Janus (moon)

Janus (moon)

: "Saturn X redirects here. For the spurious moon reported in 1905, see Themis (moon)" Infobox Planet
name = Janus

caption = Janus, as imaged by Cassini before Saturn in 2006
bgcolour = #a0ffa0
discovery = yes
discoverer = Audouin Dollfus
discovered = December 15, 1966
orbit_ref = cite journal | author= Spitale, J. N.; "et al."| title= "The orbits of Saturn's small satellites derived from combined historic and "Cassini" imaging observations"| journal= The Astronomical Journal | year= 2006 | volume= 132 | issue=2 | pages= 692–710 | url= http://www.iop.org/EJ/article/1538-3881/132/2/692/205235.html | doi= 10.1086/505206 ]
epoch = 31 December 2003 (JD 2 453 005.5)
semimajor = 151 460 ± 10 km
eccentricity = 0.0068
period = 0.694 660 342 d
inclination = 0.163 ± 0.004° to Saturn's equator
satellite_of = Saturn
physical_characteristics = yes
dimensions = 193×173×137 km cite journal | author= Porco, C. C.; "et al."| title= "Physical Characteristics and Possible Accretionary Origins for Saturn's Small Satellites"| journal= Bulletin of the American Astronomical Society| year= 2006| volume= 37| pages= 768| url=http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2289.pdf]
mean_radius = 89 ± 3
mass = 1.912 ± 0.005 ×1018 kg
density = 0.64 ± 0.06 g/cm³
surface_grav = ~0.0137 m/s2
rotation = synchronous
axial_tilt = zero
albedo = 0.71 ± 0.02 (geometric)Verbiscer, A.; French, R.; Showalter, M.; and Helfenstein, P.; [http://www.sciencemag.org/cgi/content/abstract/315/5813/815 "Enceladus: Cosmic Graffiti Artist Caught in the Act"] , Science, Vol. 315, No. 5813 (February 9, 2007), p. 815 (supporting online material, table S1)]
adjectives = Janian

Janus (pronEng|ˈdʒeɪnəs respell|JAY|nəs, or as in Greek "Ιανός)" is an inner satellite of Saturn. It is also known as Saturn X (Roman numeral X = 10). It is named after the mythological Janus.

Discovery and orbit

:"The following is a summary. For more detailed information about Janus and Epimetheus' unusual shared orbit, see Epimetheus."

Janus occupies essentially the same orbit as the moon Epimetheus. This caused some confusion for astronomers, who assumed that there was only one body in that orbit, and for a long time struggled to figure out what was going on. It was eventually realized that they were trying to reconcile observations of two distinct objects as a single object.

The discovery of Janus is attributed to its first observer: Audouin Dollfus, on December 15 1966. [ [http://cfa-www.harvard.edu/iauc/01900/01987.html IAUC 1987: "Probable New Satellite of Saturn"] 1967 January 3 (discovery)] The new object was given the temporary designation nowrap|S/1966 S 2. Previously, Jean Texereau had photographed Janus on October 29 1966 without realising it; Dollfus named it at the same occasion. [http://cfa-www.harvard.edu/iauc/01900/01995.html IAUC 1995: "Saturn X (Janus)"] 1967 February 1 (naming the moon)] On December 18, Richard Walker made a similar observation which is now credited as the discovery of Epimetheus. [ [http://cfa-www.harvard.edu/iauc/01900/01991.html IAUC 1991: "Possible New Satellite of Saturn"] 1967 January 6]

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. [Fountain, J. W.; and Larson, S. M.; [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1978Icar...36...92F&db_key=AST&data_type=HTML&format=&high=45eb6e10af21569 "Saturn's ring and nearby faint satellites"] , Icarus, Vol. 36 (October 1978), pp. 92–106] (See below for a more detailed description of their unique arrangement.)

Janus was observed on subsequent occasions and given different provisional designations. It was observed by the "Pioneer 11" probe when it passed near Saturn on September 1 1979: three energetic particle detectors observed its "shadow" (nowrap|S/1979 S 2. [ [http://cfa-www.harvard.edu/iauc/03400/03417.html IAUC 3417: "New Ring and Satellites of Saturn"] 1979 October 25] ) Janus was observed by Dan Pascu on February 19 1980 (nowrap|S/1980 S 1, [ [http://cfa-www.harvard.edu/iauc/03400/03454.html IAUC 3454: "Saturn"] 1980 February 25] ) and then by John W. Fountain, Stephen M. Larson, Harold J. Reitsema and Bradford A. Smith on the 23rd (nowrap|S/1980 S 2. [ [http://cfa-www.harvard.edu/iauc/03400/03456.html IAUC 3456: "1980 S 2"] 1980 February 29] )

The "Voyager 1" probe finally confirmed Janus' existence on March 1 1980.Fact|date=April 2007 All of these people thus share, to various degrees, the title of discoverer of Janus.


