There is a 1-in-2000 chance that Mars will get hit by a comet in October 2014. That's still pretty long odds BUT it is not negligible or zero.
The impact is estimated to carry a punch of 35 million megatons and it would drastically alter Mars. It would also have an effect on the spacecraft orbiting and roving the Red Planet.
Read about this fascinating possibility and view the cool video: http://science.nasa.gov/science-news/science-at-nasa/2013/26mar_marscomet/
Sky Guy in VA