"Discovered on January 3rd, some calculations of its orbit, ... 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."

http://www.economist.com/blogs/babbage/2013/02/comet-headed-mars

I wonder if it could melt some of the ice, and if it does, would it 'stir up' microbes there....