Mimas. Satellite image of Mimas (right), one of Saturn's moons, with the rings A, B, C and F, and part of the Cassini division (blue/white). Mimas, also known as Saturn I, was discovered by William Herschel in 1789. It has a diameter of 398 kilometres and its low density suggests it is composed mostly of water ice with only a small amount of rock. This image was taken as part of the Cassini-Huygens mission, which launched in 1997 with the purpose of studying Saturn and its moons. Image taken on 7 November 2004 at a distance of 3.7 million kilometres by a narrow angle camera aboard the Cassini orbiter. The orbiter is due to perform a flyby of Mimas on 1 August 2005.