The threat posed to Earth and Earth-orbiting spacecraft by natural space debris (asteroids, comets and meteor streams) is examined in an effort to quantify the threat and identify available, low cost mitigation measures. Our study found that the Earth resides in a swarm of natural debris that consists of at least three families of asteroids (the Apollo, Aten and Amor asteroids), several short-period comets and at least 11 active meteor streams.