Discovery image of Caliban
|Discovered by||Hale telescope|
|Discovery date||September 6, 1997|
Mean orbit radius
Average orbital speed
|36 km (estimate)|
|~16,000 km² (estimate)|
|Volume||~200,000 km³ (estimate)|
|Mass||~2.5×1017 kg (estimate)|
|~1.3 g/cm³ (assumed)|
|~0.02 m/s² (estimate)|
|~0.031 km/s (estimate)|
|Temperature||~65 K (estimate)|
Caliban (// KAL-i-ban or // KAL-i-bən) is the second-largest retrograde irregular satellite of Uranus. It 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 Sycorax and given the temporary designation S/1997 U 1.
Caliban follows a distant orbit, more than 10 times further from Uranus than the furthest regular moon Oberon. Its orbit is retrograde, moderately inclined and slightly eccentric. The orbital parameters suggest that it may belong to the same dynamic cluster as Stephano and Francisco, 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.
Somewhat inconsistent reports put Caliban in light-red category (B–V = 0.83 V–R = 0.52, B–V = 0.84 ± 0.03 V–R = 0.57 ± 0.03), redder than Himalia but still less red than most Kuiper belt objects. Caliban may be slightly redder than Sycorax. It also absorbs light at 0.7 μm, and one group of astronomers think this may be a result of liquid water that modified the surface.
Caliban is hypothesized to be a captured object: it did not form in the accretionary disk that existed around Uranus just after its formation. The exact capture mechanism is not known, but capturing a moon requires the dissipation of energy. The possible capture processes include: gas drag in the protoplanetary disk, many body interactions and the capture during the fast growth of the Uranus' mass (so-called "pull-down").