"When an asteroid exploded over the Russian city of Chelyabinsk in February, shattering windows for miles and injuring well over 1,000 people, experts said it was a rare event — of a magnitude that might occur only once every 100 to 200 years, on average."
"But now a team of scientists is suggesting that the Earth is vulnerable to many more Chelyabinsk-size space rocks than was previously thought. In research being published Wednesday by the journal Nature, they estimate that such strikes could occur as often as every decade or two.
The prospect “really makes a lot of people uncomfortable,” said Peter G. Brown, a professor of physics and astronomy at the University of Western Ontario and an author of the two studies in Nature. A third paper by other scientists describing the Chelyabinsk explosion was published online this week by the journal Science."