Charon, Pluto's companion moon, was discovered by James Christy in June, 1978. Working at the U.S. Naval Observatory in Flagstaff, Arizona, Christy noted that what appeared to be "bumps" on several photographic images taken of Pluto reappeared on a periodic basis. With this information, Christy realized that what had previously been dismissed as image distortions were really composite images of Pluto and a companion moon. Christy suggested that the new moon be named Charon, after the mythical boatman that ferried the souls of the dead across the river Styx to Hades, where Pluto, God of the underworld, sat in judgment.
Charon orbits Pluto once every 6.39 days, which is also the rate at which Pluto spins on its axis. Charon is therefore in synchronous orbit about Pluto. As seen from the satellite-facing hemisphere of Pluto, Charon hangs motionless in the sky, never setting, nor rising. The average Pluto-Charon separation is 12,196 mi (19,640 km), which is about 1/20 the distance between the Earth and the Moon.
Soon after Charon was discovered astronomers realized that a series of mutual eclipses between Pluto and its satellite would be seen from Earth every 124 years. During these eclipse seasons, which last about five years each, observes on Earth would witness a whole series of passages of Charon across the surface of Pluto. The last eclipse season ended in 1990, and the next series of eclipses will take place in 2114.
By making precise measurements of the brightness variations that accompany Charon's movement in front of and behind Pluto, astronomers have been able to construct detailed albedo (reflectivity) maps of the two bodies. They have also been able to derive accurate measurements of each components size; Pluto has a diameter of 1,413 mi (2,274 km), making the planet 1.5 times smaller than Earth's Moon, and two times smaller than Mercury. Charon has a diameter of 737 mi (1,186 km).
Since Pluto has a satellite, Kepler's third law of planetary motion can be used to determine its mass. A mass equivalent to about 1/500 that of the Earth, or about 1/5 that of the Moon has been derived for Pluto. Charon's mass is about 1/8 that of Pluto's. Given the high mass ratio of 8:1 and the small relative separation between Pluto and Charon, the center of mass about which the two bodies rotate actually falls outside of the main body of Pluto. This indicates that rather than being a planet-satellite system, Pluto and Charon really constitute a binary system, or, in other words, a double planet.
Pluto has a bulk density of about 2 g/cm3, while Charon has a lower bulk density of about 1.2 g/cm3. This difference in densities indicates that while Pluto is probably composed of a mixture of rock and ice, Charon is most probably an icy body. In general terms, Pluto can be likened in internal structure to one of Jupiter's Galilean moons, while Charon is more similar in structure to one of Saturn's moons. In fact, astronomers believe that Pluto's internal structure and surface appearance may be very similar to that of Triton, Neptune's largest moon.