Princeton, N.J. -- The odds of earth suffering a catastrophic collision with an asteroid over the next century are about one in 5,000, which is less likely than previously believed, according to research published this month.

Astronomers using data from the Sloan Digital Sky Survey found that the solar system contains about 700,000 asteroids big enough to destroy civilization. That figure is about one-third the size of earlier estimates, which had put the number at around two million and the odds of collision at roughly one in 1,500 over a one hundred-year period.

"Our estimate for the chance of a big impact contains some of the same uncertainties as previous estimates, but it is clear that we should feel somewhat safer than we did before we had the Sloan survey data," said lead researcher Zeljko Ivezic of Princeton University.

The results were published in the November issue of the Astronomical Journal.

The new estimate draws on observations of many more asteroids, particularly small faint ones, than were available in previous impact risk estimates, said Ivezic. The ability to detect faint objects in large numbers is a hallmark of the Sloan survey, a multi-institutional collaboration that is mapping one-quarter of the sky. While its main purpose is to look at objects outside our galaxy, the survey also records images of closer objects that cross the view of its telescope, which is located at the Apache Point Observatory in New Mexico.

The survey data also allowed the astronomers to gauge the size of asteroids with improved accuracy, which required categorizing the objects by their composition. Asteroids with a surface of carbon -- looking like giant lumps of coal -- are darker than those made of rock. A small rocky asteroid therefore looks just as bright as a much larger one made of carbon.

"You don't know precisely the size of an object you are looking at unless you know what type it is," Ivezic said, noting that the Sloan survey provides information about the color of objects, which allows astronomers to distinguish between carbon and rock.

Based on observations of 10,000 asteroids, the researchers estimated that the asteroid belt contains about 700,000 that are bigger than one kilometer (six-tenths of a mile) in diameter, which is the minimum size thought to pose a catastrophic risk to humans and other species. The asteroid belt is the source for a smaller group of asteroids called "near- earth objects," which have broken from the belt and have the potential to collide with earth. Although they did not specifically observe near earth objects, the researchers believe that their census of main belt asteroids reveals the likelihood of collisions with similarly sized near-earth asteroids.

Ivezic noted that the new impact risk estimate, like most previous ones, relies on assumptions about a single event 65 million years ago when a 10-kilometer asteroid collided with earth and killed the dinosaurs. The researchers assumed that such impacts occur on roughly 100 million-year intervals and used that statistic to calculate the impact odds for the more common asteroids of smaller sizes. This calculation required knowing how much more common one-kilometer asteroids are than 10-kilometer ones, which was hard to measure before the Sloan data was available.

"There is a lot of uncertainty when you have a sample of only one event," Ivezic said, referring to the dinosaur-killing impact. "But this is the best information we have."

Previous studies could detect only asteroids five kilometers or larger, so astronomers had to extrapolate to estimate the number of smaller ones, said Ivezic. The Sloan researchers found that this approach produced high estimates. When they could actually observe them, the small asteroids were not as plentiful as had been expected from observations of large ones.

The reason for this reduced number of smaller asteroids is an open question, which, if answered, may offer important clues about the history of the solar system and the factors that shaped the asteroid belts, said team member Serge Tabachnik of Princeton.

Another valuable piece of information for scientists is the observation that the rock and carbon asteroids are separated into two bands, said co-author Tom Quinn of the University of Washington. The heart of the rocky asteroid belt is 260 million miles from the sun, while the other is 300 million miles from the sun. The sun and earth, by comparison, are 93 million miles apart.

The astronomers attribute much of the success of the study to software that automatically identifies asteroids from among the millions of images observed by the Sloan survey. Independent tests by Mario Juric from the University of Zagreb, Croatia, have shown that the Sloan software finds at least nine of every ten asteroids.

"We have only five minutes to follow the motion of an asteroid as it passes in front of the telescope," said Robert Lupton, a Princeton researcher who developed the software for automatic detection of asteroids. "But we have found that we detect them very efficiently and reliably." Lupton said the team benefited greatly from software for finding the positions and relative movements of objects, developed by Jeff Pier, Jeff Munn, Robert Hindsley and Greg Hennessy of the U.S. Naval Observatory.

"The Sloan study is a major advance in our understanding of the gross asteroid belt structure," said Robert Jedicke, an asteroid expert at the University of Arizona. "Their determination of the Earth impact rate for killer asteroids agrees with soon-to-be-published results based on data from the Spacewatch Project at the University of Arizona." The Arizona team based its risk estimate on a study of near-earth objects, rather than main belt asteroids.

The Sloan Digital Sky Survey (www.sdss.org) is a joint project of the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins University, the Max-Planck-Institute for Astronomy, the Max-Planck-Institute for Astrophysics, New Mexico State University, Princeton University, the United States Naval Observatory and the University of Washington.

Funding for the survey has been provided by the Alfred E. Sloan Foundation, the participating institutions, the National Aeronautics and Space Administration, The National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho and the Max Planck Society.