The MErcury Surface, Space ENvironment, GEochemistry and Ranging (or MESSENGER for short) is a NASA mission, launched August 3, 2004, designed to study the characteristics and environment of Mercury from orbit. Specifically, the scientific objectives of the mission are to characterize the chemical composition of Mercury's surface, the geologic history, the nature of the magnetic field, the size and state of the core, the volatile inventory at the poles, and the nature of Mercury's exosphere and magnetosphere over a nominal orbital mission of one Earth year. The mission will be the first to return to Mercury in over 35 years — the only previous probe to visit Mercury was Mariner 10, which completed its mission in March, 1975.