I first heard from someone that an asteroid had a 1-in-5 chance of hitting Mars on Jan 5, 2008, that it was the size of the asteroid that impacted Earth 65 million years ago.

Just went to look for details; far less exciting than that, the asteroid is the size that exploded over Tunguska in 1908. It would have been bad if you happened to be directly under the impact; but as it happens it was a remote region that mostly flattened a lot of trees. Not the major planet changing event of 65 million years ago.

The asteroid is also scheduled to possibly hit on January 30th, not January 5th, and the odds of an impact are 1-in-75 not 1-in-5. I am so disappointed but I imagine there might be some relieved Martian bacteria.