Charon_(moon) - Wikiparency

# Charon (moon)  **Microsim — three.js (Wikitube framework):** *Charon in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Charon&embed=1" data-title="Charon in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Charon`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: set the speed to 1 day/s and watch Charon complete a circuit in about six and a half seconds while Pluto swings round a small circle of its own about the shared balance point; drag to look along the orbit plane and see the two bodies stay on opposite sides of that point; press l to toggle the labels.* **Charon** (formal designation 134340 Pluto I) is the largest of the five known moons of the [[Dwarf_planet|dwarf planet]] [[Pluto]] in the [[Kuiper_belt|Kuiper belt]], a grey, water-ice world with a mean radius of 606 km, just over half Pluto's, and about an eighth of its mass.[^nimmo2017][^fs] It was discovered in 1978 by James Christy of the United States Naval Observatory on photographic plates taken at the observatory's Flagstaff station.[^christy1978][^iauc3241] Because Charon is so massive relative to Pluto, the centre of mass of the pair lies in open space between them, and each body is tidally locked to the other, always turning the same face towards its partner.[^fs][^stern2015] The New Horizons flyby of July 2015 showed Charon as a geologically complex body: a northern hemisphere of cratered, fractured highlands, a belt of canyons crossing the equator, smooth southern plains that appear to have been resurfaced by water-rich flows, and a reddish-brown polar cap, Neverland Regio, thought to be made of organic material formed from gas escaping from Pluto.[^stern2015][^grundy2016] Its interior may once have held an ocean.[^desch2017] Among [[Trans-Neptunian_object|trans-Neptunian objects]] it is the sixth-largest known, after Pluto, [[Eris_(dwarf_planet)|Eris]], [[Haumea]], [[Makemake]] and [[Gonggong_(dwarf_planet)|Gonggong]], but the IAU classes it as a satellite rather than a dwarf planet.[^johnston][^iau-pluto] The explorer at the top of this page opens directly in Pluto's own frame at true scale, with Charon 19,596 km from Pluto's centre on its 6.39-day orbit and both bodies circling their shared barycentre; the moon's starting phase is ILLUSTRATIVE, while the distance and period come from the NASA fact sheet.[^fs] ## Discovery By the 1970s [[Pluto]] was known to vary in brightness with a period of about 6.4 days, its rotation. On 22 June 1978 James Christy, measuring Pluto's position on plates taken two months earlier with the 1.55-metre (61-inch) astrometric reflector at the Naval Observatory's Flagstaff station, noticed that the image of Pluto was sometimes elongated, with a bump that appeared on different sides at different times.[^christy1978][^dick2013] Older plates showed the same bump, back to 29 April 1965, and its cycle matched Pluto's rotation period. That match was the key argument that the bump was real rather than a flaw of the photographic emulsion: a moon orbiting in the time Pluto takes to rotate would appear in step with Pluto's light variation.[^dick2013] The International Astronomical Union announced the discovery on 7 July 1978.[^iauc3241] The find at once changed what was known of Pluto. Until then, the brightness of the pair had been attributed to Pluto alone; with a companion of substantial size, Pluto was smaller than thought, and the moon's orbit gave the combined mass of the system for the first time, about 0.2 percent of [[Earth]]'s.[^christy1978] Final confirmation came from a stroke of timing. Between 1985 and 1990 the orbital plane of the pair lay edge-on to Earth, a geometry that occurs only twice in each 248-year orbit of Pluto, and the two bodies repeatedly eclipsed and passed in front of each other. Modelling the dips in their combined light measured the sizes of both bodies and produced the first maps of their brightness patterns.[^buie1992] ## Name After its discovery the moon was given the provisional designation S/1978 P 1. Christy proposed the name Charon on 24 June 1978, as a scientific-sounding form of the nickname "Char" of his wife, Charlene; he kept it after colleagues suggested Persephone, when he learned that Charon is also the ferryman who carries souls across the river Styx to the underworld ruled by the god [[Pluto]].[^schilling2008] The IAU adopted the name in 1985, and it was announced on 3 January 1986.[^iauc4157] Because of that origin, Christy pronounced the first syllable as in "Charlene", with a "sh" sound, while the mythological name begins with a hard "k". Both forms are heard among astronomers; NASA and the New Horizons team use Christy's.