Mercury is tidally locked into a 3:2 rotation to revolution synchronization. Until radar studies in 1965 proved otherwise, it was believed that Mercury kept the same face to the sun always (like the moon does to the Earth).
MESSENGER is a MErcury Surface, Space ENvironment, GEochemistry and Ranging mission to orbit Mercury following two flybys of that planet. Launch Window: 10-Mar - 29-Mar-2004 12-May - 23-May-2004 Venus Flyby 1: 24-Jun-2004 Venus Flyby 2: 16-Mar-2006 Mercury Flyby 1: 21-Jul-2007 Mercury Flyby 2: 11-Apr-2008 Enter Mercury Orbit: 06-Apr-2009
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