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MESSENGERabbrev. for Mercury Surface, Space Environment, Geochemistry, and Ranging, a NASA mission to the planet Mercury, launched Aug. 3 2004 from Cape Canaveral by a Delta 2 Heavy rocket and forming part of the agency's Discovery Program. Although an acronym, the name recalls the planet's mythical association with Mercury the winged messenger of the gods. The MESSENGER spacecraft is NASA's first Mercury orbiter. Its chief goal is to investigate important scientific questions about Mercury's surface and interior composition and its environment. Among other things, the probe will seek to find out why Mercury's density is so high, what the composition and structure of its crust is, and whether there has been any volcanism. It will also look into the nature, dynamics and origin of Mercury's magnetic field (possibly driven by a liquid outer core) and its tenuous atmosphere. One focus of interest will be the mysterious polar deposits on Mercury, which some scientists believe could be water ice. The data for these questions will be gathered by an array of seven miniature instruments working in one of the most hostile environments in the Solar System, a mere 58 million km from the Sun.
To avoid overshooting its target and possibly falling into the Sun, the MESSENGER craft is following a 6½-year, 7.9-billion-km roundabout course to Mercury that should slow it down sufficiently to accomplish its task. The journey will involve 15 solar orbits, 6 trajectory-altering planetary flybys (including 1 of the Earth, 2 of Venus, and 3 of Mercury itself), and 6 crucial rocket firings. It will enter orbit around Mercury in March 2011 for a year of scientific data collection.