(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.