Astronomers have cataloged about 95 percent of the space objects wider than half a mile?? those that could destroy civilization.?But they have found less than 1 percent of the objects 100 feet across or larger, a class that includes the asteroid that?flitted past Earth on Friday.
Members of the Astronomical Association of Sabadell prepare to watch asteroid 2012 DA14 pass near Earth, in Sabadell near Barcelona, Spain, Feb. 15, 2013.
Gustau Nacarino/REUTERS
EnlargeThe unexpected appearance and explosion of a small asteroid over Russia's Ural Mountains on Friday highlights the challenges astronomers face as they try to get a better handle on the risk Earth faces from objects whose orbits bring them uncomfortably close to Earth, or even cross Earth's orbit.
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In their hunt to identify such near-Earth objects wider than half a mile across ? potential civilization busters if one were to strike Earth ? astronomers have cataloged about 95 percent of the objects in this size class during the past 15 years.
But they have found less than 1 percent of the objects 100 feet across or larger, a class that includes the asteroid 2012 DA14. This object flitted past Earth Friday afternoon Eastern Standard Time a scant 17,200 miles from Earth ? a record for a known object of its size.
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At 150 feet across, 2012 DA14 is comparable in size to the object that exploded over the Tunguska River in Siberia in 1908. The shock wave flattened 820 square miles of forest ? an area about the size of Greater Tampa-St. Petersburg.
The asteroid over Russia's Chelyabinsk region Friday was smaller still, estimated to span about 50 feet and weigh about 7,000 metric tons before it exploded into fragments high above the ground. The shock wave shattered windows in three major cities in the region, damaged a zinc factory, and inflicted mostly minor injuries on more than 950 people.
Nobody saw the object coming.
It's enough to make some lawmakers wince. On Friday, Lamar Alexander (R) of Tennessee and chairman of the House Science, Space, and Technology Committee, issued a statement regarding the two events that noted that the committee will hold hearings in the near future to explore ways to improve efforts to detect asteroids as well as to deal with any deemed a potential threat to the planet.
Given the size of the Chelyabinsk asteroid, astronomers estimate that an event like this occurs on average every 100 years. Yet smaller objects also can arrive with little or no warning, and explode in a loud, spectacular fashion, even with no damage on the ground. And they hit more frequently.
These surprise visitors are among the near-Earth objects that keep Kalait Ramesh awake at night.
"Historically, we've had relatively low population density. These things tended to happen in areas where nobody sees them or nobody remembers them," says Dr. Ramesh, a professor of mechanical engineering at the Johns Hopkins University whose studies of stress on various materials has led him to include asteroids in the mix of materials.
"But as our population's gone up, it's gotten to the point where these things can have a big impact. My biggest worry is that this will happen in a place where there is significant political instability or two countries on the verge of a war," he says.
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