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The Planet Pluto and Charon
Pluto is seen in the
foreground, with Charon to the left and the distant sun in the
distance.
Pluto is the ninth planet from the sun and outermost member of the solar system known to man. It was discovered by American astronomer Percival Lowell in 1905, who theorized the existence of a distant planet past Neptune that disturbed the orbit of Uranus. The search for the planet was concluded by Clyde Tombaugh, who found Pluto in 1930, near the area Lowell predicted.
Pluto revolves around the sun once every 247.7 Earth years. Its average distance from the sun is 5.9 billion kilometers. The orbit is so strange that at certain points its path is closer to the sun than Neptune. They will never collide, though, because Pluto's orbit never actually crosses Neptune's path.
Pluto is visible only through large telescopes and it seems to have a yellowish color. The Hubble Space Telescope allowed astronomers to accurately judge Pluto's size. It is about 2320 kilometers in diameters, about 2/3 the size of Earth's moon. In 1978 James Christy discovered a very large moon orbiting the planet at a distance of 19, 600 kilometers. The moon, Charon, was named after the boatman who ferried the souls of the dead across the River Styx in Greek mythology.
The Hubble Telescope imaged 85% of Pluto's surface in 1994, observing bright and dark areas of contrast. It is believed the bright areas are fields of nitrogen ice and the dark areas are methane ice colored by sunlight. The dark areas may also be valleys or impact craters. The images support the theory that ice caps form on Pluto's poles.
Pluto is twice as dense as water, and is made of much rockier material than the other outer system planets. This could be the result of chemicals combining during the formation of the planet. Many scientists believe Pluto is the former satellite of Neptune that was knocked into a separate orbit during the early days of our solar system.
Charon is the only known moon of Pluto. It orbits Pluto at a distance of only 19,600 kilometers and completes its orbit in roughly 6.4 Earth days. The orbit is circular and parallel to Pluto's equator. Charon is spherical and measures about 1270 kilometers across. It is slightly more than half as big as Pluto, which makes it the largest moon in proportion to its planet. The two are often called double planets. They orbit a common point, keeping the same face to the other always. Charon appears to be made of low-density ice, though there is hardly any information about its internal structure.
Source:
Microsoft Encarta Encyclopedia '99. "Pluto (planet)" and "Charon (astronomy)" Microsoft Corporation 1993-1998.