Caliban (moon)

Caliban /ˈkælɪbæn/ is the second-largest irregular satellite of Uranus.^[10] 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.^[1]

Caliban

Discovery image of Caliban taken by the Hale Telescope in September 1997
Discovery[1]
Discovered by	Philip D. Nicholson Brett J. Gladman Joseph A. Burns John J. Kavelaars
Discovery site	Hale Telescope at Palomar Obs.
Discovery date	6 September 1997
Designations
Designation	Uranus XVI
Pronunciation	/ˈkæləbæn/[2][3]
Named after	Caliban
Alternative names	S/1997 U 2
Adjectives	Calibanian /kæləˈbeɪniən/[4]
Orbital characteristics[5]
Epoch 27 June 2015 (JD 2457200.5)
Observation arc	17.96 yr (6,559 d)
Semi-major axis	7,163,810 km (0.0478871 AU)
Eccentricity	0.0771431
Orbital period (sidereal)	1.59 yr (579.26 d)
Mean anomaly	294.66253°
Mean motion	0° 37m 17.345s / day
Inclination	139.90814° (to the ecliptic) 140.878° (to local Laplace plane)[6]
Longitude of ascending node	175.21248°
Argument of perihelion	342.53671°
Satellite of	Uranus
Group	Caliban group
Physical characteristics
Mean diameter	42+20 −12 km[7]
Mass	~2.5×1017 kg (estimate)[8]
Mean density	~1.3 g/cm3 (assumed)[8]
Synodic rotation period	9.948±0.019 hr (double-peaked)[7] 2.66±0.04 hr (single-peaked)[9]
Albedo	0.22+0.20 −0.12[7]
Temperature	~65 K (mean estimate)
Apparent magnitude	22.0 (V)[7]
Absolute magnitude (H)	9.160±0.016[7] 9.0[5]

Designated Uranus XVI, it was named after the monster character in William Shakespeare's play The Tempest.

Orbit

Animation of Caliban's orbit around Uranus.
   Uranus  ·    Sycorax ·    Francisco  ·    Caliban  ·    Stephano  ·    Trinculo

Caliban follows a distant orbit, more than 10 times further from Uranus than Oberon, the outermost regular moon.^[1] 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 a common origin.^[11]

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.

Physical characteristics

Caliban's diameter is estimated to be around 42 km, based on thermal measurements by the Herschel Space Observatory.^[7] 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.^[7]

Somewhat inconsistent reports put Caliban in the light-red category (B–V = 0.83 V–R = 0.52,^[12] B–V = 0.84 ± 0.03 V–R = 0.57 ± 0.03^[11]), redder than Himalia but still less red than most Kuiper belt objects. Caliban may be slightly redder than Sycorax.^[9] 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.^[13]

Measurements of Caliban's light curve by the Kepler space telescope indicate that its rotation period is about 9.9 hours.^[7]

Origin

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 Uranus's mass (so-called "pull-down").^[10][11]

See also

• Moons of Uranus