Prometheus (moon)

Stewart A. Collins D. Carlson

Prometheus (also Saturn XVI) is the fifth and eleventh largest of the 62 known moons of the planet Saturn. The shepherd moon orbits the planet in the Roche- sharing within the F ring of Saturn's rings.

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface

Discovery and designation

Prometheus was discovered in mid-October 1980 by the astronomer Stewart A. Collins and D. Carlson, along with Pandora for Evaluations of recordings of the Voyager 1 spacecraft, which passed through the Saturn on 12 November 1980 already before the flyby, the probe.

On 31 October 1980, the discovery of the International Astronomical Union ( IAU) was announced; the moon first received the provisional designation S/1980 S 27 This high numbers resulted from the belief that Saturn had been 27 moons, which later turned out to be an exaggeration. Since many of the " newly discovered " moons were already known, the number was reduced to 16, which Prometheus discovered and confirmed the 16th moon of Saturn was. The simultaneous discovery of Pandora and shortly thereafter discovered Atlas Roman numbering were awarded according to the ascending order of the distances to Saturn; Prometheus was given by the number XVI, suitable for discovery as the 16th moon of Saturn.

Relatively late, on January 3, 1986 - the same time as Pandora and the Pluto Charon - the moon was named after the Titan Prometheus from Greek mythology.

The name means " forethought ". From its nickname addition Pyrphoros ( "Fire Bringer" ), among others Iapetionides ( "Son of Iapetus " ) and desmotes ( " Bound " ) known.

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Orbit

Prometheus orbits Saturn on a prograde, nearly perfectly circular orbit at an average distance of 139,380 km (about 2.313 Saturn radii ) from its center, ie 79,112 km above the cloud tops. The orbital eccentricity is 0.0022, the orbit is inclined 0.008 degrees from the equator of Saturn, that is almost exactly in the equatorial plane of the planet. Due to the low eccentricity of the track varies in distance to Saturn were only about 613 km.

The orbit of the next inner moon Atlas is located in the central 1710 km from the orbit of Prometheus, the distance of the path of the next outer moon Pandora on average is 2340 km.

Prometheus orbits Saturn in 14 hours, 42 minutes and 42.3 seconds. This corresponds almost exactly to the orbital period of Uranus moon Cupid. Prometheus needed for a round about 16 minutes longer than the inner neighboring Atlas.

The moon marks the outer end of the 2605 km wide, named after Édouard Albert Roche - division that separates the A- ring, the F ring, and runs approximately to 800 km within the 1979 by the space probe Pioneer 11 discovered narrow faint F ring around the planet.

Prometheus acts as a shepherd moon on the outside edge F- ring. It causes at its apoapsis or its approximation to the F ring deformations in the form of kinks and knots in the ring, while pulling with his gravity over short periods of material from him. This process leaves a dark channel ( so-called " Kepler shear " ) in the inner part of the ring, since the rotation time of Prometheus is shorter than the more outwardly moving material of the F- ring. Each newly produced channel is about 3.2 ° before a round previously produced channel and so leaves a continuous pattern.

On the other hand, Prometheus effective especially stabilizing effect on the F ring; he keeps from the inside and from the outside Pandora prevents the ring in its narrow form, and the union of the attractive forces of these two moons that the dust is distributed in width.

The orbit of Prometheus appears chaotic; it is a consequence of four 121:118 - track resonances with Pandora. The most noticeable changes in the orbit takes place approximately every 6.2 years when the apoapsis of Prometheus with the periapsis of Pandora is on a line and the two bodies are separated a distance of about 1,400 km. Prometheus itself disturbs the orbit of the inner neighbor Atlas significant, with whom he is in a 54:53 - orbital resonance. This leads to deviations of the Atlas track with a length of up to 600 km (~ 0.25 ° ) of the precessing Keplerian orbit with a period of roughly three years. In addition, Prometheus is near a 16:15 resonance with pan with a period of 108 days and an amplitude of about 3 km in the direction of the orbit.

In 2004, a faint thin ring of Saturn was discovered ( " Ringlet " ), which is about 480 km is located inside the orbit of Prometheus and the provisional name R/2004 S received 2.

2008, further dynamics were discovered in this system, which indicates that small " moonlets " circulate within the F ring of Saturn. This moonlets, the S 3 and S/2004 S 4 (which may be the same body and have since not been sighted ) tentatively identified as S/2004 and S/2004 S 6 are referred to, are probably clumps of ring material and happen because of perturbations by Prometheus continuously the narrow and densely core of the ring.

The moon orbits Saturn within a critical distance, the so-called Roche limit, which would bring a larger moon in this field to shatter. Probably Prometheus is preserved only because of its small size or a loose internal structure of this fate.

Rotation

The rotation period is equal to the orbital period and Prometheus has therefore, like the Earth's moon, a synchronous rotation, which therefore also takes place within 14 hours, 42 minutes and 42.3 seconds. Its axis of rotation is almost exactly perpendicular to its orbital plane.

Physical Properties

Size

Prometheus is a mean diameter of 86.2 km. Prometheus appears on the recordings of the Cassini and Voyager spacecraft as a very irregularly shaped, extremely elongated object with dimensions of 119 × 87 × 61 km ( He is more than twice as long as wide ), the longitudinal axis is aligned with Saturn. The size is specified with 94.0 ( 148 x 100 x 68 ) km; In this case, the mass would be approximately 8 x 1017.

From the size Prometheus is best compared with the Uranus moon Epimetheus or Portia.

The total area of ​​23,300 km ² Prometheus is estimated, this is something more than the area of Slovenia or Israel.

Internal construction

Its mean density is 0.47 g / cm ³ by far lower than that of the earth and even much lower than the density of Saturn; she is so low that Prometheus would float on water. This indicates that the moon is composed mainly of water ice.

That Prometheus orbits Saturn within the Roche limit, points out that he has either a very strong internal structure, or that he belongs to the so-called porous Rubble Piles, which, due to the relatively weak gravitational inside cavities. Due to the extremely low mean density, the latter hypothesis is more likely.

Surface

On the surface of Prometheus mountains, valleys and impact craters are up to 20 km in diameter. He is, however, less cratered than the neighboring moons Pandora, Epimetheus and Janus. The craters seem generally more gentle than those of its immediate neighbors, which has probably to do with the accumulation of material from the F ring, which levels the crater.

Prometheus has a relatively high albedo of about 0.60, which means that he has a very bright surface that reflects 60 % of the incident sunlight. On its surface is the gravitational acceleration 0.003 m / s ², equivalent to about 3 ‰ of the earth. The average surface temperature of Prometheus is estimated at about -199 ° C (74 K).

Research

Prometheus has an apparent magnitude of 15.5 m, which is 1:1100000 of the central planet. Since the discovery and confirmation of 1980 and the Voyager flybys Prometheus was studied by ground-based telescopes and the Hubble Space Telescope and its Bahnpararameter could be clarified.

Prometheus has so far been visited by three spacecraft, notably by the flyby probes Voyager 1 on 12 November 1980 and Voyager 2 on August 1981 and the Saturn orbiter Cassini, orbiting Saturn since July 1, 2004 25. Prometheus was repeatedly taken by Cassini targeted so that its size and shape as well as its orbital parameters are now fairly well known. Upcoming flybys of Cassini occurred during the 47th orbit around Saturn on June 28, 2007, when the probe Prometheus happened at a distance of 16,539 km and on, between December 2009 and January 2010 between the 122 and 125 round minimum in a distance of 36,000 km. It was some well-resolved images are taken.

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