Two decades ago, after carefully analyzing the Moon's cratered face, the late Eugene Shoemaker estimated that nearly 2,000 asteroids at least 1 kilometer across must exist with some chance of striking Earth. A collision of that magnitude, though rare, would have disastrous consequences for life. Small, dedicated teams of astronomers have been trying to track down these potentially hazardous asteroids ever since, but lately there's been a growing consensus that Shoemaker overestimated the impact threat. Recent studies have put the count of 1-km-wide near-Earth asteroids (NEAs) under 1,000, with one research team suggesting that it could be as low as 700.

However, a new assessment, published in the November 23rd issue of Science, has racheted the population estimate back up to near 1,250. The higher count comes from J. Scott Stuart (MIT Lincoln Laboratory), who analyzed nearly three years of NEA discoveries from the prolific LINEAR telescope near Socorro, New Mexico. Between March 1998 and February 2001, LINEAR covered almost 500,000 square degrees of sky and discovered 657 near-Earth asteroids — along with more than 110,000 new main-belt asteroids. Stuart's assessment uses the 606 NEAs found on the best nights.

His computer simulations explored possible orbital characteristics for NEAs and determined which of those orbits would have led to a discovery by LINEAR (an acronym for Lincoln Near-Earth Asteroid Research). Besides yielding a large population of 1-km NEAs, the resulting statistics argue that many of them have orbits with inclinations near 23°. This sizable tilt is shared by hundreds of objects belonging to the Phocaea and Hungaria "families" in the inner asteroid belt.

Because LINEAR has found so many more NEAs than any other search effort, asteroid specialists will likely consider Stuart's results the most definitive — if they hold up to closer scrutiny. Dynamicist Alan W. Harris (Jet Propulsion Laboratory) finds the new, higher value very plausible, in part because NEAs continue to be discovered at a fast clip. "I think it's interesting stuff," comments William Bottke (Southwest Research Institute), whose has modeled the NEA population theoretically. Bottke's results track Stuart's in most respects but don't yield so many high-inclination objects, a key discrepancy.

In 1992 a NASA-sponsored report backed a plan to locate 90 percent of all kilometer-size NEAs within a decade. After a slow start, that search is well under way, with about 550 of these large Earth-crossers now catalogued. But if the total population is well above 1,000, as Stuart suggests, then most of them await discovery. Reaching the 90-percent threshold could take LINEAR another 40 years, he estimates, because of diminishing returns over time. Harris thinks that's overly pessimistic, but he agrees that asteroid-hunters have decades of work yet ahead of them.