As if getting to Mars and establishing the first colony there wasn't dangerous enough, a new analysis of cratering rates suggests that over a three-year expedition to Mars, the planet will likely get hit by at least one asteroid large enough to cause an impact explosion in the megaton range. Good times.
A megaton impact every three years may seem like a good reason not to spend a lot of time on Mars, but Earth gets hit by crap from space all the time too. It's a little safer around here thanks to our distance from the asteroid belt, and our protective atmosphere means we have less to worry about in general. That said, we should be experiencing megaton-level events once every 15 years or so, although you have to remember that there's a lot of ocean out there: the odds of an asteroid striking (or exploding above) a populated area, while decidedly not zero, are not especially high, either. Every century, we're due for a 10-megaton event, and the last one was in 1906, so we're already overdue. For the record, one megaton is slightly smaller than the largest nuke that the United States currently has in its arsenal, which is a scary enough thought on its own.
Anyway, forget about Earth, we're talking about Mars. While the probability of an asteroid impacting Mars somewhere every three years may be pretty good, the probability of an asteroid impacting Mars somewhere you care about every three years is not that great. On the other hand, a one-megaton explosion packs a lot more punch on Mars, since the atmospheric density is a lot lower. Realistically, though, humans exploring Mars should probably not bother to worry about asteroid impacts, because we generally can't see them coming, and without seeing them coming, there's nothing that you can really do. Just chalk it up as another hazard that Martian colonists will have to deal with, along with the cold temperatures, lack of oxygen, high radiation, and aliens.
Oh, and here's a fun fact: did you know that the probability of you dying in an asteroid impact is about 1 in 200,000, even though nobody's ever died from one in recorded history? The math works out this way because in the event of a major impact, so many people will die all at once, that it becomes a serious threat to the human race, which likely includes you. Yay!