This just in...
NASA has outlined what it could do, and in what time frame, in case a quarter-mile-wide asteroid named Apophis is on a course to slam into Earth in the year 2036. The timetable was released by the B612 Foundation, a group that is pressing NASA and other government agencies to do more to head off threats from near-Earth objects.
The plan runs like this: Eight years from now, if there's still a chance of a collision in 2036, NASA would start drawing up plans to put a probe on the space rock or in orbit around it in 2019. Measurements sent back from the probe would characterize Apophis' course to an accuracy of mere yards (meters) by the year 2020.
If those readings still could not rule out a strike in 2036, NASA would try to deflect the asteroid into a non-threatening course in the 2024-2028 time frame by firing an impactor at it — using this year's Deep Impact comet-blasting probe as a model. Experts would start planning for the "Son of Deep Impact" mission even before they knew whether or not it was needed.