In the 1977 blockbuster “Star Wars,” Obi-Wan Kenobi ominously observed, "That's no moon, it's a space station," as Han Solo piloted the Millennium Falcon toward a large spherical body with a distinct circular feature.
Three years after this movie was released, science would meet pop culture when the real-life Voyager space probe observed a large crater on a distant world, revealing an uncanny similarity between this body — which really is a moon — and the “Star Wars” orb known as the Death Star. Sometimes referred to as the “Death Star Moon,” this cratered entity is one of the 62 confirmed satellites of Saturn and officially goes by the name Mimas.
German-born astronomer William Herschel discovered Mimas on Sept. 17, 1789, using his colossal 48-inch-diameter, 40-foot-long telescope mounted in Slough, England. The name Mimas was first suggested by William’s son, John, in 1847. Like his father, John was also a noted astronomer. He named not only the seven largest moons of Saturn, but also four moons of Uranus.
Mimas measures 246 miles in diameter, making it Saturn’s seventh largest moon. This is only about one-eighth the diameter of our moon and roughly equals the distance between Flagstaff and Tucson. Mimas is the smallest known astronomical body to become rounded by the force of its own gravity (scientists call this “hydrostatic equilibrium”).
Like Pluto, Mimas is too small and far away to be seen in much detail from Earth. Close-up studies by spacecraft are needed to reveal surface features of these bodies. New Horizons accomplished this last year with Pluto. Pioneer 11 first observed Mimas close-up in 1979, but not until Voyager 1 flew by the following year did the large depression — named Herschel Crater in honor of William Herschel — reveal itself.
Herschel Crater was created by a large impact that not only gouged out this depression but generated shock waves that traveled through Mimas and produced surface fractures on its opposite side.
The crater measures 81 miles across, one-third the entire diameter of Mimas. A crater on Earth, at that scale, would measure the width of the United States!
Like many other Saturnian satellites, Mimas is composed mostly of water ice and covered with numerous smaller impact craters. It is the eighth closest of Saturn’s known moons, orbiting at an average distance of 115,280 miles, which is only about half the distance between Earth and our moon. Mimas orbits Saturn in less than a day and, like our moon with respect to Earth, rotates at the same rate, meaning the same side always faces its parent planet.
Despite its small size, Mimas has enough mass, and is located in the right place, to gravitationally clear debris from a portion of Saturn’s rings, creating the co-called Cassini Division. This 3,000-mile-wide gap is visible through ground-based telescopes and is named in honor of Italian-born astronomer Giovanni Cassini, who noted the gap in 1675.