Two new moons of Uranus, provisionally known as Uranus XVI (S/1997 U 1) and Uranus XVII (S/1997 U 2), have been discovered. The names Caliban and Sycorax have been proposed by the discoverers and will provisionally accepted by the IAU. Caliban (S/1997 U 1) orbits about 7.1 million km from Uranus and is about 60 km in diameter. Sycorax (S/1997 U 2) orbits about 12.2 million km from Uranus and is about 120 km in diameter. These size estimates are based on their apparent brightness and assuming an albedo of about 7%. The orbits are retrograde and highly inclined.

   Discovered by Brett Gladman, Phil Nicholson, Joseph Burns, and JJ Kavelaars using the 200 inch Hale telescope; the first images were taken 1997 Sept 6 and 7.

   Prior to this discovery, Uranus was the only one of the gas giants not known to have irregular moons, that is, ones whose orbits are not direct and approximately in the plane of the planet's equator.

   Like the other irregular moons (eg. Jupiter's outer 8 moons, Phoebe and Nereid), these are probably captured asteroids. It is highly unlikely that they could have been formed initially in these orbits.

   The composition of the two moons, according Nicholson, "is probably a plum-pudding mixture of rocks and ice." Both moons are unusually red in color which may suggest a historical link with the Kuiper Belt.

   These are the dimmest moons ever to have been imaged with a ground-based telescope.