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Passing asteroid undetected

An asteroid streaked within 90,000 miles of Earth last month without being detected, the closest encounter on record, astronomers said over the weekend.

The rock, an estimated 30 feet in diameter, was seen by an astronomer in Arizona only after it had passed Earth and receded deeper into space on its orbit around the sun, according to the International Astronomical Union in Cambridge, Mass.

Asteroid hunter David Rabinowitz discovered the dim object, named 1993 KA2, several hours after it passed Earth while he was scanning space with the Spacewatch Telescope at Kitt Peak, Ariz. That and a subsequent view provided enough data to calculate the orbit, and from those calculations, astronomers learned just how close the encounter was. On May 20, they found, 1993 KA2 had been less than half as far from Earth as was the moon.

Scientists estimated the asteroid's mass at 6,000 tons _ about the same as a naval destroyer. They said its speed relative to Earth on May 20 was about 48,000 mph. Had it headed directly for Earth and survived a fiery plunge through the atmosphere, 1993 KA2 could have inflicted great damage, said Geoff Chester of the Einstein Planetarium at the National Air & Space Museum in Washington.

"If this thing had hit, I think it would have carved out a significant crater," Chester said.

The asteroid reached perihelion, its closest approach to the sun, in early April and was outward bound again by the time it was discovered. As close as it passed, the rock's visual magnitude at discovery was extremely dim, Rabinowitz said.