MESSENGER
MESSENGER arrived at Mercury and went into orbit around this ancient world on March 18, 2011. It was designed to map the surface composition, study the magnetic field and interior structure of our solar system's smallest and innermost planet -- Mercury. It carries eight instruments to study Mercury's polar deposits, core and magnetic dynamo, crust and mantle, magnetosphere, crustal composition, geologic evolution and exosphere. It completed three flybys of the planet and became the first spacecraft to orbit Mercury in 2011.
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Dawn
Dawn, as a mission belonging to NASA's Discovery Program, delves into the unknown, drives new technology innovations and achieves what's never been attempted before. In Dawn's case, it is orbiting one member of the main asteroid belt, Vesta, before heading to gather yet more data at a second, Ceres. The Dawn spacecraft used ion propulsion to get the additional velocity needed to reach Vesta once it left the Delta rocket. It also used ion propulsion to spiral to lower altitudes on Vesta, to leave Vesta and then cruise to Ceres, where it will use ion propulsion to spiral to a low altitude orbit.
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