Target Earth

Space -- it seems there's never enough of it, even in space. Circling the sun along with Earth and the other planets are a swarm of asteroids and comets, many of them big enough to deliver a catastrophic jolt -- which is what happened 65 million years ago, say many scientists. Luis Alvarez, a Nobel laureate physicist, and his geologist son Walter in 1980 concluded from geological evidence that a huge asteroid struck the Earth at the end of the Cretaceous Period, blanketing the planet with dust, smoke and ash that blocked the sun for months and wiped out the dinosaurs.

This theory has rapidly gained support from other scientists. Three years ago geologists found a submerged crater off the Yucatan coast of Mexico and identified it as the likely site of the impact. From the crater, scientists judged the asteroid was about 10 kilometers in diameter, with an energy release equivalent to 200 million hydrogen bombs.

A less devastating asteroid scored an Earth bullseye as recently as June 30, 1908. At least that's what scientists think happened near the Tunguska river in Siberia, when at 7:30 a.m. a huge flash lit the sky and an explosion, estimated as the equivalent of 800 Hiroshima bombs, flattened the Siberian forest for hundreds of square miles. Fortunately, the blast occurred in an unpopulated area, though it certainly startled the closest witness -- a woodcutter in front of his cabin 35 miles away. Recent analyses (by a team including Mac Low's collaborator Kevin Zahnle) conclude that the culprit was a stony asteroid about the diameter of a football field exploding five miles above the ground.

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