Whew! That Was Close

By MICHAEL D. LEMONICK

Where were you on the night of March 23? Out dancing, perhaps, or attending a PTA meeting or just sitting at home watching L.A. Law? If so, you did not realize how close you came to disaster. While you were blissfully unaware of the danger, a huge asteroid whizzed past the earth, coming closer than any other such heavenly body seen in 52 years. If the giant clump of rock -- half a mile across by one estimate -- had hit the planet, it would have packed the wallop of thousands of H-bombs and possibly killed millions of people. If it had come down in an ocean, it could have triggered tidal waves hundreds of yards high.

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.

The new 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 100 yds. across. Even if the smaller size is correct, no one would want to have 1989FC land in the backyard. A 100-yd.- wide asteroid hitting the earth at a speed of nearly 50,000 m.p.h. could dig a crater a mile or so across and several hundred feet deep -- similar in size to a gaping hole in the Arizona earth, known as Meteor Crater, that was blasted out some 40,000 years ago. Such an impact today would be enough to wipe out a major population center.

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 in a thousand lifetimes -- or before the end of next week.

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 the giant reptiles disappeared. Iridium is rare on the earth's surface but more common in asteroids.

If an enormous chunk of space rock hit the planet, the Alvarezes theorized, it would have largely disintegrated, casting a pall of iridium-rich dust and other debris over the world that could have lasted for months. Deprived of sunlight by this all-natural version of "nuclear winter," plants -- and the animals that fed on them -- would have died in droves. And when the dust finally settled, the iridium it contained would have formed just such a layer as the Alvarezes found.

Is there any way to avoid collisions with asteroids and comets? Perhaps. A nuclear warhead aimed right at a small asteroid could vaporize it, says Alan Harris, an astronomer at the Jet Propulsion Laboratory in Pasadena, Calif. But the warhead might also simply break the rock into pieces that would hit the earth anyway. A better plan, proposed by concerned scientists in the early 1980s, would be to use explosives to deflect an asteroid rather than destroy it. Properly positioned, a bomb could nudge a threatening object enough to make it miss the planet. The catch, says Harris, is that there would not be much time to react to an approaching celestial body. "With an asteroid like this one," he says, "you'd probably get a day's warning at best." In short, the most sensible thing to do about earth-grazing asteroids is try not to think about them.