Francisco (moon)

Francisco
Francisco on the discovery picture
Provisional or systematic name	S / 2001 U 3
Central body	Uranus
Properties of the orbit
Major semi-axis	4,275,910 km
Periapsis	3,690,480 km
Apoapsis	4,861,340 km
eccentricity	0.1369138
Orbit inclination	147.45993 (ecliptic) °
Orbital time	267.12 d
Mean orbital velocity	1.17 km / s
Physical Properties
Albedo	≈ 0.04-0.07
Apparent brightness	25.0 mag
Medium diameter	≈ 22 km
Dimensions	≈ 7.2 · 10 15 kg
surface	≈ 1,500 km 2
Medium density	≈ 1.3 - 1.5 g / cm 3
Acceleration of gravity on the surface	≈ 0.0025 m / s 2
Escape speed	≈ 9.4 m / s
Surface temperature	≈ −184 to −208 ° C ≈ 89–65 K
discovery
Explorer	John J. Kavelaars , Matthew J. Holman , Dan Milisavljevic , Tommy Grav , Brett J. Gladman
Date of discovery	August 13, 2001
Remarks	Physical data relatively imprecise

Francisco (also Uranus XXII ) is the nineteenth innermost of the 27 known and 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

Francisco was discovered on August 13, 2001 by a team consisting of the astronomers Matthew J. Holman , John J. Kavelaars , Dan Milisavljevic and Tommy Grav on photographic images around the same time as the outermost known Uranus moon Ferdinand . The recordings were made through the 4.0 meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile . Soon after the recordings, which also showed Ferdinand, Trinculo and actually Margaret , the astronomers unfortunately lost track of the moon again, although three more observations were made. As the orbit could therefore not be secured, the International Astronomical Union decided not to publish the discovery. It was Brett J. Gladman who found Francisco in September 2003 on recordings from 2002 with the 8.2-meter Very Large Telescope in Chile. The discovery was announced on October 7, 2003; the moon was initially given the provisional designation S / 2001 U 3 .

On December 29, 2005 the moon was officially named Francisco, like all irregular Uranus moons except Margaret after a figure in William Shakespeare's The Storm . Francisco is a nobleman who was shipwrecked with King Alonso (Ferdinand's father) and others. He tried to comfort the king because of the presumed loss of his son Ferdinand.

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 / 2001 U 3 corresponds to the system of the International Astronomical Union (IAU).

Track properties

Classification in the irregular moons

Francisco orbits Uranus on a retrograde , relatively slightly elliptical orbit for an irregular moon between 3,690,480 and 4,861,340 km from its center ( major orbit half-axis 4,275,910 km or 167,296 Uranus radii), i.e. around 4,250,000 km above its cloud ceiling. The orbital eccentricity is 0.1369138, the orbit is 147.45993 ° inclined to the ecliptic . Francisco is over 7 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 between 0.1324 and 0.1459, the orbit inclination (with respect to the ecliptic) between 145.22 ° and 147.25 ° and the major orbit half-axis between 4.2760 and 4.2829 million km. The orbit of Francisco is surprisingly circular for an irregular moon, the eccentricity is the lowest of all irregular Uranus moons , with the possible exception of Caliban , whose information on eccentricity varies greatly.

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

The orbit of the next inner moon Oberon is on average 3.7 million km away from Francisco's orbit, the distance of the orbit of the next outer moon Caliban is about 4 million km on average.

Francisco orbits Uranus in around 267 days, 2 hours and 53 minutes, or around 0.731 Earth years. The orbital period is also given as 266.56 and 267.09 days. It takes Francisco longer to make one orbit around Uranus than the planet Venus around the sun.

Physical Properties

Francisco has an estimated diameter of 22 km, 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. On its surface, the acceleration due to gravity is 0.0025 m / s ² , which corresponds to about 2 ‰ of that on earth. Francisco appears in the spectrum in gray color.

Emergence

Francisco is believed to be a trapped object of the Kuiper Belt and not arose in the accretion disk that formed the Uranus 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 Francisco belongs to the same dynamic group as Caliban and Stephano , and therefore these moons likely have a common origin.

exploration

Due to the great distance to Uranus and the weak brightness of 25.0 mag, which is 1: 63100000 compared to the central planet, Francisco was not found when the Voyager 2 space probe flew by in 1986. Since the discovery in 2001 and the rediscovery in 2003, Francisco could only be observed through earth-based telescopes and its orbital elements and its brightness could be determined.

Web links

• MPEC 2003 T29: S / 2001 U 3 October 6, 2003 (discovery)
• IAUC 8216: S / 2001 U 3 October 7, 2003 (discovery)
• IAUC 8648: Satellites of Uranus December 29, 2005 (numbering and naming)
• Polish moon page: Francisco description and further links (English)

Individual evidence

1. ^ IAU: Natural Satellites Ephemeris Service. IAU Minor Planet Center, accessed February 10, 2011 .