Earth has been struck many times by meteorites in its long history, most recently at Tungsuka in 1908. The destructive power of that event is thought to have been between 10 and 40 megatons of TNT. Sooner or later, another wandering piece of rock will cross paths with Earth, and the potential exists for all human life to be snuffed out.
So it's reassuring to learn that somebody is making plans to deflect such objects if they are a potential threat. (Although it's a little disappointg to me that the Europeans are taking the lead in this.) The basic idea is to ram the wayward rock with a spacecraft, and so alter it's trajectory.
Quotations from Chairman Barry
44 minutes ago