[^spaceref2005] Two of Pluto's smaller moons, Styx and Kerberos, were named in 2013 after other figures of the same underworld myth.[^iau2013] Moons other than Earth's have never had astronomical symbols in regular use. The software engineer Denis Moskowitz, who designed most of the symbols now proposed for the dwarf planets, suggested one for Charon that sets the orb from Pluto's bident sign above a crescent, evoking both a moon and the ferryman's boat; it has seen little use.[^bala2025] ## Orbit Charon's orbit is circular within measurement error and lies in Pluto's equatorial plane, with a radius of 19,596 km and a period of 6.3872 days, the same as the rotation periods of both bodies.[^fs][^buie2012] This is mutual tidal locking, the end state of tidal evolution: [[Tide|tidal]] friction slowed both spins until each matched the orbit. In the Earth–[[Moon]] system only the Moon is locked; [[Earth]] still turns under it. Seen from Pluto's Charon-facing hemisphere, Charon hangs fixed in the sky, and it is never visible from the other hemisphere. Among dwarf planets, [[Eris_(dwarf_planet)|Eris]] and its moon Dysnomia are confirmed to share this state, as is the Salacia–Actaea pair, and [[Orcus_(dwarf_planet)|Orcus]] and Vanth may be another example.[^szakats2023][^collyer2025][^brown2023] The balance point of the system lies outside Pluto. With a Charon-to-Pluto mass ratio of 0.122, the barycentre is about 2,130 km from Pluto's centre, some 940 km above its surface, and Charon orbits that point at a radius of about 17,470 km rather than the full 19,596 km (derived).[^fs] [[Pluto]] therefore moves on a small orbit of its own around a point in empty space, the motion the explorer shows. The orbit gives the total mass of the system through [[Kepler's_laws_of_planetary_motion|Kepler's third law]], but not the split between the two bodies. That split came only after the small moons Nix and Hydra were found in 2005: their orbits around the pair respond to how the mass is shared, and Buie and colleagues derived from them that Charon has roughly 12 percent of Pluto's mass.[^buie2006] New Horizons tracking refined the values to 1.586 × 10²¹ kg for Charon and 1.303 × 10²² kg for Pluto.[^fs][^brozovic2024] ## Formation Two main scenarios have been proposed. In Robin Canup's 2005 simulations a large [[Kuiper_belt|Kuiper belt]] body struck the proto-Pluto obliquely, and Charon either formed from the ejected debris or survived largely intact as the impactor itself, in the same way that the Moon is thought to have formed from a giant impact on the young [[Earth]].[^canup2005] A giant impact is also the favoured way to explain the four small outer moons, which could have assembled from lighter debris.[^stern2006] Charon's similar density to [[Pluto]], however, argues against a violent impact between fully differentiated bodies, which would have stripped ice into the moon and left Pluto rockier; the parent bodies may have been only partly differentiated when they met.[^stern2015] In 2025 a team at the University of Arizona proposed a gentler variant, which they called "kiss and capture": in their simulations the two bodies touched and stuck together briefly before separating, still gravitationally bound, with Charon then moving outward through tidal interaction. The heating from the contact and from the tides as the bodies separated could, on this view, have produced Pluto's subsurface ocean without a large contribution from [[Radioactive_decay|radioactive decay]].[^arizona2025] The widening of the orbit to its present size followed the same logic as the [[Moon]]'s slow retreat from Earth: [[Tide|tides]] raised on each body transfer [[Angular_momentum|angular momentum]] from spin to orbit.[^bagheri2022] ## Physical characteristics Charon's diameter is 1,212 km, just over half [[Pluto]]'s 2,377 km, making it larger than the dwarf planet [[Ceres_(dwarf_planet)|Ceres]] and similar in size to [[Uranus]]'s moons Ariel and Umbriel.[^nimmo2017][^stern2018][^jpl-sbdb][^jplsats] New Horizons images show no measurable flattening. Because Charon spins slowly, even a body in hydrostatic equilibrium would be very nearly spherical, so the round shape is consistent with equilibrium; the contrast is with [[Saturn]]'s similarly sized moon Iapetus, which kept a pronounced bulge from an early fast spin.[^nimmo2017] The mass ratio of Charon to Pluto, 0.1218 after New Horizons, is ten times that of the [[Moon]] to Earth, 0.0123 (derived), which is why the pair is sometimes called a double dwarf planet.