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Charon is the largest moon of the planet Pluto.

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Charon is a moon of Pluto and was discovered in 1978. Pluto's moon Charon is, depending on the definition employed, either the largest moon of Pluto or one member of a double dwarf planet with Pluto being the other member. With the discovery in 2005 of two other moons of Pluto (Nix and Hydra), Charon is now also referred to as Pluto I. The New Horizons mission is scheduled to visit Charon and Pluto in July 2015.

Charon should not be confused with the similarly named Chiron, a smaller object in the outer solar system.

Charon Pluto's moon.
Charon moon. Artist's concept seen from the surface of Pluto.
Discovered by James W. Christy
Discovered on June 22, 1978
Space Missions New Horizons flyby mission
to Pluto and Charon
Orbital characteristics
Semimajor axis 19,571 ± 4 km
eccentricity 0.00000 ± 0.00007
Orbital period 6.387230 ± 0.000001 d
(6 d 9 h 17 m 36.7 s ± 0.1 s)
inclination 112.78±0.02º (to the ecliptic)
0.00º±0.014º (to Pluto's equator)
119.59±0.02º (to Pluto's orbit)
Is a Satellite of Pluto
Physical characteristics
Mean diameter 1207 km ± 3 km
(0.095 Earths)
Surface area 4.58×107 km2
mass (1.52±0.06)×1021 kg
(2.54×10-4 Earths)
Mean density 1.65 ± 0.06 g/cm³
Surface gravity 0.278 m/s2
escape velocity 0.580 km/s
Surface temperature -220 degrees Celsius (53 K)
rotation period synchronous
axial tilt zero
Albedo varies 0.36 to 0.39
atmosphere none (< 0.11 µbar or 11 mPa)

Discovery of Charon.

Charon was discovered by astronomer James Christy on June 22, 1978 when he was examining highly magnified images of Pluto on photographic plates taken a couple of months before. Christy noticed that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965.

Naming of Charon.

Charon was originally known by the temporary designation S/1978 P 1, according to the then-recently instituted convention. Subsequently, Christy chose the name "Charon", which was officially accepted by the IAU in 1985. In Greek mythology, Charon was the ferryman of the dead, a figure with close ties to the god Hades (adopted by the Romans as Pluto).

Although the traditional English pronunciation of the mythological figure is with a hard k sound, Christy pronounced the ch in the moon's name as an sh sound, after his wife Charlene (nicknamed "Char"). The sh pronunciation is now common among astronomers, in spite of the pleas to follow customary usage. In European languages other than English, speakers generally follow the pronunciation established for the mythological figure, which is the ch sound.

Physical characteristics of Pluto's moon Charon.

Charon's diameter is about 1,207 km (750 miles), just over half that of Pluto, with a surface area of 4,580,000 km². Unlike Pluto, which is covered with Nitrogen and methane ices, the Charonian surface appears to be dominated by less volatile water ice.

Orbital characteristics of Pluto's moon Charon.

Charon and Pluto revolve about each other every 6.387 days. The two objects are gravitationally locked, so each keeps the same face towards the other. The average distance between Charon and Pluto is 19,570 km (12,163 mi). The discovery of Charon allowed astronomers to accurately calculate the mass of the Plutonian system, and mutual occultations revealed their sizes. However, neither indicated the two bodies' individual masses, which could only be estimated, until the discovery of Pluto's outer moons in late 2005. Details in the orbits of the outer moons reveal that Charon has approximately 11.65% of the mass of Pluto. This shows it to have a density of 1.65±0.06 g/cm³, suggesting a composition of 55±5% "rock" to 45% ice, whereas Pluto is somewhat denser and about 70% "rock".

Formation of Charon.

Pluto and Charon.
Surface of Pluto and Charon, as determined from brightness variations during mutual occultations.
Charon formation.
The formation of Pluto's moon Charon.

Simulation work published in 2005 by Robin Canup suggested that Charon could have formed by a giant impact around 4.5 billion years ago, much like the Earth and Moon. In this model a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer mantle, and Charon coalesced from the debris. However, such an impact should result in an icier Charon and rockier Pluto than we find. It is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other. The collision would have been violent enough to boil off volatile ices like methane but not violent enough to be disrupted.

Charon: Moon or dwarf planet?

The center of mass (barycenter) of the Pluto-Charon system lies outside either body. Since neither object truly rotates around the other, and they are comparable in terms of mass, it has been argued that Charon should not be considered to be a satellite of Pluto. Instead, it has been argued that they form dual dwarf planets, following the re-classification of Pluto.

In a draft proposal for the 2006 redefinition of the term, the International Astronomical Union proposed that a planet be defined as a body that orbits the sun that is large enough for gravitational forces to render the object (nearly) spherical. Under this proposal, Charon would have been classified as a planet, since the draft explicity defined a planetary satellite as one in which the barycenter lies within the major body. In the final definion, Pluto was reclassified as a Dwarf planet, but the formal definition of a planetary satellite was not decided upon, leaving Charon's status unclear. (Charon is not in the list of dwarf planets currently recognised by the IAU.)

The moons Nix and Hydra also orbit the same barycenter, but are not large enough to be spherical, and are simply considered to be satellites of Pluto (or, under the alternative viewpoint, of the Pluto-Charon system).

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