1994 image of Pluto and Charon (right)
Pluto has four known natural satellites (Charon, Nix, Hydra and S/2011 P1. (provisional name, also known as P4, identified by the Hubble Space Telescope in 2011).
The Pluto–Charon system is noteworthy for being the largest of the Solar System's few binary systems, defined as those whose barycentre lies above the primary's surface. This and the large size of Charon relative to Pluto has led some astronomers to call it a dwarf double planet.
Charon is the 12th largest moon in the Solar System.
The system is also unusual among planetary systems in that each is tidally locked to the other. Pluto and Charon are gravitationally locked, so each keeps the same face towards the other.
Because of this, the rotation period of each is equal to the time it takes the entire system to rotate around its common centre of gravity. Just as Pluto revolves on its side relative to the orbital plane, so the Pluto–Charon system does also.
Discovery
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 earlier. Christy noticed that a slight elongation appeared periodically.
Subsequent observations of Pluto determined that the bulge was due to a smaller accompanying body. The periodicity of the bulge corresponded to Pluto's rotation period, which was previously known from Pluto's light curve. This indicated a synchronous orbit, which strongly suggested that the bulge effect was real and not spurious.
Naming
Charon was originally known by the temporary designation S/1978 P1, according to the then recently instituted convention.
On June 24, 1978, Christy first suggested the name Charon as a scientific-sounding version of his wife Charlene's nickname, "Char."
Although colleagues at the Naval Observatory proposed Persephone, Christy stuck with Charon after discovering it coincidentally refers to a Greek mythological figure: Charon is the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans identified with their god Pluto.
Stats
Diameter: 1205 km
Semi-major axis: 17,530 km
Orbital Period: 6.39 days
Origin
Simulation work published in 2005 by Robin Canup suggested that Charon could have been 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 scientists have found. 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 have destroyed either body.
Physical characteristics
Unlike Pluto, which is covered with nitrogen and methane ices, the Charonian surface appears to be dominated by less volatile water ice, and also appears to have no atmosphere.
In 2007, observations by the Gemini Observatory of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.
Exploration Status
The arrival of the New Horizons Spacecraft to Pluto-Charon in 2015 is highly anticipated. Who know what new discoveries await us?
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