MESSENGER is a scientific investigation of the planet Mercury, the least explored terrestrial planet. Understanding Mercury and how it was formed is essential to understanding the other terrestrial planets and their evolution. Mercury has been visited by only one other spacecraft, Mariner 10, so we know little more than its average density (the second greatest of all the planets), the composition of its atmosphere (thinnest of the terrestrial planets), the fact that it posses a global field, and its extreme variations in temperature. MESSENGER will serve to lift some of the uncertainty about this innermost planet of our solar system.
The MESSENGER mission will orbit Mercury after making three flybys of the planet, using data collected during the flybys as an initial guide to perform a more focused scientific investigation of this mysterious world. MESSENGER will investigate six key scientific questions about Mercury's characteristics and environment with a set of miniaturized space instruments. The spacecraft will enter Mercury orbit in March 2011 and carry out comprehensive measurements for one full Earth year.
It is expensive to place an object in orbit around Earth, let alone reach another planet such as Mercury. To reduce the cost of putting the probe in orbit around Mercury, scientists and engineers will exploit the laws of physics to minimize the amount of fuel needed. Mission planners will use the gravity of Venus and Mercury to help adjust the velocity of MESSENGER so that it can enter an orbit around the inner planet. This will be done in two flybys of Venus, and three of Mercury.
During its three flybys of Mercury, MESSENGER will map nearly the entire planet in color, collecting images of most parts of the planet not seen by Mariner 10, and will take measurements of surface, atmosphere and magnetosphere composition. The flyby results will then be used to plan the yearlong portion of the mission in which MESSENGER orbits the planet.
Spacecraft design is a critical part of any mission. For each kilogram of the spacecraft, many kilograms of rocket fuel are needed to reach escape velocity. So engineers and scientists have worked to keep the spacecraft mass (including structure, engines and fuel for orbital maneuvers, instruments, and power generation, navigation and communication equipment) at an absolute minimum. MESSENGER's propulsion system is integrated into the spacecraft structure to make the most economical use of mass. The miniaturized instruments are located on a science deck that will face Mercury, while a lightweight thermal shade shields the spacecraft from the blistering light of the nearby Sun. Most of the instruments are fixed rigidly to the body of the probe, so coverage of Mercury is achieved by spacecraft motion over the planet. The imaging system uses a miniature scanning mirror so it can quickly collect image mosaics.