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Pluto is the only planet not yet visited by spacecraft. Our knowledge of it is therefore sketchy. Even the Hubble and large ground-based telescopes have difficulty imaging this small object. A spacecraft called “New Horizons” is on its way and will arrive in 2015.

Pluto has three moons, Charon being the largest. The two smaller moons were discovered recently and have been named Nix and Hydra.

Pluto has a highly elliptical orbit which takes it inside the orbit of Neptune for 20 years of its 249 year orbit. Its axis is tilted at almost 120º and has retrograde rotation. Most unusual is its large inclination to the ecliptic of 17º. It has recently been re-classified as a dwarf planet.

Being so far from the Sun Pluto is very cold, less that -200º C. The surface is mostly frozen Nitrogen with traces of frozen Methane and frozen Carbon Monoxide. From the surface the Sun would appear to be no more than a bright star. Because of its elliptical orbit Pluto's distance from the Sun varies considerably. When it is at its closest (30 AU) it has a thin atmosphere some 1/100,000 that of the Earth but as Pluto moves to its furthest distance (50 AU) from the Sun the atmosphere freezes out and falls to the surface.

Pluto (left) is seen here in this photograph taken by the Hubble Space Telescope with its moon Charon. Even with the best telescopes it is impossible to make out any detail on Pluto or Charon.

Pluto has three moons. Charon was discovered in 1978, and has diameter of 1186 km, nearly half that of Pluto. Nix and Hydra were discovered in May 2005 and were officially named in June 2005. They are much smaller than Charon and are difficult to see even with the Hubble telescope. They both have diameters of 44-50 km. As yet little is known about their physical characteristics.

Some astronomers have suggested that the three moons of Pluto may have been produced when Pluto collided with another body.