A half-mile-wide asteroid, blazing through space at nearly 50,000 mph, skirted past Earth last month in a cosmic "close call" unrivaled in a half century, NASA said.

The asteroid, called 1989FC, passed within 500,000 miles of Earth - about twice the distance to the moon - on March 23, the space agency said."On the cosmic scale of things, that was a close call," said Henry Holt, an amateur astronomer from Flagstaff, Ariz., credited with discovering the asteroid while working on a project funded by the National Aeronautics and Space Administration.

Bevan French, a scientist with NASA's solar system exploration division, said Wednesday if the asteroid had collided with Earth the impact would have been equivalent to the explosion of 20,000 hydrogen bombs, creating a crater five to 10 miles across - or "large enough to destroy a good-size city."

If it had hit an ocean, it would have created killer tidal waves that would have swept over coastal regions, French said.