Charon (moon)

Charon

Pluto and Charon
Discovery
Discovered by	James W. Christy
Discovered in	1978 April 13
Orbital characteristics
Semimajor axis	19,405 km
Eccentricity	0.0
Orbital period	6 day 9 h 18 min
Inclination	99.089 °
Is a satellite of	Pluto
Physical characteristics
Mean diameter	1186 km
Mass	1.90×1021 kg
Mean density	2.24 g/cm3
Surface gravity	0.368 m/s2
Rotation period	6 day 9 h 18 min (synchronous)
Axial tilt	unknown
Albedo	0.37
Atmosphere	none

Charon is the only known satellite of Pluto. It was discovered by astronomer James Christy on June 22, 1978 by carefully examining highly magnified images of Pluto on photographic plates taken a couple of months before and noticing that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to 1965 April 29. It received the temporary designation 1978 P 1, according to the then-recently instituted convention.

Christy named it after the Greek mythological figure Charon but pronounced it differently. The "ch" at the beginning of the moon's name is soft so it sounds like "Sharon," after the astronomer's wife Charlene, nicknamed Char, which both have soft ch sounds. The mythological figure's name is pronounced with a hard "ch" sound like the modern letter "k", like "ch" in Christy's name. The name "Charon" was officially accepted by the IAU in 1985.

The discovery of Charon allowed astronomers to more accurately calculate Pluto's mass and size. Charon revolves around Pluto in 6.387 days, the same period as Pluto's rotation. The two objects are gravitationally locked (tidal locking) so they each keep the same face towards the other.

Charon's diameter is 1,172 kilometers (728 miles), just under half the size of Pluto. It has 1/7th the mass of Pluto, and a surface area of 4,400,000 km². Unlike Pluto, which is covered in nitrogen ice, Charon appears to be coated in water ice.

Due to the unusually small difference in size between it and Pluto, Pluto and Charon are sometimes considered to be a double planet. They are also sometimes thought of as not a planet and a satellite, but as the first two Trans-Neptunian objects.