New observations by a NASA space telescope that mapped the entire sky suggest that there are fewer potentially threatening asteroids in the solar system than previously thought.

The latest survey by the Wide-field Infrared Survey Explorer, or WISE, telescope found significantly fewer near-Earth asteroids in the mid-size range than previously estimated, according to NASA.

The findings also indicate NASA has found more than 90 percent of the largest near-Earth asteroids, including those the size of the one thought to be responsible for the dinosaurs' extinction 65 million years ago.

WISE surveyed the entire celestial sphere twice in infrared light from January 2010 to February 2011. The asteroid-hunting portion of the mission, called NEO (Near Earth Object) WISE, used the data to catalog more than 157,000 asteroids in the main belt and discovered more than 33,000 new ones.

NEOWISE allowed us to take a look at a more representative slice of the near-Earth asteroid numbers and make better estimates about the whole population. It's like a population census, where you poll a small group of people to draw conclusions about the entire country, said Amy Mainzer, lead author of the new study and principal investigator for the NEOWISE project at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

The latest discovery tallies are roughly 19,500 mid-size near-Earth asteroids, down from an original estimate of 35,000 and suggests that the threat to Earth could be somewhat less than previously thought.

But most of these mid-size asteroids remain to be discovered. More research also is needed to determine if fewer mid-size objects (between 330 and 3,300 feet wide) also mean fewer that might threaten the Earth.

Meanwhile, the WISE data reveal only a small decline in the estimated numbers for the largest near-Earth asteroids, which are 3,300 feet (one kilometer) and larger. Scientists now believe there are 981 large near-Earth asteroids about the size of a small mountain, compared with the earlier estimate of 1000, of which 911 have been located and are being tracked.

None of the larger asteroids represents a threat to Earth in the next few centuries. It is believed that all near-Earth asteroids approximately six miles (10 kilometers) across, as big as the one thought to have wiped out the dinosaurs, have been found.

The risk of a really large asteroid impacting the Earth before we could find and warn of it has been substantially reduced, said Tim Spahr, the director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.

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