LYCOS RETRIEVER 
Mercury (Planet): Mariner 10
built 655 days ago
In 1991 powerful radio telescopes on Earth revealed signs of vast sheets of ice in Mercury’s polar regions, areas that had not been covered by Mariner 10. A greater understanding of the planet is the aim of the MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) probe, launched by NASA in 2004 and due to arrive in 2011. It will orbit Mercury for a year while studying the composition and structure of its crust, as well as its geological history, polar regions, atmosphere, magnetic environment, and the make-up of its core.
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Mercury's atmosphere is very tenuous and is essentially exospheric in that its atoms rarely collide with each other. The atmospheric surface pressure is 1012 times less than Earth's . Mariner 10's ultraviolet spectrometer identified hydrogen, helium, oxygen, and argon in the atmosphere, all of which are probably derived largely from the solar wind. Earth-based telescopic observations in 1985 discovered that Mercury is surrounded by a tenuous atmosphere of sodium and potassium that is probably derived from its surface.
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On its first flight past Mercury, Mariner 10 came within 470 miles (756 kilometers) of the planet and photographed about 40 percent of its surface. The probe then went into orbit around the Sun and flew past Mercury twice more in the next year before running out of fuel.
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Mercury is the closest planet to the sun, orbiting it in a highly eccentric orbit. The spacecraft Mariner 10 flew by the planet three times in 1974 and 1975. A collection of articles about the planet Mercury published in The New York Times is below.
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This photomosaic of Mercury was constructed from photos taken by Mariner 10 six hours before the spacecraft flew past the planet on March 29, 1974. These images were taken from a distance of 5,380,000 kilometers (3,340,000 miles). (Courtesy USGS, and NASA)
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Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40 to 45 percent of the planet's surface. Physically, Mercury is similar in appearance to the Moon as it is heavily cratered.
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Mariner 10