Comet 2013 A1 is currently clipping along at about 125,000 miles per hour and measures somewhere between 0.5 and 2 miles in diameter. Its trajectory is set to cross Mars’ orbit in October 2014. How likely are the two bodies to hit? Not very. The chances are a mere 1 in 2,000.
But if the comet does hit the surface of Mars, it will deliver a blow equivalent to 35 million megatons of TNT. To put that in perspective, the blast would be 800 million times the energy expelled from the meteor over Russia in February, but only a third the energy of the asteroid impact that wiped out the dinosaurs.
Even if the comet misses Mars, coming close would be a big event in and of itself. Check out this video from NASA to get all the nitty gritty details:
Image and video courtesy of NASA