12. Portia Moon Data Mass (kg) - Radius (km) 55 Density (gm/cm3) - Distance from Uranus (km) 66,100 Period of rotation (days) - Period of orbit (days) 0.513196 Orbital velocity (km/sec) 9.37 Eccentricity of orbit 0.000 Inclination of orbit (°) 0.09 Visual albedo 0.07 Visual magnitude 21.0 Discovered: 1986 Voyager Science Team
Caliban (circled) was discovered with a ground-based telescope in 1997. Image Credit: Palomar Observatory
Discovery Caliban was discovered on Sept. 6, 1997 at Palomar Observatory by B. Gladman, P. Nicholson, J. A. Burns, and J. Kavelaars. This group of astronomers also discovered another Uranian moon at the same time, which was named Sycorax after the mother of Caliban in William Shakespeare's play The Tempest.
Overview Caliban orbits Uranus in the opposite direction from the regular moons and the rotation of the planet itself (called a retrograde orbit). Its orbit is also somewhat inclined and eccentric, and very far from the planet―more than 10 times farther than Oberon, the farthest regular moon. These characteristics suggest that Caliban was an independent body that was captured by Uranus' gravity. It is thought to be about 72 km in diameter, and to be the second largest irregular satellite of Uranus (about half the size of Sycorax). The size estimate is based on the brightness of the moon and an assumed albedo of 0.04, typical of captured asteroids in the outer solar system.
How Caliban Got its Name Originally called S/1997 U1, Caliban was named for a character in Shakespeare's play, "The Tempest." Caliban was forced into labor on an island owned by Prospero. He was human, but "not honour'd with a human shape," and is referred to as a mooncalf or freckled monster.
Animation of Sycorax's orbit around Uranus. ▇ Uranus · ▇ Sycorax · ▇ Francisco · ▇ Caliban · ▇ Stephano · ▇ Trinculo
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.
Retrograde irregular satellites of Uranus 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.
Caliban's diameter is estimated to be around 42 km, based on thermal measurements by the Herschel Space Observatory. Its albedo is estimated at around 0.22, which is unusually high compared to those of other Uranian irregular satellites. Neptune's largest irregular satellite, Nereid, has a similarly high albedo as Caliban.
Somewhat inconsistent reports put Caliban in light-red category (B–V = 0.83 V–R = 0.52,[12] 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.
Measurements of Caliban's light curve by the Kepler space telescope indicate that its rotation period is about 9.9 hours.
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").
