Telescopes around the world are watching the skies for rocky remnants from outer space to determine if they are on a collision course with Earth. The presence of large impact craters on Earth provides dramatic evidence of past collisions. A NEO larger than 1 km will collide with Earth every few hundred thousand years, causing significant global disaster. A NEO larger than 6 km, a true "Planet Killer" that could cause mass extinction, can be expected to collide with Earth every hundred million years or so.