Mining some of the near-Earth asteroids will exploit the considerable constructive potential of these cosmic bodies. However, many of the NEAs have a darker, more destructive potential as well. Namely, those NEAs that cross Earth's orbit can occasionally be attracted by the planet's gravity (or the Moon's gravity) and end up striking Earth.

The reality of this threat is demonstrated by the fact that small pieces of older, disintegrated comets and asteroids enter Earth's atmosphere all the time. Indeed, tens of thousands of these meteors do so each day. Most burn up in the upper atmos phere, where at night their rapid streaks across the sky are often erroneously called "shooting stars." Occasionally, larger meteors make it through the atmosphere intact and strike the ground (in which case they become known as meteorites). A number of such...