Astronomy Picture of the Day . Mercury: A Cratered Inferno . ... Credit: Mariner 10 , NASA Explanation: Mercury's surface looks similar to our Moon's. Each is heavily cratered and made of rock. ... Mercury is the closest planet to the Sun, orbiting at about 1/3 the radius of the Earth's orbit . ... A new mission, Messenger, launched for Mercury last month and is scheduled to enter orbit around the Solar System's innermost planet in in 2011. ... Publications with keywords: Mercury - Solar System . ...