Astronomy

What is a Shepherd Moon and what does it do



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Saturn has been a source of interest and mystery since man first noticed it in the heavens. It definitely has an abundance of moons of all shapes and sizes, including what are commonly called shepherd moons. Shepherd moons are small moons that are found revolving around the outer edges of the rings of Saturn. While there are over sixty documented moons orbiting Saturn, the two most prominent of those that are shepherd moons are Prometheus and Pandora.

Thanks to the probe, Cassini, which was launched in 1997, and landed on the surface of Titan, Saturn’s largest moon, January 14, 2005, a wealth of information on this huge planet, its satellites, and its rings has been obtained. In fact, data on this planetary system has been coming back to Earth since 2004.

Many of the moons of Saturn are anything but round. These satellites, many of which are irregular and marked with craters, come in various shapes and sizes, and, as it turns out, may serve some actual purpose. While not as interesting as the two large moons of Saturn, Titan and Enceladus, shepherd moons do provide a service.

One of the most intriguing aspects of the moons of Saturn has been their contribution to the rings themselves, through their gravitational pull. Shepherd moons are named for the fact that their purpose around Saturn seems to be to keep the rings in check and “shepherd” the material within. The rings, are composed of rocks and ice. Debris in these rings can range in size anywhere from dust particles to mountains, and are believed to be the remnants of comets, asteroids, or even other moons that have been pulled into orbit by the giant Saturn.

Prometheus, which is sixty three miles across, and Pandora, fifty two miles across, maintain the ring categorized as the F Ring. Pandora is found on the exterior of the ring, and Prometheus, further inside. Together, due to their gravitational pull, they appear to be keeping the ring material within its band. Prometheus, on the inner edge of Saturn’s F Ring, has craters, and is believed to be porous and icy. While there is very little information on Pandora, the shepherd moon on the outside edge of the ring, it is very heavily cratered.

Through further data obtained by Cassini, no doubt more moons will be discovered, and more information on the rings, their formation, and the characteristics of the various satellites of Saturn.


"Frequently Asked Questions - Saturn." Cassini Solstice Mission. Web. 15 Feb. 2011. http://saturn.jpl.nasa.gov/faq/FAQSaturn/#q10.

"NASA - Close to the Shepherd Moons." NASA - Home. Web. 15 Feb. 2011. <http://www.nasa.gov/multimedia/imagegallery/image_feature_650.html>.

"Cassini Solstice Mission." Cassini-Huygens. Web. 15 Feb. 2011. <http://saturn.jpl.nasa.gov/index.cfm>.

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