Mimas is the innermost of Saturn’s major moons and the smallest known body rounded into a sphere by its own gravity. One enormous crater, Herschel, dominates an entire hemisphere - the resemblance to the Death Star was noticed the moment Voyager’s pictures came back.
Herschel crater is 139 km wide - about a third of the moon’s entire diameter.
Its gravity helps carve the Cassini Division, the great 4,700-km gap in Saturn’s rings.
Wobbles measured by Cassini hint at a geologically young liquid ocean 20-30 km beneath the ice.
The impact it barely survived
Herschel crater is 139 km across on a moon only 396 km wide. The collision that dug it came close to the energy needed to shatter Mimas entirely - fractures on the exact opposite side of the moon may be shock waves from that impact, focused through the interior.
Physical characteristics
Diameter
396 km
Mass
3.750e+19 kg
Surface gravity
0.06 m/s²
Density
1.15 g/cm³
Tidally locked
Yes
Mean temperature
-200 °C
Geometric albedo
0.96
Atmosphere and composition
Atmosphere
None.
Composition
Mostly water ice with a small amount of rock.
Discovery
Discovered by
William Herschel
Year
1789
How
His 40-foot reflector, then the largest telescope in the world
Missions
Voyager 1NASA · Flyby probe · 1977 · active
The most distant human-made object, now roughly 25 billion km from the Sun. After historic flybys of Jupiter, Saturn and Titan, Voyager 1 became the first spacecraft to enter interstellar space in 2012.
Cassini orbited Saturn for 13 years, delivering ESA’s Huygens probe to the surface of Titan in January 2005 - the first landing in the outer Solar System. It ended with a deliberate plunge into Saturn in 2017 to protect the potentially habitable moons Enceladus and Titan.