Guest Author - Lea Terry

Much as there's been controversy over whether Pluto is really a planet, there's also uncertainty about the classification of one of Pluto's satellites, Charon.

Since its discovery in 1978 by James Christy of the U.S. Naval Observatory, Charon has been considered a moon of Pluto, but it does not actually orbit the body. In fact, the two bodies are locked into a mutually synchronous orbit in which they orbit around each other (every 6.387 days), and many in the scientific community have suggested that the pair be reclassified as a dual dwarf planet system. It's thought that the two collided at some point, which forced them into orbit around each other, or that another object from the Kuiper Belt collided with Pluto and was destroyed, and that Charon formed out of the debris left behind by the collision (much how Earth's moon is thought to have formed).

Charon was Pluto's only known moon until 2005, when two additional moons, Nix and Hydra, were discovered.

Charon's diameter is a little more than half that of Pluto's, and the two bodies have an unusual orbit, in which both keep the same hemisphere facing the other at all times. This isn't unusual for a moon -- we always see the same hemisphere of our moon, for example -- but it is unusual for a planet to exhibit this characteristic.

Several images have been taken of Charon, including studies of Pluto and Charon conducted during a five-year span, between 1985 and 1990, when the two bodies entered a period of "mutual eclipses" as seen from Earth's perspective. This means astronomers could measure the planets as they passed in front of each other.

In January 2006 NASA sent its New Horizons craft to Pluto and Charon, with an expected arrival date of 2015. The New Horizons project is already organizing a search for other satellites circling Pluto, with the help of ground telescopes and potentially the Hubble Space Telescope.

Charon's name comes from Greek mythology -- Charon was the boatman who carried the souls of the dead to the underworld, which in Roman mythology was ruled by the god Pluto.

Charon's Vitals:
--Distance from Pluto: about 12,163 miles
--Diameter: 750 miles
--Mass: 1,620,000,000,000,000,000,000 kg.