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Phenomena, Comment & Notes

While Comet Shoemaker-Levy 9 is history, space scientists are just beginning to piece together the details of its pyrotechnic encounter with Jupiter last July

Both morning stars this month are, as always, actually planets: in this case Jupiter and Venus. Jupiter is so big that it can fairly be called a "failed star" (Smithsonian, September 1974). Anyone who was around last summer, however, now probably thinks of Jupiter first as the planet that was bombarded with the mile-wide fragments of a shattered comet. During November and early December, Jupiter was too close to the Sun in the sky to be seen. Now it has moved far enough away to be visible, and we will learn more about what happened to our celestial neighbor-and could happen to us.

When the 21 pieces of the "string of pearls" comet slammed into the Jovian cloudtops at speeds close to 135,000 miles per hour, the resulting explosions sent up plumes of dust and debris. High-altitude winds quickly spread the stuff into a thin blanket over large areas of the planet. A key question is how long such material stays aloft, blocking sunlight to everything beneath it. Or, if such a comet hit Earth, how long would the resulting "winter" last? For Jupiter, at least, we're about to find out.

Many of the astronomers most concerned with such questions met outside Washington, D.C. in late October and early November. (The meeting was run by the Division for Planetary Sciences of the American Astronomical Society. The parent body meets this month in Tucson, with more sessions on the fireworks.) Scientists spoke of data radioed to Earth from a satellite just days before or cited a colleague's latest findings ("I just heard an hour ago. . ."). More than a hundred ground-based observatories had watched the fireworks. Thirty-nine orbits of the Hubble Space Telescope, far and away the most difficult instrument to get time on, were devoted to watching the final hours of Periodic Comet Shoemaker-Levy 9. And the Galileo spacecraft, which will arrive at Jupiter in December along with a probe, which will travel down through that atmosphere, was in the right place to "see" the whole thing.

The comet in question had been named for its discoverers, Gene and Caroline Shoemaker and David Levy (Smithsonian, June 1994). Gravitationally captured by Jupiter, it had been in orbit around that planet (instead of the Sun) for decades and possibly a century. Working backward in time, celestial mechanicians found that the comet had made its approach to Jupiter inside the orbit of the Jovian moon Callisto by 1970 or so, and in 1992 it had passed within only 57,000 miles of the planet's cloudtops. The gravitational field of Jupiter, pulling more strongly on the part of the comet closest to it, actually pulled it apart. By May of last year, the 21 major fragments had strung out along 60,000 miles of the comet's orbit. When the collisions occurred just two months later, the objects occupied four million miles of orbit.

Astronomers knew that all the impacts would take place on the nightside of Jupiter, not visible from Earth. They feared that all they might see would be a momentary brightening of some of the inner moons. What they actually did see exceeded their wildest dreams.

In many cases, they saw two flashes before the main event. The first was believed to be either the fragment itself, heated by friction as it entered the highest atmosphere of Jupiter, or gas and dust in its wake being lit by the fragment. The second flash was believed to be the actual fireball, when the object had penetrated 200 miles or so into the atmosphere. This raised a plume thousands of miles above the cloudtops-and by this time Jupiter had rotated enough so we could study the plumes from Earth.

The best was yet to be. Plumes spread out sideways as well as rose. And then, pulled by the enormous gravity of the giant planet, the plumes slammed back down into the cloudtops, creating shock waves that instantly heated those clouds to 10,000 degrees F. Spellbound scientists marveled at flares the diameter of Earth and as bright as the Sun's surface. And from those impact zones dark clouds-"brown gunk," as one chemist called them-spread around the planet.

As spectacular as it was, this kind of thing happens all the time on Jupiter. A poll taken at the meeting showed that 90 percent of those responding thought such bombardments occurred every 100 years or even every 10 years. And it is not unknown for Jupiter to break up a comet or asteroid before it hits the planet. Astronomers at the meeting estimated it happens every 150 years or so. They pointed to linear strings of craters on the Jovian moons Callisto and Ganymede, craters we will get a good look at when Galileo flies by.

As we all know, life on Earth is nowhere near so violent. That's because Jupiter is 11 times as wide and 318 times as massive as our planet. Jupiter is not only a much larger target than Earth; its much stronger gravitational field pulls in comets and asteroids that would go speeding by Earth. Those same poll respondents thought our chances of avoiding catastrophe are a million times better. Yet we know Earth has been hit hard in the not so distant (geologically speaking) past; craters are still visible (Smithsonian, September 1989). While 12 percent thought three-mile-wide objects might strike Earth once every 100,000 years, 60 percent thought 100 million years was closer to the truth.

David Morrison of the NASA Ames Research Center supplied perspective. The postulated object that 65 million years ago caused a mass extinction, including most species of our beloved dinosaurs, would have been six or seven miles across. Comets or asteroids the size of the fragments that hit Jupiter, one or two miles across, would not cause mass extinction but would trigger global ecological catastrophe. Exploding with the force of 100,000 to 1 million megatons of TNT, just one such object would dig a crater ten miles across and blast enough dust into the atmo-sphere to block 99 percent of the sunlight for a month. Even in the Tropics temperatures would drop below freezing. Most natural ecosystems would survive, but crops would die, leading to the deaths of up to a billion people.

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