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| NASA |
On Friday (Feb. 15), an asteroid half the length of a football field will buzz close by Earth. It won't hit the planet, but if it did, the collision would create an impact large enough to level 80 million trees — or the entire city of Washington, D.C., and its suburbs.
Scientists know this because an impact by an object the size of Friday's flyby asteroid has happened in human memory. In 1908, a 220-million-pound (100-million-kilogram) hunk of meteoroid or comet fragment hurtled into the atmosphere over Tunguska, Siberia, setting the sky ablaze and releasing the same amount of energy as 185 Hiroshima bombs.
Fortunately, the impact occurred over remote, forested land, taking the lives of hundreds of reindeer but no humans. At about 130 feet (40 meters) in diameter, the Tunguska space rock is similar in size to 2012 DA14, the asteroid en route for a Friday flyby, which is estimated to be about 150 feet (45 m). For comparison, that's about the size of the White House, said Mark Boslough, a physicist at Sandia National Laboratories in New Mexico who has used computer modeling to recreate the Tunguska impact.
Asteroid 2012 DA14 will approach as close as 17,200 miles (27,700 kilometers) from Earth at approximately 2:24 p.m. Eastern Standard Time on Friday, the closest-ever predicted flyby for an object this large. The approach isn't close enough to threaten Earth, though it will pass within the zone where satellites are orbiting. Siberia in 1908 wasn't so lucky.
To read more go to: http://www.livescience.com/27000-flyby-asteroid-da14-tunguska.html
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