Editing Epimetheus (moon)

{{short description|Moon of Saturn}}
{{For|the asteroid|1810 Epimetheus}}
{{Use dmy dates|date=September 2019}}
{{Infobox planet
| name = Epimetheus
| image = PIA09813 Epimetheus S. polar region.jpg
| image_scale = 
| caption = South pole of Epimetheus imaged by ''[[Cassini–Huygens|Cassini]]'' on 3 December 2007
| pronounced = {{IPAc-en|ɛ|p|ə|ˈ|m|iː|θ|iː<!--[sic] - long /iː/ with -ευς-->|ə|s}}<ref>{{L&S|Epimetheus|ref}}</ref>
| adjectives = Epimethean {{IPAc-en|ɛ|p|ə|ˈ|m|iː|θ|iː|ə|n}}<ref>NASA (2005 March 28) [https://solarsystem.nasa.gov/resources/12454/epimethean-profile/ Epimethean Profile]</ref>
| named_after = [[Epimetheus (mythology)|Ἐπιμηθεύς]] ''Epimētheus''
| mpc_name = Saturn XI
| discoverer = [[Richard Walker (astronomer)|Richard Walker]]
| discovered = 18 December 1966
| orbit_ref =  {{sfn|Spitale Jacobson et al.|2006}}
| epoch = 31 December 2003 ([[Julian day|JD]] 2453005.5)
| semimajor = {{val|151410|10|u=km}}
| eccentricity = {{val|0.0098}}
| period = {{val|0.694333517|u=d}}
| inclination = {{val|0.351|0.004|u=°}} to Saturn's equator
| satellite_of = [[Saturn]]
| group = [[Co-orbital moon|Co-orbital]] with [[Janus (moon)|Janus]]
| dimensions = {{val|129.6|x|116.2|x|107.0}}&nbsp;km<br/>(±&nbsp;{{val|0.6|x|0.4|x|0.4}}&nbsp;km){{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| mean_diameter = {{val|117.2|0.6|u=km}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| volume = {{val|843290|2000|u=km3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| mass = {{val|5.25607|0.00081|e=17|u=kg}}{{efn|Calculated from the standard gravitational parameter ''GM'' {{=}} {{val|3.50806|0.00054|e=-2|u=km<sup>3</sup>·s<sup>–2</sup>}} given by Lainey et al. (2023), divided by the [[gravitational constant]] ''G'' {{=}} {{val|6.6743|e=-2|u=km<sup>3</sup>·kg<sup>–1</sup>·s<sup>–2</sup>}}.{{sfn|Lainey et al.|2023}}}}
| density = {{val|0.6233|0.0015|u=g/cm3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| surface_grav = {{val|0.0066|-|0.0109|u=m/s2}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=3}}
| escape_velocity = {{V2|5.25607e-4|64.8|3}}&nbsp;km/s at longest axis<br/>to {{V2|5.25607e-4|53.5|3}}&nbsp;km/s at poles<!-- V2 template uses mass in terms of e+21 kg, radius in terms of km, and number of decimal places -->
| rotation = [[Synchronous rotation|Synchronous]]
| axial_tilt = zero
| albedo = {{val|0.73|0.03}} ([[geometric albedo|geometric]]) {{sfn|Verbiscer French et al.|2007}}
| single_temperature = ≈&thinsp;78 [[kelvin|K]]
}}

'''Epimetheus''' {{IPAc-en|ɛ|p|ə|ˈ|m|iː|θ|iː|ə|s}} is an [[inner satellite]] of [[Saturn]]. It is also known as '''Saturn&nbsp;XI'''. It is named after the mythological [[Epimetheus]], brother of [[Prometheus]].

== Discovery ==
[[File:Epimetheus.jpg|thumb|left|Epimetheus]]
Epimetheus occupies essentially the same orbit as the moon [[Janus (moon)|Janus]]. Astronomers originally assumed that there was only one body in that orbit, disbelieving that two moons could share nearly identical orbits without eventually colliding.{{sfn|JPL/NASA: Epimetheus}} Thus, there was difficulty in determining their orbital characteristics. Observations were photographic and spaced widely apart in time, so that while the presence of two objects was obvious, the observations were difficult to reconcile with a reasonable orbit.{{sfn|Phys.org}}

