The miss was one of the nearest ever recorded for an object of that size, scientists said Thursday. "It was a close shave," said Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
The asteroid would have caused "considerable loss of life" if it had struck Earth in a populated area, said Grant Stokes, the principal investigator for the Lincoln Near Earth Asteroid Research Project, whose New Mexico observatory spotted the object last week.
"The energy release would be of the magnitude of a large nuclear weapon," Stokes said.
The asteroid was not detected until three days after it sped past Earth on June 14. When such asteroids are detected, they are usually spotted far from Earth, when they are approaching or on their way out.
The asteroid, provisionally named 2002 MN, was traveling at more than 23,000 mph when it was spotted, Stokes said in a phone interview from Lexington, Mass., where he is associate head of the aerospace division of MIT Lincoln Laboratory.
Light in weight but with a diameter of between 50 and 120 yards, 2002 MN was big enough to have caused the kind of devastation wreaked in Siberia in 1908, when an asteroid that exploded above Tunguska flattened nearly 800 square miles of forest.
That asteroid's air blast was believed to have done the damage, since no crater was found.
The size of asteroids is estimated by measuring their brightness, without knowing their composition. In general, damage on the ground depends on what an asteroid is made of, varying from solid metal to a loosely bound aggregate.
"Looking statistically at the asteroid population, maybe 50 times a year a 100-meter-class asteroid passes within a lunar distance of Earth," Stokes said. "But only a handful of such asteroids that have penetrated the Moon's orbit have been spotted by asteroid search programs."
Benny Peiser, an expert on near earth objects at Liverpool John Moore's University in England, said that most asteroids do not come so close, but noted the latest "reminder" comes as Britain tests telescopes on the Spanish island of La Palma to search for the objects.
"Such near misses do highlight the importance of detecting these objects," he said.
Currently, there is no program dedicated to searching for objects of 2002 MN's size. NASA concentrates its efforts on bodies bigger than .62 miles across, which would cause worse devastation.
"NASA has a goal of discovering and obtaining good orbits for all the near earth objects with diameters larger than 1 kilometer," said Thomas Morgan, a scientist at NASA headquarters in Washington. "Asteroids of this size could potentially destroy civilization as we know it."
Such asteroids could theoretically hit Earth every million years, or at longer intervals.
Asteroids the size of 2002 MN are estimated to hit the Earth every 100 to several hundred years, causing local damage but no disaster to civilization or the planet's ecosystem, Stokes said.
"It's something the public should know about, but shouldn't get nervous about," he said. "Civilization has to get used to them on some level."