Doomsday Scenarios, episode 1: Can an asteroid destroy earth?

We're starting a mini series on KYW Newsradio In Depth called Doomsday Scenarios, where we ask very smart people if this, that, or the other thing could destroy life as we know it. Today: could an asteroid strike earth with enough mass and force to kill the planet? How much should we be concerned about this actually happening? How big would something have to be to do catastrophic damage to the planet? Could we actually do anything to prevent this? Our guest is Dr. Eric Jensen, Professor of astronomy at Swarthmore College and friend of the podcast.