[[Audouin Dollfus]] observed a moon on 15 December 1966,{{sfn|IAUC 1987}} which he proposed to be named "Janus".{{sfn|IAUC 1995}} On 18 December, [[Richard Walker (astronomer)|Richard Walker]] made a similar observation which is now credited as the discovery of Epimetheus.{{sfn|IAUC 1991}} However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.{{sfn|JPL/NASA: Epimetheus}}

Twelve years later, in October 1978, [[Stephen M. Larson]] and [[John W. Fountain]] realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits.{{sfn|Fountain|Larson|1978}} This was confirmed in 1980 by ''[[Voyager 1]]'',{{sfn|Leverington|2003|p=454}} and so Larson and Fountain officially share the discovery of Epimetheus with Walker.{{sfn|JPL/NASA: Epimetheus}} A moon that was probably Epimetheus appeared in two ''[[Pioneer 11]]'' images and was designated '''S/1979S1''', though there is uncertainty because the two images were not enough to allow a reliable orbit to be calculated.<ref name=Ulivi150>{{cite book |last1=Ulivi |first1=Paolo |last2=Harland |first2= David M |date=2007 |title=Robotic Exploration of the Solar System Part I: The Golden Age 1957–1982  |publisher=Springer |page=150 |isbn=9780387493268 }}</ref>

Epimetheus received its name in 1983.{{efn|name=mythology}} The name Janus was approved by the [[International Astronomical Union|IAU]] at the same time, although the name had been used informally since Dollfus proposed it shortly after the 1966 discovery.{{sfn|JPL/NASA: Epimetheus}}

== Orbit ==
[[File:PIA08170 Epimetheus and Janus.jpg|thumb|left|Epimetheus (lower left) and [[Janus (moon)|Janus]] (right) seen on 20 March 2006, two months after swapping orbits. The two moons appear close only because of [[foreshortening]]; in reality, Janus is about 40,000&nbsp;km farther from ''Cassini'' than Epimetheus.]]
[[File:Epimetheus-Janus Orbit.png|thumb|250px|[[Rotating reference frame|Rotating-frame]] depiction of the [[horseshoe orbit]]s of Janus and Epimetheus]]
[[File:Animation of Epimetheus orbit - Rotating reference frame.gif|thumb|250px|right|Animation of Epimetheus{{'s}} orbit – Rotating reference frame<br>{{legend2| Darkkhaki| Saturn}}{{·}}{{legend2| Lime| Janus }}{{·}}{{legend2|Magenta|Epimetheus}}]]
Epimetheus's orbit is [[Co-orbital configuration|co-orbital]] with that of [[Janus (moon)|Janus]]. Janus's [[semimajor axis|mean orbital radius]] from Saturn is, as of 2006 (as shown by the green color in the adjacent picture), only 50&nbsp;km less than that of Epimetheus, a distance smaller than either moon's mean radius. In accordance with [[Kepler's laws of planetary motion]], the closer [[orbit]] is completed more quickly. Because of the small difference, it is completed in only about 30 seconds less. Each day, the inner moon is an additional 0.25° farther around Saturn than the outer moon. As the inner moon catches up to the outer moon, their mutual gravitational attraction increases the inner moon's momentum and decreases that of the outer moon. This added momentum means that the inner moon's distance from Saturn and [[orbital period]] are increased, and the outer moon's are decreased. The timing and magnitude of the [[Gravity assist|momentum exchange]] is such that the moons effectively swap orbits, never approaching closer than about 10,000&nbsp;km. At each encounter Janus's orbital radius changes by ~20&nbsp;km and Epimetheus's by ~80&nbsp;km: Janus's orbit is less affected because it is four times more massive than Epimetheus. The exchange takes place approximately every four years; the last close approaches occurred in January 2006,{{sfn|JPL/NASA: The Dancing Moons}} 2010, 2014, and 2018. This is the only such orbital configuration of moons known in the [[Solar System]]{{sfn|El Moutamid et al|2015}} (Though asteroids are known to participate in horseshoe orbits).

