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How dangerous are near-Earth asteroids? 5 key questions answered.

On Feb. 15, asteroid 2012 DA14, discovered a year ago, cleared Earth by a scant 17,200 miles. The same day, a smaller, unrelated asteroid that no one saw coming exploded 12 to 15 miles above Russia’s Chelyabinsk region. The shock wave shattered windows, injuring more than 1,000 people. Events that day highlight the risk that near-Earth objects (NEOs) can pose – although to some extent, humans can counter them.

1. What are near-Earth objects, and how big are they?

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    This image shows a simulation of asteroid 2012 DA14 approaching from the south as it passes through the Earth-moon system, last Friday.
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NEOs are asteroids and comets whose orbits bring them close to Earth. They range in size from about three feet to several miles across. The asteroid or comet that punched a 110-mile-wide crater in the Yucatán Peninsula 65 million years ago, doing in the dinosaurs, has been estimated at six miles across.

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