Pluto’s Moon Charon

Planet Charon — Charon is the only known satellite of Pluto.

Charon was discovered by astronomer James Christy in 1978 using photographic plates which showed a bulge moving around Pluto. 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” (or more properly like the German “ch” in “Bach”), like in Christy’s name.

The discovery of Charon allowed astonomers 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. It has 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.



Discovered by James Christy
Discovered in 1978

Orbital characteristics

Semimajor axis 19,405 km
Eccentricity 0.0
Orbital period 6d 9h 18m
Inclination 99.089

Physical characteristics

Mean radius 586 km
Mass 1.901021 kg
Mean density 2.24 g/cm3
Surface gravity 0.368 m/s2
Rotation period 6d 9h 18m (synchronous)
Axial tilt unknown
Albedo 0.37
Atmosphere none



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