MESSENGER uses gravity assists from Earth, Venus and Mercury to lower its speed relative to Mercury at orbit insertion. In a gravity assist, a spacecraft flies close by a planet and picks up (or loses) a tiny amount of the planet's angular momentum around the Sun. The planet is so massive (compared with the spacecraft) that its orbit does not change. But each gravity assist changes the shape, size and tilt of MESSENGER's orbit until the propellant onboard is sufficient to insert the spacecraft into its planned scientific orbit around Mercury. "Mercury orbit insertion" is the mission planners' term for the maneuver that will move MESSENGER from an orbit around the Sun to an orbit around Mercury. MESSENGER launched from Cape Canaveral Air Force Station, Fla., on August 3, 2004. It will return to Earth for a gravity boost in August 2005, then fly past Venus twice, in October 2006 and June 2007. The spacecraft uses the tug of Venus gravity to resize and rotate its trajectory closer to Mercurys orbit. Three Mercury flybys, each followed about two months later by a course correction maneuver, put MESSENGER in position to enter Mercury orbit in March 2011. During the flybys set for January 2008, October 2008 and September 2009 MESSENGER will map nearly the entire planet in color, image most of the areas unseen by Mariner 10, and measure the composition of the surface, atmosphere and magnetosphere. It will be the first new data from Mercury in more than 30 years and invaluable for planning MESSENGERs yearlong orbital mission.
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