Incident At Tunguska

The fireball that rose over a conifer forest in the remote Stony Tunguska River basin in central Siberia on the morning of June 30, 1908, reached an altitude of twelve miles, and the blast was heard hundreds of miles away. Those closest to the explosion, the townspeople of Vanavara, 40 miles away, felt a wave of intense heat; windows cracked, objects fell from walls, and one man sitting on his porch was thrown several yards and knocked unconscious. Trees were flattened and scorched over an area of several hundred square miles, their felled trunks all pointing away from the epicenter.

Even now, more than three-quarters of a century after the spectacular event at Tunguska, scientists are certain only that a celestial intruder was responsible. Some argue that it was an asteroid as large as 500 ft. across and weighing 7 million tons, which rapidly heated as it entered the earth's atmosphere and exploded about five miles above ground. Others believe it was a small comet. Whatever the cause, the destructive power of the object from space rivaled that of a very large nuclear warhead; scientists gauge the explosion at twelve megatons.

Although both comets and asteroids can wreak considerable havoc if they collide with the earth, they are of very different natures and origins. Asteroids are rocky chunks that range in size from pebbles to a mammoth named Ceres that astronomers estimate to be as much as 600 miles across. Most of them orbit the sun in a belt between Mars and Jupiter and are thought to be either remnants of a planet that disintegrated early in the life of the solar system or celestial building blocks that never quite coalesced into a planet. Occasionally an asteroid is slowed in its travels, probably by the gravity of nearby Jupiter, and tugged into an orbit that sends it closer to the sun and the inner planets, including earth. And sometimes collisions occur. About 100 of the earth's largest known craters are believed to have been caused by huge asteroids dropping from the heavens.

Comets are more mysterious. Unlike asteroids, which reveal themselves in the form of meteorites that can be pried from the ground and analyzed in laboratories, comets have left no known remnants on earth; they have been studied only from afar--through telescopes. (Scientists hope to unlock more cometary secrets when Soviet, Japanese and the European Space Agency probes fly past Halley's comet in March 1986.)

Most experts agree with Harvard Astronomer Fred Whipple, who characterized comets as "dirty snowballs" consisting largely of ice and mineral-rich dust. Comets are thought to originate in the Oort cloud, a distant shell of icy debris believed to surround the solar system and extend out some 10 trillion miles from the sun. Passing stars sometimes dislodge snowballs from the cloud, which can sprout the classic luminous tails of gas and dust as they plunge toward the sun. Most comets whip around the sun and head back out of the solar system. Some, like Halley's, periodically return. But others crash into the sun or the planets, a fact confirmed in 1979 when a Defense Department satellite photographed a comet plunging to a fiery death in the sun.

Although comets are more ethereal than asteroids, usually quite small and probably never more than several miles in diameter, they often pass earth at speeds greater than 100,000 m.p.h. and thus have great destructive power. Despite the fact that no direct evidence of a cometary collision has been , found, the earth has undoubtedly been hit many times by the icy missiles.

If comets do strike in barrages every 26 million years or so, as the new cyclic theory holds, inhabitants of the earth 13 million years from now are in for some trouble. But it is also possible, even probable, that long before that time, astronomers will spot a random, incoming comet or asteroid nudged by the gravity of an outer planet into a direct collision course with the earth.