"Asclepius passed by Earth on March 22, 1989, at a distance of 680,000 km (0.68 Gm). Although this exceeds the Moon's orbital radius, the close pass received attention at that time, especially since the asteroid passed through the exact position of Earth only six hours earlier
. "On the cosmic scale of things, that was a close call," said Dr. Henry Holt. Geophysicists estimate that collision with Asclepius would release energy comparable to the explosion of a 600 megaton atomic bomb.
Subsequent discoveries revealed that a whole class of such objects exists, and that an object the size of the one that just missed Earth in March 1989, probably comes by undetected once every two or three years. Asclepius will continue to approach Earth within 30 Gm a dozen times each century. The next pass to come within 1 Gm will not take place until 2189."