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Saturn’s Small Moons

saturn's small moons

 

Saturn’s Small Moons

 

Epimetheus

 

Orbit: 527,040 km from Saturn
Orbital period: 17 hours
Gravity: 0.0064 m/s²
Diameter: 115 km
Mass: 5.6e17 kg

Epimetheus is an inner satellite of Saturn. It is also known as Saturn XI.

Epimetheus was the son of Iapetus and brother of Prometheus and Atlas;  husband   of Pandora.  “Epimetheus” is Greek for “hindsight”.

Saturn's small moon Epimetheus

Epimetheus was first observed by Walker in 1966. But the  situation was confused since Janus is in a very similar  orbit.   So Walker officially shares the discovery of Epimetheus with Fountain  and Larson who showed in 1977 that there were two satellites involved.   The  situation was clarified in 1980 by Voyager  1.

The dark line across the surface in the Voyager image to the left is  actually the shadow of the Saturn’s F-ring.

Epimetheus and Janus are “co-orbital”.

There are several 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.

Pan

 

Orbit: 133,583 km from Saturn
Orbital period: 14 hours
Gravity: 0.0001 m/s²
Diameter: 20 km
Mass: 4.95e17 kg

Discovered by Mark R. Showalter  in 1990 from Voyager photos taken in 1981; reconfirmed by images from Cassini in 2005.

Pan is the innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn’s A Ring. Small moons near the rings produce wave patterns in the rings.  Prior to     the discovery of Pan, an analysis of the patterns in the edge of Saturn’s A ring    predicted the size and location of a small moon.  Pan was discovered by reexamining the 10 year old Voyager photos at the predicted spot.

Atlas

 

Orbit: 137,670 km from Saturn
Orbital period: 14 hours
Gravity: 0.0002 m/s²
Diameter: 30 km
Mass: 6.60e17 kg

Atlas is an inner satellite of Saturn. Atlas was discovered by Richard Terrile in 1980 from Voyager photos and was designated S/1980 S 28. Atlas seems to be a shepherd satellite of  the A ring.

Prometheus

 

Orbit: 139,350 km from Saturn
Orbital period: 15 hours
Gravity: 0.0013 m/s²
Diameter: 91 km
Mass: 2.7e17 kg

 

Prometheus is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 27. In late 1985 it was officially named after Prometheus, a Titan in Greek mythology.

Prometheus is the inner shepherd  satellite of the F ring.

Prometheus has a number of ridges and valleys and several craters about 20 km  in diameter but appears to be less cratered than the neighboring moon  Pandora,   Janus and  Epimetheus.

From their very low densities and relatively high albedos, it seems likely  that  Prometheus, Pandora, Janus and Epimetheus are very porous icy bodies.  (Note,  however, that there is a lot of uncertainty in these values.)

The 1995/6 Saturn Ring Plane  Crossing observations found that Prometheus was lagging by 20 degrees from  where it should have been based on Voyager 1981 data. This is much more than can  be explained by observational error. It is possible that Prometheus’s orbit was  changed by a recent encounter with the F ring, or it may have a small companion  moon sharing its orbit.

Pandora

 

Orbit: 141,700 km from Saturn
Orbital period: 15 hours
Gravity: 0.0026 m/s²
Diameter: 84 km
Mass: 2.2e17 kg

Pandora is an inner satellite of Saturn. It was discovered in 1980 from photos taken by the Voyager 1 probe, and was provisionally designated S/1980 S 26. In late 1985 it was officially named after Pandora from Greek mythology. Pandora is the outer shepherd satellite  of the F ring.  The image at right shows both Pandora, the faint F ring, Prometheus and part of the A ring. (Click the image for a  much finer image from Cassini)Craters formed on this object by impacts appear to be covered by debris, a  process that probably happens rapidly in a geologic sense. The grooves and small  ridges on Pandora suggest that fractures affect the overlying smooth material.

Janus

 

Orbit: 151,472 km from Saturn
Orbital period: 15 hours
Gravity: 0.011 m/s²
Diameter: 178 km
Mass: 2.01e18 kg

 

Discovered by the French astronomer Audouin  Dollfus in 1966.

Dollfus is credited with the discovery of Janus but it’s not really certain whether the object he saw was Janus or   Epimetheus and his observations led to a       spurious orbit.  (Walker discovered it independently but his telegram  arrived a few hours after   Dollfus’.)   Larson and Fountain determined  in 1978 that there are in fact two moons  at about 151000 km from Saturn.   This was confirmed in 1980 by   Voyager  1.

Saturn's small moon Janus

Janus and Epimetheus   are “co-orbital“.  The  orbital radii of Janus and Epimetheus differ by only 50 km, less than       the  diameter of either. Their orbital velocities are thus very nearly       equal  and the lower, faster one slowly overtakes the other.  As they  approach  each other they exchange a bit of momentum the end result       of which is to  boost the lower one into a higher orbit and to drop the       higher one to a  lower orbit. They thus exchange places. The exchange       takes place about  once every four years.  The orbital data given here is as of the time of  the Voyager encounters.

Janus is extensively cratered with several craters larger than 30 km but few  linear features.   Its surface appears to be older than  Prometheus but younger than Pandora’s.

Telesto

 

Orbit: 294,660 km from Saturn
Orbital period: 46 hours
Gravity: ?
Diameter: 29 km
Mass: ?

It was discovered by Smith, Reitsema, Larson and Fountain in 1980 from ground-based observations, and was provisionally designated S/1980 S 13

 

Calypso

 

Orbit: 294,660 km from Saturn
Orbital period: 46 hours
Gravity: ?
Diameter: 26 km
Mass: ?

Calypso is a moon of Saturn. It was discovered in 1980, from ground-based observations, by Dan Pascu, P. Kenneth Seidelmann, William A. Baum, and Douglas G. Currie, and was provisionally designated S/1980 S 25.

Calypso is in Tethys’ trailing  Lagrange point. Calypso and Telesto are among the  smallest moons in the solar system.

Helene

 

Orbit: 377,400 km from Saturn
Orbital period: 66 hours
Gravity: ?
Diameter: 33 km
Mass: ?
Helene is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory, and was designated S/1980 S 6. Helene is in Dione’s leading Lagrange point and  hence was sometimes referred to as “Dione B”.

 

 

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Saturn’s Small Moons

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