Strom, Robert G. Department of Planetary Sciences, Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona.
Last reviewed:February 2019
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- Mercury (planet), published October 2017:Download PDF Get Adobe Acrobat Reader
- Mercury (planet), published June 2014:Download PDF Get Adobe Acrobat Reader
- Polar radar features
- Magnetic field
- Geologic history
- Related Primary Literature
- Additional Reading
The planet closest to the Sun, as well as the smallest planet in the solar system. Mercury (Fig. 1) is the innermost planet of the solar system. It is visible to the unaided eye only shortly after sunset or shortly before sunrise, when it is near its greatest angular distance from the Sun (28°). Mercury is the smallest planet (because Pluto is no longer classified as one), having a diameter of 4878 km (3031 mi). Its mass is 3.30 × 1023 kg (7.28 × 1023 lb), or 0.055 times the mass of the Earth. Most of the detailed knowledge of Mercury is derived from data returned by the Mariner 10 spacecraft, which flew by the planet three times in 1974 and 1975, and the MESSENGER (MErcury Surface, Space, ENvironment, GEochemistry, and Ranging) spacecraft that orbited Mercury from 2011 to 2015. Mariner 10 imaged only about 45% of the surface at an average resolution of about 1 km (0.6 mi), and less than 1% at resolutions between about 91 and 457 m (300 and 1500 ft). MESSENGER flew by Mercury three times before entering orbit in March 2011. It imaged the entire planet at a resolution of about 250 m (820 ft) and targets of interest from about 12 to 24 m (39 to 78 ft). MESSENGER also studied the magnetic field, determined the composition of the surface and exosphere, mapped the topography of the northern hemisphere, and studied the gravity field and interior. See also: Earth; MESSENGER mission; MESSENGER mission results; Planet; Pluto; Solar system; Space probe
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