A shepherd moon can do more to define ring structures than just keep the flock of particles in line, as Cassini spacecraft images such as this have shown.

Prometheus (102 kilometers, or 63 miles across) is seen here with two long streamers of material that it has pulled out of the F ring. When Prometheus comes close to the F ring in its orbit, the moon's gravity tugs on the ring particles. The disturbed particles, now pulled into orbits slightly closer to Saturn and therefore faster, shear out during successive orbits, creating the long and delicate streamers seen here.

This view looks toward the unlit side of the rings from about 31 degrees above the ringplane.

The image was taken in visible light with the Cassini spacecraft narrow-angle camera on Sept. 29, 2006 at a distance of approximately 1.7 million kilometers (1 million miles) from Prometheus and at a Sun-Prometheus-spacecraft, or phase, angle of 160 degrees. Image scale is 10 kilometers (6 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colo.

For more information about the Cassini-Huygens mission visit http://saturn.jpl.nasa.gov/home/index.cfm. The Cassini imaging team homepage is at http://ciclops.org.