Small asteroids discovered with plenty of warning could be nudged aside with a “gravity tractor”—a plasma-powered craft that would hover near the NEO and use its own gravity to divert the target. (If you slow or accelerate an asteroid by just one centimetre per second, and you start ten years before the projected impact, the cumulative divergence will be sufficient.) For larger or closer NEOs, or those requiring a greater degree of deflection, you’d use “kinetic impactors,” spacecraft loaded with lead or copper, to ram the target—speeding nails aimed at a speeding hammer. For NEOs larger than half a kilometre across, or those only a few years out, you’d need nuclear weapons. Such is the confused state of affairs that the Air Force would have to borrow a warhead from the Department of Energy—but couldn’t, by law, let NASA stow it on a rocket.

Tad Friend, “Vermin of the Sky”