by Traci Watson

For the first few hours, July 16, 1994, seemed a day like any other in the swirl of clouds and gas at the surface of planet Jupiter. Then, out of the planet's southern skies, a massive chunk of rock and ice came hurtling through the Jovian heavens at 134,000 mph. The most spectacular celestial bombardment ever witnessed had begun. When the fragment collided with the planet's thick gaseous cover, a massive plume of gas spewed from the impact site like a geyser, forming a brilliant fireball 1,000 miles high and 4,000 miles wide. Soon, astronomers on Earth saw the first clear pictures of the faraway cataclysm -- and reached for the champagne. 

        The ill-fated projectile, which scientists labeled fragment A, was in itself an astronomer's dream come true. But the fireworks that followed over the next six days were enough to have Galileo himself cheering from the heavens. Twenty-odd chunks of the so-called Shoemaker-Levy 9 comet, lined up like pearls on a celestial string stretching millions of miles, followed fragment A into Jupiter's depths. The planet outdid even the most fantastic predictions, igniting in fireballs and breaking out in dark blotches at the collision sites. Sleepless planetary scientists around the world watched in astonishment as the monster planet revealed secrets almost daily. There was sulfur; no one was sure it would be there, but it could explain the multicolored clouds enshrouding the planet. Water, which astronomers had expected to find in abundance, seems to be relatively scarce. Data that couldn't be analyzed immediately were feverishly being downloaded from many of the Earth's best telescopes for study in the months ahead. Out of the trove of information, scientists hope to discover everything from the behavior of the planet's atmosphere and its chemical makeup to its interior structure. The awesome event may also illuminate the nature of comets and even the origin and evolution of the solar system. 

        Demolition derby. That there was anything at all to see came as a great relief to scientists. Since May 1993, when predictions about the coming crashes were first made, astronomers had been arguing about what the collisions would look like. Indeed, many predicted there would be no fireworks at all. "The Big Fizzle is coming," sneered a leading scientific journal two days before the first crash. But no one is scoffing now. "We're all flabbergasted by what we are seeing," says MIT astronomer Heidi Hammel, head of a key comet observation team. No wonder: Astronomers have never seen a show like this, let alone predicted it a year in advance. 

        Among the most remarkable sights were the immense gas plumes, or fireballs, generated by many of the comet pieces. Fragment G, one of the biggest in the train of comet chunks, produced a 5,000-mile-wide plume that towered 1,200 to 1,600 miles above Jupiter's surface. Even though the crash took place on the side of Jupiter facing away from Earth, the fireball rose so high that its tip peeked over Jupiter's rim and became visible to Earth-based sensors. Astronomers estimate that G's impact had the force of 6 million megatons of TNT, 100,000 times the power of the largest nuclear bomb ever exploded on Earth. Says Glenn Orton, an astronomer at California's Jet Propulsion Laboratory: "It was like God striking the planet." 

        The fireballs proved dramatic even in their demise. The G fragment fireball, for example, reached its maximum height and glory in a few minutes -- and immediately began to collapse, pulled downward by gravity. As it fell, it cooled from thousands of degrees Fahrenheit to 350 degrees below zero, deflating into a dim, puddlelike cloud floating high over Jupiter's swirling surface. Astronomers reported seeing strange chemicals in these clouds, organic molecules that could either have shot out of Jupiter's interior in the plume or been deposited by the comet itself. 

        Among the other star players in the celestial show were Jupiter's new spots, described by one scientist as planetary "bruises and black eyes." Especially clear were the scars left by the G piece: a dark blotch; a thin, dark circle around the blotch, and a lighter, crescent-shaped smear bigger than the Earth. These features were so prominent that even amateurs could see them through small telescopes. 

        Planetary black eye. Despite all the sophisticated calculations performed before the collisions, no one predicted these ultradark spots. Even more puzzling, they're the wrong color: New markings on Jupiter are usually light, not dark. Scientists speculate that some of the spots are made of Jovian soot generated by the immense heat of the fireballs. Or the "black eyes" could be composed of cometary rubble. One analysis of the A spot found no Jovian gases, leading some astronomers to suspect that the mark is formed from rocky comet dust. 

        The fireballs and spots aren't just pyrotechnics. When scientists studied the light absorbed by the G fragment fireball, for instance, they determined that the fireball contained ammonia gas, implying that the G chunk must have blasted down into Jupiter's ammonia layer some 25 miles below the visible surface. Similarly, the G fragment's blotch appears to contain sulfur, indicating that the G chunk may have drilled as deep as the sulfur-containing layer of Jupiter's atmosphere, 40 miles down. Scientists have long suspected that Jupiter's bright stripes were colored by sulfur, but until now no one had ever actually seen it on the planet. 

        The fireballs even say something about the comet shards' strength. Some experts hypothesized that the comet pieces were so weakened by cracks that when they got close to Jupiter they would disintegrate into bits of rubble -- leading not to fireballs but to a meteor shower probably invisible on Earth. The evidence from last week suggests that comets are glued together more powerfully than expected. 

        Ultimately, of course, scientists hope to learn far more than how Jupiter reacts when hit with a huge comet. The planet remains an enigma nearly 400 years after the astronomer Galileo first observed its moons with a crude telescope. Bigger and more dazzling than any other planet in the solar system, it is a mass of contradictions. The Jovian clouds churn wildly, but the planet's winds blow in orderly patterns. Giant cyclones score the planet, but many of them have held steady for centuries. 

