Atlas (moon)

Richard John Terrile

Atlas (also Saturn XV) is the fourth and one of the smaller of the 62 known moons of the planet Saturn. The moon orbits the planet in the Roche- division outside the outer edge of the A ring of Saturn's rings.

  • 3.1 size
  • 3.2 Internal structure
  • 3.3 Surface

Discovery and designation

Atlas was by astronomer Richard John Terrile, discovered shortly before 12 November 1980 evaluations of recordings of the Voyager 1 spacecraft, which passed through the Saturn on said day.

A day later, on 13 November, the discovery of the International Astronomical Union ( IAU) was announced; the moon first received the provisional designation S/1980 S 28 This high numbers resulted from the belief that Saturn had been 28 moons, which later turned out to be an exaggeration. Since many of the " newly discovered " moons were already known, the number was reduced to 17, which Atlas was the 17th moon of Saturn discovered and confirmed. Since about the same time or recently discovered lower numbered moons Prometheus and Pandora also proven to be away from Saturn, Atlas received the Roman numbering XV.

On September 30, 1983, the moon was named after the Titan Atlas holding the world on his shoulders in Greek mythology. Atlas was the son of Iapetus and the Oceanid Asia, the brother of Menoitios, Prometheus and Epimetheus, and the father of the Hyades, Pleiades and the Hesperides ( to Homer ) of the nymph Calypso. Hyginus Mythographus, who wanted to emphasize the primordial shape, Atlas made ​​the son of Aether and Gaia.

Zeus punished Atlas by the titanic struggle for his loyalty to Kronos ( Roman Saturn ) by him - rather than banish him like most other Titans in Tartarus - to sentenced to Gaia ( the Earth) to stand westernmost known point and its enormous power for all time their husband Uranus ( the sky ) to wear. However, the goal was not so much the punishment of Atlas, but Zeus wanted to prevent Uranus broke on the earth because so many battles had taken place there. A special punishment also got Atlas ' brothers Prometheus and Epimetheus and Kronos itself, since Atlas was rude to Perseus later, this petrified him. Captured with the head of Medusa

The name Atlas ( Greek " carrier ( He ) " for ) was for the Atlas Mountains used ( the Atlas itself is supposed to represent ), for the star atlas, the Atlas rocket, the Atlas vertebrae, as well as the Atlantic Ocean. However, the name does not stand for the atlas as a map series or for the legendary island of Atlantis.

Web properties

Orbit

Atlas orbits Saturn on a prograde, nearly perfectly circular orbit at an average distance of 137,670 km (about 2.284 Saturn radii ) from its center, ie 77,402 km above the cloud tops. The orbital eccentricity is 0.0012, the orbit is inclined 0.003 degrees from the equator of Saturn, that is almost exactly in the equatorial plane of the planet. Due to the low eccentricity of the distance to Saturn varies by only about 30 km.

The orbit of the next inner moon Daphnis is in the middle distance 1165 km of Atlas ' orbit, the distance of the path of the next outer moon Prometheus is an average of 1710 km.

Atlas orbits Saturn in 14 hours, 26 minutes and 26.4 seconds. This roughly corresponds to the orbital period of Uranus moon Cupid. Atlas needed for a round about 11 minutes longer than the inner neighboring Daphnis.

The moon runs in the 2605 km wide, named after Édouard Albert Roche - division around the planet, which separates the A- ring of F- ring and is only 895 km from the outer edge of the A ring removed. It has long been believed that Atlas acts as a shepherd moon with its gravity on the inner A- ring; instead is now known that the A-ring outer edge is in a 7:6 orbital resonance, with larger, more distant moons Epimetheus and Janus.

The orbit of Atlas is the larger Prometheus (with whom he is in a 54:53 - orbital resonance ) significantly disturbed and to a lesser extent of Pandora, leading to variations in the length of up to 600 km (~ 0.25 ° ) leads from the precessing Keplerian orbit with a period of about 3 years. Since the orbits of Prometheus and Pandora are messy, this could be true for Atlas also.

In 2004, a faint thin ring of Saturn was discovered ( " Ringlet " ), which is located along the path of Atlas and the provisional name R/2004 S received 1.

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 Atlas is saved from this fate only because of its small size or a loose internal structure.

Rotation

The rotation period is equal to the orbital period and Atlas has it ( like the Earth's moon ) to a synchronous rotation, which thus also takes place within 14 hours, 26 minutes and 26.4 seconds. Its axis of rotation is almost exactly perpendicular to its orbital plane.

Physical Properties

Size

Atlas has a mean diameter of 30.6 km. Atlas appears as an irregularly shaped elongated object with dimensions of 46 × 38 × 19 km, wherein the longitudinal axis is aligned with Saturn on the images of the Cassini-Huygens probe.

High-resolution images taken by the Cassini spacecraft in June 2005, show that Atlas is not round, but at the equator is much wider than at the poles. He gets by the appearance of a flying saucer. It is believed that the most pronounced form it created that Atlas has collected at its equatorial region of very thin material R/2004 S 1- ring. In fact, the size of the equatorial bulge of the expected size of the Roche lobe of Atlas is comparable. This would mean that no other particles may accumulate at the equator more without being transported by the centrifugal force, which overcomes the low gravity Atlas, back into the space.

The total area of ​​3700 km ² Atlas is estimated, which corresponds approximately to the area of ​​Transnistria.

Internal construction

The mean density of Atlas is 0.44 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 Atlas would float on water. This indicates that the moon is composed mainly of water ice.

That Atlas 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 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

The surface of the moon has hardly any impact craters, indicating a relatively young age. Atlas having a relatively high albedo of about 0.4, which means that it has a bright surface, which reflects 40% of the incident sunlight. On its surface is the gravitational acceleration is 0.0017 m / s ², equivalent to about 2 ‰ of the earthly. The average surface temperature of Atlas is estimated at about -192 ° C ( 81 K).

Research

Due to the small size and apparent magnitude of 18.1 m ( 1:20900000 that of the central planet is ) as well as the close proximity to Saturn and the fact that he is outshone by this is, atlas with ground-based telescopes not make out; It is not surprising therefore that Atlas for 24 years could not be found again.

Atlas has since been visited by three spacecraft, notably by the flyby probes Voyager 1 on 12 November 1980 and Voyager 2 on August 25, 1981, and the Saturn orbiter Cassini, orbiting Saturn since July 2004. Atlas was repeatedly taken by Cassini targeted so that its orbital parameters are now fairly well known. The next flyby of Cassini occurred during the 46th orbit around Saturn on June 12, 2007, when the probe Atlas happened at a distance of 40,000 km. Here some shots could be made with good resolution.

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