Sycorax (moon)

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For other uses, see Sycorax (disambiguation).
Discovery image of Sycorax
Discovered by using the Hale telescope
Discovery date September 6, 1997
Orbital characteristics
Mean orbit radius
12,179,000 km[2]
Eccentricity 0.5224
1288.28 d
Inclination 159° (to the ecliptic)[3]
Satellite of Uranus
Physical characteristics
Mean radius
75 km (estimate)[4][5]
~70,000 km² (estimate)
Volume ~1,800,000 km³ (estimate)
Mass ~2.3×1018 kg (estimate)
Mean density
~1.3 g/cm³ (assumed)
3.6 h[6]
Albedo 0.04 (assumed)[4]
Temperature ~65 K (estimate)
20.8 (V)[7]

Sycorax (/ˈsɪkəræks/ SIK-or-aks) is the largest retrograde irregular satellite of Uranus. 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, and given the temporary designation S/1997 U 2.[1]

Retrogade irregular satellites of Uranus

Officially confirmed as Uranus XVII, it was named after Sycorax, Caliban's mother in William Shakespeare's play The Tempest.


Sycorax follows a distant orbit, more than 20 times further from Uranus than the furthest regular moon, Oberon.[1] 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.[8]

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[edit]

The diameter of Sycorax is estimated at 150 km (assuming an albedo of 0.04),[4][5] 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,[9] B–V = 0.78 ± 0.02 V–R = 0.62 ± 0.01,[8] B–V = 0.839 ± 0.014 V–R = 0.531 ± 0.005[6]), 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.[7]

The rotation period of Sycorax is estimated at about 3.6 hours. Rotation causes periodical variations of the visible magnitude with the amplitude of 0.07.[6]


Sycorax is hypothesized to be a captured object: it did not form in the accretionary disk, which 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's mass (so called pull-down).[10][6]

See also[edit]


  1. ^ a b c Gladman Nicholson et al. 1998.
  2. ^ Jacobson, R.A. (2003) URA067 (2007-06-28). "Planetary Satellite Mean Orbital Parameters". JPL/NASA. Retrieved 2008-01-23. 
  3. ^ Sheppard, Jewitt & Kleyna 2005, p. 523, Table 3.
  4. ^ a b c Sheppard, Jewitt & Kleyna 2005, p. 523, Table 3 ... ri (km) ... 75 ... i Radius of satellite assuming a geometric albedo of 0.04.
  5. ^ a b "Planetary Satellite Physical Parameters". JPL (Solar System Dynamics). 2008-12-20. Retrieved 2009-03-10. 
  6. ^ a b c d Maris, Michele; Carraro, Giovanni; Parisi, M.G. (2007). "Light curves and colours of the faint Uranian irregular satellites Sycorax, Prospero, Stephano, Setebos, and Trinculo". Astronomy&Astrophysics 472 (1): 311–319. arXiv:0704.2187. Bibcode:2007A&A...472..311M. doi:10.1051/0004-6361:20066927. 
  7. ^ a b Romon, J.; de Bergh, C. et al. (2001). "Photometric and spectroscopic observations of Sycorax, satellite of Uranus". Astronomy&Astrophysics 376 (1): 310–315. Bibcode:2001A&A...376..310R. doi:10.1051/0004-6361:20010934. 
  8. ^ a b Grav, Holman & Fraser 2004.
  9. ^ Rettig, Walsh & Consolmagno 2001.
  10. ^ Sheppard, Jewitt & Kleyna 2005.

External links[edit]