(Reuters) - About 1,000 asteroids big enough to cause catastrophic damage if they hit Earth are orbiting relatively nearby, a NASA survey shows.
In a project known as Spaceguard, the U.S. space agency was ordered by Congress in 1998 to find 90 percent of objects near Earth that are 1 km (0.62 of a mile) in diameter or larger.
The survey is now complete, with 93 percent of the objects accounted for, astronomer Amy Mainzer of NASA's Jet Propulsion Laboratory in Pasadena, California, said at the American Geophysical Union conference in San Francisco on Tuesday.
Using NASA's recently retired Wide-field Infrared Survey Explorer (WISE) telescope, scientists also found about 20,500 smaller asteroids near Earth.
Previous studies estimated there were 36,000 to 100,000 of these objects, which have a diameter of about 100 meters (110 yards).
"They could still pack quite a punch," Mainzer told Reuters, adding that "any impact is not a very likely event."
But a major asteroid strike could and has happened.
An asteroid or comet between 5 and 10 km (3.1 and 6.2 miles) in diameter is believed to have smashed into Earth some 65 million years ago, triggering global climate changes that led to the extinction of dinosaurs and other animals.
"We know something that big could wipe out mostly all life on Earth," Mainzer said.
Scientists are now using archived WISE observations to home in on potentially hazardous objects whose orbits come within about 4.6 million miles (7.4 million km) of Earth.
So far, there is no plan about what to do if an asteroid was discovered to be on a collision course with Earth.