Editing Janus (moon)

{{short description|Moon of Saturn}}
{{redirect|Saturn X|the spurious moon reported in 1905|Themis (hypothetical moon)}}
{{Infobox planet
| name = Janus
| image = PIA12714 Janus crop.jpg
| image_scale =
| caption = Janus as imaged by ''Cassini'' on 7 April 2010: highest-resolution full-disk image to date
| pronounced = {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}}<ref>{{OED|Janus}}</ref>
| adjectives = Janian {{IPAc-en|ˈ|dʒ|eɪ|n|i|ə|n}}<ref>[https://web.archive.org/web/20090123141712/http://saturn.jpl.nasa.gov/science/moons/janus/ JPL (ca. 2008) ''Cassini Equinox Mission: Janus'']</ref><ref>Carter (1919) ''The gates of Janus''</ref>
| named_after = [[Janus|Jānus]]
| mpc_name = Saturn X
| discoverer = [[Audouin Dollfus]]
| discovered = 15 December 1966
| orbit_ref = {{sfn|Spitale Jacobson et al.|2006}}
| epoch = 31 December 2003 ([[Julian day|JD]] 2&nbsp;453&nbsp;005.5)
| semimajor = {{val|151460|10|u=km}}
| eccentricity = {{val|0.0068}}
| period = {{val|0.694660342|u=d}}
| inclination = {{val|0.163|0.004|u=°}} to Saturn's equator
| satellite_of = [[Saturn]]
| group = [[Co-orbital moon|Co-orbital]] with [[Epimetheus (moon)|Epimetheus]]
| dimensions = {{val|203.4|x|185.8|x|149.0}}&nbsp;km<br/>(±&nbsp;{{val|1.8|x|0.6|x|0.6}}&nbsp;km){{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| mean_diameter = {{val|178.0|1.0|u=km}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=2}}
| volume = {{val|2953010|950|u=km3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| mass = {{val|1.89388|0.00028|e=18|u=kg}}{{efn|Calculated from the standard gravitational parameter ''GM'' {{=}} {{val|0.126403|0.000019|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|0.6413|0.0002|u=g/cm3}}{{sfn|Lainey et al.|2023}}{{rp|page=4}}
| surface_grav = {{val|0.0111|-|0.0169|u=m/s2}}{{sfn|Thomas & Helfenstein|2020}}{{rp|page=3}}
| escape_velocity = {{V2|1.89388e-3|101.7|3}}&nbsp;km/s at longest axis<br/>to {{V2|1.89388e-3|74.5|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|0.71|0.02}} ([[geometric albedo|geometric]]) {{sfn|Verbiscer French et al.|2007}}
| single_temperature = 76 [[Kelvin|K]]
}}

'''Janus''' {{IPAc-en|ˈ|dʒ|eɪ|n|ə|s}} is an [[inner satellite]] of [[Saturn]]. It is also known as '''Saturn X'''. It is named after the mythological [[Janus (mythology)|Janus]]. This natural satellite was first identified by Audouin Dollfus on December 15, 1966, although it had been unknowingly photographed earlier by Jean Texereau. Further observations led to the realization that Janus shares a unique orbital relationship with another moon, [[Epimetheus (moon)|Epimetheus]]. The discovery of these two moons' peculiar co-orbital configuration was later confirmed by Voyager 1 in 1980.

==History==

=== Discovery ===
Janus was identified by [[Audouin Dollfus]] on 15 December 1966{{sfn|IAUC 1987}} and given the temporary designation '''{{nowrap|S/1966 S 2}}'''. Previously, {{ill|Jean Texereau|fr}} had photographed Janus on 29 October 1966 without realising it. On 18 December, Richard Walker observed an object in the same orbit as Janus, but whose position could not be reconciled with the previous observations. 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}} Walker is now credited with the discovery of [[Epimetheus (moon)|Epimetheus]].{{sfn|IAUC 1991}} ''[[Voyager 1]]'' confirmed this orbital configuration in 1980.{{sfn|Solar System, NASA: Janus}}

===Observational history===
Janus was observed on subsequent occasions and given different [[provisional designation in astronomy|provisional designations]]. ''[[Pioneer 11]]''{{'s}} three energetic-particle detectors detected its "shadow" when the probe flew by Saturn on 1 September 1979 ('''{{nowrap|S/1979 S 2}}''').{{sfn|IAUC 3417}}<!--- [[Tom Gehrels]] and [[James A. van Allen]] ---> Janus was observed by Dan Pascu on 19 February 1980 ('''{{nowrap|S/1980 S 1}}'''),{{sfn|IAUC 3454}} and then by John W. Fountain, Stephen M. Larson, [[Harold J. Reitsema]] and Bradford A. Smith on 23 February 1980 ('''{{nowrap|S/1980 S 2}}''').{{sfn|IAUC 3456}}

=== Name ===
Janus is named after the two-faced [[Roman mythology|Roman]] god [[Janus (mythology)|Janus]]. Although the name was informally proposed soon after the initial 1966 discovery,{{sfn|IAUC 1995}} it was not officially adopted until 1983,{{efn|name=mythology}} when [[Epimetheus (moon)|Epimetheus]] was also named.

