Charon is the largest satellite of the dwarf planet Pluto. It was discovered in 1978 at
the United States Naval Observatory Flagstaff Station. Following the 2005 discovery of two
other natural satellites of Pluto, and two more in 2011 and 2012, Charon may also be
referred to as Pluto I. The New Horizons mission is scheduled to visit
Charon and Pluto in July 2015.
Discovery
Charon was discovered by United States Naval Observatory astronomer James Christy, using
the 1.55-meter telescope at NOFS, and was formally announced to the world via the International
Astronomical Union on July 7, 1978. On June 22, 1978, he had been examining highly magnified
images of Pluto on photographic plates taken at the 1.55-meter Flagstaff telescope two
months prior. Christy noticed that a slight elongation 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. All doubts were erased when Pluto and Charon
entered a five-year period of mutual eclipses and transits 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 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 Telescope in the 1990s. Later, the development of adaptive optics
made it possible to resolve Pluto and Charon into separate disks using ground-based telescopes.
Physical characteristics Charon's diameter is about 1,207 kilometres,
just over half that of Pluto, with a surface area of 4,580,000 square kilometres. 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. 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 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 that 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, whereas
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, because solar radiation would have degraded older ice to an amorphous
state after roughly thirty thousand years. Photometric mapping of Charon's surface shows
a latitudinal trend in albedo, with a bright equator band and darker poles. In particular
the south polar region seems darker than the north.
Orbital characteristics Charon and Pluto revolve about each other
every 6.387 days. The two objects are both gravitationally locked to the other, so each
keeps the same face towards the other. This is a mutual case of tidal locking compared
to for instance Earth and the Moon, where the Moon always shows the same face to Earth,
but not vice versa. The average distance between Charon and Pluto is 19,570 kilometres. 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. 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 impact around 4.5 billion
years ago, much like Earth and the 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 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. Classification as a moon or dwarf planet
The center of mass of the Pluto–Charon system lies outside either body. Because neither
object truly orbits the other, and Charon has 11.6% the mass of Pluto, it has been argued
that Charon should be considered to be part of a binary system. The International Astronomical
Union states that Charon is considered to be just a satellite of Pluto, but the idea
that Charon might be classified a dwarf planet in its own right may be considered at a later
date. In a draft proposal for the 2006 redefinition
of the term, the IAU proposed that a planet be defined as a body that orbits the sun that
is large enough for gravitational forces to render the object spherical. Under this proposal,
Charon would have been classified as a planet, because 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. Charon is not in the list of dwarf planets currently recognized
by the IAU. Had the draft proposal been accepted, even the Moon would have been classified as
a planet in billions of years when the tidal acceleration that is gradually moving the
Moon away from Earth takes it far enough away that the center of mass of the system no longer
lies within Earth. The other moons of Pluto, Nix, Hydra, Kerberos
and Styx, orbit the same barycenter, but they are not large enough to be spherical, and
they are simply considered to be satellites of Pluto.
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 Observatory proposed Persephone, Christy stuck with Charon
after discovering it coincidentally refers to a Greek mythological figure: Charon is
the ferryman of the dead, closely associated in myth with the god Hades, whom the Romans
identified with their god Pluto. Official adoption of the name by the IAU waited 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 many 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, after his wife Charlene. Because of this, as an acknowledgement of
Christy and sometimes as an in-joke or shibboleth, 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. In popular culture
The Mass Effect video game franchise is built around the premise of a series of relays that
allow faster-than-light travel around the Milky Way without time dilation. In the series,
Charon is discovered in 2149 not to be a moon or a dwarf planet, but to be a chunk of ice
containing an intricate piece of technology built by an ancient civilization that allows
this travel to occur, leading to the discovery of other planetary systems and enabling their
colonization. In the 1974 sci-fi novel The Forever War written
by Joe Haldeman, a group of elite military recruits train for combat on a fictional icy
planetoid called Charon that orbits the Sun at a distance of 80 AU. Being that the story
was published 4 years before the discovery of Pluto’s largest moon, it is unclear whether
the fictional Charon of the story somehow factored into the naming of Pluto’s moon
or if the two share a name purely by coincidence. The Jonathan Coulton song "I'm Your Moon"
from the Thing A Week Four album is a love song sung to Pluto by Charon.
See also Moons of Pluto
Notes
References
External links Charon Profile at NASA's Solar System Exploration
site James W. Christy and Robert S. Harrington,
"The satellite of Pluto," The Astronomical Journal 83 1005
Marc W. Buie, Phases of Charon as seen from Pluto, Lowell Observatory
Buie, Surface of Charon and Pluto Hubble reveals new map of Pluto, BBC News,
September 12, 2005 IAU Circular No. 3241 describing the discovery
Measuring the Size of a Small, Frost World M. J. Person et al.: Charon’s Radius and
Density from the Combined Data Sets of the 2005 July 11 Occultation
Cryovolcanism on Charon and other Kuiper Belt Objects
New Horizons Camera Spots Pluto’s Largest Moon - July 10, 2013