Charon (moon)

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**Charon**
Discovery
Image:Charon 2.jpg Artist's concept of Charon seen from the surface of Pluto.
Discovered by	James W. Christy
Discovered on	June 22, 1978
Space Missions	New Horizons flyby mission to Pluto and Charon
Orbital characteristics <ref name="Buie06">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.(http://arxiv.org/abs/astro-ph/0512491)</ref>
Semimajor axis	19,571 ± 4 km
Eccentricity	0.00000 ± 0.00007
Orbital period	6.387230 ± 0.000001 d (6 d 9 h 17 m 36.7 s ± 0.1 s)
Inclination	112.78±0.02° (to the ecliptic) 0.00°±0.014° (to Pluto's equator) 119.59±0.02° (to Pluto's orbit)
Is a satellite of	Pluto
Physical characteristics
Mean diameter	1207 km ± 3 km <ref name="Sicardy06">B. Sicardy et al (2006). "Charon’s size and an upper limit on its atmosphere from a stellar occultation". Nature 439: 52.</ref> (0.095 Earths)
Surface area	4.58×10⁷ km²
Mass	(1.52±0.06)×10²¹ kg <ref name="Buie06"/> (2.54×10⁻⁴ Earths)
Mean density	1.65 ± 0.06 g/cm³
Surface gravity	0.278 m/s²
Escape velocity	0.580 km/s
Surface Temperature	-220 degrees Celsius (53 K)
Rotation period	synchronous
Axial tilt	zero
Albedo	varies 0.36 to 0.39
Atmosphere	none (< 0.11 μbar or 11 mPa)

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.

1 Discovery
2 Name
3 Physical characteristics
4 Orbital characteristics
5 Formation
6 Moon or dwarf planet?
7 References
8 External links

[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

Image:Plutoncharon1.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 (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[1]. 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

Image:Pluto Moon Form.jpg

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. 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.<ref>Schriber (2005). "Charon's Shadow Illuminates Its True Nature". ScienceNOW.</ref>

[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 argued 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 definion, 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 recognised 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). <ref>http://www.boulder.swri.edu/plutomoons/</ref>

[edit] References

[edit] External links

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Pluto

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 [2] describing the discovery
Measuring the Size of a Small, Frost World (ESO press release January 2006) [3]
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)

Image:Pluto symbol.svg

Pluto's natural satellites

edit

Charon · Nix · Hydra

See also : Pronunciation key

Solar System Natural Satellites v • d • e</div>
Planetary satellites:	Terran • Martian • Jovian • Saturnian • Uranian • Neptunian
Other satellite systems:	Plutonian • Eridian • Asteroid satellites

Largest satellites:	Ganymede • Titan • Callisto • Io • Earth's Moon • Europa • Triton Titania • Rhea • Oberon • Iapetus • Charon • Umbriel • Ariel • Dione • Tethys
See also: inner satellites • trojans • irregulars • list by diameter • discovery timeline • naming

v • d • e</span>

Trans-Neptunian objects

Plutinos : Pluto* | 1993 RO | 1993 RP | 1993 SB | 1993 SC | 1994 TB | 1995 QZ₉ | 1996 SZ₄ | 1996 TP₆₆ | 38083 Rhadamanthus | 38628 Huya | 28978 Ixion | 2003 VS₂ | 90482 Orcus | Unnumbered: 2003 AZ₈₄

Cubewanos: 1998 WW₃₁ | 1992 QB₁ | 1994 GV₉ | 1994 JQ₁ | 1994 VK₈ | 1996 TO₆₆ | 19521 Chaos | 53311 Deucalion | 2002 AW₁₉₇ | 20000 Varuna | 50000 Quaoar | 2002 MS₄ | 2002 TX₃₀₀ | 2002 UX₂₅ | 58534 Logos | 2004 GV₉| 2003 EL₆₁ | 2005 FY₉ | Unnumbered: | 2003 QW₉₀ | 2005 RN₄₃

Twotinos: Unnumbered: 1996 TR₆₆ | 1998 SM₁₆₅ | 1997 SZ₁₀ | 1999 RB₂₁₆ | 2000 JG₈₁

Scattered disc objects: Eris* | 2002 TC₃₀₂ | 1995 TL₈ | 1996 GQ₂₁ | 1996 TL₆₆ | 2000 OO₆₇ | 2000 OM₆₇ | 2001 KC₇₇ | 2001 UR₁₆₃ | 2002 CY₂₂₄ | 2002 GX₃₂ | 90377 Sedna** Unnumbered: 2004 XR₁₉₀

Unclassified Objects : 1994 JS | 1994 JR₁ | 1995 DA₂ | 1995 SM₅₅ | 1996 TQ₆₆ | 1997 CR₂₉ | 1997 CS₂₉ | 1997 CU₂₉ | 1997 QJ₄ | 1998 HJ₁₅₁ | 1998 HK₁₅₁ | 1998 HP₁₅₁ | 1998 HM₁₅₁ | 1998 KR₆₅ | 1998 SM₁₆₅ | 1998 SN₁₆₅ 1998 US₄₃ | 1998 VG₄₄ | 1998 WW₂₄ | 1998 WA₃₁ | 1998 WU₃₁ | 1998 WA₂₅ | 1999 CP₁₃₃ | 1999 CL₁₅₈ | 1999 CC₁₅₈ | 1999 DF₉ | 1999 HT₁₁ | 1999 HB₁₂ | 1999 HC₁₂ | 1999 KR₁₆ | 1999 OY₃

* - Also classified as a dwarf planet ** - Currently classified as an SDO, though may be part of the Inner Oort Cloud

v • d • e</span>

Large trans-Neptunian objects

Kuiper belt: Orcus | Pluto (Charon) | Ixion | 2002 UX₂₅ | Varuna | 2002 TX₃₀₀ | 2003 EL₆₁ | Quaoar | 2005 FY₉ | 2002 AW₁₉₇

Scattered disc: 2002 TC₃₀₂ | Eris | 2004 XR₁₉₀ | Sedna

See also Triton, astronomical objects and the solar system's list of objects, sorted by radius or mass.
For pronunciation, see: Centaur and TNO pronunciation.

The Solar System v • d • e</div>
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The Sun · Mercury · Venus · Earth · Mars · Ceres · Jupiter · Saturn · Uranus · Neptune · Pluto · Eris
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies: Meteoroids · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.