A shepherd moon orbits near the outer edges of a gas giant’s rings, or sometimes even within the rings themselves. These moons appear to be responsible for defining sharp boundaries and even gaps in those rings.
The gravity of the shepherd moon “herds” wayward ring particles and keeps them from straying. Some wayward particles are captured and fall on the moon, some are deflected back into the rings proper, and still others are ejected entirely from the ring system.
Shepherd moons often come in pairs: one to keep the ring from spreading outward and the other to keep the particles from falling inward. Some shepherd moons are singletons; however, we don’t know whether they will stay that way or whether there is another shepherd moon or moonlet out there, waiting to be discovered. There is still much we don’t know about the gravitational dynamics of shepherd moons and ring systems.
We do know that each of the gas giants in our solar system has at least one shepherd moon which maintains at least part of their ring structure. Many of these shepherd moons are inside the Roche limit, the distance at which a planet or other celestial body will cause an orbiting object to break apart due to tidal forces. We don’t know why they have not broken apart yet. Current thought suggests that several of these moons will break apart in the next few million years, although we don’t know enough about the density of some of these objects to say this with any degree of certainty.
Tiny Pan, the innermost moon of Saturn and well within the Roche limit, orbits inside the Encke Gap in Saturn’s rings and sweeps it free of ring particles, apart from the Pandean Ringlet, where Pan maintains some ring particles in horseshoe orbits. The area it clears is ten times wider than the moon itself.
Galatea, Neptune’s only known shepherd moon, is suspected to be the reason why Neptune’s rings were thought to be ring arcs until Voyager 2 took closer pictures. Even with that closer observation, most of the particles in the Adams ring have been confirmed to occupy a series of well-defined arcs, the cause of which remains unexplained.
More typical are Cordelia and Ophelia, the inner and outer shepherds of Uranus’ Epsilon ring. As with Galatea, Pan, and many other shepherd moons; their orbits are slowly decaying due to tidal deceleration, and they are expected eventually to impact Uranus’ atmosphere or be torn into a ring of their own.
Most fascinating and bewildering are Prometheus and Pandora, which shepherd Saturn’s F-ring (and to a lesser extent the A-ring) and cause its particles to aggregate into clumps, strands, and even braid-like structures. Even their orbits are chaotic and currently unpredictable: each time they get close to each other, their orbits seem to change. The best we can do is to define a mean motion resonance of 121:118 between Prometheus and Pandora: and consequently larger changes in orbit approximately every 6.2 years.