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• At the same time, NASA and the Federal Emergency Management Agency are looking into the need for evacuations in the case of an asteroid headed for Earth and how to keep the public informed without scaring people.
• Those issues came up after the two agencies quietly held a disaster drill last spring in Washington that was meant to simulate what would happen if a space rock slightly bigger than the Chelyabinsk one threatened the East Coast.
• During the drill, when it looked as if the meteor would hit just outside the nation's capital, experts predicted 78,000 people could die. But when the mock meteor ended up in the ocean, the fake damage featured a 49-foot tsunami and shortages of supplies along the East Coast, according to an after-action report obtained by The Associated Press.
• 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.)

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