An asteroid exploded over Chelyabinsk, Russia in February 2013 with the energy of a large nuclear warhead, injuring over a thousand people. In 1908, above the Tunguska river in Siberia, an even larger asteroid exploded and flattened 800 square miles of forest. How often do asteroids or comets collide with Earth? Why do some explode in the atmosphere and some crater the surface? How big a threat do these pose to our civilization? And finally, what can we do about it, what are we doing about it, and what should we do about it?
Chris Chyba is a MacArthur Fellow, former co-director of Stanford's Center for International Security and Cooperation, former holder of the Carl Sagan Chair at the SETI Institute, and currently director of the Program on Science and Global Security at the Woodrow Wilson School at Princeton.