The revised calculation comes from data gathered by NASA's Near-Earth Asteroid Tracking System (NEAT) and published in the January 13 issue of the journal Nature.

"Until now, scientists thought the population of large, near-Earth asteroids was between 1,000 and 2,000, but we've downgraded that figure significantly," said Dr. David Rabinowitz, of Yale University, New Haven, CT, lead author of the article and NEAT co-investigator. "We now believe there are between 500 and 1,000 near-Earth asteroids larger than one kilometer (about 0.6 miles) in diameter.

"This newer estimate was made possible by the computerized technology of the NEAT camera," Rabinowitz said. The NASA-funded system began tracking near Earth asteroids and comets in 1995 with a charge-coupled device camera mounted on a 1-meter (39-inch) Air Force telescope atop Mount Haleakala on Maui, HI.

The new figures may represent good news in the quest to achieve NASA's stated goal of finding 90-percent of all large, near-Earth asteroids by 2010, according to Pravdo. "Right now we know of 322 large, near-Earth asteroids," he said. "That was a fairly small fraction of the 2,000 asteroids in our previous estimate. With our new calculations of between 500 and 1,000 such objects, this 322 figure represents a large chunk."

While stressing that we must learn more about potential hazards from asteroids, Rabinowitz said, "None of the asteroids we've observed will hit Earth anytime in the near future."

"This new analysis reduces by half the estimated number of these potential hazards to Earth," Pravdo said.

"In the past, we relied on humans poring over photographic plates of the nighttime sky," Rabinowitz said. "The problem was, they didn't know how many asteroids they were missing, because they couldn't see faint objects. People's eyes also became tired and teary and they overlooked some objects. Machines don't get tired."

"With this computerized technology, we can find asteroids more easily and count them more accurately," said NEAT project manager Dr. Steven Pravdo of NASA's Jet Propulsion Laboratory, Pasadena, CA, a co-author of the Nature article. "It's important to know your observational limits, and with that information, we can develop models for what we are not able to see. This makes our estimates even more accurate."

Additional co-authors on the Nature article are Eleanor Helin of JPL, NEAT principal investigator, and Kenneth Lawrence, also of JPL. Helin was also principal investigator of the Palomar Planet-Crossing Asteroid Survey, a photographic search program, conducted for almost 25 years until it was discontinued and replaced by NEAT, the electronic detection program, in 1995. Her efforts were key to the organization of the NEAT program.

Data gathered by the asteroid tracking system are processed at Haleakala, and then undergo post-processing and analysis at JPL. This latest asteroid estimate is based on data collected between 1995 and 1998. The asteroid tracking system has been on hiatus for the past year, but plans are in the works to re-activate the system in February using an upgraded 1.2 meter (48 inch) Air Force telescope on Haleakala. In addition, later this year, NEAT scientists will begin using the 1.2 meter (48 inch) Oschin telescope at Palomar Observatory near San Diego, CA.