[^stern2018][^nasa-fs] With four small moons orbiting both, the Pluto system has also been used as a test case for the stability of planets orbiting binary stars.[^sutherland2019] ### Internal structure Charon's density is 1.70 g/cm³, slightly less than Pluto's 1.85 g/cm³, which implies a somewhat higher fraction of ice.[^stern2018][^fs] New Horizons found clear signs that the moon is differentiated into a rocky core and an icy mantle and that it once had a subsurface ocean. Two families of models explain the surface. In the "hot start" of Desch and Neveu, Charon accreted quickly from impact debris and partly melted; an ocean formed early and lasted for a long period before freezing, possibly feeding the eruptions that resurfaced the southern plain Vulcan Planitia, and a later ocean of water–ammonia melt may have built features such as Kubrick Mons. Freezing of these oceans would expand the moon and crack its crust, forming the equatorial canyons.[^desch2017] In the "cold start" of Malamud, Perets and Schubert, Charon began as a porous, undifferentiated body; radiogenic heat later compacted and differentiated it, and an ocean formed some 100–200 million years after formation. Alternating episodes of contraction and expansion in this model could produce both the canyon belt and the arcuate ridges of the north.[^malamud2017] Bagheri and colleagues modelled the coupled tidal and thermal evolution of the pair and found that an early ocean on Charon is consistent with the observed tectonics.[^bagheri2022] ### Surface Charon's surface is dominated by [[Water|water]] ice, much less volatile than the [[Nitrogen|nitrogen]] and methane ices that cover [[Pluto]].[^stern2015] In 2007 near-infrared [[Spectroscopy|spectra]] taken at the Gemini Observatory showed ammonia hydrates and crystalline water ice. Crystalline ice should turn amorphous under radiation within tens of thousands of years, so its presence was taken as a sign of recent resurfacing, perhaps by cryovolcanism.[^cook2007][^gemini2007] New Horizons found no active plumes, and later work suggested that the ammonia could be replenished more passively.[^holler2017] The most striking feature is Neverland Regio, the dark, reddish north polar region nicknamed "Mordor" by the New Horizons team. The favoured explanation involves Pluto's atmosphere: some of the methane that escapes from Pluto reaches Charon, and during the century-long polar winter, when temperatures fall to about −258 °C, it freezes onto the cold pole. Ultraviolet light converts the frozen methane into heavier, reddish organic compounds, called tholins, that survive when the pole warms to about −213 °C in summer and the remaining volatiles sublimate away; over millions of years the residue builds up into a dark cap.[^grundy2016][^howett2015] An alternative proposes that the methane came from Charon's own interior during past cryovolcanism.[^menten2022] The northern hemisphere is heavily cratered and cut by fractures, while the southern plain has fewer craters and is less rugged, which points to a large resurfacing event, possibly linked to the freezing of an internal ocean.[^beatty2015] A belt of grabens and scarps, including Serenity Chasma, runs around the equator for at least 1,000 km; Argo Chasma may be as deep as 9 km.[^keeter2016] ### Hypothesized exosphere Charon has no significant atmosphere. A stellar occultation observed in 2005 set an upper limit on any gas at its surface, and New Horizons found none.[^sicardy2006][^stern2015] Charon's [[Gravity|gravity]] is weak, so gas escapes much faster than on Pluto, and the stream of nitrogen and methane leaving Pluto reaches Charon only in small amounts. Tucker, Johnson and Young calculated that Pluto's escaping gas could sustain a very tenuous exosphere around Charon, held close to its surface.[^tucker2015] Teolis and colleagues modelled how methane from Pluto would migrate across Charon's surface and argued that seasonal exospheres, forming briefly as polar frost sublimates and absent for centuries between, could help explain the red pole.[^teolis2022] ## Observation and exploration From [[Earth]], [[Pluto]] and Charon are never more than about 0.9 arcseconds apart (derived), too little for ordinary telescopes to split through the atmosphere. The Hubble Space Telescope first imaged them as distinct discs in the 1990s; a 1994 image with its Faint Object Camera, taken when the system was 4.4 billion km from Earth, showed two clearly separated bodies.[^esa1994] Ground-based adaptive optics later achieved the same, and in 2008 a group of Italian amateur astronomers resolved Charon with a 14-inch (36 cm) telescope.