MErcury Surface, Space ENvironment, Geochemistry, and Ranging (MESSENGER) is a NASA Discovery mission to study the planet Mercury. It will be the first to orbit Mercury. Like the NASA Mariner 10 mission which flew past Mercury, MESSENGER takes advantage of solar pressure to save propellant. While Mariner 10 used solar pressure for attitude control, MESSENGER uses it for course correction. Like Mariner 10, this capability was added to the original mission.