Gravitational Shear At the F-Ring. Here is the effect on the F-ring of the passage of one of its shepherding moons. Prometheus, which is seen inside the ring, is orbiting from the right to the left of the picture. It's just leaving a gravitational wake behind, which is disturbing some of the small particles which the F-ring is made of. Hence the shear. Prometheus is a small, 63-mile (102-km) accross, Saturnian moon. It's one of these small moons which are linked to the ring system. It's call a shepherding moon because of its action upon one of the constituents of the ring, namely the F-ring. The F-ring is a faint ring, invisible from Earth, located outside the well-known ring of Saturn. This picture was taken by the joint European ESA/NASA/Italian Space Agency Cassini mission which is currently performing a four-year work in the Saturnian world, studying Saturn, the ring and the planet's environment. The white edge at the lower right of the picture is the outermost part of the A ring. picture NASA/JPL/Space Science Institute