Prospero was discovered July 18, 1999 by Matthew J. Holman, John J. Kavelaars, Brett J. Gladman, Jean-Marc Petit, and Hans Scholl using the Canada-France-Hawaii Telescope at the Mauna Kea Observatory on the island of Hawaii. They discovered Setebos and Stephano at the same time.
At about 31 miles (50 kilometers) in diameter, Prospero is a small, dark, irregular moon that orbits Uranus in the opposite direction from the regular satellites and the rotation of the planet, itself (known as a retrograde orbit). Its orbital characteristics are similar to those of Sycorax and Setebos, suggesting a common origin. But its gray color differs from the light red color of Sycorax, implying a different origin.
Originally called S/1999 U3, Prospero was named for a sorcerer in William Shakespeare's play, "The Tempest." Prospero, rightfully Duke of Milan, has his dukedom usurped by his brother Antonio with the help of Alonso, King of Naples. He is forced to flee with his daughter Miranda to an island in the Mediterranean, where he perfects his magical powers, hoping to use them to regain his dukedom.