WASHINGTON - An asteroid the size of a soccer field whizzed by Earth at a distance much nearer than the Moon, the biggest such space rock in decades to get this close, scientists said on Thursday.
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Asteroid 2002MN was not detected until Monday, three days after its closest approach on June 14, when it got within 75,000 miles (120,000 km) of Earth and was traveling at a speed of some 23,000 miles per hour (10 km per second), astronomers said.
It is now several million miles away, according to Brian Marsden of the Harvard-Smithsonian Center for Astrophysics' Minor Planet Center, which tracks asteroids.
"It's the largest (asteroid) we've seen at that distance in the last several decades," Marsden said in a telephone interview.
The last time any asteroid came this close was in 1994, according to the Near Earth Object Information Center in Britain.
The big rock, with a diameter of roughly 50 yards (meters) to 120 yards (metres), would not have caused global catastrophe if it had struck Earth. That would take an asteroid of several miles (km) diameter.
However, if it had hit Earth, it had the potential to cause as much local devastation as a 1908 hit in Tunguska, Siberia, which flattened some 800 square miles (2,000 square km) of forest.
Asteroid 2002MN was first spotted by the Lincoln Near Earth Asteroid Research (LINEAR) program, based in Socorro, New Mexico.
"It's a good thing it missed the Earth, because we never saw it coming," Steve Maran of the American Astronomical Society said in a telephone interview. "The asteroid wasn't discovered until three days after it passed its closest approach to our planet."
LINEAR is part of the National Aeronautics and Space Administration's initiative to find 90 percent of all near-Earth objects, including asteroids, that measure .62 mile (1 km) or more in diameter by 2008.
An asteroid the size of 2002MN may hit Earth about once every hundred years or so, and the planet may not have seen the last of this one, Marsden said.
"There is a slim chance it could hit in 2061," he said, putting that chance at about one in 100,000. "At some level, it behooves us to look out for these things," he added.
Asteroid 2002 MN will be observable by some telescopes but it is getting fainter as it moves away, Marsden said.