Space rocks big enough to destroy a city hit the Earth much more often than thought, according to an estimate by a private group devoted to preventing disaster from such orbital killers.

It took a space rock the size of San Francisco to finish off the dinosaurs 65 million years ago, but a decent-sized metropolis could be reduced to smoldering ruins by a boulder that could fit inside a soccer field. The strike rate for such large space rocks, properly known as asteroids, has been estimated at once every 3,000 years, but the B612 Foundation, a planetary defense group, says the true figure could be as high as once a century. Outside scientists say that frequency is plausible but could well be too high.

“There are people who say, ‘Oh, once every million years we have something we have to worry about.’ That couldn’t be more wrong,” says physicist and former space shuttle astronaut Ed Lu, chief executive officer of the B612 Foundation. “Eventually you’re going to get hit, because it’s just a matter of time.”

….

Read more on USA Today