The world will have to live with surprise asteroid attacks on the scale of Friday's Russian fireball, at least for a while. The meteor that exploded over the Russian city of Chelyabinsk without warning Friday (Feb. 15), damaging hundreds of buildings and wounding more than 1,000 people, was caused by a space rock about 50 feet (15 meters) wide, researchers said. Asteroids of this size are both difficult to detect and incredibly numerous, so it will take a long time for astronomers to find and map out the orbits of all the potentially dangerous ones. Besides, researchers have bigger fish to fry.



Observations by NASA's WISE space telescope, for example, suggest that about 4,700 asteroids at least 330 feet (100 m) wide come uncomfortably close to our planet at some point in their orbits. To date, astronomers have detected less than 30 percent of these objects, which could destroy an area the size of a state if they slammed into Earth. And researchers have spotted less than 1 percent of asteroids at least 130 feet (40 m) wide, according to officials with the B612 Foundation, a nonprofit organization dedicated to predicting and preventing catastrophic asteroid strikes. Space rocks of this size can cause severe damage on a local scale, as the 1908 "Tunguska Event" shows. That year, a 130-foot-wide object exploded over Siberia's Podkamennaya Tunguska River, flattening roughly 825 square miles (2,137 square km) of forest.

A space rock in this size class gave Earth a close shave Friday. The 150-foot-wide (45 m) asteroid 2012 DA14 � which was just discovered in February 2012 � cruised within 17,200 miles (27,000 km) of our planet, marking the closest approach of such a big space rock that was ever predicted in advance. Overall, scientists think 1 million or more near-Earth asteroids are lurking out there, and just 9,600 have been identified to date.