Want to demolish Earth? A new Web site allows you to choose the form of the destructor. Just type in a few variables such as diameter, density, and velocity and Impact: Earth! will send a comet or asteroid hurtling toward our planet. Lest you be unsatisfied with a simulation of a massive rock barreling down on us, the Web site also provides data on the aftermath, including the size of the crater, the extent of the fireball, and even the height of the tsunami wave, should the object crash into the ocean. The Web site is an update of an impact calculator created in 2002 for use by NASA and homeland security. The new version is more visual and user-friendly, allowing even average Joes to smash up the planet.