Comet C/2013 A1 (Siding Spring), discovered just this January, is expected to come within 100,000 km of Mars in October 2014, as O.M. reports (as Babbage). Perhaps even closer: the track of a comet is not as predictable as that of an asteroid, as the flying snowball ejects mass on its approach to the sun. Astronomers, professionals and amateurs alike, are looking at the possibility of an even closer approach, and an actual impact is not out of the question at this point.
Update: More reporting on the story, with a fabulous hed and subhed for us Cole Porter fans.