The "very small" asteroid missed the Earth on March 31 by the wafer-thin gap of 6,500 kilometers (4,000 miles), according to an email circulated by the international network NEO News (NEO stands for Near-Earth Object).

Robot telescopes operated by NASA in New Mexico under the Spaceguard Survey to track potential threats from asteroids, spotted the object, 2004 FU 162, just a few hours before the flyby.

"It would have exploded harmlessly in the upper atmosphere had it hit," the circular said.

Given the large population of asteroids of this size -- a population estimated to be a couple of hundred million in the Solar System -- the statistical chance of a flyby is "more than once a year" and the risk of an actual impact is rated once every several years.

"This event is not particularly rare, except that (the telescopes) had the good fortune to notice it," NEO News said.

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