 Pluto is the furthest planet from the sun, and is the only planet in our Solar System which has not been visited by a spacecraft from Earth. (In 2006 it was decided that Puto is not really a planet at all; it has been reclassified as a dwarf planet). What little information we have learned about Pluto, mostly in the last ten years or so, has been through images taken by the orbiting Hubble telescope. Pluto is so far away ... 4 billion miles ... that the image shown above, of Pluto and its moon Charon, is the best we have. But mathematical analysis of the orbits of these bodies reveals some interesting facts.
The surface temperature on Pluto varies between about -235° and -210° C. The 'warmer' regions roughly correspond to the darker areas visible in the Hubble telescope photographs. Pluto's composition is unknown, but its density suggests that it is probably a mixture of 70% rock and 30% water ice, again much like Triton. The brighter areas of the surface may be covered with ices of nitrogen, and smaller amounts of solid methane, ethane and carbon monoxide. The gravity on the surface of Pluto is about one fifteenth of what it is on Earth. By comparison, the force of gravity on Earth's Moon is a sixth of Earth's. Pluto has a very thin atmosphere, but it may exist as a gas only when Pluto is closest to the sun in its orbit. We are able to detect it because Pluto has just passed this neaerst point, during its long 248 year orbit.  Pluto's orbit is highly inclined to the plane of the solar system, and is very elongated. In fact, during its 248 year long orbit, Pluto actually gets closer to the sun than Neptune. It was inside Neptune's orbit for a few years, and has just recently moved further out.The diagram at the right also indicates the rotational axis of each of the bodies. Note that Pluto, like Uranus, rotates 'on its side', compared to the sun and other planets. Its 'day' is just over 6 earth days long.  Pluto is not very big; it is about 4660 km in diameter. Pluto's moon Charon is slightly over 2400 km wide. Because Charon is over half of Pluto's diameter, this makes it the largest satellite relative to its planet. (The next closest pair is the earth/moon system). Watching Charon disappear behind Pluto during its approximately 6 day 'month', and Pluto being partially blocked by Charon as it orbits in front of Pluto, helps scientists learn about Pluto's atmosphere and surface features. Interestingly, Charon is tidally locked to Pluto ... it rotates around Pluto, and rotates itself, in the same time it takes Pluto to revolve.
 The image at the right shows the best view we have of Pluto; it's a photograph taken by the Hubble space telescope, which orbits the earth. No spacecraft have visited Pluto. Pluto is at a point in its orbit which would make it relatively easy for a spacecraft to be launched towards the planet sometime in the next ten years, before Pluto begins to move out of the orbital plane of the other planets. The Pluto-Kuiper Express Mission being planned at the Jet Propulsion Laboratory in the USA is attempting to design and build two spacecraft to explore Pluto and Charon, possibly as a joint American-Russian project, using Russian launch vehicles known as Protons. The spacecraft would take 7 to 9 years to reach Pluto.
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