Are almost nonexistent:
[I]t’s vanishingly unlikely that air defense systems would be able to even make the shot. The Chelyabinsk meteor was traveling at something like 32,000 miles per hour. (A 747′s typical cruising speed? 567 miles per hour.) By the time you notice it, it’s too late to stop it.
Not that you would notice it. Meteors like the one in Chelyabinsk are going to pass through the detection systems that humans have. Telescopes pointed to space are only going to be able to see a ginormous asteroid. Missile warning and air-defense radars run via software that ignores things that aren’t planes and missiles. And the eyes of U.S. military satellites are pointed the wrong way — down toward Earth. The Defense Support Program satellite constellation, for instance, is looking for launches of things like intercontinental ballistic missiles that threaten America, using infrared. But the asteroid is cold until it enters the atmosphere.
How we could prevent a major asteroid strike:
[W]e don’t have to hit the incoming meteor with a nuclear space missile, and we don’t have to deploy a crew of tough-but-sentimental action heroes to the meteor’s desolate surface. We just have to launch something heavy and unmanned to float next to the object long enough to [send] it a fraction of a degree in a different direction so that it misses the “keyhole” approach.