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The Cassini Spacecraft has caught a very nice picture of a Shepherd Moon in action. These are small, moon like bodies amid the saturnian rings, their larger gravity pulls on the crumb to melon sized debris that makes up the ring, and shepherds the ring material into shape. In the picture, Prometheus has just grazed the visible F ring, and as is evident in the photo, perturbed the material, creating that razor-feature in the picture.

Pretty keen.