On Sept. 17, 1789, the British astronomer William Herschel discovered Saturn's "Death Star" moon, Mimas. [‘On This Day in Space’ Video Series on Space.com]( br Of course, "Star Wars" wasn't a thing at the time, and no one had ever heard of something called a "Death Star." But there's no denying that this moon looks just like it. Anyway, Herschel was a guy who liked to build telescopes and discover things, like the planet Uranus, tiny moons around Jupiter and Saturn, and other stuff out there in space. Shortly after he invented a huge new kind of reflecting telescope called the Herschelian telescope, he spotted Mimas orbiting Saturn. Mimas is super tiny with a diameter of less than 250 miles. It is the smallest known spherical body in space that is held together by self-gravitation.