Charon, discovered in 1978 at the
United States Naval Observatory Flagstaff StationThe United States Naval Observatory Flagstaff Station , is an astronomical observatory operated as a dark-sky observing site by the United States Naval Observatory...
, is the largest
satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
of the
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
. Following the 2005 discovery of two other natural satellites of Pluto (
NixNix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005 by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A...
and
HydraHydra is the outer-most natural satellite of Pluto. It was discovered along with Nix in June, 2005 by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot...
), Charon may also be referred to as
Pluto I. The
New HorizonsNew Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, and Hydra. NASA may also approve flybys of one or more other Kuiper Belt Objects.New Horizons was launched...
mission is scheduled to visit Charon and Pluto in July 2015.
Charon should not be confused with the similarly named
Chiron2060 Chiron is a planetoid in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first known member of a new class of objects now known as centaurs, with an orbit between those of Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...
, a smaller object in the outer solar system.
Charon was discovered by astronomer James Christy on June 22, 1978, when he was examining highly magnified images of
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
on
photographic platePhotographic plates preceded photographic film as a mean of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
s taken a couple of months before.
Charon, discovered in 1978 at the
United States Naval Observatory Flagstaff StationThe United States Naval Observatory Flagstaff Station , is an astronomical observatory operated as a dark-sky observing site by the United States Naval Observatory...
, is the largest
satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
of the
dwarf planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
. Following the 2005 discovery of two other natural satellites of Pluto (
NixNix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005 by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A...
and
HydraHydra is the outer-most natural satellite of Pluto. It was discovered along with Nix in June, 2005 by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot...
), Charon may also be referred to as
Pluto I. The
New HorizonsNew Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, and Hydra. NASA may also approve flybys of one or more other Kuiper Belt Objects.New Horizons was launched...
mission is scheduled to visit Charon and Pluto in July 2015.
Charon should not be confused with the similarly named
Chiron2060 Chiron is a planetoid in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first known member of a new class of objects now known as centaurs, with an orbit between those of Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...
, 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
PlutoPluto, formal designation 134340 Pluto, is the second-largest known dwarf planet in the Solar System and the tenth-largest body observed directly orbiting the Sun...
on
photographic platePhotographic plates preceded photographic film as a mean of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...
s taken a couple of months before. Christy noticed that a slight bulge appeared periodically. The discovery was announced on July 7, 1978. 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 curveIn astronomy, a light curve is a graph of light intensity of a celestial object or region, as a function of time. The light is usually in a particular frequency interval or band. Light curves can be periodic, as in the case of eclipsing binaries, cepheid variables and other variables, or...
. This indicated a
synchronous orbitA synchronous orbit is an orbit in which an orbiting body has a period equal to the average rotational period of the body being orbited , and in the same direction of rotation as that body.-Properties:...
, 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 planeThe orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded. Three non-colinear points in space suffice to define the 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.
Observations
Images showing Pluto and Charon resolved into separate disks were taken for the first time by the
Hubble Space TelescopeThe Hubble Space Telescope is a space telescope that was carried into orbit by the space shuttle in April 1990. It is named after the American astronomer Edwin Hubble. Although not the first space telescope, the Hubble is one of the largest and most versatile, and is well-known as both a vital...
in the 1990s. Later, the development of
adaptive opticsAdaptive optics is a technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems...
made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes.
Physical characteristics
Charon's diameter is about 1207
kmThe kilometre , symbol km is a unit of length in the metric system, equal to one thousand metres and is therefore exactly equal to the distance travelled by light in free space in of a second....
(750 miles), just over half that of Pluto, with a surface area of 4 580 000
km²Square kilometre , symbol km2, is a decimal multiple of the SI unit of surface area, the square metre, one of the SI derived units.1 km2 is equal to:* 1,000,000 m2...
. Unlike Pluto, which is covered with
nitrogenNitrogen is a chemical element that has the symbol N and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
and
methaneMethane is a chemical compound with the chemical formula . It is the simplest alkane, and the principal component of natural gas. Methane's bond angles are 109.5 degrees. Burning methane in the presence of oxygen produces carbon dioxide and water. The relative abundance of methane and its clean...
ices, the Charonian surface appears to be dominated by less
volatileVolatile means changing or changeable. It can refer to:"something likely to cause trouble"In general:* Volatility, a measure of instabilityIn economics:* Volatility , a measure of the risk in a financial instrument...
water ice, and also appears to have no atmosphere. In 2007, observations by the
Gemini ObservatoryThe Gemini Observatory is an astronomical observatory consisting of two telescopes at different sites. The Northern Operations Center is located in Hilo, Hawaii, and the Southern Operations Center is in La Serena, Chile. The Gemini telescopes were built and are operated by a consortium consisting...
of patches of ammonia hydrates and water crystals on the surface of Charon suggested the presence of active cryo-geysers.(see also
CryovolcanoA cryovolcano is, literally, an icy volcano. Cryovolcanoes form on icy moons,and possibly on other low-temperature astronomical objects ....
)
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
spectroscopicallyA spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
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 ObservatoryThe Gemini Observatory is an astronomical observatory consisting of two telescopes at different sites. The Northern Operations Center is located in Hilo, Hawaii, and the Southern Operations Center is in La Serena, Chile. The Gemini telescopes were built and are operated by a consortium consisting...
