Sycorax (moon)

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Discovery image of Sycorax
Discovered by using the Hale telescope
Discovery dateSeptember 6, 1997
Orbital characteristics
Mean orbit radius
12,179,000 km[2]
1288.28 d
Inclination159° (to the ecliptic)[3]
Satellite ofUranus
Physical characteristics
Mean radius
~80,000 km² (estimate)
Volume~2,000,000 km³ (estimate)
Mass~2.5×1018 kg (estimate)
Mean density
~1.3 g/cm³ (assumed)
~0.025 m/s² (estimate)
~0.064 km/s (estimate)
3.6 h[5]
Temperature~65 K (estimate)
20.8 (V)[6]

Sycorax (/ˈsɪkəræks/ SIK-ər-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]

Retrograde irregular satellites of Uranus

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


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

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.[7]

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 165 km based on the thermal emission data from Spitzer and Herschel Space telescopes[4] 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,[8] B–V = 0.78 ± 0.02 V–R = 0.62 ± 0.01,[7] B–V = 0.839 ± 0.014 V–R = 0.531 ± 0.005[5]), 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[clarification needed] and finally becomes neutral at the longer wavelengths.[6]

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.[5]


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).[9][5]

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 d Lellouch, E.; Santos-Sanz, P.; Lacerda, P.; Mommert, M.; Duffard, R.; Ortiz, J. L.; Müller, T. G.; Fornasier, S.; Stansberry, J.; Kiss, Cs.; Vilenius, E.; Mueller, M.; Peixinho, N.; Moreno, R.; Groussin, O.; Delsanti, A.; Harris, A. W. (September 2013). ""TNOs are Cool": A survey of the trans-Neptunian region. IX. Thermal properties of Kuiper belt objects and Centaurs from combined Herschel and Spitzer observations" (PDF). Astronomy & Astrophysics. 557: A60. arXiv:1202.3657. Bibcode:2013A&A...557A..60L. doi:10.1051/0004-6361/201322047. Retrieved 7 November 2014.
  5. ^ 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.
  6. ^ 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.
  7. ^ a b Grav, Holman & Fraser 2004.
  8. ^ Rettig, Walsh & Consolmagno 2001.
  9. ^ Sheppard, Jewitt & Kleyna 2005.

External links[edit]