Pluto

  In Roman mythology, Pluto, Greek: Hades, is the god of the underworld. The planet received this name, after many other suggestions, perhaps because it is so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percial Lowell.

  Pluto is usually the furthest planet from the and by far the smallest. Pluto is even smaller than seven of the moons in our solar system; smaller than the Moon, Europa, Io, Callisto, Ganymede, Titan, and Triton.

  Pluto was discovered by fortunate accident in 1930. Error in calculations had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Not knowing about the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned out to be Pluto anyway.

  After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbit of the other planets. The search for Planet X contunued, but nothing was found. It is not likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 spacecraft is used. So there is no planet number ten.

  Pluto is the only planet in the solar system that has never been visited by a spacecraft. Even the Hubble Space Telescope can resolve only the largest surface features, because of its distance.

  Pluto has one satellite, Charon. By a very good fortune, Charon was discovered, in 1978, just before its orbital plane moved edge on toward the inner solar system. It was therefor possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.

  The radius of Pluto is not well known. JPL´s value of 1137 is given with an error of +/-8, almost one percent.

  Though the sum of the masses of Pluto and Charon is known pretty well, the individual masses of Pluto and Charon are difficult to detirmine because that requires much finer measurements, they are so small and far away that even the Hubble Space Telescope has difficulty. The ratio of their masses is probably somewhere between 0.084 and 0.157; more observations are underway but we won´t get really accurate data until a spacecraft is sent.

  Pluto is the second most contrast body in the Solar System, after Iapetus. Exploring the origin of that contrast is one of the high-priority goals for the proposed Pluto Express mission.

  Some think that Pluto would be better classified as a large asteroid or even comet rather than as a planet. Some consider it to be the largest Kuiper Belt object (also known as Trans-Neptunian Objects). There is considerable merit to the later position, but historically Pluto has been classified as a planet and it is likely to remain so.

  Pluto´s orbit is highly eccentric. At times it is closer to the Sun than Neptune. It was nearer to the Sun than Neptune from January 1979 thru February 11 1999. Pluto rotates in the opposite direction from most of the other planets.

  Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto´s orbital period is exactly 1.5 times longer than Neptune´s. Its orbital inclination is also much higher than the other planets´. Thus though it appears that Pluto´s orbit crosses Neptune´s, it really doesn´t and they will never collide.

  The plane of Pluto equator is at almost right angles to the plane of its orbit.

  Pluto is very cold planet due to its distance. The surface temperature on Pluto is  not well known but it is probably between -228°C to -238°C.

  The composition of Pluto is unknown, but its density indicates that it is probalby a mixture of 70% rock and 30% water ice much like Triton. The bright areas of the surface seem to be covered with ices of nitrogen with smaller amounts of solid methane, ethane and carbon monoxide. The compositions of the darker areas of Pluto´s surface is unknown but may be due to primordial organic materal or photochemical reactions driven by cosmic rays.

  Very little is known about Pluto´s atmosphere, but it probably consists of nitrogen with some carbon monoxide and methane. It is extremely tenuous the surface pressure being only a few microbars. Pluto´s atmosphere may exist as a gas only when Pluto is near its perihelion; for the majority of Pluto´s long year, the atmospheric gases are frozen into ice. Near perihelion, it is likely that some of th atmosphere escapes to space perhaps even interacting with Charon. The Pluto Express mission planners want to arrive at Pluto while tha atmosphere is unfrozen.

  Pluto can be seen with an amateur telescope but it is not easy. The maximum magnitude of Pluto is +14.

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