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Moon of Saturn
Mimas
Saturn's little ice moon with a giant crater that makes it look like the Death Star
Key facts
- Diameter
- About 245 miles / 394 km (slightly egg-shaped: 207 x 197 x 191 km)
- Distance from Saturn
- About 116,000 miles / 186,000 km (mean)
- One orbit
- 22 hours 36 minutes (about 0.94 Earth days)
- Rotation
- Tidally locked: one rotation per orbit, same side always faces Saturn
- Density
- About 1.15 g/cm³, almost entirely water ice
- Surface temperature
- About 64 K (roughly minus 209 °C)
- Discovered
- 17 September 1789 by William Herschel
- Named after
- Mimas, a Giant in Greek mythology; name suggested by John Herschel
- Surface
- Heavily cratered ice; Herschel crater is about 80.8 miles / 130 km wide with a 6 km central peak
- Hidden ocean?
- Cassini orbit data (2024) suggest a young ocean 12.4 to 18.6 miles / 20 to 30 km below the ice
- Visited by
- Pioneer 11 (1979), Voyager 1 (1980), Voyager 2 (1981), Cassini (2004 to 2017)
- Saturn's known moons
- 293 as of August 2026
About Mimas
Mimas is a small moon of Saturn. It is made mostly of ice. It looks a bit like a snowball. One side has a giant hole. The hole is a crater. A big rock hit Mimas long ago. The crater is called Herschel. Some people think Mimas looks like a space station. The moon is very cold. It goes round Saturn in one day. Saturn has many moons. Mimas is one of the closest. Hugo would love to see it. Would you like to visit Mimas?
Mimas is a small, icy moon that orbits the giant planet Saturn. It is about 394 kilometres across, which is tiny compared with our own Moon. Its gravity is just strong enough to pull it into a rough sphere. It orbits close to Saturn, about 186,000 kilometres from the planet's centre.
One trip around Saturn takes only 22 hours and 36 minutes. The same side of Mimas always faces Saturn, just like our Moon faces Earth. The most famous feature on Mimas is a huge crater called Herschel. It is about 130 kilometres wide, a third of the whole moon.
The crash that made it must have nearly broken Mimas apart. Herschel has a central peak that stands about 6 kilometres tall. Because of this crater, many people say Mimas looks like the Death Star. Mimas is made almost entirely of water ice, so it is very light.
Scientists studying its orbit think there may be a hidden ocean under the ice. William Herschel discovered Mimas in 1789 using a very large telescope. The Voyager and Cassini spacecraft have taken close-up pictures of this strange little world.
Mimas is the smallest and innermost of Saturn's seven large, round moons. NASA gives its mean diameter as about 394 km (JPL's Solar System Dynamics group lists a mean radius of 198.2 km), and it is noticeably ovoid, measuring roughly 207 by 197 by 191 km. It orbits Saturn at a mean distance of just over 186,000 km, well inside the orbit of Enceladus, and completes one circuit in 22 hours 36 minutes. It is tidally locked, so the same hemisphere always faces Saturn. Its mean density of about 1.15 g/cm³ shows it is composed almost entirely of water ice, which is the only material ever detected on its surface, and its surface temperature is roughly 64 K (about minus 209 °C).
The moon's defining feature is Herschel, an impact crater about 130 km across, a third of the moon's diameter, with walls about 5 km high and a central peak rising about 6 km. An impact of that scale probably came close to shattering Mimas, and fractures on the far side may be related to it. The rest of the surface is saturated with craters, some more than 40 km across. Herschel's resemblance to the Death Star from Star Wars is a coincidence: the first clear images arrived from Voyager 1 in 1980, three years after the film was released.
Mimas also shapes Saturn's rings. Ring particles at the inner edge of the Cassini Division orbit twice for every one orbit of Mimas, and this 2:1 resonance helps clear the gap. Mimas is in a 2:1 resonance with the larger moon Tethys as well.
Pioneer 11 passed at about 104,000 km in 1979, Voyager 1 and 2 imaged the moon in 1980 and 1981, and Cassini made several close approaches, including a 9,500 km flyby on 13 February 2010, while orbiting Saturn from 2004 to 2017. In 2024, analysis of Cassini astrometry showed that the moon's orbit precesses more slowly than a solid body's would, pointing to a global liquid ocean 20 to 30 km beneath the ice and, surprisingly, less than 25 million years old. That result is a model inference, not a direct detection, and Mimas shows none of the surface activity seen on Enceladus.
William Herschel discovered Mimas on 17 September 1789. It was named by his son John Herschel after a Giant from Greek mythology.
Did you know?
- Herschel crater is about 130 km wide, one third of the width of Mimas. The impact that made it very nearly smashed the moon to pieces.
- A Cassini temperature map of Mimas looked so much like Pac-Man that scientists gave it that nickname, with Herschel crater as the power pellet.
- Mimas helps carve the Cassini Division, the big gap in Saturn's rings. Ring particles there orbit exactly twice for every one orbit of Mimas.
Neighbours around Saturn
Questions children ask
Why does Mimas look like the Death Star?
It is a coincidence. Mimas has an enormous crater called Herschel that sits on one side like a giant eye. Star Wars came out in 1977, but the first clear pictures of Mimas were not taken until Voyager 1 flew past in 1980.
Is there water on Mimas?
Yes, but almost all of it is frozen. Mimas is made nearly entirely of water ice. In 2024 scientists studying its orbit found signs of a liquid ocean hidden 20 to 30 km below the ice, though nobody has seen it directly.
How long is a day on Mimas?
Mimas goes round Saturn once every 22 hours and 36 minutes, and it spins once in exactly the same time. That means the same side always faces Saturn, and a day on Mimas is a little shorter than a day on Earth.
Who discovered Mimas and how did it get its name?
William Herschel found Mimas on 17 September 1789 using a huge telescope in England. His son John later suggested naming Saturn's moons after the Titans and Giants of Greek myth, and Mimas was a Giant who fought the gods.
Quick quiz
Three questions for space cadets. Nothing is recorded.
- What is the name of the giant crater on Mimas?
- What is Mimas mostly made of?
- How long does Mimas take to orbit Saturn?
Enceladus
The Moon
Saturn