Asteroids may call to mind lots of images: the Earth shrouded in dust, dinosaurs dying, crowds running, Aerosmith singing, "I Don't Want to Miss a Thing." Asteroids have made plenty of disaster-movie plots, all of which go like this: Don't panic, but a giant rock is headed for the Earth.
Here's how it could go: Either a comet (if an outer planet's gravity pulled one closer to the Earth) or an asteroid could cross Earth's orbit or pass nearby [source: Morrison]. NASA watches for these near-Earth objects and plans to find most of the ones 1 kilometer (0.6 miles) in diameter and larger, as well as learn which ones might collide with Earth. Why 1 kilometer? Anything with that diameter or bigger could do terrible damage [source: Morrison].
Movies like "Deep Impact," "Armageddon" and "Asteroid" get a lot wrong. For example, in "Armageddon" a comet collides with an asteroid, knocking an asteroid the size of Texas at Earth. No asteroid in our solar system is that big, and no comet could knock an asteroid that big at us [source: Plait].
Real plans to divert an asteroid are more like sketches than ready-to-use plans. With 10 years of warning and a medium-sized asteroid, we might plant a nuclear bomb on or near the asteroid. With 20 years of warning and a small asteroid, we might collide an unmanned spacecraft with the asteroid to verify its location and slow it and divert it [source: Yeomans]. But as Carl Sagan points out in "Pale Blue Dot," if an asteroid were headed toward Earth anyway, and we had mastered how to intercept and deflect it, nations could try to knock the asteroid at one another.
The next weapon is as gentle as a butterfly.