Phenomena, Comment & Notes

While Comet Shoemaker-Levy 9 is history, space scientists are 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.

(If such an object hit the ocean rather than land, we would not be as lucky as one might at first think. Jack Hills of Los Alamos National Laboratory said after the talk that a two-mile-wide object coming down in the middle of the Atlantic would set off tsunamis that would reach all the way to the Appalachians. Washington, D.C. would be under 500 feet of water.)

In Morrison's view, there is a 1-in-5,000 chance that such an event will occur during anyone's lifetime. Or, in the language of the poll, he estimates it happens once in 100,000 to 300,000 years. Very rare, he concedes, but when it does happen the results are apocalyptic. "It is the only natural hazard that we could-in principle-do something about," he continued. If we picked up one coming in, we could set off a nuclear bomb nearby, nudging it into a slightly different orbit, one that would miss Earth by a comfortable margin.

Smaller objects would do catastrophic damage locally. Chris Chyba of the White House Office of Science and Technology Policy said that the comet or asteroid that exploded over Siberia in 1908 was about 200 feet across and exploded with the force of 12 megatons of TNT set off six miles above the ground. It blew down all the trees within 850 square miles, an area about the size of Washington, D.C. and neighboring counties. The strength of shock waves required to blow down trees, Chyba pointed out, is that needed to explode frame houses. He estimated that such objects hit Earth about once every 300 years. Over land, he added, it happens once every thousand years.

Still smaller bits and pieces hit us more often. I had not known that in 1965 one exploded in the air near Revelstoke, British Columbia, with the force of a few tens of kilotons, the magnitude of the Hiroshima bomb. Trappers found meteorite fragments on top of a snowfield.

Morrison said that worldwide, only about 15 people or, as he put it, a work force about the size of one shift at a McDonald's, are trying to catalog all the objects that could hit Earth. They estimate there are 2,500 short-period comets or asteroids whose orbits cross Earth's. It could take 10 to 20 years to plot 95 percent of them, he said, "and a very long time" to get the last one. Gene Shoemaker heads a committee that will present its recommendations on the matter to Congress later this month.

Emphasizing that he was speaking personally, Chyba responded to the thinly veiled appeals for more money by noting that it will be difficult to find the funds for an expanded effort. Other scientists want comparable sums to study equally important global problems, he said, and there is not enough money to properly fund everything.

Meanwhile, we still have lots to learn from the pyrotechnics of last July. For scientists the excitement is in the chase. Take the most basic question: What was that damn thing? Just because everybody has been calling it a comet does not mean that it was one. In that same poll, taken by Prof. Andrew Ingersoll of Caltech, the respondents split on the nature of Shoemaker-Levy 9. Thirty-two percent thought the progenitor (whatever was there before Jupiter broke it into 21 pieces) had been solid; another 32 percent said no, it had been unconsolidated gravel. Not far behind, 22 percent voted for fluffy snow. On the identity of the dark clouds of debris around impact sites, nearly 60 percent thought they were hydrocarbons, but about 30 percent considered them to be silicates. "Will there be recognizable remnants of the impacts when Galileo arrives?" Ingersoll asked. Forty-seven percent said yes, while 51 percent said no.

This is real science: not magisterial pronouncements of fact for us unwashed layfolk, but the fever of the hunt, with the added enticement of not being certain of the nature of the prey. In the matter of Shoemaker-Levy 9, the fun is only beginning. Stay tuned. And in the meantime, remember to look up every once in a while. Just in case.

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