Charon (moon)

Charon (; also , as in Χάρων), discovered in 1978, is either the largest moon of Pluto or the smaller member of a double dwarf planet with Pluto, depending on the definition employed. 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 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. The discovery was announced on 1978 July 7. Later, the bulge was confirmed on plates dating back to April 29, 1965.

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.

Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses between 1985 and 1990. This occurs when the Pluto-Charon orbital plane is edge-on as seen from Earth, which only happens at two intervals in Pluto's 248-year orbital period. It was fortuitous that one of these intervals happened to occur so soon after Charon's discovery.

Images showing Pluto and Charon resolved into separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. Later, the development of adaptive optics made it possible to also resolve Pluto and Charon into separate disks using ground-based telescopes.


Charon was originally known by the temporary designation S/1978 P 1, 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. Official adoption of the name by the IAU would wait until late 1985, and was announced on 3 January 1986.

In Greek mythology, Charon is the ferryman of the dead, a figure with close ties to the god Hades, which the Romans identified with their god Pluto. Although in English the mythological figure Charon is pronounced with a hard k sound, Christy pronounced the ch in the moon's name as sh (IPA [ʃ]), after his wife Charlene. The sh pronunciation is customary among astronomers when speaking English. Speakers of languages other than English, and some English-speaking astronomers, follow the pronunciation established for the mythological figure.

Physical characteristics

Charon's diameter is about 1207 km, 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, 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.

Mutual eclipses of Pluto and Charon in the 1980s allowed astronomers to take spectra of Pluto and then the combined spectrum of the pair. By subtracting Pluto's spectrum from the total, astronomers were able to spectroscopically determine the surface composition of Charon.

Charon's volume and mass allow calculation of its density from which it can be told that Charon is largely an icy body and contains less rock by proportion than its partner Pluto. This supports the idea Charon was created by a giant impact into Pluto's icy mantle (see Formation below.) There are two conflicting theories about Charon's internal structure: some scientists believe it to be a differentiated body like Pluto with a rocky core and an icy mantle while others believe Charon to be of uniform composition throughout. Evidence in support of the former position was found in 2007, when 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. The fact that the ice was still in crystalline form suggested it had been recently deposited, as solar radiation would have degraded older ice to an amorphous state after 30 000 years or so.

Orbital characteristics

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. 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".


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 what 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.

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 Charon has 11.6% the mass of Pluto, it has been argued that Charon should not be considered to be a satellite of Pluto. Instead, it has been suggested 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 explicitly defined a planetary satellite as one in which the barycenter lies within the major body. In the final definition, 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 recognized 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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