- 400km diameter satellite of Saturn discovered by William Herschel in 1789. Mimas has a 186 000km orbit taking 0.9 days to travel, not far beyond the rings of Saturn. Mimas is revealed by Voyager photography as having an immense 100km crater a quarter of the diameter of Mimas itself. Its surface is densely cratered and presumably old. Its low density of about 1.2 gm/cc indicates an interior dominated by ice rather than by rock.