At Saturn…
Saturn's outermost visible ring, the F ring, is curiously thin. When Voyager 1 arrived at Saturn in 1980 the reason for this became apparent. There were two tiny moons orbiting Saturn in the plane of the rings, one on either side of the F ring. The action of these two moons prevents the F ring from spreading in either direction. In essence they “shepherd” the particles that make up the F ring into a thin band, and so they were collectively referred to 'Shepherd Moons'.
The inner moon was named Prometheus and the outer moon was named Pandora. The Cassini Saturn orbiter which will be arriving at Saturn on July 1 managed to capture an image of the shepherd moons on April 15th. (Visit the article Prometheus and Pandora at the Cassini-Huygens home page for more information about this image.)
These moons pass so closely to each other that they affect each other's orbit strongly and in hard to predict ways causing chaotic orbits.
At Uranus…
In 1986, another pair of shepherd moons were discovered, this time at the planet Uranus, which also has rings. The ring system of Uranus is much darker, and the rings are much thinner and therefore not visible from Earth.
The Voyager 2 probe returned the image shown at left of Ophelia and Cordelia, shepherd moons which kept the epsilon ring of Uranus thin and focussed. An interesting sidenote was that not long after Voyager 2's relatively short visit at Uranus (2 weeks), astronomers lost track of these two little moons, and they were not seen again for the next 14 years. In 2000, using the Hubble Space Telescope, the moons were found again. Visit the article Shepherd Moons, Lost and Found for more information about that story.
At Neptune and Jupiter…
All of the gas giants have rings…
Jupiter's faint ring is thought to be formed almost entirely of ash which erupts from the volcanically active moon Io, and/or surface material from the small moons Adrastea and Metis. There do not appear to be any shepherds keeping this ring focused so in theory if Io stopped spewing ash, eventually the ring would dissipate.
Neptune also has thin rings of uneven thickness. Originally they were thought to be incomplete ring-arcs but more detailed photography and analysis has revealed they are complete circles, but much thicker in some areas than in others. To date one moon discovered in the ring system there, Galatea, is thought to be a shepherd, and is also thought to be responsible for the clumpiness of the rings. To date, no companion shepherd has been discovered for Galatea.
In Music…
In 1991, Enya released a wonderful CD called “Shepherd Moons” (review) which was indeed named for the Voyager 1 discovery of Prometheus and Pandora. Many of the tracks on the album take their names and/or lyrics from astronomical themes. Enya was said (link) to have been fascinated by the many images of strange satellites that came to us in the 80's from the Voyager probes which between them visited all of the gas giants.
From the forward to the Shepherd Moons music book:
Shepherd Moons:
Two tiny new moons had been discovered. Shepherd Moons orbiting a ring of Saturn. Numbered, not named. Working ceaselessly, it seemed, to keep the particles of the ring together, much as a shepherd would guide his flock. Protectors both, 1980S26 and 1980S27 – either side of the F rings. Voyager had performed like a dream and sent their pictures to Earth. I marvelled at their diligence, a new mystery, poignant and beautiful. An unspoken glory to the rings of Saturn. The world had not known but all the time they were there, working tirelessly. Everyone had appreciated the spectacular beauty of Saturn, but where would the planet be without those precious, mis-shapen, miniature moons?
Further Information and Pictures…
SeaSky.com offers a very accessible “tour” of the solar system that will give you a great deal of information about most of the solar system's known bodies.
NASA's “Planetary Photojournal” is also a great source of images and information.
I'll close out this article with links to pictures of the heavenly bodies mentioned here:
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