Gravitational interactions between planets and small bodies in the solar system can theoretically send asteroids and comets onto a collision course with Earth. This happens persistently in the fall of meteorites to Earth, though with masses that only rarely do any damage. The finding of craters on almost all solid bodies in the solar system when spacecraft image their surfaces brings home the fact that no planet is spared collisions. Asteroids larger than 50 m across collide with Earth, on average, once every 600 years. In 1967, an interdepartmental, systems engineering project conducted at the Massachusetts Institute of Technology developed a concept to avoid a collision of an asteroid with Earth. Conferences on the subject began in the 1980s. After the fall of the Soviet Union, nuclear weapons experts teamed with asteroid scientists to defend Earth against comets and asteroid impacts.