Word: asteroids
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Before you become alarmed, however, you should understand that this was a close encounter only in a relative sense. At its closest, the asteroid was about 450,000 miles away, roughly twice the distance between the earth and the moon. Still, in cosmic terms it was virtually a direct hit. No asteroid has been sighted so near since 1937, when Hermes, a minor planet nearly half a mile in diameter, passed by at about the same distance...
...asteroid, called 1989FC in accord with the official numbering system of the International Astronomical Union, was first detected by Henry Holt, an adjunct professor of geology at Northern Arizona University. That was in late March, after it was already moving safely away from earth. Holt spotted the speeding intruder in photographs taken through an 18-in. telescope at the Palomar Observatory in Southern California, during a systematic search for asteroids passing close by, which scientists call earth grazers. Holt figures that 1989FC may be in Hermes' league, but other astronomers dispute the claim, saying the new asteroid may be only...
Ominously, astronomers say 1989FC will be back. Like the earth, the asteroid orbits the sun, but it takes about 380 days to do so, instead of 365. When the asteroid passes by again next April, it will probably be at a safer distance from the earth. The next time earthlings need to worry, says astronomer Brian Marsden of the Harvard-Smithsonian Observatory, who calculated the orbit based on Holt's observations, is 2015. "If our figures are correct," he says, "the asteroid will have made 25 orbits to earth's 26, and we will meet again...
That could mean a direct hit or, more probably, another nerve-jangling near miss. But even if 1989FC never strikes earth, a similar asteroid is destined to do so eventually. It has happened so many times before, in fact, that the earth's surface would be as pockmarked as the moon's were it not for the cosmetic effects of erosion caused by the oceans and atmosphere. Half-mile asteroids are a dime a dozen in the solar system, and they run into the planet once every 100,000 years, on average. That means the next one could strike...
Then there are the really big asteroids -- masses of rock and iron five or ten miles across that hit every 10 million to 100 million years. The half- milers are bad enough, but these giant ones pose a threat to the entire planet. It was such an asteroid (or an equivalent-size comet) that many scientists believe caused the extinction of dinosaurs some 65 million years ago. The primary evidence, discovered by the late physicist Luis Alvarez and his son Walter, a geologist, is a layer of the element iridium laid down in sedimentary rock at about the time...