Stephano (moon)

Brett J. Gladman, Matthew J. Holman, John J. Kavelaars, Jean -Marc Petit, Hans Scholl

Stephano (also Uranus XX ) is the twenty-first of the 27 known and the third of the outer retrograde irregular moons of the planet Uranus. He is one of the smaller natural satellite of the planet.

Discovery and designation

Stephano was discovered on July 18, 1999 a team of astronomers Brett J. Gladman, Matthew J. Holman, John J. Kavelaars, Jean -Marc Petit and Hans Scholl on photographic images together with the Uranian moons Prospero and Setebos. The recordings were made by the 3.6-meter Canada -France - Hawaii Telescope on Mauna Kea in Hawaii (USA). The discovery was announced on 27 July 1999; the moon first received the provisional designation S/1999 U 2

On 21 August 2000 the moon will then receive the official name Stephano, like all irregular moons of Uranus except Margaret, after a figure in William Shakespeare's The Tempest. Stephano is a drunken servant, a castaway who is chosen by Caliban as masters and new god to kill the sorcerer Prospero, Caliban caught holding on an island. Finally, Caliban realizes that the alcoholic Stephano, Prospero is not equal and submits to this again.

So far, all the moons of Uranus are named after characters from Shakespeare or Alexander Pope. The first four moons discovered Uranus ( Oberon, Titania, Ariel, Umbriel ) were proposed by John Herschel, the son of Uranus discoverer William Herschel, named. Later, the tradition of naming was retained.

The provisional designation S/1999 U 2 corresponds to the classification of the International Astronomical Union ( IAU).

Web properties

Stephano Uranus rotates on a retrograde irregular moon for a relatively slightly elliptical orbit 6804300-9100340 km from its center ( Large semi-major axis 7.95232 million kilometers or 311 136 Uranus radii ), or about 7.9268 million kilometers above the cloud tops. The orbital eccentricity of 0.1443629, the orbit is inclined 141.87372 ° to the ecliptic. Stephano is about 13 times as far from Uranus as the outermost regular moon Oberon.

Due to the large distance to Uranus and gravitational disturbances caused by the sun and other factors, the orbital parameters are thus possibly variable; the moon could perhaps get ( back ) into a heliocentric orbit. The eccentricity is therefore also with 0.2292, the orbital inclination ( with respect to the ecliptic ) between 141.81 ° and 144.10 ° and the Great semi-major axis between 8,002 und 8.004 million kilometers indicated. The orbit of Stephano is surprisingly circular for an irregular moon.

Stephano is a member of Caliban group, a subgroup of the irregular moons with moderate eccentricity and high orbital inclinations of 140 to 170 °, which also includes Francisco, Caliban and Trinculo are.

The orbit of the next inner moon Caliban is the average distance about 800,000 km from Stephanos orbit, the distance of the path of the next outer moon Trinculo is an average of about 500,000 km.

Stephano orbits Uranus in around 677 days 11 hours and 31 minutes or about 1,855 Earth years. The orbital period is also specified with 676.5 and 677.37 days. Stephano needed for one orbit around Uranus so much longer than the earth around the sun.

Physical Properties

Stephano has a diameter estimated at 32 km, based on the assumed for him albedo of 4 %, which may be as well as 7%. The surface is thus in any case very dark. Its density is estimated to be between 1.3 and 1.5 g/cm3. So the moon is likely to be composed predominantly of water ice and silicate rock. On its surface, the gravitational acceleration is 0.0041 m/s2, representing about 4% of the earthly. Stephano appear in the spectrum in gray.

Formation

It is believed that Stephano is a captured Kuiper belt object and is not in the accretion disk that formed the Uranus system, created. It is conceivable that the moon of a Kuiper Belt object first became a centaur and was subsequently captured by Uranus. The exact trapping mechanism is not known, but the entrapment of a moon requires the dissipation of energy. The hypotheses range from withdrawal of gas from the protoplanetary disk, interactions within the framework of the multi- body problem and capture by the strongly growing mass of Uranus. The orbital parameters suggest that Stephano belongs to the same group dynamic as Caliban and Francisco and these moons therefore likely to have a common origin.

Research

Because of the great distance to Uranus and weak brightness of 22.4 like the opposite to the central planet is 1:27500000 and the second lowest of the entire system is Uranus, Stephano was 2 1986 not found during the flyby of the Voyager spacecraft. Since the discovery in 1999 Stephano could only be observed by ground-based telescopes, while its orbital elements and its brightness can be determined.

748053
de