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.
Orbital characteristics
Charon and Pluto revolve about each other every 6.387 days. The two objects are
gravitationally lockedTidal locking occurs when the gravitational gradient makes one side of an astronomical body always face another; for example, one side of the Earth's Moon always faces the Earth. A tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. This...
, 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
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
s 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. 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 impactThe giant impact hypothesis is the currently favored scientific hypothesis for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body that is sometimes called Theia for the mythical Greek Titan who ruled the Sun...
around 4.5
billion1,000,000,000 is the natural number following 999,999,999 and preceding 1,000,000,001.In scientific notation, it is written as 10
9....
years ago, much like the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
and
MoonThe Moon is Earth's only natural satellite and the fifth largest satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is , about thirty times the diameter of the Earth. The common centre of mass of the system is located at about —a quarter the Earth's...
. In this model a large
Kuiper beltThe Kuiper belt , sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive...
object struck Pluto at high velocity, destroying itself and blasting off much of Pluto's outer
mantleThe mantle is a part of an astronomical object. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core. Earth's mantle is about 2,970 km thick rocky shell that...
, 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?
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 .
In a draft proposal for the 2006 redefinition of the term, the
International Astronomical UnionThe International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
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 planetA dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass to...
, 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
NixNix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005 by the Hubble Space Telescope Pluto Companion Search Team, composed of Hal A. Weaver, S. Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot F. Young, and Leslie A...
and
HydraHydra is the outer-most natural satellite of Pluto. It was discovered along with Nix in June, 2005 by the Hubble Space Telescope's Pluto Companion Search Team, which is composed of Hal A. Weaver, Alan Stern, Max J. Mutchler, Andrew J. Steffl, Marc W. Buie, William J. Merline, John R. Spencer, Eliot...
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).
Name
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 ObservatoryThe United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...
proposed
PersephoneIn Greek mythology, Persephone was the embodiment of the Earth's fertility at the same time that she was the Queen of the Underworld, the korē , and the parthenogenic daughter of Demeter and, in later Classical myths, a daughter of Demeter and Zeus...
, Christy stuck with
Charon after discovering it coincidentally refers to a Greek mythological figure:
CharonIn Greek mythology, Charon or Kharon was the ferryman of Hades who carried souls of the newly deceased across the River Styx that divided the world of the living from the world of the dead. A coin to pay Charon for passage, usually an obolus or danake, was sometimes placed in or on the mouth of a...
is the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans identified with their god
PlutoPluto was the Roman god of the underworld, known in Latin as Tertius, the counterpart of the Greek Hades.-Overview:Pluto was God of the underworld and its riches...
. Official adoption of the name by the
IAUThe International Astronomical Union is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
would wait until late 1985, and was announced on January 3, 1986.
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. 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.
However, Christy himself pronounced the
ch in the moon's name as
sh (IPA ), after his wife Charlene. Because of this, as an acknowledgement of Christy and sometimes as an
in-jokeAn in-joke is a joke whose humor is clear only to those people who are "inside" a social group, occupation or other community of common understanding; an esoteric joke. It is only humorous to those who know the situation behind it...
or
shibbolethShibboleth is any distinguishing practice which is indicative of one's social or regional origin.It usually refers to features of language, and particularly to a word whose pronunciation identifies its speaker as being a member or not a member of a particular group.-Origin:The term originates from...
, the initial
sh pronunciation is common among astronomers when speaking English,
and this is the prescribed pronunciation at NASA and of the New Horizons Pluto mission team.
External links
- Charon Profile by NASA's Solar System Exploration
- James W. Christy
James Walter Christy is an American astronomer.Working at the United States Naval Observatory, on June 22, 1978 he discovered that Pluto had a moon, which he named Charon shortly afterwards...
and Robert S. HarringtonRobert Sutton Harrington was an American astronomer who worked at the United States Naval Observatory . He should not be confused with Robert G. Harrington, who was also an astronomer, but was born earlier and worked at the Palomar Observatory.Harrington was born near Newport News, Virginia. His...
, "The satellite of Pluto," The Astronomical Journal 83 (1978) 1005
- Marc W. Buie
Marc W. Buie is an astronomer at Lowell Observatory in Flagstaff, Arizona. He grew up in Baton Rouge, Louisiana and went on to get a B.S. in Physics from Louisiana State University in 1980. After that he switched fields and earned his Ph.D. in Planetary Science from the University of Arizona in...
, Phases of Charon as seen from Pluto, Lowell ObservatoryLowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory is among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....
- Buie, Surface of Charon and Pluto
- Hubble reveals new map of Pluto, BBC
The British Broadcasting Corporation, usually referred to by its abbreviation as the "BBC", is the longest established and largest broadcaster in the world...
News, September 12, 2005
- IAU Circular No. 3241 describing the discovery
- Measuring the Size of a Small, Frost World (ESO
ESO, as a three-letter abbreviation, may stand for:*European Southern Observatory*Ensemble Studios Online*English Symphony Orchestra*Edmonton Symphony Orchestra*Executive Stock Options...
press release January 2006)
- 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
The Astronomical Journal is a monthly scientific journal published by Institute of Physics Publishing on behalf of the American Astronomical Society. It is one of the premier journals for astronomy in the world...
, February 3, 2006)
- Cryovolcanism on Charon and other Kuiper Belt Objects