        Nature's lab. The celestial smashups should help explain some of these puzzles, a prospect that scientists view with glee. "In astronomy, normally you can't do experiments," MIT's Hammel says. "Here, nature is providing us with one." For instance, the clouds formed by the collapse of the fireballs have settled high above Jupiter. By watching the clouds' motion, astronomers will learn the speed of the winds in the planet's upper atmosphere. Scientists also hope to resolve whether the many dark pockmarks on Jupiter's surface were caused by comet collisions; if the scars left by the current crashes disappear quickly, astronomers will have to rethink the origin of the older spots. 

        But even if these leads don't pan out, the comet fragments may provide other insights into the Jovian character. Some astronomers had predicted that a very large comet chunk plunging to its doom would create tiny circular ripples in Jupiter's surface, and indeed that appears to have been the case. The dark ring that resulted from impact G was the first ripple to be identified late in the week, and scientists are hopeful that they'll see more after processing all their data. 

        Any ripples that are sighted will be used as planetary divining rods. For example, the faster they spread through the atmosphere, the more water there is on the planet, says Andrew Ingersoll, a planetary scientist at the California Institute of Technology. Jupiter's water is key to understanding the birth of the solar system. Astronomers now think that the planets and sun all formed from the same vast sheet of gas. Jupiter resembles a smaller, colder sun: Both bodies are made mostly of hydrogen and helium, and both radiate heat. Scientists know from their measurements of the sun how much water Jupiter should contain, but so far they have not been able to find it; they are waiting anxiously to see whether the missing water will show up. If there isn't enough water, the reigning theory about the solar system's beginnings will come into serious question. 

        Scientists also spied a different kind of ripple that should help them get a glimpse at Jupiter's unplumbed depths. Astronomers have long believed that a layer of an unusual form of hydrogen lies far below Jupiter's clouds. If this layer exists, it is the most likely source of Jupiter's intense magnetic field. Seismic waves resulting from the comet's furious passage appear to be moving toward the planet's core. The waves should bounce back from the center and then reappear at the surface as tiny changes in the temperature. Scientists are hoping they can use these readings to explore the Jovian interior -- and perhaps locate the magnetic source. 

        After the storm. Scientists will have barely begun to digest the glut of information they've received from last week's impact when they'll be treated to yet another encounter with the giant planet -- this one of their own making. Late next year, a 750- pound robot probe will slice into the Jovian atmosphere near the planet's equator at more than 100,000 miles per hour. The tiny probe, deployed from the robot explorer Galileo, is expected to unfurl a parachute and descend some 125 miles through the cloud layers. The device will sample the composition of Jupiter's clouds and look for signs of electrical activity, relaying the information back to Galileo for a little over an hour before it succumbs to the atmosphere's enormous pressures. Meanwhile, Galileo will go into orbit around Jupiter, passing close to several moons and taking readings of the planet's electricity and magnetism. The data from these sensors should give scientists a clearer picture of the giant planet's composition. 

        More than anything, last week's celestial fireworks offered a dramatic demonstration that the solar system is a dynamic, evolving place. "In textbooks, we learn that the solar system was completed 4 billion years ago," says David Levy, a co-discoverer of the star-crossed comet. "Well, now we know that's not true. If we were to go around Jupiter, there would be a little yellow construction fence there. It would say, `Danger. Keep out. Solar system under construction.'"

Bombs away 1. The Plunge Last week, more than 20 pieces of a shattered comet smashed into the planet Jupiter at 134,000 mph. The biggest chunks may have plunged 40 miles into the Jovian clouds before exploding or sputtering to a halt.

2. The Fireball The energy released by the crash superheated the surrounding clouds, sending hot gas ballooning upward in a giant fireball hundreds of miles high. The crash also sent seismic waves surging into Jupiter's interior and atmospheric ripples through the surface clouds, giving clues tothe planet's interior and atmostphere.

3. The Black Eye The bigger impacts left mysterious scars, some big enough to see with backyard telescopes. Many scientists think the dark splotch (1) is Jovian material thrown upward by the fireball. Others think the splotch is formed from rocky cometary rubble. The ring (2) is a ripple in Jupiter's atmosphere.  Most astronomers think the unexpected bruise (3) is more fireball fallout, perhaps material that didn't rise as high as the ring's jetsam. Or it could be the scar where descending debris from the fireball set off a second shock wave.

Up next: Galileo On December 7, 1995, a probe launched from the Galileo robot explorer will slam into the Jovian atmopshere at more than 100,000 miles per hour, sampling the clouds with an array of instruments as it descends. Meanwhile, Galileo will orbit Jupiter, taking readings of the planet's electricity and magnetism.

Planetary plummet Unfurling a parachute, the probe will descend 125 miles through the cloud layers, measuring the temperature and the atmospheric pressure, and looking for signs of electrical flashes. The probe will relay the information back to Galileo for a little over an hour before it succumbs to the atmosphere's enormous pressures.

Probe Weight: 750 lbs. Height: 34 inches Diameter: 49 inches 

Upper atmosphere Probe decelerates, small parachute opens

17 miles Top cover removed, main parachute opens

26 miles Heat shield drops off

125 miles Pressure destroys probe


USN&WR -- Basic data: NASA, Jet Propulsion Laboratory