Mars may get hit by a comet in 2014
March 4, 2013
Assuming it does hit, while the nucleus size is not well known, it may be as small as 15 kilometers (9 miles) or as big as 50 km (30 miles). Even using the small number means Mars would be slammed by an unimaginable impact. The comet will be moving at a speed of about 55 kilometers per second (120,000 miles per hour!) upon impact, he says.
Doing a rough calculation, Plait gets an explosive yield of roughly one billion megatons: That’s a million billion tons of TNT exploding. Or, if you prefer, an explosion about 25 million times larger than the largest nuclear weapon ever tested on Earth.
UPDATE Mar. 6, 2013 — wording improved.