Prometheus (moon)
| Prometheus | |
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| Trailing hemisphere of Prometheus in reflected light from Saturn (Cassini, January 27, 2010, distance 34,000 km) | |
| Provisional or systematic name | S / 1980 S 27 |
| Central body | Saturn |
| Properties of the orbit | |
| Major semi-axis | 139,380 ± 10 km |
| Periapsis | 139,073 km |
| Apoapsis | 139,686 km |
| eccentricity | 0.0022 |
| Orbit inclination | 0.008 ± 0.004 ° |
| Orbital time | 0.612990038 d |
| Mean orbital velocity | 16.54 km / s |
| Physical Properties | |
| Albedo | 0.60 |
| Apparent brightness | 15.5 mag |
| Medium diameter | 86.2 ± 4.0 (119 × 87 × 61) km |
| Dimensions | 1.566 ± 0.019 x 10 17 kg |
| surface | ≈ 23,300 km 2 |
| Medium density | 0.47 ± 0.07 g / cm 3 |
| Sidereal rotation | 0.612990038 |
| Axis inclination | 0.0 ° |
| Acceleration of gravity on the surface | ≈ 0.003 m / s 2 |
| Escape speed | ≈ 20.47 (mean) m / s |
| Surface temperature | ≈ −199 ° C (74) K |
| discovery | |
| Explorer | |
| Date of discovery | October 1980 |
| Remarks | F ring inner shepherd moon |
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| The positions of the inner moons of Saturn in Saturn's ring system, from inside to outside Pan, Atlas, Prometheus, Pandora, Janus & Epimetheus, Mimas, Enceladus, Tethys, Dione and Rhea | |
Prometheus (also Saturn XVI ) is the fifth and eleventh largest of the 82 known moons of the planet Saturn (counting from the inside) . As a so-called shepherd's moon, it orbits the planet in the Roche division within the F ring of Saturn's rings and stabilizes it together with the partner moon Pandora, which is circling outside .
The delicate F-ring connects directly to the bright A-ring visible in the telescope, but cannot be observed from Earth.
Discovery and naming
Prometheus was discovered in mid-October 1980 by the astronomers Stewart A. Collins and D. Carlson, at the same time as Pandora, when evaluating images from the space probe Voyager 1 , which passed Saturn on November 12, 1980 , before the probe flew by.
On October 31, 1980, the discovery was announced by the International Astronomical Union (IAU); the moon was initially given the provisional designation S / 1980 S 27 . This high numbering resulted from the opinion that Saturn had 27 moons so far, which later turned out to be an exaggeration. Since many of the "newly discovered" moons were already known, the number was reduced to 16, making Prometheus the 16th discovered and confirmed Saturn's moon . Due to the simultaneous discovery of Pandora and the Atlas , which was discovered shortly afterwards , the Roman numbering was assigned according to the ascending order of the distances to Saturn; This gave Prometheus the number XVI , matching the discovery as the 16th Saturn moon.
Relatively late, on January 3, 1986 - at the same time as Pandora and the Pluto moon Charon - the moon was then named after the Titan Prometheus from Greek mythology .
The name means "the one who thinks ahead". Besides Pyrphoros (" Bringer of Fire "), among other things Iapetionides ("Son of Iapetos") and Desmotes ("Fettered") are known from his nicknames .
Track properties
Orbit
Prometheus orbits Saturn on a prograde , almost perfectly circular orbit at an average distance of 139,380 km (approx. 2,313 Saturn radii) from its center, i.e. 79,112 km above its cloud ceiling. The orbit eccentricity is 0.0022, the orbit is inclined 0.008 ° to the equator of Saturn , so it lies almost exactly in the equatorial plane of the planet. Due to the low eccentricity, the orbit varies in distance to Saturn by only around 613 km.
The orbit of the next inner moon Atlas is on average 1,710 km from the orbit of Prometheus, the distance of the orbit of the next outer moon Pandora is on average 2,340 km.
Prometheus orbits Saturn in 14 hours, 42 minutes and 42.3 seconds. This corresponds almost exactly to the orbital period of the Uranus moon Cupid . Prometheus takes about 16 minutes longer to make one orbit than its inner neighbor, Atlas.
The moon marks the outer end of the 2,605 km wide Roche division , named after Édouard Albert Roche , which separates the A-ring from the F-ring , and runs about 800 km within the narrow, faint F-ring discovered in 1979 by the Pioneer 11 spacecraft around the planet.
Prometheus acts as a shepherd's moon on the outer F-ring. During its apoapsis or its approach to the F-ring, it causes deformations in the form of kinks and knots in the ring, while its gravity pulls material out of it over short periods of time. This process leaves a dark channel (so-called “Kepler shear”) in the inner part of the ring, as the orbital period of Prometheus is shorter than the material of the F-ring running further out. Each newly produced canal is located about 3.2 ° in front of the previously produced canal and thus leaves a continuous pattern.
On the other hand, Prometheus has a stabilizing effect on the F-ring; he keeps the ring in its narrow form from within and Pandora from without, and the union of the attractive forces of these two moons prevents the dust from spreading out.