== Orbit ==
[[File:PIA08170 Epimetheus and Janus.jpg|thumb|left|[[Epimetheus (moon)|Epimetheus]] (lower left) and 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 km farther from ''Cassini'' than Epimetheus.]]
Janus's orbit is co-orbital with that of [[Epimetheus (moon)|Epimetheus]]. Janus's [[semimajor axis|mean orbital radius]] from Saturn was, as of 2006, 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 in exchange 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 as massive as Epimetheus. 
[[File:Epimetheus-Janus_Orbit.png|thumb|250x250px|[[Rotating reference frame|Rotating-frame]] depiction of the [[Horseshoe orbit|horseshoe orbits]] of Janus and Epimetheus]]
The exchange takes place close to every four years; the last close approaches occurred in January 2006,{{sfn|JPL/NASA: The Dancing Moons}} 2010, 2014, 2018, and 2022 and the next in 2026. This is the only such orbital configuration known in the [[Solar System]].{{sfn|El Moutamid et al|2015}}

The orbital relationship between Janus and Epimetheus can be understood in terms of the [[Three-body problem#Circular restricted three-body problem|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 ==

Janus is extensively cratered with several [[Impact crater|craters]] larger than 30 km and has few linear features. Janus's surface appears to be older than [[Prometheus (moon)|Prometheus]]'s but younger than [[Pandora (moon)|Pandora]]'s. [[File:Animation_of_Epimetheus_orbit_-_Rotating_reference_frame.gif|right|thumb|250x250px|Animation of Epimetheus{{'s}} orbit – Rotating reference frame

{{legend2|Darkkhaki|Saturn}}{{·}}{{legend2|Lime|Janus}}{{·}}{{legend2|Magenta|Epimetheus}}]]
Janus has a very low [[density]] and relatively high [[albedo]], meaning that it is likely icy in composition and structurally a [[rubble pile]] object.

===Features===
Craters on Janus, like those on Epimetheus, are named after characters in the legend of [[Castor and Pollux]].<ref>[https://web.archive.org/web/20040811075205/http://planetarynames.wr.usgs.gov/saturn/januTOC.html USGS: Janus nomenclature]</ref>

{| class="wikitable"
|+ Named Janian craters
|-
! Name !! Pronunciation !! Greek 
|-
| [[Castor and Polydeuces|Castor]] || {{IPAc-en|ˈ|k|æ|s|t|ər}} || Κάστωρ
|-
| [[Idas]] || {{IPAc-en|ˈ|aɪ|d|ə|s}} || Ἴδας
|-
| [[Lynceus of Messenia|Lynceus]] || {{IPAc-en|ˈ|l|ɪ|n|s|iː|ə|s}} || Λυγκεύς
|-
| [[Phoebe of Messenia|Phoibe]] (of Messenia) || {{IPAc-en|ˈ|f|oi|b|iː}} || Φοίβη
|}

== Interactions with rings ==

A faint dust ring is present around the region occupied by the orbits of Janus and Epimetheus, 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 km.{{sfn|JPL/NASA: Moon-Made Rings}} Its source is particles blasted off their surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.{{sfn|JPL/NASA: Creating New Rings}}

Along with Epimetheus, Janus 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 widths="240px" heights="200px" perrow="3" style="clear:both; margin-left:auto; margin-right:auto;">
File:Janus - Voyager 2.jpg|Janus as viewed by ''[[Voyager 2]]'' (1981-08-25).
File:Saturn's moons Janus and Prometheus PIA08192 (NASA).jpg|Janus and [[Prometheus (moon)|Prometheus]] lie above and below [[Rings of Saturn|Saturn's rings]] (2006-04-29).
File:PIA18353-SaturnRingsMoons-JanusTethys-20151027.jpg|Janus and [[Tethys (moon)|Tethys]] (foreground) near [[Rings of Saturn|Saturn's rings]] (2015-10-27).
File:Janus against Saturn PIA08296.jpg|Janus in front of Saturn as imaged by ''[[Cassini–Huygens|Cassini]]'' (2006-09-25).
File:Janus-PIA09872.jpg|Janus as imaged by ''[[Cassini–Huygens|Cassini]]'' (2008-02-20).
File:PIA10447 Janus.jpg|Crescent Janus (2008-06-30).
</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'']], September 30, 1983)
}}

}}

== References ==
=== Citations ===
{{reflist|20em}}

=== Sources ===
{{More footnotes|date=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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== External links ==

{{Commons category inline|bullet=none}}
{{Spoken Wikipedia|Janus (moon).ogg|date=2010-01-13}}
* [https://web.archive.org/web/20021126213902/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Janus Janus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.planetary.org/explore/topics/saturn/janus.html The Planetary Society: Janus]
* {{cite book|last1=Murray|first1=Carl D.|last2=Dermott|first2=Stanley F.|title=Solar System Dynamics|url=https://books.google.com/books?id=aU6vcy5L8GAC&pg=PP1|year=1999|publisher=Cambridge University Press|isbn=978-0-521-57597-3 |author1-link=Carl D. Murray }}
* [https://web.archive.org/web/20140903190527/http://ssdbook.maths.qmw.ac.uk/animations/Coorbital.mov QuickTime illustration 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/JANUS/target Janus nomenclature] from the [http://planetarynames.wr.usgs.gov USGS planetary nomenclature page]

{{Saturn}}
{{Moons of Saturn}}

{{DEFAULTSORT:Janus (Moon)}}
[[Category:Janus (moon)| ]]
[[Category:Moons of Saturn]]
[[Category:Co-orbital moons]]
[[Category:Astronomical objects discovered in 1966|19661215]]
[[Category:Moons with a prograde orbit]]