What If a Comet Hits Earth?

Imagine that one of Shoemaker-Levy 9’s bigger pieces — a mile or two in diameter — is streaking in at 130,000 m.p.h., except that the target is not Jupiter but Earth. The mammoth chunk of rock and ice tears through the atmosphere and smashes into the ground with the force of 6 million H-bombs, gouging out a crater the size of Rhode Island and throwing so much pulverized real estate into the stratosphere that the sun is blocked for months and Earth goes into a worldwide deep freeze. If the comet hits an ocean, a pall of dust rises from underwarter sediment, and a tidal wave several thousand feet high races across the sea and hundreds of miles inland.

If that sounds like science fiction, think again. Comets and asteroids have crashed into Earth in the past. Craters marking the points of impact are mostly hidden by vegetation, their edges softened by erosion. But the size of some of the holes suggests that Earth has been hit by intruders at least seveal miles in diameter, as big as S-L 9 before it broke up.

It was probably such an object that wiped out the dinosaurs 65 million years ago. Realizing that a deadly collision could happen again, astronomer Eugene Shoemaker decided nearly two decades ago to use a small but powerful telescope to look for comets and asteroids headed this way. Five years ago, a member of Shoemaker’s team saw a chunk of rock perhaps a third of a mile across that had just zipped by the planet at a distance of only 450,000 miles. There are about 2,000 large bodies that cross the orbit of Earth and could, in theory, hit us. That is why Shoemaker and his colleagues have for years been urging a stepped- up program to search for Earth-crossing comets and asteroids. Now S-L 9 has spurred Congress to listen. Last week the House Subcommittee on Science, Technology and Space added an amendment to the NASA appropriation bill requiring the space agency to come up with a plan to find and catalog all menacing heavenly intruders within a decade.

What if one turns out to be on a collision course with Earth? Star Wars scientists think a nuclear warhead sent out to blow the comet off course might work, but others doubt it. Even if no one has a good answer yet, lawmakers have taken the first step: acknowledging that the threat is real.