NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, launched on 3 August 2004, is well into its voyage to initiate a new era in our understanding of the terrestrial planets. The mission, spacecraft, and payload are designed to answer six fundamental questions regarding the innermost planet during three flybys and a one-year-long, near-polar-orbital observational campaign. The cruise phase to date has been used to commission the spacecraft and instruments and begin the transition to automated use of the instruments with on-board, time-tagged commands. An Earth flyby one year after launch, a large propulsive maneuver in December 2005, and Venus flybys in October 2006 and June of this year began the process of changing MESSENGER'S heliocentric motion. The second Venus flyby was also used to complete final rehearsals for Mercury flyby operations in January 2008 while coordinating observations with the European Space Agency's Venus Express mission. The upcoming Mercury flyby will be the first since that of Mariner 10 in 1975. Along with the second and third MESSENGER flybys in October 2008 and September 2009, that flyby will provide images of the hemisphere of Mercury never before seen by spacecraft as well as the first high-resolution information on Mercury's surface mineralogy. These three flybys, interspersed with deep space maneuvers, finish adjusting the spacecraft motion sufficiently for Mercury orbit injection to follow in March 2011. In the orbital phase, the spacecraft's nominal periapsis latitude of 60°N will gradually drift northward as the periapsis altitude of 200 km gradually drifts upward due to solar gravitational perturbations. The 12-hour period and 80° inclination are maintained while the altitude is readjusted downward to 200 km every Mercury revolution about the Sun. The profile enables mapping of the entire planet and acquiring detailed elemental and topographic data over the northern hemisphere. After conclusion of the nominal mission in March 2012, an additional year of data analysis and archiving is planned before the conclusion of the MESSENGER project. To broaden scientific participation in the mission, NASA has established a Participating Scientist Program, bringing a full complement of international scientific researchers into the mission. The MESSENGER team is also continuing its informal interaction with members of the BepiColombo project to maximize the overall scientific return from both missions.
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