• (written as Mercury)
    The innermost planet of the solar system, orbiting the Sun in 88 days at an average distance of 58 million km. Mercury has a radius of 2439 km, making it intermediate in size between the Moon and Mars, although its mean density of 5.42 gm/cc is nearer to those of Venus and the Earth. Mercury has the least circular orbit of the major planets, with an eccentricity of 0.2. Almost all our knowledge of Mercury was gained by three flybys carried out by Mariner 10 in 1974 and 1975, in an exquisite piece of celestial snooker. Mariner showed Mercury as a cratered planet like the Moon without the maria, although there is one major basin structure aptly called the Caloris (hot) Basin. There are also the inter-crater plains where there are few large craters, perhaps indicating a more recent surface. Mercury has a magnetic field about 1 per cent as strong as the Earth’s, indicating a possible metal core, although this seems not to be hot enough to be liquid. Mercury also rotates very slowly, in a special version of captured rotation. Its day of 57 Earth days means that it makes three turns on its axis for every two orbits of the Sun. (Telescopic maps of Mercury were drawn until the 1960s on the basis of true captured rotation, assuming an 88-day spin, and so were wildly inaccurate.) Mercury has an exceptionally thin atmosphere, about a thousand trillionth as dense as the Earth’s, dominated by hydrogen and helium, possibly captured from the solar wind, as well as sodium and other components. Its eccentric orbit and spin provide hot and cold poles. At the cold poles the Sun is at its highest when Mercury is farthest from the Sun, and at the warm poles when it is nearest Surface temperatures on Mercury can go to over 400° and fall to perhaps –200° during the mercurian night, since Mercury has too little atmosphere to moderate either the huge influx of solar energy or the cold of deep space by moving heat to the parts of the planet where no solar energy is arriving. It is even possible that ice is present in the coldest zones. Mercury has no satellites.
  • (written as Mercury)
    First US manned spaceflight campaign, involving four manned orbital flights, and one carrying a monkey, along with suborbital and test flights, between 1959 and 1963
  • A dense metallic chemical element whose atomic number is 80. It is a poisonous silvery liquid, is a very good electrical conductor, and has extremely high surface tension. Mercury has over 30 known isotopes, of which several are stable. Its applications are many, including its use in vapor lamps, arc lamps, switches, amalgams, as neutron absorbers in nuclear plants, and in thermometers and other instruments such as barometers and vacuum pumps. Its chemical symbol, Hg, is taken from the Latin word for liquid silver: hydragyrum. Also called quicksilver.
  • chemical symbol Hg

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