LYCOS RETRIEVER 
Mercury (Planet): Spacecraft
built 627 days ago
A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004. The MESSENGER spacecraft will make several close approaches to planets to place it onto the correct trajectory to reach an orbit around Mercury. The first fly-by of Mercury occurred on January 14, 2008. Two more fly-bys of Mercury are scheduled, in October 2008 and September 2009. Most of the hemisphere not imaged by Mariner 10 will be mapped during the fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011.
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Mercury is one of the least-studied of the solar system's planets. It was visited by only one spacecraft, Mariner 10, which flew past it on three occasions in 1974 and 1975. Only 45% of the planet's surface was mapped, and it is located too near the sun for existing telescopes to conduct further mapping from Earth. Until radar observations in 1965 proved otherwise it was thought that Mercury was tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the sun at all times. Instead, Mercury is in a 3:2 resonance, rotating three times for every two revolutions around the sun. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in it's 3:2 resonance, so showing the same face, which would be ... the case if it was totally locked.
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Reaching Mercury from Earth poses significant technical challenges, since the planet orbits three times closer to the Sun than the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun's gravitational potential well. From a stationary start, a spacecraft would require no delta-v or energy to fall towards the Sun. However, starting from the Earth with an orbital speed of 30km/s, the spacecraft's significant angular momentum resists sunward motion. Hence, the spacecraft must change its velocity considerably to enter into a Hohmann transfer orbit that passes near Mercury.
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Between January 9 and 13, 2008, as the MESSENGER probe approached Mercury for its first flyby, the Narrow Angle Camera, part of the Mercury Dual Imaging System (MDIS), acquired a series of images of the planet in support of spacecraft navigation. These images have been put together as frames in a movie. The final frame of the movie has the highest spatial resolution (20 km/pixel, 12 miles/pixel) and was recorded when the spacecraft was at a distance of about 760,000 kilometers (470,000 miles) from Mercury. Mercury is about 4,880 kilometers (about 3,030 miles) in diameter.
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The only spacecraft to approach Mercury has been the NASA Mariner 10 mission ( 1974 – 75 ). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury, and it provided the first close-up images of Mercury's surface. It made three close approaches to Mercury, the closest of which took it to within 327km of the surface. Unfortunately, the same face of the planet was lit at each close approach, resulting in the restriction of images to less than 45% of the planet's surface. Mariner 10 ... found the first evidence for Mercury's magnetic field, and measured temperatures across its surface http://nssdc.gsfc.nasa.gov/nmc/tmp/1973-085A.html.
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The Narrow Angle Camera of the Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft obtained high-resolution images of the floor of the Caloris basin on January 14, 2008. Near the center of the basin, an area unseen by Mariner 10, this remarkable feature - nicknamed "the spider" by the science team - was revealed. A set of troughs radiates outward in a geometry unlike anything seen by Mariner 10. The radial troughs are interpreted to be the result of extension (breaking apart) of the floor materials that filled the Caloris basin after its formation. Other troughs near the center form a polygonal pattern. This type of polygonal pattern of troughs is ... seen along the interior margin of the Caloris basin.
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