Sycorax (moon)


Sycorax /ˈsɪkɒræks/ 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]

Animation of discovery images taken by the Hale Telescope in September 1997
Discovered by
Discovery siteHale Telescope at Palomar Obs.
Discovery date6 September 1997
Uranus XVII
Named after
S/1997 U 2
AdjectivesSycoraxian /sɪkɒˈræksiən/[4]
Orbital characteristics[6]
Epoch 31 July 2016 (JD 2457600.5)
Observation arc32.37 yr (11,815 d)
Earliest precovery date2 June 1984
12,193,230 km (0.0815067 AU)
3.52 yr (1,286.28 d)
0° 16m 47.56s / day
Inclination153.22796° (to the ecliptic)
159.403° (to local Laplace plane)[5]
Satellite ofUranus
Physical characteristics
Mean diameter
Mass~2.5×1018 kg (estimate)[5]
Mean density
~1.3 g/cm3 (assumed)[5]
6.9162±0.0013 hr (double-peaked)[7]
3.6 hr (single-peaked)[9]
Temperature~65 K (mean estimate)
20.8 (V)[10]
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  ·    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.[11]

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 characteristicsEdit

Full discovery image of Sycorax, located at the top-right of the image

The diameter of Sycorax is estimated at 165 km based on the thermal emission data from Spitzer and Herschel Space telescopes[8] 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,[12] B–V = 0.78 ± 0.02 V–R = 0.62 ± 0.01,[11] B–V = 0.839 ± 0.014 V–R = 0.531 ± 0.005[9]), 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.[10]

The rotation period of Sycorax is estimated at about 6.9 hours.[7] Rotation causes periodical variations of the visible magnitude with the amplitude of 0.12.[7] The rotation axis of Sycorax is unknown, though measurements of its light curve suggest it is being viewed at a near equator-on configuration. In this case, Sycorax may have a north pole right ascension around 356° and a north pole declination around 45°.[7]


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).[13][9]

See alsoEdit


  1. ^ a b c Gladman Nicholson et al. 1998.
  2. ^ Shakespeare Recording Society (1995) The Tempest (audio CD)
  3. ^ Benjamin Smith (1903) The Century Dictionary and Cyclopedia
  4. ^ Goldberg (2004) Tempest in the Caribbean
  5. ^ a b c Jacobson, R.A. (2003) URA067 (2007-06-28). "Planetary Satellite Mean Orbital Parameters". JPL/NASA. Retrieved 2008-01-23.
  6. ^ "M.P.C. 102109" (PDF). Minor Planet Circular. Minor Planet Center. 14 November 2016.
  7. ^ a b c d e f g Farkas-Takács, A.; Kiss, Cs.; Pál, A.; Molnár, L.; Szabó, Gy. M.; Hanyecz, O.; et al. (September 2017). "Properties of the Irregular Satellite System around Uranus Inferred from K2, Herschel, and Spitzer Observations". The Astronomical Journal. 154 (3): 13. arXiv:1706.06837. Bibcode:2017AJ....154..119F. doi:10.3847/1538-3881/aa8365. S2CID 118869078. 119.
  8. ^ 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.
  9. ^ a b c 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. S2CID 12362256.
  10. ^ 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.
  11. ^ a b Grav, Holman & Fraser 2004.
  12. ^ Rettig, Walsh & Consolmagno 2001.
  13. ^ Sheppard, Jewitt & Kleyna 2005.

External linksEdit

  • Sycorax Profile (by NASA's Solar System Exploration)
  • David Jewitt pages
  • Uranus' Known Satellites (by Scott S. Sheppard)
  • MPC: Natural Satellites Ephemeris Service