times Earth's. In this infrared image the bright spot (lower left) is the fireball ignited by fragment Q1. Bright spots near the South Pole are the hot, glowing remnants of earlier impacts.
PHOTO: Within 12 minutes after fragment G hit, the impact site rotated into Earth's view, revealing a 6 million-megaton explosion whose plume rose hundreds of miles above the Jovian cloudtops.