The shepherd moons of the F-ring, Prometheus and Pandora were discovered during the Voyager mission.
The moons are small - Prometheus is 102 kilometers across and Pandora is slightly smaller.
Prometheus maintains the inner edge of the F-ring and Pandora patrols its outer edge.
These taken by Cassini in 2004 show that:
the F-ring is multi-stranded and kinked
Prometheus has an oblong, potato-like shape.
(Higher resolution images are hoped for later in the mission.)
A faint strand of material appears to connect Prometheus to the ringlets.
Scientists speculate that the moon is gravitationally pulling material
away from the F-ring. Discontinuities, kinks and gaps in the strands
at various places around the ring may be caused by Prometheus.