Researchers estimate that there are about 1,000 NEOs larger than 1 kilometer, the minimum size considered capable of causing global devastation. Though no one knows for sure, such objects are suspected of hitting Earth every 100,000 to 300,000 years.

If one were found to be headed our way, experts say it's possible the rock could be deflected or destroyed by detonating nuclear explosives on or near it. The technology needed to mount such a mission has yet to be developed.

For now, nonetheless, the search is on.

NASA has a congressionally mandated goal to find 90 percent of these large NEOs by 2008. Roughly 500 have been found by various individuals and international research teams. But as more are discovered, those that remain become statistically harder to root out, and most astronomers don't expect NASA's goal to be met on time.

Meanwhile, critics have long maintained that not enough is being done to find smaller NEOs, which could cause regional destruction if they hit Earth. Others worry that the cost and resources needed to find all these small space rocks, which number in the millions, is prohibitive, at least in the near term.

Other institutions inside and outside the United States contribute to the search and research of NEOs. The Minor Planet Center, in Cambridge, Massachusetts, serves as the clearinghouse for all data collected on space rocks.

The two new centers represent an increased internationalization of the effort, but it's not yet clear what role they will ultimately play.