Janus is named after Janus, the two-faced Roman god. Although the name was informally proposed soon after the initial 1966 discovery, it was not officially given this name until 1983. [Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (confirms Janus, names Epimetheus, Telesto, Calypso) (mentioned in [http://cfa-www.harvard.edu/iauc/03800/03872.html IAUC 3872: "Satellites of Jupiter and Saturn"] , 1983 September 30)] Epimetheus received its name at the same time.

According to the "OED", the adjectival form of the moon's name is "Janian".

Orbital relationship between Epimetheus and Janus

Janus and Epimetheus are "co-orbital:" Janus' mean orbital radius from Saturn is currently only 50 km less than that of Epimetheus. This is smaller than either moon's diameter. Since closer orbits have higher velocities, the two moons must inevitably approach each other, and it would seem at first glance that a collision would be inevitable. But as the inner moon catches up with the outer moon their mutual gravitational attraction boosts the inner moon's momentum and raises its orbit, causing it to slow down. At the same time, the outer moon equally loses momentum and drops into a lower orbit. The moons thus "trade" orbits and begin to move apart again, since the forward moon is now in the lower, faster orbit. The nearest they ever approach is some 10 000 km. The exchange takes place about once every four years; the last close approach was on 21 January, 2006, [NASA JPL, [http://saturn.jpl.nasa.gov/multimedia/images/image-details.cfm?imageID=2103 "Cassini-Huygens Multimedia: The Dancing Moons"] , May 3, 2006] the next will be in 2010. At that time, Janus' orbital radius will increase by ~20 km, while Epimetheus' decreases by ~80 km; Janus' orbit is less affected because it is 4 times more massive than Epimetheus. As far as it is currently known, this arrangement is unique in the solar system.

The orbital relationship between Janus and Epimetheus can be understood in terms of the 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. Other examples of the three-body problem include the Lagrangian points, Trojan asteroids and Trojan moons, the "horseshoe" orbit of Cruithne with respect to Earth, and potentially dozens of other objects in similar orbits. [Lloyd, R.; [http://www.space.com/scienceastronomy/solarsystem/second_moon_991029.html "More Moons Around Earth? Its Not So Loony"] , Space.com, 29 October 1999]

Physical characteristics

Janus is extensively cratered with several craters larger than 30 km but few linear features. The Janian surface appears to be older than Prometheus' but younger than Pandora's. From its very low density and relatively high albedo, it seems likely that Janus is a very porous icy body. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.


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 spacecraft in 2006. The ring has a radial extent of about 5000 km. NASA Planetary Photojournal, [http://photojournal.jpl.nasa.gov/catalog/PIA08328 "PIA08328: Moon-Made Rings"] ] Its source is particles blasted off the moons' surfaces by meteoroid impacts, which then form a diffuse ring around their orbital paths.Cassini-Huygens press release, [http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=698 "NASA Finds Saturn's Moons May Be Creating New Rings"] , 11 October, 2006]


Prometheus lie above and below Saturn's rings in this "Cassini" image from 2006-04-29.
Voyager 2" on 1981-08-25.

=Janus in literature= The moon Janus features prominently in the science fiction novel "Pushing Ice" by Alastair Reynolds. In the novel, Janus is a camouflaged alien spaceship which unexpectedly takes off in the direction of a giant structure orbiting the star Spica. The novel tells the story of the crew of a human mining ship who establish a colony on the moon as it leaves the solar system.

ee also

* List of geological features on Saturn's smaller moons
* Saturn's moons in fiction


External links

* [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]
* [http://ssdbook.maths.qmw.ac.uk/ 'Solar System Dynamics' by Murray and Dermott] The standard text on the subject, describes the orbits in detail.
* [http://ssdbook.maths.qmw.ac.uk/animations/Coorbital.mov Quicktime illustration of co-orbital motion] from Murray and Dermott
* [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.

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