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Near-Earth Asteroids Lurk in Obscure Orbits
By Robert Roy Britt
Senior Science Writer
posted: 09:26 am ET
25 October 2000
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****EMBARGOED FOR October 24, 2000**** PASADENA, Calif. In what has become almost a ritual of revision, the estimated number of asteroids that could one day come close to Earth has been changed again. This time the estimation of near-Earth asteroids, or NEAs as they are called, has gone back up, according to a researcher at the Massachusetts Institute of Technology who says scientists have been looking in the easiest places for NEAs, and therefore haven't accounted for at least 200 of them. Near-Earth asteroids are space rocks larger than 0.6 mile (1 kilometer) whose orbits come close to that of Earth. Gravitational effects of Earth and other planets could alter the course of such asteroids, so scientists study their trajectories and estimate how their travels might change many decades into the future, all in an effort to learn if they will ever smash into our planet. Of course they have to find them first. While none of the more than 400 known NEAs are currently on a course that would actually hit our planet, researchers have been unable to agree on how many there are, and hence how many are left to find. The last estimate, made in January, ranged from 700 to 900. Prior to that researchers had calculated there might be as many as 2,000.Charting a different course But scientists may have wrongly assumed that most NEAs orbited the Sun in roughly the same plane as Earth. Instead, there could be many rocks roaming the solar system in orbits that cut through a different plane, one inclined to the plane of Earth's orbit, said graduate student Scott Stuart, who developed the idea with colleagues at the MIT Lincoln Laboratory. Because the inclinations of known NEAs are not representative of the entire population, there may be more undetected NEAs out there, Stuart said, figuring that there are around 1,100 of them. "Were finding a significant number of asteroids at higher inclinations," said Stuart, in presenting his findings Tuesday at the American Astronomical Societys Division for Planetary Sciences meeting in Pasadena, California. Brian Marsden, director of the Minor Planet Center at the Smithsonian Astrophysical Observatory, said the latest revision would not have much of a material effect on NASA's goal of finding 90 percent of all NEAs over the next decade."It doesnt alter the mission, it doesnt alter what were doing. The work wont change," said Marsden, adding that it takes very little to revise the estimates. "There are so many free parameters in this whole mission," he said. Why the rush to tally the rocks? Asteroids bigger than 1 kilometer are thought to be capable of causing extensive damage on a global scale, possibly even rendering human life difficult or impossible. Regardless which number is right, the odds of one of these rocks destroying civilization in any given year range from about 1-in-100,000 to 1-in-300,000, experts say. The sooner we pin down the locations and orbits of all NEAs, the sooner we can rest easy or, if the case may be, worry about what to do.
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