Pan

Pan is the innermost named moon of Saturn. It is a small, walnut-shaped moon approximately 35 kilometres across and 23 km wide that orbits within the Encke Gap in Saturn's A Ring. Pan is a ring shepherd and is responsible for keeping the Encke Gap free of ring particles. It is sometimes described as having the appearance of a ravioli. It was discovered by Mark R. Showalter in 1990 from analysis of old Voyager 2 probe photos and received the provisional designation because the discovery images dated back to 1981. Prediction and discovery The existence of a moon in the Encke Gap was first predicted by Jeffrey N. Cuzzi and Jeffrey D. Scargle in 1985, based on wavy edges of the gap which indicated a gravitational disturbance. In 1986 Showalter et al. inferred its orbit and mass by modeling its gravitational wake. They arrived at a very precise prediction of 133,603 ± 10 km for the semi-major axis and a mass of 5–10 Saturn masses, and inferred that there was only a single moon within the Encke gap. The actual semi-major axis differs by 19 km and the actual mass is 8.6 of Saturn's. The moon was later found within 1° of the predicted position. The search was undertaken by considering all Voyager 2 images and using a computer calculation to predict whether the moon would be visible under sufficiently favorable conditions in each one. Every qualifying Voyager 2 image with resolution better than ~50 km/pixel shows Pan clearly. In all, it appears in eleven Voyager 2 images.