Charon (moon)
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 Charon's Pluto-facing hemisphere (determined from brightness variations during Pluto-Charon occultations) |
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| Discovery | |
|---|---|
| Discovered by: | James W. Christy |
| Discovery date: | June 22, 1978 |
| Orbital characteristics [1] | |
| Epoch 2,452,600.5 | |
| Semi-major axis: | 19,571 ± 4 km (relative to Pluto centre) |
| Eccentricity: | 0.00000 ± 0.00007 |
| Orbital period: | 6.3872304 ± 0.0000011 d (6 d 9 h 17 m 36.7 ± 0.1 s) |
| Inclination: | 0° (to Pluto's equator) 119.591 ± 0.014° (to Pluto's orbit) 112.783 ± 0.014° (to the ecliptic) |
| Longitude of ascending node: | 223.046 ± 0.014° (to vernal equinox) |
| Satellite of: | Pluto |
| Physical characteristics | |
| Mean radius: | 603.5 ± 1.5 km [2] (0.095 Earths) |
| Surface area: | 4.58×106 km² |
| Mass: | (1.52 ± 0.06)×1021 kg[1] (2.54×10−4 Earths) |
| Mean density: | 1.65 ± 0.06 g/cm³[1] |
| Equatorial surface gravity: | 0.278 m/s2 |
| Escape velocity: | 0.580 km/s |
| Rotation period: | synchronous |
| Axial tilt: | zero? |
| Albedo: | varies between 0.36 and 0.39 |
| Temperature: | -220°C (53 K) |
Charon (shair'-ən or kair'-ən (key), IPA /ˈʃɛərən, ˈkɛərən/, Greek Χάρων), discovered in 1978, 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.
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[edit] 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 before. Christy noticed that a slight bulge appeared periodically. Later, the bulge was confirmed on plates dating back to April 29, 1965.
[edit] Name
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 (IPA [ʃ]), 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.
[edit] Physical characteristics
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.
[edit] 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 (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.[3] 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".
[edit] Formation
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 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 be disrupted.[4]
[edit] 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 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). [5]
[edit] References
- ^ a b c 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: 290. arXiv:astro-ph/0512491.
- ^ B. Sicardy et al (2006). "Charon’s size and an upper limit on its atmosphere from a stellar occultation". Nature 439: 52.
- ^ http://arxiv.org/PS_cache/astro-ph/pdf/0512/0512491.pdf
- ^ Schriber (2005). "Charon's Shadow Illuminates Its True Nature". ScienceNOW.
- ^ http://www.boulder.swri.edu/plutomoons/
[edit] External links
- James W. Christy and Robert S. Harrington, "The satellite of Pluto," The Astronomical Journal 83 (1978) 1005
- Marc W. Buie, Phases of Charon as seen from Pluto, Lowell Observatory
- Hubble reveals new map of Pluto, BBC News, 12 September 2005
- IAU Circular No. 3241 [1] describing the discovery
- Measuring the Size of a Small, Frost World (ESO press release January 2006) [2]
- M. J. Person et al.: Charon’s Radius and Density from the Combined Data Sets of the 2005 July 11 Occultation (submitted to the Astronomical Journal, 3 February 2006)
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