Shepherd moon

For the album by Enya, see Shepherd Moons.
Prometheus (right) and Pandora (left) both orbit near Saturn's F ring, but only Prometheus is thought to act as a shepherd.
Operation of a shepherd moon– particles are located in front or behind the Moon in its orbit, so these are either accelerated in the direction of the moon and thrown to the outside, or they are slowed on their path and pulled inwards.

A shepherd moon is a small natural satellite that clears a gap in planetary ring material or keeps particles within a ring contained. The name is a result of their limiting the "herd" of the ring particles as a shepherd.

Due to their gravitational influence, shepherd moons deflect ring particles from their original orbits due to proximity or through orbital resonances. This can carve gaps in the ring system, such as the Encke Gap maintained by Saturn's moon Pan, or lead to the confining of narrow ringlets, such as Saturn's F ring.