Look at this:
4581 Asclepius is a small asteroid of the Apollo group that can approach Earth’s orbit to within 600,000 km; it was discovered in 1989 by the American astronomers Henry E. Holt (1929-) and Norman G. Thomas (1930-), and named after the Greek demigod of Medicine and Healing. Asclepius (1989 FC) passed by the Earth on March 22, 1989 at a distance of some 700,000 km, causing some media attention at that time.
The asteroid passed through the exact position where the Earth was only 6 hours before. Astronomers say that had the asteroid hit Earth, the impact would have been equivalent to one Hiroshima-sized atomic bomb detonating every second for 50 days. Subsequent discoveries revealed that a whole class of such objects exists, and that an object the size of the one which just missed Earth in March 1989, probably comes by undetected once every two or three years. "On the cosmic scale of things, that was a close call," said Dr. Henry Holt.
Also, there's a 1 in 45000 chance (very low) that an asteroid will hit Earth on the 12th/13th April 2036
http://en.wikipedia.org/wiki/File:2037_ ... f_Risk.jpg
And another big asteroid may wipe out civilisation in... 2880
I'll update it later when I have more time but I was fascinated by these objects