Information about Charon (planet)

Charon

Charon's Pluto-facing hemisphere (determined from brightness variations during Pluto-Charon occultations)
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
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:	4.58106 km²
Mass:	(1.52 ± 0.06)1021 kg[1] (2.5410−4 Earths)
Mean density:	1.65 ± 0.06 g/cm³[1]
Equatorial surface gravity:	0.278 m/s2
Escape velocity:	0.580 km/s .36 mi/s
Rotation period:	synchronous
Axial tilt:	zero?
Albedo:	varies between 0.36 and 0.39
Temperature:	-220°C (53 K)
Apparent magnitude:	16.8 [3]
Angular size:	55 milli-arcsec[4]

Charon (shair'-ən (), IPA: /ˈʃeərən/, also kair'-ən, /ˈkeərən/; 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.

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.

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 good luck 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.

Name

Charon was originally known by the temporary designation S/1978 P 1, according to the then recently instituted convention. Christy immediately chose the name "Charon", but official adoption by the IAU would wait until late 1985, and was announced on 3 January 1986.^[5]

In Greek mythology, Charon was 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 (nicknamed "Char"). The sh pronunciation is customary among astronomers when speaking English. Speakers of languages other than English, which use the name Charon, follow the pronunciation established for the mythological figure.

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, 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.^[6]

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 us to calculate its density; from this we can tell Charon is largely an icy body and contains less rock by proportion than its partner Pluto, supporting 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.^[7]

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.^[8] 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".

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.^[9]

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).^[10]

References

External links

• Charon Profile by NASA's Solar System Exploration
• 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 http://cfa-www.harvard.edu/iauc/03200/03241.html describing the discovery
• Measuring the Size of a Small, Frost World (ESO press release January 2006) http://www.eso.org/outreach/press-rel/pr-2006/pr-02-06.html
• 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)

•  • [ e] Moons of Pluto and Eris
Pluto Charon Nix Hydra Eris Dysnomia

•  • [ e] Natural satellites of the Solar System
Planetary satellites	Terrestrial Martian Jovian Saturnian Uranian Neptunian
Other satellite systems	Plutonian Eridian Asteroid satellites
Largest satellites	Ganymede Titan Callisto Io Moon Europa Triton Titania Rhea Oberon Iapetus Charon Umbriel Ariel Dione Tethys Enceladus Miranda Proteus Mimas
Inner satellites • Trojans • Irregulars • List • List by diameter • Timeline of discovery • Naming

•  • [ e] Trans-Neptunian objects
Plutinos	Pluto* |1993 SB| |1993 SC| 1994 JR1 |1994 TB| 1995 QZ9 1996 SZ4 1996 TP66 1996 TQ66 1997 QJ4 1998 HK151 1998 US43 1998 VG44 1998 WW24 1998 WU31 38083 Rhadamanthus 1999 TC36 38628 Huya 28978 Ixion 2002 KX14 2002 VR128 2003 VS2 90482 Orcus Unnumbered: 1993 RO 1993 RP 2003 AZ84 2001 QF298
Cubewanos:	1992 QB1 1994 GV9 1994 JQ1 1994 VK8 1995 SM55 1996 TO66 58534 Logos 1997 CS29 1997 CU29 1998 HJ151 1998 HP151 1998 HM151 1998 KR65 19521 Chaos 1998 WA25 1999 DF9 1999 HT11 53311 Deucalion 20000 Varuna 2002 AW197 50000 Quaoar 2002 TX300 2002 UX25 2003 EL61 2003 OP32 2004 GV9 2005 FY9 2005 RN43 Unnumbered: 1998 WW31 2002 MS4 2003 MW12 2003 QW90
Twotinos:	1996 TR66 1998 SM165 1999 RB216 2000 JG81 2002 WC19 Unnumbered: 1997 SZ10
Other Orbital Resonances:	|1994 JS| 1995 DA2 1998 WA31 1999 CP133 1999 DE9 1999 HB12 2001 KC77 2001 KP77 2002 TC302 2003 LG7
Scattered disc objects:	Eris* 1995 TL8 1996 GQ21 1996 TL66 1999 CC158 2000 EE173 2000 OO67 2000 OM67 2001 UR163 2002 CY224 2002 GX32 2002 RP120 90377 Sedna** 2005 RM43 2000 CR105 Unnumbered: 2004 XR190 2005 TN74 2006 QH181
Unclassified Objects:	1997 CR29 1998 SN165 1999 CL158 1999 HC12 1999 KR16 1999 OY3 2003 FY128 2004 SB60 2004 TY364 2005 RR43
Natural satellites:	Charon (Pluto) Hydra (Pluto) Nix (Pluto) Dysnomia (Eris) S/2000 (1998 WW31) 1 S/2005 (2003 EL61) 1 S/2005 (2003 EL61) 2 (58534) Logos I Zoe S/2005 (79360) 1
* - Also classified as a dwarf planet   ** - Currently classified as an SDO, though may be part of the Inner Oort Cloud

•  • [ e] trans-Neptunian dwarf planets and candidates
Kuiper belt:	Orcus Pluto 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.

•  • [ e] Footer SolarSystem
The Sun Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Pluto Eris
Planets Dwarf planets Moons: Terrestrial Martian Jovian Saturnian Uranian Neptunian Plutonian Eridian
Small bodies:   Meteoroids Asteroids/Asteroid moons (Asteroid belt) Centaurs TNOs (Kuiper belt/Scattered disc) Comets (Oort cloud)
See also astronomical objects, the solar system's list of objects, sorted by radius or mass, and the