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Pluto is, on average, the most distant planet from the Sun.
Pluto's very close satellite named Charon was discovered in 1978. Charon orbits Pluto at a distance of 20,000 kilometres in 6.4 days. From these facts we can determine that Pluto has a mass only 0.2% of the Earth. Its diameter is about 2500 kilometres and so Pluto has a density much less than the Earth. It is also very black and it has been supposed by some astronomers that it is more like a giant comet nucleus than a planet. Its surface temperature is about -230°C, too cold for there to be much of an atmosphere.
From recent observations in the infrared Pluto is known to have on its surface solid ices of nitrogen, methane and carbon monoxide. This implies that there will be a thin atmosphere of these gases around the planet. The only information about surface detail comes from an analysis of the variation in the observed brightness over 5 years during which Pluto's satellite, Charon, occulted differing parts of the surface. From these measures it has been deduced that Pluto's south pole has recently received a new layer of methane ice giving high reflectivity of about 90 percent whereas other parts of the surface only reflect less than 30 percent of the sunlight.
Due to its great distance no space probes have visited the Pluto-Charon system. NASA is now in the process of planning such a mission, the Pluto-Charon Express. It will use Jupiter’s gravity to accelerate the satellite towards Pluto, arriving there after 2010 and then continuing into the Edgeworth-Kuiper Disk (see our leaflet on 'The Furthest Object in the Solar System’). This contains many minor planets which have yet to be investigated.
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