Charon is the largest of Pluto’s three moons, nearly half the size of the former ninth planet. With Nix and Hydra, Pluto’s other two moons, the dwarf planet and Charon make up what is known as a double dwarf planet system. Because of their similarity in size and the strangeness of their orbits about one another, Charon and Pluto may have been one body in the past, acted upon by a large object impacting Pluto and splitting it nearly in half. (Sobel, Planets, 217)
Pluto’s largest moon was discovered in 1978, and observations using the Hubble Space Telescope revealed within a decade that Charon and Pluto occult (hide) one another from Earth view twice during each 248-year revolution around the Sun. Close observation of their orbital characteristics reveals that Charon and Pluto are tidally locked, that is, each shows to the other the same face at all times, like Earth’s Moon.
What this means is that Charon’s orbit around Pluto takes 6.4 days, while Pluto’s rotation period is also 6.4 days. An observer standing on the bitterly cold surface of either of these tiny worlds would see only one face of Charon or Pluto during their entire orbit around the Sun. A photograph of Pluto and Charon held by NASA may be seen here.
By 2015 the space agency expects to gather better information about the outer planets using the instruments built into New Horizons, a spacecraft presently winging its way toward the Kuiper Belt and Pluto. New Horizons is expected to study atmospheres and composition of Kuiper Belt Objects, Pluto, and Pluto’s faraway neighbors until 2026.
The New Horizons spacecraft may allow NASA to answer such questions as why the surface of Charon is a different color from that of its dwarf planet, Pluto. Scientists also want to discover the nature of Charon’s atmosphere, if any; has the moon lost its atmosphere due to its small size? Are Charon’s two tiny companion moons (discovered in 2005), both of which share the plane of its orbit, actually additional pieces of Pluto knocked away by the impact which may have formed Charon?
Physical characteristics aside, Charon was named for the boatman on the River Styx, who ferries the souls of the dead to the underworld. That underworld is ruled by Pluto, a Roman god, and it was an 11-year-old girl who named the dwarf planet after Clyde Tombaugh discovered it in 1930.
Planets, The. (2005). Dava Sobel. New York: Byron Preiss Visual Publications, Inc.