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Pan (moon)
Pan ( , (ギリシア語:Πάν)) is the second-innermost moon of Saturn. It is a walnut-shaped small moon about 35 kilometres across and 23 km high that orbits within the Encke Gap in Saturn's A Ring. Pan acts as a ring shepherd and is responsible for keeping the Encke Gap free of ring particles.
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.〔(IAUC 5052: ''Saturn'' ) July 16, 1990 (discovery)〕
==Prediction==
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.〔Cuzzi, J. N.; and Scargle, J. D.; (''Wavy Edges Suggest Moonlet in Encke's Gap'' ), Astrophysical Journal, Vol. 292 (May 1, 1985), pp. 276–290〕 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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