A new study estimates 900 large asteroids cruise orbital "superhighways" where they someday might strike Earth, causing widespread death and destruction. Yet so far, astronomers have found only 40 percent of the big space rocks, each at least 0.6 miles (1 kilometer) wide.

The new calculation by U.S. and French scientists -- published in the June 23 issue of the journal Science -- is almost 30-percent larger than an estimate of 700 near-Earth asteroids outlined in January in the journal Nature.

A small asteroid just 80 feet (24 meters) across created Meteor Crater in Arizona 20,000 to 50,000 years ago.

"The fewer large ones there are to keep an eye on, the more comforting it is," said Don Yeomans, who manages NASA's near-Earth object program at Jet Propulsion Laboratory in Pasadena, California.

Cornell University planetary scientist William Bottke Jr., who led the new study, said preliminary calculations derived from the research suggest Earth may get smacked by a kilometer-sized asteroid once every 500,000 to 1 million years. That's less than the once every 100,000 to 300,000 years cited by other scientists and based on the rate of impact cratering on the moon.