Astronomer Phil Plait gives a 14 minute TED Talk in which he describes the problem posed by asteroids whose orbits take them into a collision course with earth. He also discusses some current strategies for dealing with these threats if they're discovered in time.
Mr. Plait tries a bit too hard to be funny and his audience seems to have a low humor threshhold, both of which are somewhat irritating, but if you can tolerate that sort of thing, his talk is interesting: