LYCOS RETRIEVER 
Mercury (Planet): Mariner 10
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A much more ambitious mission to Mercury called MESSENGER was launched by the National Aeronautics and Space Administration (NASA) in 2004. MESSENGER will orbit the planet to conduct an in-depth study of its entire surface, in contrast to Mariner 10’s quick flybys. MESSENGER is an acronym for MErcury Surface, Space ENvironment, GEochemistry, and Ranging—instruments on board include detectors to help analyze the planet’s mineral composition, topography, geological processes, possible ice, internal structure, and the origin of its magnetic field. It will take MESSENGER nearly seven years to adjust its path and lose enough energy to be captured into orbit around Mercury. This process involves a complicated series of flybys of Venus and Earth, and includes three flybys of Mercury in 2008 and 2009, during which the mapping of the planet will begin. MESSENGER’s orbital phase will start in 2011 when it begins orbiting the planet for at least one Earth year, equivalent to four Mercurian years.
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The surface of Mercury superficially resembles that of the Moon (see illustration). It is heavily cratered, with large expanses of younger smooth plains (similar to the lunar maria) that fill and surround major impact basins. Unlike the Moon surface, Mercury's heavily cratered terrain is interspersed with large regions of gently rolling intercrater plains, the major terrain type on the planet. Also unlike the Moon surface, a system of thrust faults, unique in the solar system, transects the surface viewed by Mariner 10. The largest structure viewed by Mariner 10 is the 800-mi-diameter (1300-km) Caloris impact basin. Infrared temperature measurements from Mariner 10 indicate that the surface is a good thermal insulator and, therefore, must be covered with porous soil or rock powder like the lunar regolith.
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* Much about Mercury remains mysterious. There is of course the half of the planet not observed by Mariner 10. In addition, current models of the creation of the planets ... suggest that it shouldn't have been able to acquire such a large iron core. This implies that a better knowledge of Mercury would lead to a better understanding of the origins of the other planets of the Solar System.
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Little else was known about Mercury until the U.S. space probe Mariner 10 photographed the planet in 1975. Mariner first approached the planet Venus in February 1974, then used that planet's gravitational field to send it around like a slingshot in the direction of Mercury. The second leg of the journey to Mercury took seven weeks.
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This photomosaic of the planet Mercury was assembled from individual high-resolution images taken by Mariner 10 shortly before closest approach in 1974. The sun is shining from the right, and the terminator is at about 100 degrees west longitude. Crater Kuiper, named after astronomer Gerard P. Kuiper, can be seen just below the center of the planet's illuminated side. The landscape is dominated by large craters and basins with extensive plains between craters.
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Messenger — short for Mercury Surface, Space Environment, Geochemistry and Ranging — is the first spacecraft to visit the planet since NASA’s Mariner 10 made three flybys between 1974 and 1975. During those sweeps, Mariner 10 mapped only 45 percent of the planet, leaving an entire hemisphere a mystery until now.
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Mariner 10