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...
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