A 1,000-foot diameter Near-Earth asteroid misses the Earth by 400,000 miles - March 23, 1989

Literally hundreds of thousands of asteroids have been discovered over the past century of space exploration and, approximately, 5000 more are discovered every month. Near-earth asteroids are classified as asteroids whose orbits are near to the Earth’s orbit. Some of these near-earth asteroids pose a threat due to a possible collision with the Earth. Two near-earth asteroids have actually been explored by spaceships.

On March 23rd, 1989 an asteroid 1,000 feet in diameter, named 4581 Asclepius, missed the earth by a mere 400,000 miles. The asteroid crossed the position where the earth had been only six hours after the Earth has passed through. If the asteroid had hit it would have made the largest explosion ever in recorded history. Scientists determined that the asteroid was traveling at 46,000 miles per hour and weighed over 50 million tons. Earlier in the century, in 1908, an asteroid almost a third smaller than 4581 Asclepius, exploded in mid-air over Siberia and wiped out over a half a million acres of forest-land.

http://en.wikipedia.org/wiki/4581_Asclepius