Nasa Mission: Messenger
MESSENGER stands for Mercury Surface, Space Environment, Geochemistry and Ranging.
On the 18th March 2011 the Messenger became the first spacecraft to orbit Mercury. During a series of flybys that edged it closer to orbit insertion, the spacecraft revealed more than has ever seen before. Images and data reveal Mercury has 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.
MESSENGER lifted off from Cape Canaveral Air Force Station, Fla., on a three-stage Boeing Delta II expendable launch vehicle on August 3, 2004.
On Aug. 2, 2005, MESSENGER flew past Earth at a distance of 2,348 kilometers (1,459 miles), using our planet's gravity to redirect itself toward Venus. The flyby also allowed mission controllers to calibrate part of MESSENGER's science payload.
MESSENGER's path to Mercury took it past Venus twice. The spacecraft used the tug of Venus' gravity to resize and rotate its trajectory closer to Mercury's orbit. On Oct. 24, 2006, MESSENGER flew past Venus at a distance of about 2,987 kilometers (1,856 miles), reducing its orbit's perihelion and aphelion (minimum and maximum distance from the Sun) and increasing the orbit inclination - the tilt angle relative to Earth's orbit around the Sun. The approaching spacecraft viewed a brightly sunlit Venus. The departure view was mostly dark .