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.

Launch and Cruise

MESSENGER launched from Cape Canaveral Air Force Station, Fla., on August 3, 2004. It returned to Earth for a gravity boost on August 2, 2005, then it will 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 Mercury’s 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 MESSENGER’s yearlong orbital mission.

During the 4.9-billion (4,900,000,000) mile (about 7.9-billion kilometers) journey from launch to Mercury orbit, the spacecraft will experience a range of extreme conditions. One such extreme includes repeated passes at a distance 70% closer to the Sun than the Earth's average distance from the Sun. At this distance, the spacecraft receives about 11 times more of the Sun's thermal energy than Earth-orbiting satellites. Shortly after each of the first two Mercury flybys, the spacecraft will approach the ultimate speed record for all spacecraft by achieving speeds of over 140,000 miles per hour (over 225,300 kilometers per hour) relative to the Sun - about eight times faster than the speed of NASA's Space Shuttle in low-Earth orbit. The Helios-2 spacecraft achieved at least 150,000 miles per hour relative to the Sun on April 17, 1976.

The image above shows MESSENGER's entire trajectory, looking down on Earth's orbit plane. Click on it for a detailed explanation of MESSENGER's path.