There are widespread reports today of an Tunguska-sized asteroid on a collision course with Mars. The odds are 1 in 75 that 2007 WD5 will hit the Red Planet in late January. While this would be exciting to observe, it probably won't happen. The asteroid was discovered in late November and has been tracked for barely a month. This means its orbit is not well determined. As more observations in the weeks ahead refine the asteroid's path, the chances of a Mars collision will almost certainly evaporate. Of course, sometimes the unlikely does happen.