Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Saturn: Prometheus Link for sharing this page on Facebook
     Prometheus is one of the inner "shepherd" satellites embedded in Saturn's ring system. Along with Pandora, which is in almost the same orbit, it traps dust particles ejected from the two moons by meteorite impacts to form the F ring. The inner of the two moons gradually gains on the outer one (since smaller orbits have shorter orbital periods), and when it laps the outer one their gravitational interaction causes small changes ("perturbations") in each others' orbits. As a result, Pandora and Prometheus' orbits are slightly "chaotic", varying irregularly around their average orbital elements.
     Because of its small size it is hard to determine the mass of Prometheus, and as a result its density and surface gravity are essentially unknown. Current estimates are that it has a density less than half the density of water. If correct, that would mean that like Hyperion it is very porous, consisting of as much empty space as icy material. The January 2010 close approach of Cassini to Prometheus should have improved its mass and density estimates (though given the fact that this paragraph was originally written prior to that flyby, the results will be posted in the next update of this page; however, an image from that approach is shown below). But given its irregular shape the surface gravity of Prometheus must vary by a considerable amount from place to place, whatever its average value might be.

Prometheus as imaged by the Cassini spacecraft on January 27, 2010 (distance about 21 thousand miles). Part of the moon is lit by the Sun, and part by light reflected from Saturn. (modified version of image by Cassini Imaging Team, ISS, JPL, ESA, NASA, apod100405)

Prometheus as seen by Voyager 2 on August 25, 1981 (distance about 150,000 miles)
(NASA, JPL, Voyager 2)

     Prometheus, as seen from one end by the Cassini spacecraft on June 7, 2005. The moon rotates synchronously (as do almost all of Saturn's moons), with the long axis always pointing at Saturn. This view shows the end which points away from Saturn, toward the F ring, which is shepherded by Promethus and Pandora. (NASA, Cassini Imaging Team)

     One of several dozen images of Prometheus taken by the Cassini spacecraft on December 26, 2009. This and other "raw" images will be compared, run through software analysis, and be used to produce better quality images; but depending upon the priority given the analysis, it is often several months before the final images are released, whereas raw images are made available on the Cassini site almost immediately. (SSI, JPL, ESA, NASA, Cassini Imaging Team)

     A "raw" image of Prometheus at one of the closest Cassini approaches (only 23,000 miles distance) on January 27, 2010. Most of the white dots and streaks are caused by cosmic rays hitting the imaging CCD, and will be removed when the image has been fully processed. The faint dark streaks on the upper left "point" are shadows of the F ring, which Prometheus and Pandora "shepherd". (Cassini Imaging Team, SSI, JPL, ESA, NASA, apod100201)

(Image credit NASA/JPL/Space Science Institute, Planetary Photojournal)
     Prometheus' interaction with the F ring pulls material toward it, but most of the time, as it moves past the ring particals (its smaller orbit gives it a higher physical and angular velocity), very little of the material actually reaches it. Most of the ring particles surge toward Prometheus then gradually fall backward and behind the moon towards the ring, creating waves and ripples in the ring. As a result of this process, the pull of the moon tends to actually concentrate material in the ring, rather than spreading it out. This paradoxical result is due to the interaction of the ring particles with the small temporary pull of Prometheus and the much larger permanent pull due to Saturn's gravity (see how satellites shepherd rings).

(Image credit NASA/JPL/Space Science Institute, CICLOPS)
     Although as noted above the interaction between Prometheus and the F ring tends to keep the ring stable over long periods of time, some very interesting effects occur once in each orbital period (every 14.7 hours).
     As is true of all orbital motions, the orbit of Prometheus is an ellipse, which carries it closer to and further away from Saturn at opposite points in its orbit. When at its furthest point from Saturn (apoapse), it almost comes into contact with the inner edge of the F ring. At that point its gravity, though weak, pulls material away from the ring toward the moon, forming a bright streak which follows Prometheus. However, as discussed in how satellites shepherd rings, pulling the material toward Saturn changes the effect of the planet on the ring particles. This is most easily seen by examining the dark streaks formed in the hazy inner part of the ring, where material has been pulled toward Prometheus, leaving a relatively empty area in its wake.
     Since the orbital period of Prometheus is less than that of the ring particles, it reaches apoapse a little further along the ring's circumference (about 1% further along) in successive orbits. On the far left of the image above we see Prometheus pulling a new bright streak (and forming a new dark streak) away from the ring, fairly directly toward the moon (and Saturn). To its right we see older streaks and rings from previous apoapse encounters. Note that the hazy portion of the ring is broader near the moon's current position but thinner and closer to the bright central arc of the ring in the regions where previous passages occurred. This is due to the differential motion of the particles (caused by Saturn's gravity) spreading out the ring particles, making the bright and dark streaks more and more parallel to their original positions, and returning the ring to its original appearance. It is the complicated interplay of ring particles, moons and Saturn that produces the complex structure of the rings.

Data for Prometheus

Discovered by S.Collins and others in 1980, from Voyager 1 photographs
Named after one of the sons of Uranus and Gaea
Orbital size 139,353 km (about 86,600 miles) (just inside the F ring)
Orbital eccentricity 0%
Orbital inclination 0 degrees
Orbital period 14.7 hours
Diameter about 150 x 100 x 70 km (about 92 x 62 x 42 miles) (very elongated)
Mass, density and surface gravity all very uncertain
Albedo (reflectivity) 60%
Inner of two shepherd moons (outer is Pandora) which control Saturn's F ring