Sun-scorched Mercury is only slightly larger than Earth's Moon. Like the Moon, Mercury has very little atmosphere to stop impacts, and it is covered with craters. Mercury's dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing. Ice may even exist in craters. Mercury's egg-shaped orbit takes it around the sun every 88 days.
Featured Mission: MESSENGER
NASA's MESSENGER orbiter is the second spacecraft to visit Mercury. It made three flybys before working its way into orbit about Mercury in March 2011. MESSENGER is the first spacecraft to orbit Mercury.
Read More About Mercury