Final answer:
Shepherd satellites, or shepherd moons, are small moons orbiting a planet that stabilize and maintain the structure of planetary rings through their gravitational forces. Saturn's F Ring is confined by the shepherd moons Pandora and Prometheus, while Uranus's Epsilon Ring is maintained by Cordelia and Ophelia.
Step-by-step explanation:
A shepherd satellite, or shepherd moon, is a small moon that orbits near the edge of a planet's ring system and exerts gravitational forces that help maintain the shape and sharpness of planetary rings. In the case of Saturn's F Ring, the shepherd moons Pandora and Prometheus, each about 100 kilometers in diameter, confine the ring particles to a narrow ribbon, thus stabilizing the ring. Similarly, Uranus's Epsilon Ring is shepherded by two moons named Cordelia and Ophelia, which are each about 50 kilometers in diameter and orbit approximately 2000 kilometers inside and outside the ring.
These shepherd moons essentially act as barriers that prevent ring particles from spreading out into space, and through their gravitational influence, can induce waves and other structures within the rings. Although not all shepherd moons have been directly observed, their existence is theorized based on the narrow and well-defined rings present in the uranian and neptunian systems.