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Definition: Mercury (astronomy) from The Macmillan Encyclopedia

The innermost and second smallest (4880 km diameter) planet, orbiting the sun every 88 days at a mean distance of 57.9 million km. Its long period of axial rotation, 58.6 days, is two-thirds of its orbital period. Mercury can only be seen low in the twilight and early morning sky and, like the moon, exhibits phases. Its surface is heavily cratered, with intervening lava-flooded plains. It has a very thin atmosphere, mainly helium and argon. See also planetary probe.

Summary Article: Mercury
from Space Exploration and Humanity: A Historical Encyclopedia

Mercury is the closest planet to the Sun. Known to the classical Greeks in antiquity, Mercury's orbit became the focus of attention in the nineteenth century as the precession of its perihelion could not be explained by Newtonian gravitation theory. While some astronomers searched for a planet closer to the Sun, notionally called Vulcan, that perturbed Mercury's orbit, eventually its orbital anomalies were explained by Albert Einstein's general theory of relativity in the early twentieth century.

By the 1950s astronomers still knew little about the planet. The consensus was that the planet's rotation period matched its solar orbit period of 88 days, that it had little or no atmosphere, and that its surface was volcanic. In 1965 radar astronomy studies showed that Mercury's rotation period was 58.65 days, or two thirds of its orbital period.

Mariner 10 flew by Mercury three times in 1974–75, photographing 45 percent of the planet, uncovering a heavily cratered planet with no atmosphere, but with a magnetic field. The magnetic field was a surprise to scientists, because Mercury's small size implied more rapid internal cooling than the other inner planets, and hence a solid core with no magnetic field. In 2007 results of ground-based radar studies indicated the possibility of a molten core. In 1991 radar studies provided evidence that Mercury's permanently shadowed polar craters could contain water ice, which rekindled interest in the planet.

NASA launched the MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) spacecraft in August 2004. The first of three scheduled flybys occurred on 14 January 2008, when MESSENGER passed 200 km above Mercury. The data revealed a dynamic system with a history of volcanism and an iron-rich liquid outer core. At least 60 percent of the planet's mass is in the core, twice as much as the other terrestrial planets. Cooling of the core has led to contraction of the entire planet, and creates a dipolar magnetic field that interacts with the surface and exosphere. The second flyby on 6 October 2008 revealed 30 percent of the surface previously unseen, at which point spacecraft had imaged 95 percent of the planet. The remaining flyby will shape MESSENGER’s orbit to allow it to enter orbit around Mercury in March 2011.

See also: Mariner 10

  • Applied Physics Laboratory MESSENGER.
  • Leverington, David , Babylon to Voyager and Beyond (2003).
  • Special Issue Science, 4 July 2008.
  • Berryhill, Katie J.
    Copyright 2010 by ABC-CLIO, LLC

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