Dealing with Threats From Space

By Michael Lemonick.

It is a sunny afternoon in Karachi, and the streets of Pakistan's largest city are crowded with shoppers, apparently unconcerned about the rising tension between Pakistan and India. Suddenly, a second sun bursts into view overhead, so bright it temporarily blinds thousands and so hot it blisters the skin. Thirty seconds later, the shock wave hits, crumbling buildings and throwing people to the ground. To the Pakistanis, only one explanation is possible for the tremendous blast: India has launched a nuclear attack. They immediately order their bombers, armed with atomic bombs, to strike back at India, which responds in kind. Only later do the surviving officials learn of their mistake. The object that exploded over Karachi was not a nuclear weapon but a large meteor hurtling in from outer space.

Though this scenario sounds like the plot for a made-for-TV movie, Eugene Shoemaker, a respected U.S. Geological Survey scientist, is concerned that just such an event--and an unwarranted reaction--could occur. Shoemaker expressed his fears at a recent Baltimore meeting of the American Geophysical Union (AGU): "The effect of a meteor blast appears the same as a high- altitude nuclear explosion," he said. "If this happens in the wrong place, people will think they've been nuked."

Meteors, which are asteroids or cometary debris that has entered the atmosphere, continually shower the earth. Most of them are small and either break up or are burned to ash by frictional heat generated by their plunge through the atmosphere. But, explains Shoemaker, the incineration of larger asteroids is far more violent. An asteroid 80 ft. across, striking the atmosphere at 50,000 m.p.h., compresses the air in its path so much that in effect the asteroid is stopped dead in its tracks, converting kinetic energy almost instantaneously into heat, light and a powerful shock wave. That causes a tremendous explosion, in this case equivalent to the blast of a one-megaton bomb.

If a meteor were to burst in the atmosphere tomorrow, Shoemaker says, "the Soviets and the U.S. would know what it was" and not react militarily. Their detectors could distinguish between a nuclear explosion, which generates million-degree temperatures, X rays and gamma rays, and an exploding meteor, which would produce considerably lower temperatures and no deadly radiation. But smaller nations, unaware of the nature of the blast, might react violently. Says Shoemaker: "Suppose it happens over Syria or Pakistan?" He proposes that the U.S. immediately try to determine whether the explosion was of cosmic origin and notify the affected nation.

Since 1973, Shoemaker has been photographing the sky in search of asteroids that periodically cross the earth's orbit and thus pose the danger of a collision. To date, he says, 57 such asteroids at least 1 km (.62 mile) in diameter have been cataloged. In addition, about three earth-crossing comets are detected each year. From the rate at which new earth crossers are discovered, Shoemaker estimates that there are some 2,000 asteroids in this category and that 100 comets intersect the earth's orbit every year.

His calculations suggest that asteroids packing the explosive energy of one megaton should enter the atmosphere on an average of once every 30 years, larger asteroids with a 20-megaton punch every 400 years, and a 1 km, 10,000- megaton comet or asteroid once in 100,000 years.

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."

With reporting by Jon D. Hull/Los Angeles