Astronomer Chris Impey surveys the possible causes of earth's extinction. Whether it come from an asteroid or the sun's implosion, the rock we live on is by no means an eternal home. "What kind of catastrophe would it take to end the world? Astronomical intruders provide a potentially serious threat. Impacts can be caused by stray rubble from the Asteroid Belt and the rocky snowballs that travel in highly elliptical orbits in the comet cloud. There are many fewer large bits of debris than small bits, so the interval between large impacts is much longer than the interval between small impacts." Impey goes on to discuss less plausible scenarios—if only because a killer asteroid is more likely in the short term—like a hypernova to scorch the earth and the inevitable break down of the universe's most fundamental elements.