Fewer giant asteroids are out in space than once thought and most of the biggest near-Earth asteroids have been found, leaving little threat of one smashing into the planet, NASA said Thursday.

The latest data from the US space agency's Wide-field Infrared Survey Explorer, or WISE telescope, shows that 93 percent of asteroids 3,300 feet (one kilometer) and larger have been found.

Scientists used to think there were about 1,000 of them, but they have revised that number down to 981, of which 911 have been located and are being tracked.

"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, Massachusetts.

Where the challenge remains is with mid-sized near-Earth asteroids 330 feet (100 meters) and larger. The WISE data shows there are about 19,500 -- not 35,000 as previous data suggested -- and that only 5,200 of these are being tracked.

That leaves nearly 15,000 yet to be found, NASA said.

A near-Earth asteroid is defined as a space rock that orbits within 120 million miles (195 million kilometers) of the sun into Earth's orbital vicinity.

Amy Mainzer, lead author of the latest research which appears in the Astrophysical Journal, said WISE has given astronomers a better sense of what is out there, and what is not.

"It's like a population census, where you poll a small group of people to draw conclusions about the entire country," she said.

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