Charon, discovered in 1978 at the United States Naval Observatory Flagstaff Station, is the largest satellite of the dwarf planet Pluto. Following the 2005 discovery of two other natural satellites of Pluto (Nix and Hydra), Charon may also be referred to as Pluto I.[1] 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.


File:Charon Discovery.jpg

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 July 7, 1978.[2] 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.

All 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.


File:Pluto and charon.jpg

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 resolve Pluto and Charon into separate disks using ground-based telescopes.

Physical characteristicsEdit


Charon's diameter is about 1207 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, 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.[3][4] (See also Cryovolcano.)

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 determined 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. 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.[3]

Orbital characteristicsEdit

File:Charon 2.jpg

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.[5] 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? Edit


Charon compared with Eris, Pluto, Makemake, Haumea, Orcus, 2007 OR10, Quaoar, their moons and Earth. All to scale

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[citation needed].

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.) Had the draft proposal been accepted even Earth's moon may have been classified as a planet in billions of years.[6]

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).[7]


Pluto system 2006

The Plutonian system contains 4 known objects

File:Discovery Charon.jpg

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:[8] Charon is the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans identified with their god Pluto. Official adoption of the name by the IAU would wait until late 1985, and was announced on January 3, 1986.[9]

There is minor debate over the preferred pronunciation of the name. The practice of following the classical pronunciation established for the mythological ferryman Charon is used by major English-language dictionaries such as the Merriam-Webster and Oxford English Dictionary.[10][11] These indicate only one pronunciation of "Charon" when referring specifically to Pluto's moon: with an initial "k" sound. Speakers of languages other than English, and many English-speaking astronomers as well, follow this pronunciation.[12]

However, Christy himself pronounced the ch in the moon's name as sh (IPA [ʃ]), after his wife Charlene. Because of this, as an acknowledgement of Christy and sometimes as an in-joke or shibboleth, the initial sh pronunciation is common among astronomers when speaking English,[13][12][14][15] and this is the prescribed pronunciation at NASA and of the New Horizons Pluto mission team.[16][17]



  1. [1]
  2. "IAUC 3241: 1978 P 1; 1978 (532) 1; 1977n". Archived from the original on 2006-04-22. Retrieved on 2008-06-22.
  3. 3.0 3.1 "Charon: An ice machine in the ultimate deep freeze". Gemini Observatory (2007). Retrieved on 2007-07-18.
  4. Cook et al. (2007). "Near-Infrared Spectroscopy of Charon: Possible Evidence for Cryovolcanism on Kuiper Belt Objects". The Astrophysical Journal 663 (2): 1406–1419. doi:10.1086/518222, 
  5. Marc W. Buie, William M. Grundy, Eliot F. Young, Leslie A. Young, S. Alan Stern (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". Astronomical Journal 132 (1): 290–298. doi:10.1086/504422. arΧiv:astro-ph/0512491, . a, i, e per JPL (site updated 2008 Aug 25)
  6. Robert Roy Britt (2006-08-18). "Earth's moon could become a planet". CNN Science & Space. Retrieved on 2009-11-25.
  7. Stern, Alan (2005-05-15). "Background Information Regarding Our Two Newly Discovered Satellites of Pluto". Planetary Science Directorate (Boulder Office). Retrieved on 2006-08-30.
  8. Govert Shilling, "A Bump in the Night" in Sky & Telescope (June 2008) pp. 26-27. Prior to this, Christy had considered naming the moon Oz.
  9. "IAU Circular No. 4157" (January 3, 1986). Retrieved on 2007-04-10.
  12. 12.0 12.1 Pronounced "KAIR en" or "SHAHR en" per "Charon". Retrieved on 2008-10-03.
  13. Astronomer Mike Brown can be heard pronouncing it [ˈʃɛɹɪn] in ordinary conversation on the KCET interview "Julia Sweeney and Michael E. Brown". Hammer Conversations: KCET podcast (2007). Retrieved on 2008-10-01. at 42min 48sec. Being a long-time resident of California, he does not distinguish the /ær/ vowel of the name Sharon and the /ɛər/ vowel of the classical pronunciation of Charon.
  14. Pronounced 'with a soft "sh" ' per "Welcome to the solar system, Nix and Hydra!". The Planetary Society Weblog. Archived from the original on 2006-07-08. Retrieved on 2008-10-03.
  15. US Naval Observatory spokesman Jeff Chester, when interviewed on the NPR commentary "Letters: Radiology Dangers, AIDS, Charon". Morning Edition (2006-01-19). Retrieved on 2008-10-03. (at 2min 49sec), says Christy pronounced it [ˈʃɛɹɒn] rather than classical [ˈkɛɹɒn]. In normal conversation, the second vowel is reduced to a schwa: /ˈkɛərən/ in RP (ref: OED).
  16. Pronounced "Sharon" /ˈʃærən/ per "NASA New Horizons: The PI's Perspective—Two for the Price of One". Retrieved on 2008-10-03. and per "New Horizons Team Names Science Ops Center After Charon's Discoverer". Retrieved on 2008-10-03.
  17. Hal Weaver, who led the team that discovered Nix and Hydra, also pronounces it [ˈʃɛɹɪn] on the Discovery Science Channel documentary Passport to Pluto, premiered 2006-01-15.

External linksEdit

Template:Pluto Template:Moons of plutoids


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