COMET MAY HIT MARS IN 2014: “It would be an extinction-level impact, were there any life on Mars.”
Science fiction idea: An alt-history story about this where Mars has been colonized for a couple of decades. Kind of a Martian Lucifer’s Hammer.
UPDATE: Charlie Martin emails to suggest that a comet strike on Mars could jumpstart terraforming efforts by delivering a big load of volatiles (water, methane, CO2 and so on), plus a lot of heat energy that could release water and other volatiles from the martian soil. That’s right, though planets are huge by comparison to comets: In his Terraforming: Engineering Planetary Environments, Martyn Fogg calculates how many comet impacts it would take to give Mars a useful atmosphere and it’s quite a lot. You don’t gain by making the comets really big, either, as beyond a certain size the impact becomes so energetic that you start losing atmosphere to space.
Another reader, in an allusion to The World According To Garp, thinks that a cometary strike on Mars would make it an attractive destination because of presumed safety — how likely is it that something like that would happen again any time soon? Hmm.