Prometheus' orbit appears chaotic ; it is a consequence of four 121: 118 orbit resonances with Pandora. The most noticeable changes in orbit occur roughly every 6.2 years, when the apoapsis of Prometheus is in line with the periapsis of Pandora and the two bodies are separated by a distance of about 1,400 km. Prometheus himself significantly perturbs the orbit of its inner neighbor Atlas , with whom he is in a 54:53 orbital resonance. This leads to deviations of the Atlas orbit in the length of up to 600 km (~ 0.25 °) from the precessing Kepler orbit with a period of roughly 3 years. In addition, Prometheus is close to a 16:15 resonance with Pan with a period of 108 days and an amplitude of about 3 km towards the orbit.
In 2004, a faint, thin ring of Saturn was discovered ("Ringlet"), which is located about 480 km within the orbit of Prometheus and was given the provisional name R / 2004 S 2 .
In 2008 further dynamics were discovered in this system, which indicate that small "moonlets" orbit Saturn within the F-ring. These moonlets, tentatively referred to as S / 2004 S 3 and S / 2004 S 4 (which may be the same body and have not been sighted since) and S / 2004 S 6 , are likely clumps of ring material and happen because of orbital disruptions by Prometheus continuously the narrow and densest core of the ring.
The moon orbits Saturn within a critical distance, the so-called Roche limit , which would cause a larger moon in this area to burst. Prometheus is likely to be saved from this fate only because of its small size or loose internal structure.
rotation
The rotation time is the same as the orbital time and Prometheus shows, like the earth's moon , a synchronous rotation , which thus also takes place within 14 hours, 42 minutes and 42.3 seconds. Its axis of rotation is almost exactly perpendicular to its plane .
Physical Properties
size
Prometheus has a mean diameter of 86.2 km. On the images of the Cassini and Voyager probes , Prometheus appears as a very irregularly shaped, extremely elongated object with dimensions of 119 × 87 × 61 km (it is more than twice as long as it is wide), with the longitudinal axis oriented towards Saturn . The size is also given as 94.0 (148 × 100 × 68) km; in this case the mass would be about 8 · 10 17 kg.
In terms of size, Prometheus can best be compared with Epimetheus or the Uranus moon Portia .
The total area of Prometheus is estimated to be 23,300 km², which is slightly more than the area of Slovenia or Israel .
internal structure
Its mean density of 0.47 g / cm³ is far lower than that of Earth and even considerably lower than the density of Saturn; it is so low that Prometheus would swim on water. This indicates that the moon is predominantly composed of water ice .
The fact that Prometheus Saturn orbits within the Roche limit indicates that it either has a very solid internal structure or that it belongs to the porous so-called rubble piles , which have cavities inside due to the comparatively weak gravity. Because of the extremely low mean density, the latter hypothesis is more likely.
surface
On the surface of Prometheus there are ridges, valleys and impact craters up to 20 km in diameter. However, it is less cratered than the neighboring moons Pandora , Epimetheus and Janus . The craters generally appear gentler than those of its immediate neighbors, possibly due to the build-up of material from the F-ring that levels the craters.
Prometheus has a relatively high albedo of around 0.60, which means that it has a very bright surface that reflects 60% of the incident sunlight . On its surface, the acceleration due to gravity is 0.003 m / s², which corresponds to about 3 ‰ of that on earth. The mean surface temperature of Prometheus is estimated to be around −199 ° C (74 K ).
exploration
Prometheus has an apparent magnitude of 15.5 meters , which is 1 / 1,100,000 that of the central planet. Since its discovery and confirmation in 1980 and the Voyager flyby, Prometheus has been studied by earth-based telescopes and the Hubble space telescope , and its orbit parameters have been refined.
Prometheus has so far been visited by three space probes , namely by the flyby probes Voyager 1 on November 12, 1980 and Voyager 2 on August 25, 1981 and the Saturn orbiter Cassini , which has orbited Saturn since July 1, 2004. Prometheus was targeted by Cassini several times, so its size and shape, as well as its orbital parameters, are now fairly well known. Cassini's next flyby occurred during the 47th orbit around Saturn on June 28, 2007, when the Prometheus probe passed at a distance of 16,539 km and between December 2009 and January 2010 between the 122nd and 125th orbits at a minimal distance of 36,000 km. Some well-resolved recordings could be made.
media
Web links
- IAUC 3532: Satellites of Saturn October 31, 1980 (discovery)
- IAUC 4157: Satellites of Saturn and Pluto January 3, 1986 (designation)
- How Prometheus controls the F-ring
- The peculiar shapes of Saturn's small inner moons (English)
Individual evidence
- ↑ Paul Rincon: Saturn overtakes Jupiter as planet with most moons. BBC , October 7, 2019, accessed March 20, 2020 .
- ^ The Orbits of Saturn's small satellites. Retrieved February 15, 2011 .
| further inside | Saturn moons | further outside |
| Atlas |
Semi- major axis (km) Prometheus 139.400 |
Pandora |