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Pluto has a highly eccentric orbit that takes it from 29.7 AU to 49.3 AU over its 248 year period. Neptune, by contrast follows a very circular orbit at about 30.1 AU. Between 1979 and 1999, Pluto was as close to the Sun as it ever gets and inside the orbit of Neptune. Pluto is now moving away from the Sun, and its thin atmosphere will soon freeze out onto its surface. Happily, the New Horizons spacecraft arrived in time to investigate the atmosphere before it fully disappeared.
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