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Charon (moon)


Pluto and Charon
Discovered byJames Christy
Discovered in1978
Orbital characteristics
Semimajor axis19,405 km
Orbital period6d 9h 18m
Is a satellite ofPluto
Physical characteristics
equatorial diameter1028 km
Mass1.90×1021 kg
Mean density2.24 g/cm3
Surface gravity0.368 m/s2
Rotation period6d 9h 18m (synchronous)
Axial tiltunknown

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", like "ch" in Christy's name.

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.