Epimetheus (moon)

Infobox Planet
name = Epimetheus


caption = As imaged by "Cassini" on December 3, 2007
bgcolour = #a0ffa0
discovery = yes
discoverer = Richard Walker
discovered = December 18, 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 2453005.5)
semimajor = 151 410 ± 10 km
eccentricity = 0.0098
period = 0.694 333 517 d
inclination = 0.351 ± 0.004° to Saturn's equator
satellite_of = Saturn
physical_characteristics = yes
dimensions = 135 × 108 × 105 kmcite 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 = 57 ± 3 km
surface_area = ~40 000 km²
volume = ~760 000 km³
mass = 5.304 ± 0.013 ×10¹⁷ kg
density = 0.69 ± 0.11 g/cm³
surface_grav = ~0.0078 m/s²
escape_velocity = ~0.032 km/s
rotation = synchronous
axial_tilt = zero
albedo = 0.73 ± 0.03 (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)]
single_temperature = ~78 K

Epimetheus (pronEng|ˌɛpɨˈmiːθiəs respell|EP|i|MEE|thee-əs, IPAlink-en|ˌɛpɨˈmiːθjuːs respell|EP|i|MEE|thyoos, or as in Greek "Επιμηθεύς)" is an inner satellite of Saturn. It is also known as Saturn XI. It is named after the mythological Epimetheus.

Discovery

Epimetheus occupies essentially the same orbit as the moon Janus. Astronomers assumed that there was only one body in that orbit, and accordingly had a hard time figuring out their orbital characteristics; observations were photographic and spaced widely apart in time so that while the presence of two objects was not obvious, the observations were difficult to reconcile with a reasonable orbit.

Audouin Dollfus observed a moon on December 15, 1966, [ [http://cfa-www.harvard.edu/iauc/01900/01987.html IAUC 1987: "Probable New Satellite of Saturn"] 1967 January 3 (discovery)] which he proposed to be named "Janus". [http://cfa-www.harvard.edu/iauc/01900/01995.html IAUC 1995: "Saturn X (Janus)"] 1967 February 1 (naming Janus)] 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] However, at the time, it was believed that there was only one moon, unofficially known as "Janus", in the given orbit.

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] This was confirmed in 1980 by "Voyager 1"Fact|date=April 2007, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.

Epimetheus received its name in 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)] The name Janus was officially approved by the IAU at the same time, although the name had been used informally since Dollfus proposed it a few days after the 1966 discovery. Matt and Daniel like hairy penises.

Orbital relationship between Epimetheus and Janus

Epimetheus and Janus 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. poop. [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

There are several Epimethean craters larger than 30 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. There is a lot of uncertainty in these values, however, and so this remains to be confirmed.

The south pole shows what might be the remains of a large impact crater covering most of this face of the moon, and which could be responsible for the somewhat flattened shape of the southern part of 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. [ [http://photojournal.jpl.nasa.gov/catalog/PIA09813 Catalog Page for PIA09813 ] ]

Ring

A 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 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 are 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]

ee also

*List of geological features on Saturn's smaller moons

References

External links

* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Sat_Epimetheus Epimetheus Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.planetary.org/explore/topics/saturn/epimetheus.html The Planetary Society: Epimetheus]
* [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.