Planet Mercury. NASA mission Messenger is leaving next summer to Mercury. Mercury is solar system Sun closest planet. It was last visited by mission Mariner 10 thirty years ago. Mercury remained ill-known until space age as it is an elusive planet as seen from Earth. Mercury is one third larger than Earth's Moon and is orbiting Sun at a distance of 36 million miles (58 million km). It is very similar in composition and aspect to Moon as its surface is mainly composed of craters, basins and lava flows. Like what is hypothesized about Moon, Mercury might have some water-ice at one of its poles. Messenger mission (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is scheduled to reach a definitive orbit in April 2009, after two Venus (2004, 2006) and one Earth (2005) gravity assist flybys. Messenger is bound to a Mercury thorough study, performing a very elliptical orbit which will bring it from 120 miles above planet to 9,000 miles away. picture courtesy U.S. Geological Survey