Saturn's Pandora Flirts with 'Death Star' Moon
Saturn's moons Mimas and Pandora as seen by the NASA Cassini Solstice mission. Click to enlarge.NASA/JPL-Caltech/Space Science Institute
Saturn's moon Mimas looms ominously in this photo from NASA's Cassini mission with the smaller moon Pandora passing behind. Nicknamed the "Death Star moon," Mimas sports a huge 86 mile (139 kilometer) wide circular crater called Herschel resembling the Star Wars sci-fi superweapon. Fortunately for Pandora, this Death Star isn't fully operational.
Cassini imaged this stunning view on May 14 through the spacecraft's narrow angle camera in blue light when the probe was 690,000 miles (1.1 million kilometers) from Mimas. Pandora was 731,000 miles (1.2 million kilometers) from Cassini at the time. Mimas and Pandora were therefore separated by approximately 41,000 miles (66,000 kilometers).
Mimas and Pandora are radically different shapes and provide a wonderful example as to how gravity affects planetary bodies of different masses.
Mimas is 246 miles (or 396 kilometers) wide and is massive enough for hydrostatic equilibrium to dominate its shape. Hydrostatic equilibrium causes planetary bodies to form into a sphere. For smaller Pandora, however, its mass isn't large enough to create a gravitational field strong enough to achieve hydrostatic equilibrium -- its internal structure resists becoming a sphere, so it remains elongated like a potato, measuring 50 miles (or 81 kilometers across) at its longest axis.