The exercise and the studies show there's a risk from smaller space rocks that strike before they are detected — not just from the giant, long-seen-in-advance ones like in the movie "Armageddon," said Bill Ailor, a space debris expert at the Aerospace Corporation who helped coordinate the drill.

"The biggest hazard from asteroids right now is the city-busting airbursts, not the civilization-busting impacts from 1-kilometer-diameter objects that has so far been the target of most astronomical surveys," Purdue University astronomer Jay Melosh, who wasn't part of the studies, wrote in an email.

"Old-fashioned civil defense, not Bruce Willis and his atom bombs, might be the best insurance against hazards of this kind."

Chodas said the Chelyabinsk rock surprised astronomers because it was coming from the direction of the sun and was not detectable. Telescopes can see some space rocks as small as 3 feet wide, but some are simply too dark to spot, he said.

Scientists said a 1908 giant blast over Siberia, a 1963 airborne explosion off the coast of South Africa, and others were of the type that is supposed to happen less than once a century, or in the case of Siberia, once every 8,000 years, yet they all occurred in a 105-year timespan.

Because more than two-thirds of Earth is covered with water and other vast expanses are uninhabited deserts and ice, other past fireballs could have gone unnoticed.

Just this week, NASA got a wake-up call on those bigger space rocks that astronomers thought they had a handle on, discovering two 12-mile-wide asteroids and a 1.2-mile-wide one that had escaped their notice until this month. However, NASA said the three objects won't hit Earth.

Asteroids are space rocks that circle the sun as leftovers of failed attempts to form planets billions of years ago. When asteroids enter Earth's atmosphere, they become meteors. (When they hit the ground, they are called meteorites.)

The studies said the Chelyabinsk meteor probably split off from a much bigger space rock.

What happened in the Russian city of 1 million people is altering how astronomers look at a space rocks. With first-of-its-kind video, photos, satellite imagery and the broken-up rock, scientists have been able to piece together the best picture yet of what happens when an asteroid careens into Earth's atmosphere. It's not pretty.

"I certainly never expected to see something of this scale or this magnitude," said University of Western Ontario physicist Peter Brown, lead author of one study. "It's certainly scary."

Scientists said the unusually shallow entry of the space rock spread out its powerful explosion, limiting its worst damage but making a wider area feel the effects. When it burst it released 500 kilotons of energy, scientists calculated.

"We were lucky. This could have easily gone the other way. It was really dangerous," said NASA meteor astronomer Peter Jenniskens, co-author of one of the papers. "This was clearly extraordinary. Just stunning."