[^atkinson2008] [[NASA]]'s New Horizons spacecraft photographed Charon on approach through June and July 2015 and flew through the system on 14 July 2015, imaging the Pluto-facing hemisphere in detail, measuring its composition with its spectrometers and looking for an atmosphere.[^stern2015] It remains the only spacecraft to have visited Charon. ## Classification Because the barycentre lies outside [[Pluto]] and Charon is round, some astronomers have argued that the pair should be treated as a binary dwarf planet. A draft of the IAU's 2006 definition of a planet would have done so, since it defined a satellite as a body whose system barycentre lies inside the primary. That clause was dropped from the adopted [[IAU_definition_of_planet|definition]], which makes no formal definition of a satellite, and the IAU lists Charon as a moon of Pluto while noting that its status could be revisited.[^iau-pluto] Under the draft rule the [[Moon]] would itself one day have become a planet, since its slow tidal retreat will eventually move the Earth–Moon barycentre outside Earth.[^britt2006] The four small moons, Nix, Hydra, Kerberos and Styx, orbit the same barycentre but are far too small to be round and are simply satellites of the Pluto–Charon pair.[^stern2005nh] ## See also - [[Pluto]] - [[Dwarf_planet]] · [[IAU_definition_of_planet]] - [[Triton_(moon)]] - [[Kuiper_belt]] · [[Trans-Neptunian_object]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the NASA fact-sheet values: the mass ratio 1.586 × 10²¹ / 1.303 × 10²² ≈ 0.122; the barycentre's distance from Pluto's centre 19,596 × 0.122 / 1.122 ≈ 2,130 km, or about 940 km above Pluto's 1,188 km radius; Charon's distance from the barycentre 19,596 − 2,130 ≈ 17,470 km; the Moon-to-Earth mass ratio 0.07346 / 5.9722 ≈ 0.0123; and the largest separation seen from Earth, 19,596 km at Pluto's minimum distance of 4.28 × 10⁹ km, 4.6 × 10⁻⁶ rad ≈ 0.94 arcseconds. ## References [^fs]: Williams, D. R. "Pluto Fact Sheet" (includes Charon). NASA NSSDCA, last updated 11 January 2024. https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html (fetched 2026-09-18). [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^nimmo2017]: Nimmo, F.; Umurhan, O.; Lisse, C. M.; et al. (2017). "Mean radius and shape of Pluto and Charon from New Horizons images". *Icarus* 287: 12–29. https://doi.org/10.1016/j.icarus.2016.06.027 [^christy1978]: Christy, J. W.; Harrington, R. S. (1978). "The satellite of Pluto". *The Astronomical Journal* 83: 1005–1008. https://doi.org/10.1086/112284 [^iauc3241]: Central Bureau for Astronomical Telegrams (7 July 1978). "IAUC 3241: 1978 P 1". http://www.cbat.eps.harvard.edu/iauc/03200/03241.html [^iauc4157]: Central Bureau for Astronomical Telegrams (3 January 1986). "IAUC 4157: Satellites of Saturn and Pluto". http://www.cbat.eps.harvard.edu/iauc/04100/04157.html [^stern2015]: Stern, S. 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"The orbit of Charon is circular". *The Astronomical Journal* 144: 15. https://doi.org/10.1088/0004-6256/144/1/15 [^szakats2023]: Szakáts, R.; Kiss, Cs.; Ortiz, J. L.; et al. (2023). "Tidally locked rotation of the dwarf planet (136199) Eris discovered from long-term ground based and space photometry". *Astronomy & Astrophysics* 669: L3. https://doi.org/10.1051/0004-6361/202245234 [^collyer2025]: Collyer, C.; Fernández-Valenzuela, E.; Ortiz, J. L.; et al. (2025). "Synchronous rotation in the (120347) Salacia–Actaea system". *The Planetary Science Journal* 6: 270. https://doi.org/10.3847/PSJ/ae0b6a [^brown2023]: Brown, M. E.; Butler, B. J. (2023). "Masses and densities of dwarf planet satellites measured with ALMA". *The Planetary Science Journal* 4: 193. https://doi.org/10.3847/PSJ/ace52a [^buie2006]: Buie, M. W.; Grundy, W. M.; Young, E. F.; Young, L. A.; Stern, S. A. (2006). 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Southwest Research Institute. http://www.boulder.swri.edu/plutomoons/ ## External links - NASA Science: Charon. https://science.nasa.gov/dwarf-planets/pluto/moons/charon/ - NASA NSSDCA Pluto Fact Sheet (Charon parameters). https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html - New Horizons mission (JHUAPL). https://pluto.jhuapl.edu/ - USGS/IAU Gazetteer of Planetary Nomenclature: Charon. https://planetarynames.wr.usgs.gov/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Charon_(moon)) : [Wikitube](https://en.wikitube.io/wiki/Charon_(moon)) · pinned revision [1374241186](https://en.wikipedia.org/w/index.php?oldid=1374241186) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-069 · explorer state `?obj=Charon`.*