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Pluto |
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Note: You can click on each image below to view a larger version.
The search was done by taking thousands of long-exposure photographs of the sky and then comparing images of the same area taken on different nights, using a "blink comparator". This is essentially a simple projector, but with a mechanism to allow the user to swap rapidly from one image to another. Although a planet at this distance would look just like a star on the photograph, it would move from day to day. So, stars would be in the same place on each photographic plate, but a planet would be in different places. When Tombaugh flicked between the two images in the blink comparator, anything that had moved would appear to "blink" on and off. Pluto ceased to be classified as a planet in August 2006, when the International Astronomical Union adopted a new definition of the term "planet". However its planetary status had long been in question because of its small size (only 2390km in diameter). Some astronomers argued that it should be classified as a Kuiper Belt object, a debate that was only intensified by the discovery of Sedna. On the other hand, the fact that Pluto's largest moon Charon has a diameter almost half that of Pluto (1172km) also made it possible to see it as a double planet. Charon orbits Pluto in 6.4 days, the same amount of time it takes the planet to rotate and the same amount of time that Charon itself takes to rotate about its axis. So the two always present the same face to each other, and since Charon orbits over Pluto's equator it would always appear in the same place in the sky. Despite this it would still have phases. Two much smaller moons were found in 2005 and named Nix and Hydra. 
ABOVE: A map of Pluto derived from Hubble Space Telescope images. Each of the two large pictures shows one hemisphere and the small images inset into them show the original photographs from which the map was made. Even these pictures are enough to show that there must be a varied landscape on Pluto, with a very strong contrast between the light and dark areas. The brighter areas appear to be frozen nitrogen with some methane and carbon monoxide ice, but the composition of the darker regions is unknown. Charon appears to be covered in water ice. Pluto has an extremely tenuous atmosphere, which probably consists of nitrogen, methane and carbon monoxide. It has a very elliptical orbit which at times takes it closer to the Sun than Neptune, and the atmosphere probably freezes out into surface ice during the more distant parts of Pluto's orbit. Pluto was at its closest point to the Sun (perihelion) in 1989, so scientists are hoping to get a space probe to it before the atmosphere disappears again. The New Horizons mission was launched in January 2006, to reach Pluto in July 2015 and then go on to a fly-by with one or more Kuiper Belt objects. To see Pluto requires a powerful telescope and a very detailed star chart, since it would just appear as a faint point of light indistinguishable from a star. |
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Last Updated: 1 Oct 07 |  |
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