Last February 15, at 9:20 in the morning, residents of Chelyabinsk, Russia, looked out their windows and saw a giant ball of fire fly through the sky.
The Chelyabinsk meteor was a 65-foot hunk of space rock that entered the Earth's atmosphere at about 12 miles per second before exploding with a force equal to 600,000 tons of TNT, enough to level buildings and send 1,200 people to local hospitals.
The scary thing is: No one saw it coming, says the author of a paper, out today in the journal Science by researchers who studied the Chelyabinsk event.
"Chelyabinsk was not detected in space at all," said Peter Jenniskens, an astronomer with NASA Ames and the SETI Insitute. "It couldn't be detected because it was coming from the direction of the sun."