Caliban (moon)

Caliban is Uranus’s second-largest irregular moon. A team of astronomers—Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars—spotted it on September 6, 1997, through the 200-inch Hale Telescope, alongside another moon called Sycorax. Its orbit sits more than ten times farther from Uranus than Oberon, the outermost regular moon, and it moves in a retrograde direction with a moderate tilt and a slight oval shape. Based on its orbital traits, Caliban might share a common origin with Stephano and Francisco, possibly belonging to the same dynamic cluster.

The moon’s diameter is roughly 42 kilometers, according to a single thermal measurement by the Herschel Space Observatory, though an alternative estimate of 50 kilometers with a different albedo is also plausible. Its albedo of about 0.22 is unusually high for a Uranian irregular satellite, similar to Neptune’s largest irregular moon, Nereid.

Color readings place Caliban in the light-red range, redder than Jupiter’s moon Himalia but less red than most Kuiper belt objects, and it may be slightly redder than Sycorax. It absorbs light at 0.7 micrometers, which some astronomers think could indicate liquid water once modified its surface. Data from the Kepler space telescope suggest Caliban rotates once every 9.9 hours.

Caliban likely didn’t form in the accretion disk around Uranus; instead, it’s thought to be a captured object. Exactly how it was captured isn’t known, but possible mechanisms include gas drag in the protoplanetary disk, interactions among multiple bodies, or a “pull-down” capture during Uranus’s rapid mass growth.