Sycorax is the largest irregular satellite of Uranus. It was discovered in September, 1997 on the Hale Telescope in California. Sycorax's orbit is retrograde, irregular, and much more distant than that of Oberon, the furthest of Uranus' regular moons. With a diameter of over 150 km, it is the largest irregular moon of Uranus. It has been theorized that Sycorax is a captured object, as opposed to one formed with Uranus.

Discovery

Sycorax was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J. Kavelaars using the 200-inch Hale Telescope, together with Caliban. At the time, it was given the temporary designation S/1997 U 2. Officially confirmed as Uranus XVII, it was named after Sycorax, Caliban's mother in William Shakespeare's play The Tempest. This follows the trend that all Uranian moons are named after Shakespearean characters or those from Alexander Pope's The Rape of the Lock.

Orbit

Sycorax follows a distant orbit, more than 20 times further from Uranus than the furthest regular moon, Oberon. Its orbit is retrograde, moderately inclined and eccentric. The orbital parameters suggest that it may belong, together with Setebos and Prospero, to the same dynamic cluster, suggesting common origin. The diagram illustrates the orbital parameters of the retrograde irregular satellites of Uranus (in polar co-ordinates) with the eccentricity of the orbits represented by the segments extending from the pericentre to the apocentre.

Physical characteristics

The diameter of Sycorax is estimated at 165 km, based on the thermal emission data from Spitzer and Herschel Space telescopes making it the largest irregular satellite of Uranus, comparable in size with Puck and with Himalia, the biggest irregular satellite of Jupiter. The satellite appears light-red in the visible spectrum (colour indices B–V = 0.87 V–R = 0.44, B–V = 0.78 ± 0.02 V–R = 0.62 ± 0.01, B–V = 0.839 ± 0.014 V–R = 0.531 ± 0.005 ), redder than Himalia but still less red than most Kuiper belt objects. However, in the near infrared, the spectrum turns blue between 0.8 and 1.25 μm and finally becomes neutral at the longer wavelengths. The rotation period of Sycorax is estimated at 6.9 hours. Rotation causes periodical variations of the visible magnitude with the amplitude of 0.12. The rotation axis of Sycorax is unknown, though measurements of its light curve suggest it is being viewed at a near equator-on configuration. In this case, Sycorax may have a north pole right ascension around 356° and a north pole declination around 45°.

Origin

It is hypothesized that Sycorax is a captured object; it did not form in the accretion disk which existed around Uranus just after its formation. No exact capture mechanism is known, but capturing a moon requires the dissipation of energy. Possible capture processes include gas drag in the protoplanetary disk and many-body interactions and capture during the fast growth of Uranus's mass (so-called pull-down).