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The surface of Pluto is mostly icy. With a density between 1.8 and 2.1 g/cc, Pluto is believed to be 50% to 75% rock; the rest composed of ice. Nitrogen (98%), solid methane and carbon monoxide make up the surface. Because of the solid methane, scientists believe that the surface temperature of Pluto never reaches above 70 Kelvin. Pluto's temperature depends greatly on its orbit. Pluto comes as close as 30 A.U. to the Sun and as far as 50 A.U. away from the Sun. As Pluto's orbit takes it away from the Sun, its atmosphere actually freezes and falls to the ground. Obviously, mankind has never reached Pluto. However, in 2001, NASA plans to launch the Pluto Express to study Pluto.
The idea of a distant ninth planet was first documented in 1905 by an American astronomer named Percival Lowell. Lowell proceeded to continue his search for his mysterious planet, going so far as predicicting a possible area the planet could be found. However, it wasn't until the year 1930 that the planet was finally discovered by Clyde W. Tombaugh. (For more information on Pluto's discovery, click here.) In honor of Lowell, who had passed away in 1916 without fulfilling his dream, Tombaugh named the planet after the Roman god of the dead. In 1978, James Christy, an astronomer at the U.S. Naval Observatory in Flagstaff, Arizona, discovered a moon for Pluto. Named Charon, the moon had a diameter of only 740 miles (1,190 kilometers). (For more information on Charon, click here.)
Pluto's orbit takes it 249 years to complete. Usually, its orbit puts it as the farthest planet from the Sun. However, for 20 years of its trip around the Sun, its orbit takes it inside Neptune's orbit. For these 20 years, Pluto is the eighth planet from the Sun. The most recent orbit crossing took place on January 21, 1979 when Pluto came inside Neptune's orbit. They switched back on February 11, 1999. This will not occur again until September 2226. Pluto and Neptune never collide as their orbits cross because as Pluto reaches perihelion, the closest it ever comes to the Sun, it is at its maximum degree of inclination. When the two planets pass by each other, Pluto is usually well below or above the plane Neptune's orbit is on. The closest the planets come together is 18 A.U.
The Hubble Telescope took this picture of Pluto and Charon.