Goals: MESSENGER 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.
Accomplishments: On 18 March 2011 (UTC), MESSENGER became the first spacecraft to orbit Mercury. During a series of flybys that edged it closer to orbit insertion, the spacecraft revealed more of Mercury than has ever been seen before. Images and data reveal Mercury as a unique, geologically diverse world with a magnetosphere far different than the one first discovered by Mariner 10 in 1975.
MESSENGER solved the decades-old question of whether there are volcanic deposits on the planet's surface. MESSENGER orbital images have revealed volcanic vents measuring up to 25 kilometers (15.5 miles) across that appear to have once been sources for large volumes of very hot lava that, after eruption, carved valleys and created teardrop-shaped ridges in the underlying terrain.
The spacecraft also found Mercury has an unexpectedly complex internal structure. Mercury's core is huge for the planet's size, about 85% of the planetary radius, even larger than previous estimates. The planet is sufficiently small that at one time many scientists thought the interior should have cooled to the point that the core would be solid. However, subtle dynamical motions measured from Earth-based radar combined with parameters of the gravity field, as well as observations of the magnetic field that signify an active core dynamo, indicate that Mercury's core is at least partially liquid.