Writing about something that is definitely unlikely, but the precise unlikeliness of which is uncertain, is an interesting thing. I assume the chances of Comet C/2013 A1 (Siding Spring) hitting Mars on my beloved niece Lucy's birthday next year are really low -- thousands to one or worse. But the possibilities that would be opened up if it did hit would be remarkable. Here's a post on the subject from The Economist's Babbage blog:
A PAIR of middle-aged tourists (see previous post) are not the only thing headed for Mars. Comet C/2013 A1 (Siding Spring) is also on its way. Discovered on January 3rd, some calculations of its orbit, according to Phil Plait, the rather good “Bad Astronomer”, have it passing 37,000km above the surface of the planet in October 2014—roughly the height at which communication satellites orbit Earth, and a remarkably close shave by cosmic standards. An official NASA website puts the most likely “close-approach” distance between the comet and Mars at something more like 100,000km.
But the minimum close-approach distance is zero.
Update: as this later piece notes, the chances of an impact are now put at zero, though the approach might still be spectacularly ...