Spencer Ackerman in Wired with bad news:

All the advanced air defenses that humanity has invested in? The interceptor missile that are (sometimes) able to stop an adversary missile from impacting? The early-warning monitoring systems that are supposed to give humanity enough time to plan a response? They are useless, useless against a meteorite onslaught….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.

On the other hand:

Within the United Nations Special Committee on the Peaceful Uses of Outer Space is a subcommittee on science and technology. That subcommittee has an ad hoc advisory team of dozens of space experts from around the world who try to “figure out how to coordinate detections, warnings and response, and possible deflection” of asteroids that might impact Earth, Weeden says. They’re mostly focused on the big asteroids, the ones larger than 100 meters in diameter or bigger. And among their ideas — the one that “physics says should work,” in Weeden’s phrase — is something called a Gravity Tractor.
The idea is to launch a spaceship near a nefarious asteroid. The presence of the spaceship’s inherent gravitational field should impact the asteroid’s, to the point where it might be able to shift the asteroid’s trajectory and get it to avoid the path of Earth’s celestial journey. It’s untested — and apparently it won’t even host astronauts: it’s unmanned, so Bruce Willis can take a knee. But Weeden has faith the Gravity Tractor will work….

Faster please (unless you are an asteroid)