The Dark Mission

Near-Earth asteroids are ones which orbit the Sun on a similar sort of orbit to the Earth within a certain... Yeah, or at least they're in orbits that cross the Earths. Those are the worrying ones. If the Earth happens to be in the wrong place as these asteroids are swinging back and forth, then we have a collision. This is why we see meteors and shooting stars and get the occasional meteorite fall. The idea is that we'd be able to see these things coming so that if we had our complete catalogue of 100-meter or so objects, then we could do something about it.