In 134 years, watch out for comet Swift-Tuttle comet to be tracked for chance it might hit Earth

October 20, 1992|By Newsday

New calculations suggest a tiny chance that a large comet that is now nearing the sun may take a swipe at Earth when it returns again in 134 years.

Although the chances of comet Swift-Tuttle hitting Earth in the year 2126 are extremely small, a collision could perhaps snuff out much of the life on Earth.

The Earth's surface is marked by craters from such collisions. It is thought that such an incident 60 million years ago erased much of the life on Earth, including the dinosaurs.

The comet is thought to be a chunk of ice and rock about 6 miles in diameter, moving in an orbit that takes it close to the sun every 130 years.

Brian Marsden, director of the International Astronomical Union's Central Telegram Bureau, in Cambridge, Mass., alerted the world's astronomers to the tiny hint of the danger yesterday. He said "it appears prudent to attempt to follow Swift-Tuttle for as long as possible" after its current trip close to the sun.

Tracking Swift-Tuttle with telescopes as it leaves the sun might help define its orbit, letting astronomers estimate chances of collision. "That is our greatest defense, to follow it out as far as we can with telescopes in the southern hemisphere," Mr. Marsden said. "That would let us get enough observations to be reasonably sure it won't hit us."

Mr. Marsden said his calculations of Swift-Tuttle's orbit indicate it will return to visit the sun July 11, 2126. But because of irregularities in the comet's travels, a change of only 15 days in the timing of its return "could cause the comet to hit the Earth on Aug. 14, 2126."

Chances are much, much larger, however, that the comet will not hit Earth. "The odds are that it will miss. There's a much greater chance it will miss us by a respectable amount, tens of millions of miles," Mr. Marsden said. As it passes this time, Swift-Tuttle will be about 110 million miles from Earth.

Astronomer Fred Whipple, former director of the Harvard-Smithsonian Center for Astrophysics, also in Cambridge, thinks the danger of collision is essentially nil.

"The Earth is a pretty small target when you look at it from an astronomical point of view," Mr. Whipple said. "I wouldn't consider doing anything about it, except watch for it when it comes back next time. There's no reason for people to be scared."

Comet and asteroid specialist Donald Yeomans, at the Jet Propulsion Laboratory in California, said, "The orbit [of the comet] is so poorly known, and the Earth's diameter is so small, that I frankly don't know why" Mr. Marsden notified astronomers.

Mr. Marsden voiced concern about Swift-Tuttle mostly because of its unpredictability. Astronomers had expected it to return in 1981, but it arrived 11 years late.

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