Researchers at Purdue University have updated their popular Impact Earth! online calculating tool. If you've a morbid interest in just how big an asteroid it would take to end civilization (and who doesn't?), click on over and type in size, density, angle of impact data to find out.
I am a proponent of the Strong Gaia hypothesis. The Gaia hypothesis views the Earth as a single organism that regulates conditions to sustain life on the planet. The Strong Gaia hypothesis accepts this notion, but further hypothesizes that Gaia finally got so pissed off at being whacked by roving asteroids that She evolved big brained primates as an immune system to fend off asteroids using nuclear bombs. Perhaps the Purdue researchers will add nuclear asteroid defense as a feature later.
See my column on planetary defense here.