Stephano (moon)
Irregular moon of Uranus, member of the Caliban group.
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Stephano (Uranus XX, formerly S/1999 U 2) is an irregular moon of Uranus. Discovered on 18 July 1999 by Brett J. Gladman and his team, it was named after the drunken butler from Shakespeare's The Tempest. It belongs to the Caliban group, the first recognized cluster of irregular Uranian moons with similar orbits.
The moon orbits Uranus at an average distance of roughly 8 million km (5.0 million mi), completing one lap every 677 Earth days (about 1.8 Earth years). Its orbit is retrograde, with an inclination near 144° and an eccentricity of 0.200. These orbital elements shift over time due to solar and planetary gravitational pulls. The Caliban group's orbits range from 7–8 million km (4.3–4.9 million mi) with inclinations between 141–144°.
Physical details are scarce. Assuming a low albedo of 0.04, typical of other Uranian irregulars, its brightness suggests a diameter of about 32 km (20 mi). In visible light, Stephano appears reddish, with color indices B–V = 0.97 ± 0.07 and V–R = 0.66 ± 0.05, nearly as red as many trans-Neptunian objects. It may be redder than Caliban and Sycorax, but more observations are needed to confirm.
Stephano likely did not form near Uranus but was captured later. As a Caliban group member, it is thought to be a fragment of a larger parent body shattered by a collision, forming a collisional family.
No space probe has imaged Stephano up close. The proposed Uranus Orbiter and Probe (UOP) plans to observe all Uranian irregular moons from a distance, measuring their rotation periods and shapes by monitoring brightness changes. However, a close flyby of Stephano is unlikely because its orbit lies far from the ecliptic plane.
Background
Stephano was discovered on 18 July 1999, together with Prospero and Setebos, by Brett J. Gladman et al. When the discovery 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.
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 and inclinations between 141–144°.
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 32 km. 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. Stephano probably did not form near Uranus but was captured later. It is a member of the Caliban group, whose members are thought to be fragments of an earlier parent body that was shattered in a collision, making them a collisional family.
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Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: Stephano (moon) (CC BY-SA 4.0).
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