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This century has already seen a major meteorite blast. In 1908, either an asteroid or a comet exploded about five miles above the remote Stony Tunguska River region of Siberia, igniting and flattening trees over hundreds of square miles. From descriptions of the blast and photographs of the damage, scientists have estimated that the object was at least 200 ft. across and caused a twelve-megaton explosion.
Depending on their velocity, size and composition, some meteors survive their fiery trip through the atmosphere and hit the ground, at which point they are dubbed meteorites. Most are in the form of pebbles or small rocks, but occasionally they are much larger. Scientists think it was a 130-ft. hunk of meteoric iron that hit Arizona with a force of 15 megatons between 20,000 and 50,000 years ago, digging a crater three-quarters of a mile across and 600 ft. deep.
But even greater menace lurks in the darkness of space. Scientists have speculated that objects as large as several miles across have crashed into the earth, spewing millions of tons of debris into the atmosphere, blotting out the sun for months or years, and causing mass extinctions of life--including, many believe, the dinosaurs. Of the known larger earth crossers, none seem to pose a threat in the near future. But, says Shoemaker, "until we have tracked all of them, something could sneak up on us."
What if a large asteroid or comet is discovered heading toward the earth? At the AGU meeting, Shoemaker and Colleague Alan Harris, of the Jet Propulsion Laboratory in Pasadena, Calif., suggested that the intruder could be diverted by landing a thrusting device on it. As a last-ditch effort, they say, a small nuclear warhead could be detonated on or near it. Says Shoemaker: "We have the technology to do that right now." But if the explosion simply broke the meteorite into large chunks, the danger would only be multiplied. "The more prudent solution," says Harris, "is to burrow a substantial charge into the object and blow it to smithereens."