Safe passage. The asteroid Toutatis will pass Earth in 2004.

A Little Respect for the Asteroid Threat

ARLINGTON, VIRGINIA--A decent-sized-asteroid could end civilization on Earth. At a workshop here last week, researchers heard mixed tidings about this threat. The good news is that the search for civilization-ending asteroids seems to be on schedule. On the other hand, astronomers haven't found many of the tens of thousands of smaller bodies that could still wreak havoc.

Asteroids whose paths could cross ours are called near-Earth asteroids (NEAs). Anything 1 kilometer in diameter or larger could disrupt the environment badly enough to deal civilization a death blow. At the Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids, astronomer Alan Harris of NASA's Jet Propulsion Laboratory in Pasadena, California, reported that the current discovery rate of about nine NEAs per month now indicates a range of 1000 to 1200 NEAs that size. So far, 635 have been discovered and tracked. Only one looks to have any chance of ever hitting Earth (Science, 5 April, p. 27), and the odds are 1-in-300 at most in 2880. Harris says the project has a good chance of finding 90% of the big NEAs by 2008.

Many researchers, however, think more needs to be done. Monster 1-kilometer asteroids jolt Earth only every few hundred thousand years, but a still-formidable 300-meter body strikes every 60,000 years or so, they point out. If such an impactor hit within hundreds of kilometers of the U.S. Atlantic coast, it could send a 100-meter tsunami into Boston and New York City. As telescopic imaging technology has improved, surveying such 200- or 300-meter "subkilometer" objects might soon be practicable.

But it wouldn't be cheap. A subkilometer survey would cost considerably more than the $4 million per year NASA is spending on its current search. In the past 2 years, the National Research Council has twice recommended that NASA and the National Science Foundation (NSF) jointly fund a survey facility such as the ground-based Large-Aperture Synoptic Survey Telescope (LSST) currently under study by NSF (Science, 19 July, p. 317). With something like a $95 million start-up cost, LSST could find 90% of 300-meter NEAs in 10 years, Harris says.

Related sites
Workshop Web site
Session organizer Erik Asphaug's site
Near-Earth Object Program at NASA's Jet Propulsion Laboratory
LSST Web site

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