GEochemistry and Ranging (MESSENGER) probe is a 485-kilogram (1,070 lb) robotic American space probe in orbit around the planet Mercury. It was launched by NASA in August 2004 to study the chemical composition, geology, and the magnetic field of Mercury. It became the second mission to reach Mercury successfully when it made a flyby in January 2008, followed by a second flyby in October 2008,and a third flyby on September 2009. (The first space probe to reach Mercury was Mariner 10 in 1975.) MESSENGER is the first spacecraft to orbit the planet Mercury. Inserting into orbit around Mercury is difficult because a satellite approaching on a direct path from Earth would be accelerated by the Sun's gravity and pass Mercury far too quickly to orbit it.
The instruments carried by MESSENGER performed well on a complex series of flybys of Earth (once), Venus (twice), and Mercury itself (three times). These allowed the craft to be slowed relative to Mercury with minimal fuel. MESSENGER successfully entered Mercury orbit on 18 March 2011 with no reported problems. The craft's science instruments were reactivated 24 March, with a first photo returned from Mercury orbit on 29 March. Its formal science data collection mission is planned to begin 4 April 2011.
In 1973 Mariner 10 was launched to make multiple flyby encounters of Venus and Mercury. Mariner 10 provided the first detailed data of Mercury, mapping 40-45% of the surface. The final flyby of Mercury by Mariner 10 occurred on March 16, 1975, ending close-range observations of the planet for over thirty years. Being the least explored terrestrial planet with no future planned mission, a study, published in 1998, detailed a proposed mission to send an orbiting spacecraft to Mercury. In the years since the Mariner 10 mission, subsequent mission proposals to revisit Mercury had appeared too costly, requiring large quantities of propellant and a heavy lift launch vehicle. However, using a trajectory designed by Chen-wan Yen in 1985, the study showed it was possible to seek a Discovery-class mission by using multiple, consecutive gravity assist, 'swingby' maneuvers around Venus and Mercury, in combination with minor propulsive trajectory corrections, to gradually slow the spacecraft and thereby minimize propellant needs.
The primary science objectives of the mission include:
determine accurately the surface composition of Mercury
characterize the geological history of the planet
determine the precise strength of the magnetic field and its variation with position and altitude
investigate the presence of a liquid outer core by measuring Mercury's libration
determine the nature of the radar reflective materials at Mercury’s poles
investigate the important volatile species and their sources and sinks on and near Mercury.
The spacecraft was designed and built at the Johns Hopkins University Applied Physics Laboratory. Science operations, managed by Dr. Sean Solomon as principal investigator, and mission operations are also conducted at JHU/APL. The contrived acronym MESSENGER was chosen because Mercury was the messenger of the gods according to Roman mythology.