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xkcd #2908: Moon Armor Index
(imgs.xkcd.com)
Alt text:
Astronomers are a little unsure of the applicability of this index, but NASA's Planetary Protection Officer is all in favor.
Pluto-charon shouldn't count since it is more akin to a binary planetary system than a planet-moon system. Charon and Pluto are so close in size that the system has a barycenter outside of Pluto. This means instead of Charon orbiting a point inside Pluto, Charon and Pluto orbit a point about 800 km above Pluto's surface.