NASA Probe Counts Space Rock Impacts On Mars

Scientists using images from NASA's Mars Reconnaissance Orbiter (MRO) have estimated that the planet is bombarded by more than 200 small asteroids or bits of comets per year forming craters at least 12.8 feet (3.9 meters) across.

via NASA Breaking News

http://www.nasa.gov/home/hqnews/2013/may/HQ_13-142_Mars_MRO_Craters.html