The orbital relationship between Janus and Epimetheus can be understood in terms of the [[Three-body problem#Circular|circular restricted three-body problem]], as a case in which the two moons (the third body being Saturn) are similar in size to each other.{{sfn|Llibre and Ollé 2011}}

== Physical characteristics ==
There are several Epimethean craters larger than 30&nbsp;km in diameter, as well as both large and small ridges and grooves. The extensive cratering indicates that Epimetheus must be quite old. Janus and Epimetheus may have formed from a disruption of a single parent to form co-orbital satellites, but if this is the case the disruption must have happened early in the history of the satellite system. From its very low density and relatively high [[albedo]], it seems likely that Epimetheus is a very porous icy body.{{sfn|JPL/NASA: Epimetheus}} There is considerable uncertainty in these values, however, and so this remains to be confirmed.{{Citation needed|date = October 2019}}

The south pole is dominated by a large, flat basin with a central peak that covers much of the moon's southern hemisphere, which may be the remains of a large impact crater.{{sfn|JPL/NASA: Epimetheus}}

There appear to be two terrain types: darker, smoother areas, and brighter, slightly more yellowish, fractured terrain. One interpretation is that the darker material evidently moves down slopes, and probably has a lower ice content than the brighter material, which appears more like "bedrock". Nonetheless, materials in both terrains are likely to be rich in water ice.{{sfn|JPL/NASA: Epimetheus Revealed}}

===Features===
Craters on Epimetheus, like those on Janus, are named after characters in the legend of [[Castor and Pollux]].<ref>[https://planetarynames.wr.usgs.gov/Page/EPIMETHEUS/target USGS: Epimetheus nomenclature]</ref>

{| class="wikitable"
|+ Named Epimethean craters
|-
! width=100 | Name !! Pronunciation !! Latin or Greek 
|-
| [[Hilaeira]] || {{IPAc-en|h|ɪ|l|eɪ|ˈ|ɪər|ə}}<!--per alt spelling Hilaëra--> || Ἱλάειρα
|-
| [[Castor and Polydeuces|Pollux]] || {{IPAc-en|ˈ|p|ɒ|l|ə|k|s}} || Pollūx 
|}

The first has been misspelled 'Hilairea' at USGS, which would presumably be pronounced {{IPA|/hɪˈlɛəriə/}}.

== Interactions with rings ==
A [[Rings of Saturn#Janus/Epimetheus Ring|faint dust ring]] is present around the region occupied by the orbits of Epimetheus and Janus, as revealed by images taken in forward-scattered light by the [[Cassini–Huygens|''Cassini'' spacecraft]] in 2006. The ring has a radial extent of about 5000&nbsp;km.{{sfn|JPL/NASA: Moon-Made Rings}} Its source are particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.{{sfn|Williams et al. 2011}}{{sfn|JPL/NASA: Creating New Rings}}

Along with Janus, Epimetheus acts as a [[shepherd moon]], maintaining the sharp outer edge of the [[A Ring]] in a 7:6 [[orbital resonance]]. The effect is more obvious when the more massive Janus is on the resonant (inner) orbit.{{sfn|El Moutamid et al|2015}}

==Gallery==
<gallery align="center">
File:Epimetheus - Voyager 1.jpg|Epimetheus crossed by the shadow of the [[F Ring]], as imaged by ''[[Voyager 1]]'' (NASA)
File:PIA17208-SaturnMoon-Epimetheus-CloseFlyby-20151206.jpg|Epimetheus - flyby<br>(6 December 2015)
File:Epimetheus_Mosaic.png|Mosaic of five images during February 2017 flyby
</gallery>

== Notes ==

{{notelist|notes=

{{efn
| name = mythology
| Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned in [[#{{sfnRef|IAUC 3872}}|IAUC 3872: ''Satellites of Jupiter and Saturn'']] 1983 September 30)
}}

