Stephano (moon)

Stephano
Discovery picture by Stephano
Provisional or systematic name	S / 1999 U 2
Central body	Uranus
Properties of the orbit
Major semi-axis	7,952,320 km
Periapsis	6,804,300 km
Apoapsis	9,100,340 km
eccentricity	0.1443629
Orbit inclination	141.87372 (ecliptic) °
Orbital time	677.48 d
Mean orbital velocity	0.86 km / s
Physical Properties
Albedo	≈ 0.046-0.07
Apparent brightness	24.1 mag
Medium diameter	≈ 32 km
Dimensions	≈ 6.0 · 10 15 -2.2 · 10 16 kg
surface	≈ 3,000 km 2
Medium density	≈ 1.3-1.5 g / cm 3
Acceleration of gravity on the surface	≈ 0.0016-0.0041 m / s 2
Escape speed	≈ 7.1-13 m / s
Surface temperature	≈ −184 to −208 ° C; 89-65 K
discovery
Explorer	Brett J. Gladman , Matthew J. Holman , John J. Kavelaars , Jean-Marc Petit , Hans Scholl
Date of discovery	July 18, 1999
Remarks	Physical data relatively imprecise

Stephano (also Uranus XX ) is the seventh outermost of the 27 known and the third innermost of the outer retrograde irregular moons of the planet Uranus . It is one of the smaller natural satellites on the planet.

Discovery and naming

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

On August 21, 2000, the moon was officially named Stephano, like all irregular Uranus moons except Margaret after a character in William Shakespeare's comedy The Tempest . Stephano is a drunken servant, a shipwrecked man who is chosen by Caliban as the new Lord and God to kill the magician Prospero who is keeping Caliban prisoner on an island. Finally, Caliban realizes that the alcoholic Stephano Prospero is not equal and submits to this again.

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

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

Track properties

Classification in the irregular moons

Stephano orbits Uranus on a retrograde , for an irregular moon, relatively slightly elliptical orbit between 6,804,300 and 9,100,340 km from its center ( major orbit half-axis 7,952,320 km or 311,136 Uranus radii), i.e. around 7,926,800 km above its cloud ceiling. The orbital eccentricity is 0.1443629; the web is 141.87372 ° relative to the ecliptic inclined . Stephano is over 13 times as far from Uranus as the outermost regular moon Oberon .

Due to the great distance to Uranus and gravitational disturbances from the sun and other factors, the orbit parameters are possibly variable; the moon could perhaps also enter a heliocentric orbit (again). The eccentricity is therefore also given as 0.2292, the orbit inclination (compared to the ecliptic) between 141.81 ° and 144.10 ° and the major orbit half-axis between 8.002 and 8.004 million km. Stephano's orbit is surprisingly circular for an irregular moon.

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

The orbit of the next inner moon Caliban is on average about 800,000 km from Stephano's orbit, the distance of the orbit of the next outer moon Trinculo is on average about 500,000 km.

Stephano orbits Uranus in around 677 days, 11 hours and 31 minutes or around 1,855 earth years. The orbital period is also given as 676.5 and 677.37 days. Stephano therefore needs much longer for one orbit around Uranus than the earth around the sun.

Physical Properties

Stephano has an estimated 32 km in diameter, based on the assumed reflectivity of 4%, which can, however, also be 7%. In any case, the surface is extremely dark. Its density is estimated to be between 1.3 and 1.5 g / cm ³ . The moon is therefore likely to be composed mainly of water ice and silicate rock. The acceleration due to gravity on its surface is 0.0041 m / s ² , which corresponds to around 4% of that on earth. Stephano appears in the spectrum in gray color.

Emergence

Stephano is believed to be a trapped object of the Kuiper Belt and not originated in the accretion disk that formed the uranium system. It is conceivable that the moon first went from a Kuiper belt object to a centaur and was then captured by Uranus. The exact trapping mechanism is unknown, but trapping a moon requires dissipating energy. The hypotheses range from the entry of gas into the protoplanetary disk , interactions in the context of the multibody problem and capture by the rapidly increasing mass of Uranus. The orbital parameters indicate that Stephano belongs to the same dynamic group as Caliban and Francisco , and therefore these moons likely have a common origin.

exploration

Due to the great distance to Uranus and the weak brightness of 22.4 mag, which is 1: 27500000 compared to the central planet and is the second lowest in the entire Uranus system, Stephano was not found when the Voyager 2 space probe flew by in 1986. Since its discovery in 1999, Stephano could only be observed through earth-based telescopes and its orbital elements and its brightness could be determined.

Web links

• IAUC 7230: Probable new Satellites of Uranus July 27, 1999 (discovery)
• IAUC 7248: Probable new Satellites of Uranus September 4, 1999 (discovery)
• IAUC 7473: S / 1999 U 2 August 5, 2000 (rediscovery)
• IAUC 7479: Satellites of Uranus August 21, 2000 (designation)
• Polish moon page: Stephano description and further links (English)

Individual evidence

1. ^ IAU: Natural Satellites Ephemeris Service. IAU Minor Planet Center, accessed February 11, 2011 .