An asteroid called 2011MD the size of a tractor trailer passed by this morning 32 times closer than the moon. The asteroid was discovered as recently as June 22, 2011, there would have been very little time to prepare if it where to enter earth’s atmosphere. It’s worth noting the Tunguska Event of 1908 where a similar sized meteor exploded 3-6 miles above the earth’s surface, Wikipedia says:
Although the meteoroid or comet burst in the air rather than hitting the surface, this event is still referred to as an impact. Estimates of the energy of the blast range from 5 to as high as 30 megatons of TNT (21–130 PJ), with 10–15 megatons of TNT (42–63 PJ) the most likely—roughly equal to the United States’ Castle Bravo thermonuclear bomb tested on March 1, 1954, about 1,000 times more powerful than the atomic bomb dropped on Hiroshima, Japan, and about one-third the power of the Tsar Bomba, the largest nuclear weapon ever detonated. The explosion knocked over an estimated 80 million trees covering 2,150 square kilometres (830 sq mi). It is estimated that the shock wave from the blast would have measured 5.0 on the Richter scale. An explosion of this magnitude is capable of destroying a large metropolitan area. This possibility has helped to spark discussion of asteroid deflection strategies. The Tunguska event is the largest impact event over land in Earth’s recent history. Impacts of similar size over remote ocean areas would most likely have gone unnoticed[dubious – discuss] before the advent of global satellite monitoring in the 1960s and 1970s.
Check out the wikipedia articles: