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Saturn's Satellites

Saturn has 34 named satellites:Of those moons for which rotation rates are known, all but Phoebe and Hyperion rotate synchronously.The three pairs Mimas-Tethys, Enceladus-Dione and Titan-Hyperion interact gravitationally in such a way as to maintain stable relationships between their orbits: the period of Mimas' orbit is exactly half that of Tethys, they are thus said to be in a 1:2 resonance; Enceladus-Dione are also 1:2; Titan-Hyperion are in a 3:4 resonance.

      Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- --------- --------- --------- --------- ---------
Atlas 138 14 ? Terrile 1980
Prometheus 139 46 2.70e17 Collins 1980
Pandora 142 46 2.20e17 Barnard 1980
Epimetheus 151 57 5.60e17 Walker 1980
Janus 151 89 2.01e18 Dollfus 1966
Mimas 186 196 3.80e19 Herschel 1789
Enceladus 238 260 8.40e19 Herschel 1789
Tethys 295 530 7.55e20 Cassini 1684
Telesto 295 15 ? Reitsema 1980
Calypso 295 13 ? Pascu 1980
Dione 377 560 1.05e21 Cassini 1684
Helene 377 16 ? Laques 1980
Rhea 527 765 2.49e21 Cassini 1672
Titan 1222 2575 1.35e23 Huygens 1655
Hyperion 1481 143 1.77e19 Bond 1848
Iapetus 3561 730 1.88e21 Cassini 1671
Phoebe 12952 110 4.00e18 Pickering 1898

Saturn's Rings

  Radius Radius   approx. approx.
Name inner outer width position mass (kg)
---------- ----------- ----------- ------------- ------------ -------------
D-Ring 67,000 74,500 74,500 (ring)  
Guerin Division          
C-Ring 74,500 92,000 17,500 (ring) 1.1e18
Maxwell Division 87,500 88,000 500 (divide)  
B-Ring 92,000 117,500 25,500 (ring) 2.8e19
Cassini Division 115,800 120,600 4,800 (divide)  
Huygens Gap 117,680 (n/a) 285-440 (subdiv)  
A-Ring 122,200 136,800 14,600 (ring) 6.2e18
Encke Minima 126,430 129,940 3,500 29%-53%  
Encke Division 133,410 133,740      
Keeler Gap 136,510 136,550      
F-Ring 140,210   30-500 (ring)  
G-Ring 165,800 173,800 8,000 (ring) 1e7?
E-Ring 180,000 480,000 300,000 (ring)  

* distance is kilometers from Saturn's center
* the "Encke Minima" is a slang term used by amateur astronomers, not an official IAU designation
Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.
orbit:2,870,990,000 km (19.218 AU) from Sun
diameter:51,118 km (equatorial)
mass: 8.683e25 kg
Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' South Pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' Polar Regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

Like Jupiter, Saturn is about 75% hydrogen and 25% helium with traces of water, methane, ammonia and "rock", similar to the composition of the primordial Solar Nebula from which the solar system was formed.

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer. Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

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