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Charon (shair'-ən or kair'-ən, IPA /ˈʃɛərən, ˈkɛərən/, Greek Χάρων), or Pluto I, is the largest satellite of Pluto. It was discovered by astronomer James Christy on June 22, 1978 by carefully examining highly magnified images of Pluto on photographic plates taken a couple of months before and noticing that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965. It received the temporary designation S/1978 P 1, according to the then-recently instituted convention. It is not to be confused with the similarly named 2060 Chiron, another object in the outer solar system with an orbit between those of Saturn and Uranus. With the discovery in 2005 of two other moons of Pluto (S/2005 P 1 and S/2005 P 2), Charon is now also referred to as Pluto I.
Due to the unusual similarity in size between Charon and Pluto, they are sometimes considered a double planet. They are also sometimes thought of as the first known Trans-Neptunian objects.
Christy chose the name "Charon" for the moon, and this 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 Pluto. The ch of the mythological figure is pronounced as a k sound, like the ch in Christy's name. However, 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 of classicists.
Charon's diameter is 1,205 km (748.8 miles), just over half that of Pluto, with a surface area of 4,560,000 km2. Unlike Pluto, which is covered with nitrogen and methan ices, the Charonian surface appears to be dominated by less volatile water ice.
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 19,130 km (11,889 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 one-eighth the mass of Pluto. This shows it to have a density of 1.71 g/cc, suggesting a composition of 63% ±5% rock to 37% ice, whereas Pluto is somewhat denser and about 70% rock.
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, and it is now thought that Pluto and Charon may have been two bodies that collided before going into orbit about each other, violently enough to boil off volatile ices like nitrogen but not violently enough to be disrupted.[2]
The moon, along with Pluto, is to be visited by the New Horizons mission in July 2015.
References
- ↑ Buie, M. W., W. M. Grundy, E. F. Young, L. A. Young, S. A. Stern. "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1 and S/2005 P2". Astrophisics.
- ↑ Schriber (2005). "Charon's Shadow Illuminates Its True Nature". ScienceNOW.
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