``We know now where most of them are and where most of them are going. That really has reduced our risk'' of an impact, said Amy Mainzer of the NASA Jet Propulsion Laboratory.

NASA researchers also downgraded their estimate of the number of medium-sized asteroids, saying there are 44 percent fewer than previously believed. The downside is that scientists have yet to find many of these mid-sized asteroids, which could destroy a metropolitan city.

``Fewer does not mean none,'' Mainzer said. ``There are still tens of thousands out there that are left to find.''

The updated census comes from data from NASA's sky-mapping spacecraft named Wide-field Infrared Survey Explorer, or WISE, which launched in 2009 to seek out near-Earth objects, galaxies, stars and other cosmic targets.

Unlike previous sky surveys, WISE has sensitive instruments that can pick out both dark and light objects, allowing it to get the most accurate count yet of near-Earth asteroids. The spacecraft takes a small sample of asteroids of varying sizes and then estimates how large the population would be.

For the largest asteroids _ bigger than 3,300 feet (1,000 meters) across _ NASA said 911 of the 981 thought to exist have been found. None poses a threat to Earth in the near future, the space agency said.

Previous estimates put the number of medium-sized asteroids at 35,000, but WISE data indicate there are about 19,500 between 300 feet (91 meters) and 3,300 feet (1,000 meters)wide. Only about 5,200 have been found and scientists said there is still a lot of work left to identify the potentially hazardous ones.

Results were published in the Astrophysical Journal.

WISE is not equipped to detect the more than a million smallest asteroids that could cause damage if they impact Earth. The spacecraft recently ran out of coolant and is currently in hibernation.