Stephano, also known as Uranus XX and previously as S/1999 U 2, is an irregular moon of Uranus. It was discovered on 18 July 1999 by Brett J. Gladman et al. and it was named after the drunken butler in William Shakespeare's play The Tempest. Stephano orbits Uranus at an average distance of about and takes about 677 Earth days (1.8 Earth years) to complete one orbit. It is a member of the Caliban group, the first identified group of irregular Uranian moons that share similar orbital characteristics. Stephano is estimated to have a diameter of , though most of its physical properties are unknown.

Discovery and naming

Stephano was discovered on 18 July 1999, together with Prospero and Setebos, by Brett J. Gladman et al.

When the discovery of Stephano was announced, it was given the temporary provisional designation S/1999 U 2 by the Minor Planet Center. It was later named and given the Roman numeral designation Uranus XX by the International Astronomical Union's Working Group for Planetary System Nomenclature on 21 August 2000.

The moon was named after the drunken butler in William Shakespeare's play The Tempest.

Orbit

Stephano orbits Uranus at a mean distance of approximately 8 million km and completes one revolution in about 677 days. Its orbit is retrograde, moderately inclined with an inclination of approximately 144° and eccentricity of 0.200. Its orbital elements are subject to continuous variations caused by solar and planetary perturbations.

It is a member of the Caliban group, which orbits Uranus at orbital distances between 7–8 million km (4,3–4.9 million mi) and inclinations between 141–144°.

Physical characteristics

Very little is known about its physical properties. Assuming Stephano has a low albedo of 0.04 like other Uranian irregular moons, its brightness suggests a diameter of about .

Stephano appears reddish in visible light (colour indices B–V = 0.97 ± 0.07, V–R = 0.66 ± 0.05 and is nearly as red as many trans-Neptunian objects. It may even be redder than Caliban and Sycorax; however, additional observations are required to confirm this.

Origin

Stephano probably did not form near Uranus but was captured by Uranus later. It is a member of the Caliban group, whose members—including Stephano—are thought to be fragments of an earlier parent body that was shattered in a collision, making them a collisional family.

Exploration Stephano has not been imaged up close by a space probe. All Uranian irregular moons including Stephano are planned to be distant observation targets for the proposed Uranus Orbiter and Probe (UOP), which will measure the Uranian irregular moons' rotation periods and shapes by watching their brightness change over time. The UOP may not be able to do a close flyby of Stephano because the moon does not orbit near the ecliptic plane.

See also

Moons of Uranus Other Uranian irregular moons discovered in 1999: Prospero Setebos Irregular satellites

References

Gladman, B. J.; Kavelaars, J. J.; Holman, M. J., Petit, J.-M.; Scholl, H.; Nicholson, P. D.; and Burns, J. A.; The Discovery of Uranus XIX, XX, and XXI, Icarus, 147 (2000), pp. 320–324

Marsden, Brian G.; Probable New Satellites of Uranus, IAUC 7230, 1999 July 27

Marsden, Brian G.; Probable New Satellites of Uranus, IAUC 7248, 1999 September 4

Marsden, Brian G.; S/1999 U 1, S/1999 U 2 and S/1999 U 3, IAUC 7385, 2000 March 24

Marsden, Brian G.; S/1999 U 2, IAUC 7473, 2000 August 5

Marsden, Brian G.; Satellites of Uranus, IAUC 7479, 2000 August 21

.

External links Stephano Profile by NASA's Solar System Exploration David Jewitt pages Uranus' Known Satellites (by Scott S. Sheppard)