The European Space Agency said that events of the magnitude of the Chelyabinsk meteor blast "are expected once every several of tens to 100 years."

It calculates that the meteoroid burst and disintegrated about 15 to 20 kilometers (nine to 12 miles) above the ground.

"The terminal part of the explosion probably likely occurred almost directly over Chelyabinsk," said Detlef Koschny, of the European Space Agency. "This was perhaps the single greatest contributor to the blast damage."