}}

== References ==

=== Citations ===

{{Reflist|20em}}

=== Sources ===

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  |display-authors=etal|bibcode = 2016Icar..279..125E |doi = 10.1016/j.icarus.2015.10.025 | s2cid = 51785280
 }}<!--| access-date = 17 January 2016-->
* {{cite journal| doi = 10.1016/0019-1035(78)90076-3| last1 = Fountain| first1 = J. W.| last2 = Larson| first2 = S. M.| year = 1978| title = Saturn's ring and nearby faint satellites| journal = Icarus| volume = 36| issue = 1| pages = 92–106| bibcode = 1978Icar...36...92F
}}
* {{cite journal
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|bibcode=1967IAUC.1987....1D 
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* {{cite journal
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* {{cite web
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* {{cite journal| doi = 10.1086/505206| last1 = Spitale| first1 = J. N.| last2 = Jacobson| first2 = R. A.| last3 = Porco| first3 = C. C.| last4 = Owen| first4 = W. M. Jr. | s2cid = 26603974| year = 2006| title = The orbits of Saturn's small satellites derived from combined historic and ''Cassini'' imaging observations| journal = The Astronomical Journal| volume = 132| issue = 2| pages = 692–710| bibcode = 2006AJ....132..692S| ref = {{sfnRef|Spitale Jacobson et al.|2006}}| doi-access = free}}
* {{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = 4 September 2015| archive-date = 23 December 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}
* {{cite journal| doi = 10.1126/science.1134681| last1 = Verbiscer| first1 = A.| last2 = French| first2 = R.| last3 = Showalter| first3 = M.| last4 = Helfenstein| first4 = P.| title = Enceladus: Cosmic Graffiti Artist Caught in the Act| journal = Science| volume = 315| issue = 5813| page = 815| date = 9 February 2007| pmid = 17289992| bibcode = 2007Sci...315..815V| s2cid = 21932253| ref = {{sfnRef|Verbiscer French et al.|2007}}| url = https://www.science.org/doi/abs/10.1126/science.1134681| access-date = 20 December 2011}} (supporting online material, table S1)
* {{cite journal| doi = 10.1016/j.icarus.2010.11.038| last1 = Williams| first1 = G.A.| author2-link = Carl D. Murray| last2 = Murray| first2 = Carl D.| title = Stability of co-orbital ring material with applications to the Janus-Epimetheus system| journal = Icarus| volume = 212| issue = 1| page = 275| date = March 2011| bibcode = 2011Icar..212..275W| ref = {{sfnRef|Williams et al. 2011}}}}
* {{cite journal | doi=10.1051/0004-6361:20011274 | title=The motion of Saturn coorbital satellites in the restricted three-body problem | first1=J. | last1=Llibre | first2=M. | last2=Ollé | journal=Astronomy and Astrophysics | volume=378 | issue=3 |pages = 1087–1099| date=11 November 2011 | ref={{sfnRef|Llibre and Ollé 2011}} |bibcode = 2001A&A...378.1087L | hdl=2117/1193 | doi-access=free | hdl-access=free }}
* {{cite web | url=http://phys.org/news/2015-09-moons-saturn.html | title=The moons of Saturn | first=Matt | last=Williams | date=14 September 2015 | publisher=Phys.org | work=Universe Today | access-date=17 December 2016 | ref={{sfnRef|Phys.org}} }}
* {{cite journal
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  |title      = The small inner satellites of Saturn: Shapes, structures and some implications
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* {{cite journal
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{{NASA}}

== External links ==
{{Commons category}}
{{Spoken Wikipedia|Epimetheus_(moon).ogg|date=2010-01-11}}
* [https://web.archive.org/web/20070610003029/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Epimetheus Epimetheus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [https://web.archive.org/web/20060831083534/http://www.planetary.org/explore/topics/saturn/epimetheus.html The Planetary Society: Epimetheus]
* [http://ciclops.org/search.php?x=20&y=7&search=Epimetheus ''Cassini'' Images of Epimetheus] ({{Webarchive|url=https://web.archive.org/web/20200820235938/http://ciclops.org/search.php?x=20&y=7&search=Epimetheus |date=20 August 2020 }})
* [http://ssdbook.maths.qmw.ac.uk/ "Solar System Dynamics" by Murray and Dermott]—The standard text on the subject, describes the orbits in detail.
* [https://web.archive.org/web/20140903190527/http://ssdbook.maths.qmw.ac.uk/animations/Coorbital.mov QuickTime animation of co-orbital motion] from Murray and Dermott
* [https://web.archive.org/web/20061002211034/http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2103 ''Cassini'' image] of Janus and Epimetheus near the time of their orbital swap.
* [http://planetarynames.wr.usgs.gov/Page/EPIMETHEUS/target Epimetheus nomenclature] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]

{{Saturn}}
{{Moons of Saturn}}
{{Solar System moons (compact)}}

{{DEFAULTSORT:Epimetheus (Moon)}}
[[Category:Astronomical objects discovered in 1966|19661218]]
[[Category:Co-orbital moons]]
[[Category:Horseshoe orbit]]
[[Category:Moons of Saturn]]
[[Category:Moons with a prograde orbit]]