The planet Mercury is the smallest of the inner planets (4,880 km/3,032 mi in diameter), and the closest to the Sun (58 million km/36 million mi - or 3.2 light minutes). It was visited by the Mariner 10 spacecraft twice in the 1970s, and about 45% of the surface was mapped. On August 3rd, 2004, NASA launched a new mission to Mercury, the MErcury Surface, Space ENvironment, GEochemistry and Ranging probe (or MESSENGER). MESSENGER is now in the last stages of multiple gravity-assist flybys of Earth, Venus and Mercury, en route to an insertion into orbit around Mercury in March of 2011. In just two flyby encounters, MESSENGER has already greatly increased our knowledge about Mercury's surface features. As you look at Mercury in the new images below, keep in mind that it has minimal atmosphere, gravity about 1/3 of Earth's, and surface temperatures ranging from -183 C (-297 F) in some polar craters to 427 C (801 F) at high noon (Mercury's solar day lasting 176 Earth days). ( 20 photos total