Motherboard reports that on December 30th, the White House "quietly released" a 25-page Near-Earth Object Preparedness Strategy, explaining how the United States would react in the event that an asteroid was on a collision-course with planet Earth. The goal is to improve detection of Near-Earth Objects (NEO), work out deflection systems, and how to prepare if a hit is inevitable — which it just might be. Reportedly, we would need almost a decade of warning to do something about an approaching NEO, assuimg we even even had a way of knowing one was coming.
Finally, should all else fail, the report also considers what to do in an impact scenario. Although asteroid deflection is definitely possible given enough time, this requires a lot of advance warning. A recent joint exercise between NASA and FEMA ran a simulation in which there was only a four year advance warning of an impending asteroid impact. This might seem like a lot of time, but the simulation found that it wasn’t enough time to launch a deflection mission—at best, NASA would be able to send a satellite to take pictures of the asteroid as it approached Earth. In many cases though, giant asteroids have passed uncomfortably close to Earth and astronomers weren’t even aware of them until a few days in advance.