Heads Up

EVERY AUGUST THE EARTH PASSES THROUGH THE orbital path of Comet Swift-Tuttle. If the comet ever happened to be there, the 10-km-wide (6-mile) chunk of ice and rock could slam into the planet, carving an enormous crater, generating tidal waves and throwing up a worldwide pall of dust that could block sunlight for months. Plants would be largely wiped out, and so would many species that ultimately depend on plants for food -- including, perhaps, the human race. Just such a disaster, many scientists believe, killed off the dinosaurs 65 million years ago. A smaller strike in the Atlantic 35 million years ago, described in a U.S. Geological Survey report released last week, sent a 300-m- high (1,000-ft.) wall of water over much of the U.S. East Coast.

Luckily, planet and comet have never been in the same place at the same time. The only visible effect of the crossed paths is the annual Perseid meteor shower, caused when lingering comet dust burns up in the earth's atmosphere. But humanity may not be so lucky for long: there is a chance that the next time Swift-Tuttle comes around, probably in the year 2126, it will fall to earth. The odds are small -- 10,000 to 1 against -- according to the International Astronomical Union's Brian Marsden. But the downside is so great that Marsden has urged his colleagues to keep careful track of Swift-Tuttle so its orbit can be more precisely calculated. If it really is on a collision course, the only answer may be to blast it from afar with nuclear warheads.