Mimas, also designated Saturn I, is a natural satellite of Saturn that has the second largest crater on any moon in the Solar System, named Herschel. The Herschel crater measures across, about one-third of Mimas's mean diameter (), and is believed to be formed from an extremely energetic impact event. The crater's name is derived from the discoverer of Mimas, William Herschel, in 1789.
Mimas is the smallest astronomical body known to be roughly rounded in shape due to its own gravity. Mimas's low density, 1.15 g/cm3, indicates that it is composed mostly of water ice with only a small amount of rock. The moon's presence has created one of the largest 'gaps' in Saturn's ring, named the Cassini Division, due to orbital resonance destabilizing the particles' orbit there.
Mimas was discovered by the astronomer William Herschel on 17 September 1789. He recorded his discovery as follows: "I continued my observations constantly, whenever the weather would permit; and the great light of the forty-feet speculum was now of so much use, that I also, on the 17th of September, detected the seventh satellite, when it was at its greatest preceding elongation."
The 40-foot telescope was a metal mirror reflecting telescope built by Herschel, with a aperture. The 40 feet refers to the length of the focus, not the aperture diameter as more common with modern telescopes.
Mimas is named after one of the Giants in Greek mythology, Mimas. The names of all seven then-known satellites of Saturn, including Mimas, were suggested by William Herschel's son John in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope. Saturn (the Roman equivalent of Cronus in Greek mythology) was the leader of the Titans, the generation before the Gods, and ruler of the world for some time. The Giants were the subsequent generation, and each group fought a great struggle against the Gods.
The customary English pronunciation of the name is 'maɪməs, though some people attempt a more 'authentic' pronunciation, 'miːməs.