Sycorax is the largest retrograde

Retrograde motion

Retrograde motion is motion in the direction opposite to the movement of something else, and is the contrary of direct or prograde motion. This motion can be the orbit of one body about another body or about some other point, or the rotation of a single body about its axis, or other phenomena such...

 irregular

Irregular satellite

In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

 satellite

Natural satellite

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

 of Uranus

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

. Sycorax was discovered on 6 September 1997 by Brett J. Gladman

Brett J. Gladman

Brett J. Gladman is a Canadian astronomer and a full professor at the University of British Columbia's Department of Physics and Astronomy in Vancouver, British Columbia. He holds the Canada Research Chair in Planetary Astronomy.-Career:...

, Philip D. Nicholson, Joseph A. Burns

Joseph A. Burns

Joseph Burns is a professor at Cornell University with a dual appointment in the Department of Theoretical and Applied Mechanics and the Astronomy department. His primary area of research is dynamics in planetary sciences.- Professional biography :...

, and John J. Kavelaars

John J. Kavelaars

John J. Kavelaars, better known as JJ Kavelaars, is a Canadian astronomer who was part of a team that discovered several moons of Jupiter, Saturn, Uranus, and Neptune....

 using the 200-inch Hale telescope

Hale telescope

The Hale Telescope is a , 3.3 reflecting telescope at the Palomar Observatory in California, named after astronomer George Ellery Hale. With funding from the Rockefeller Foundation, he orchestrated the planning, design, and construction of the observatory, but did not live to see its commissioning...

, together with Caliban

Caliban (moon)

Caliban is the second largest retrograde irregular moon of Uranus. It was discovered on 6 September 1997 by Brett J. Gladman, Philip D. Nicholson, Joseph A. Burns, and John J...

, and given the temporary designation S/1997 U 2.

Officially confirmed as Uranus XVII, it was named after Sycorax

Sycorax (Shakespeare)

Sycorax , an unseen character in William Shakespeare's play The Tempest , is a powerful witch and the mother of Caliban, one of the few native inhabitants of the island on which Prospero, the hero of the play, is stranded....

, Caliban's

Caliban

Caliban is a character in William Shakespeare's play The Tempest.Caliban may also refer to:* Caliban , a moon of Uranus* Caliban , a metalcore band from Germany* Caliban , an acoustic Celtic folk duo...

 mother in William Shakespeare

William Shakespeare

William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

's play The Tempest.

Orbit

Sycorax follows a distant orbit, more than 20 times further from Uranus than the furthest regular moon Oberon

Oberon (moon)

Oberon , also designated ', is the outermost major moon of the planet Uranus. It is the second largest and second most massive of the Uranian moons, and the ninth most massive moon in the Solar System. Discovered by William Herschel in 1787, Oberon is named after the mythical king of the fairies...

. Its orbit is retrograde, moderately inclined

Inclination

Inclination in general is the angle between a reference plane and another plane or axis of direction.-Orbits:The inclination is one of the six orbital parameters describing the shape and orientation of a celestial orbit...

 and eccentric. The orbital parameters suggest that it may belong, together with Setebos

Setebos (moon)

Setebos is one of the outermost retrograde irregular satellites of Uranus. It was discovered on 18 July 1999 by John J. Kavelaars et al. and provisionally designated S/1999 U 1....

 and Prospero

Prospero (moon)

Prospero is a relatively small retrograde irregular satellite of Uranus discovered on 18 July 1999 by the astrophysicist Matthew Holman and his team, and given the provisional designation S/1999 U 3...

, to the same dynamic cluster, suggesting common origin.

The diagram illustrates the orbital parameters of the retrograde irregular satellite

Irregular satellite

In astronomy, an irregular moon is a natural satellite following a distant, inclined, and often eccentric and retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form in situ....

s of Uranus (in polar co-ordinates) with the eccentricity of the orbits represented by the segments extending from the pericentre

Apsis

An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

 to the apocentre

Apsis

An apsis , plural apsides , is the point of greatest or least distance of a body from one of the foci of its elliptical orbit. In modern celestial mechanics this focus is also the center of attraction, which is usually the center of mass of the system...

.

Physical characteristics

The diameter of Sycorax is estimated at 150 km (assuming albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 of 0.04) making it the largest irregular satellite of Uranus, comparable in size with Puck

Puck (moon)

Puck is an inner moon of Uranus. It was discovered in December 1985 by the Voyager 2 spacecraft. The name Puck comes from Celtic mythology and English folklore. The orbit of Puck lies between the rings of Uranus and the first of Uranus' large moons, Miranda. Puck is approximately spherical in...

 and with Himalia

Himalia (moon)

Himalia is the largest irregular satellite of Jupiter, the sixth largest overall in size, and the fifth largest in mass. It was discovered by Charles Dillon Perrine at the Lick Observatory on 3 December 1904 and is named after the nymph Himalia, who bore three sons of Zeus .- Discovery...

, the biggest irregular satellite of Jupiter.

The satellite appears light-red in the visible light (colour indices

Color index

In astronomy, the color index is a simple numerical expression that determines the color of an object, which in the case of a star gives its temperature...

  , , ) redder than Himalia but still less red than most Kuiper belt

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 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.

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.

Origin

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

Dissipation

In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...

 of energy. The possible capture processes include: gas drag in the protoplanetary disk

Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star, or Herbig Ae/Be star...

, many body interactions and the capture during the fast growth of the Uranus' mass (so called pull-down).

External links

• Sycorax Profile (by NASA's Solar System Exploration)
• David Jewitt pages
• Uranus' Known Satellites (by Scott S. Sheppard
  Scott S. Sheppard
  Scott S. Sheppard is an astronomer in the Department of Terrestrial Magnetism at the Carnegie Institution for Science. Starting as a graduate student at the Institute for Astronomy at the University of Hawaii, he was credited with the discovery of many small moons of Jupiter, Saturn, Uranus, and...
  
  )
• MPC: Natural Satellites Ephemeris Service