Uranus' satellitesPrior to Voyager 2's flyby in 1986, five moderate-sized satellites were known to exist. The two largest, Titania and Oberon, are nearly the same size with diameters about 1550 km. Ariel and Umbriel are also nearly the same size with diameters about 1160 km, whereas Miranda is only 472 km across. Voyager 2 discovered 10 very small satellites in 1986, most having diameters of less than 100 km. In 1997 two further moons were discovered by astronomers at Mount Palomar. These, given the provisional names Caliban (S/1997 U1) and Sycorax (S/1997 U2), were the first irregular satellites of Uranus to be identified. Subsequently, in 1999, a further satellite was identified by the Lunar and Planetary Lab at the University of Arizona from data collected by Voyager 2. This is a regular satellite, so far unnamed (S/1986 U10). Also in 1999, three more irregular satellites were discovered by astronomers using the Canada-France-Hawaii Telescope at Maunder Kea – Setebos (S/1999 U1), Stephano (S/1999 U2), and Prospero (S/1999 U3). Trinculo (S/2001 U1) was discovered in August 2001 by M. Holman and others, along with two other satellites as yet unnamed. With a further three satellites discovered in 2003, this gives Uranus a total of 27 satellites in all.
The average density of the Uranian satellites is about 1.5 g cm–3, which is consistent with all 27 being composed of a mixture of rock and ice. Voyager data suggests the larger satellites are 50% water ice, 20% carbon- and nitrogen-based materials, and 30% rock. In addition, Uranus' satellites all have low albedos. Umbriel is one of the darkest moons in the Solar System with an albedo of 0.18. The most recent impact craters on Oberon and Titania appear to have exposed fresh undarkened ice, which implies the darkness is caused by a coating of dark sootlike material. See also Table 2, backmatter.