Collisions between space objects are a vital part of the evolution of our Solar System. Most of Earth's impact craters have been wiped away due to plate tectonics, but evidence of such cosmic catastrophes, such as Arizona's 50,000-year-old meteor crater, do remain. When is Earth due for another major blast? Meet the professional and amateur astronomers who may be the first to know: first at LINEAR, a near-earth asteroid detection facility in New Mexico, and then at the Smithsonian's Minor Planet Center, where orbits of near-earth objects are tracked for possible hits and misses.