A NASA Discovery mission to conduct the first orbital study
of the innermost planet
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MESSENGER used 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 changed the shape, size and tilt of MESSENGER's orbit until the onboard propellant was sufficient to insert the spacecraft into its planned scientific orbit around Mercury. "Mercury orbit insertion" is the mission planners' term for the maneuver that moved 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 flew past Venus twice, in October 2006 and June 2007. The spacecraft used 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 – which occurred in January 2008, October 2008 and September 2009 – MESSENGER mapped nearly the entire planet in color, imaged most of the areas unseen by Mariner 10, and measured the composition of the surface, atmosphere and magnetosphere. The three successful flybys of Mercury by the MESSENGER spacecraft returned the first new data from Mercury in more than 30 years. This data was invaluable as plans were made for 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 has experienced 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 received 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 approached 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.

The final maneuver dates and velocity change (ΔV) values for the mission’s deep space maneuvers (DSMs) and trajectory correction maneuvers (TCMs) that took place during the cruise phase are shown in the table below. The DSM designation identifies the TCMs with the largest velocity change, which include all but one of the maneuvers (TCM-3) required in the pre-launch planned cruise-phase trajectory. Contingency maneuvers TCM-4, TCM-7, TCM-8, TCM-14, and TCM-17 were never implemented due to the achievement of sufficient accuracy with the prior TCM. After TCM-19 only DSMs were required before Mercury orbit insertion, based on the effective implementation of a strategy to orient the spacecraft sunshade and solar panel  for sufficient durations to use sunlight particles to correct small TCM implementation errors (known as “solar sailing”),

Primary Maneuvers

Maneuver Alternate Name Calendar Date Delta-V (m/s) Purpose
DSM-1 TCM-9 12 Dec 2005 315.6 target the first Venus flyby
DSM-2 TCM-18 17 Oct 2007 227.4 target the first Mercury flyby and test the spacecraft configuration to be used in future maneuvers; component maneuver
DSM-3 TCM-23 19 Mar 2008 72.2 target the second Mercury flyby and test spacecraft rotation during a maneuver as required for Mercury orbit insertion
DSM-4 (part 1) TCM-29A 4 Dec 2008 222.1 target the third Mercury flyby
DSM-4 (part 2) TCM-29B 8 Dec 2008 24.7 target the third Mercury flyby and test spacecraft rotation during a maneuver
DSM-5 TCM-35 24 Nov 2009 177.75 target Mercury arrival for orbit insertion

Secondary Maneuvers

Maneuver Calendar Date Delta-V (m/s) Purpose
TCM-1 24 Aug 2004 17.9 correct most inaccuracies resulting from launch
TCM-2 24 Sep 2004 4.6 correct all remaining inaccuracies resulting from launch
TCM-3 18 Nov 2004 3.2 target the Earth flyby
TCM-5 23 Jun 2005 1.1 target the Earth flyby
TCM-6 21 Jul 2005 0.2 target the Earth flyby
TCM-10 22 Feb 2006 1.3 target the first Venus flyby
TCM-11 12 Sep 2006 1.7 target the first Venus flyby; component maneuver
TCM-12 5 Oct 2006 0.5 target the first Venus flyby
TCM-13 2 Dec 2006 25.6 target the second Venus flyby; component maneuver
TCM-15 25 Apr 2007 0.6 target the second Venus flyby
TCM-16 25 May 2007 0.2 target the second Venus flyby
TCM-19 19 Dec 2007 1.1 target the first Mercury flyby

Click here for detailed information on all of MESSENGER's propulsive activity from launch to the present.

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