Setebos (moon)

Setebos (Uranus XIX) is among the largest irregular moons orbiting Uranus. It was discovered on July 18, 1999, by John J. Kavelaars and his team, and was initially designated S/1999 U 1. The moon takes its name from the god Setebos, who is worshipped by Caliban and Sycorax in William Shakespeare's play The Tempest. (A crater on the Uranian moon Umbriel shares this name, though spelled Setibos.)

The moon travels around Uranus at an average distance of 16,256,000 km, completing one orbit in 1,978 days. Its orbit is retrograde, inclined about 152° to the ecliptic, and highly eccentric (0.445). These orbital parameters shift continuously due to gravitational influences from the Sun and other planets.

Setebos is estimated to be 47 kilometers in diameter, assuming an albedo of 4%. In visible light, it appears neutral grey, with color indices of B−V=0.77 and R−V=0.35—similar to the moon Prospero. Light curve measurements from the Kepler space telescope show that Setebos rotates once every 4 hours and 15.3 minutes, making it one of the fastest-spinning moons in the Solar System.

The moon likely did not form near Uranus but was captured later. Its orbital characteristics suggest it may belong to the same dynamic cluster as Sycorax and Prospero, hinting at a shared origin. However, Sycorax is much redder than the other two, which are grey.

No space probe has imaged Setebos up close. The proposed Uranus Orbiter and Probe (UOP) plans to observe all Uranian irregular moons, including Setebos, from a distance, measuring their rotation periods and shapes by monitoring brightness changes over time. A close flyby of Setebos may not be possible because its orbit lies far from the ecliptic plane.