Analysis of new observations from NASA's infrared space telescope shows that there are fewer near-Earth asteroids that are large enough to destroy the planet than previously thought.

The latest survey by the Wide-field Infrared Survey Explorer, or WISE, telescope significantly lowers the number of midsize near-Earth asteroids to 19,500 from an original estimate of 35,000 and suggests that the threat to Earth could be somewhat less than previously thought, according to NASA.

But most midsize asteroids remain to be discovered. More research is needed to determine if fewer midsize objects (between 330 and 3,300 feet wide) also mean fewer that might be a threat to the Earth
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 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 1,000, of which 911 have been located and are being tracked.

The NASA report also suggests none of the larger asteroids represent any sort of serious threat to the planet, at least over the next few centuries. It is believed that all near-Earth asteroids, approximately six miles (10 kilometers) across, big enough to have wiped out the dinosaurs, have been found.

"The risk of a really large asteroid impacting the Earth before we find and warn of it has been substantially reduced," said Tim Spahr, director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass.
NEOWISE observations indicate that there are at least 40 percent fewer near-Earth asteroids in total

 that are larger than 330 feet, or 100 meters.

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.

The technology has enabled the research team to capture far more accurate data than its previous attempts, largely because the infrared detectors could distinguish between both dark and light objects.

"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," said Amy Mainzer, lead author of the new study and principal investigator for the NEOWISE project at NASA's Jet Propulsion Laboratory.

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