SOUTHERN ASTRONOMICAL DELIGHTS
U R A N U S
Part 3 : MOONS & RINGS


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The MOONS of URANUS

Uranus has presently twenty-seven (27) known moons (2006), which can be roughly divided into three groups; the five classical main moons, the inner moons associated with the dark rings surrounding the planet, and those well quite some distance from the planet. Our knowledge about these satellites are limited by the single spacecraft visit by Voyager 2 in January 1986. On of the biggest problems is the slow changing tilt of the satellite orbital plane, which causes each little change in the aspects of the sun-lit portions visible to us. Many of the images obtained by Voyager 2 mean we only have knowledge of half the surface features. For us to see the opposing sides will require a future expedition to Uranus around 2016. However, no missions are planned at the moment.


The Five Main Uranian Moons


These five main moons are all smaller, at least when compared to largest moons of the other outer planets, but they are all in a similar range around 1 000 km. apiece. Each have classical names derived from the ancient world, while the other moons have names that have broken from the normal tradition, each adopted from characters in either the writings of Alexander Pope or the Shakespearian plays.

Oberon is the outermost and brightest satellite that was discovered by Sir William Herschel in 11th January 1787, only six years after Uranus itself. He found the slightly fainter Titania on the same night. Oberon and Titania are very similar in size, being about 1 500 km. in diameter, with Titania being some 50 km larger. Although appearing quite similar, Oberon contains more slightly more ice, as seen in the 1.6 g.cm-3. density versus 1.72g.cm-3. Both are mixtures of ice and silicates, though about one-fifth the volume is likely made of methane or similar organic compounds frozen in the cold depths of the outer Solar System. Titania is generally darker but is scored by powdery white scratches from either meteor impacts or geological ice flows. Titania also has a deep canyon feature along its equator being similar to what is seen on Tethys — one of Saturns moons. Both Oberon and Titania, like the Earths are locked in to a synchronous orbit and always facing the same direction towards Uranus.

Oberon and Titania remained for some years until William Lassell discovered the two closer moons of Umbriel and Ariel on 24 October 1851. Both of these moons are also similar in size. Umbriel is likely one of the dimmest and darkest place in the Solar System. The last of the traditional five moons was the smallest and closest to Uranus, Miranda, which was discovered by Gerard Kuiper in 1948.

A 20cm telescope can see both Oberon and Titania when they are at maximum elongation from Uranus, whose magnitude at opposition can reach 13.7v and 13.9v magnitude, respectively. Both Umbriel and Ariel and are little more problematic, as they are closer to the planet and slightly fainter at 14.8v and 14.1v, respectively. These latter two moon can be seen with 30cm with care. Closest of all is Miranda, which is far too difficult for most amateur telescopes.

The most interesting moon from the Voyager 2 flyby was Miranda with its very odd terrain. Twenty-three satellites are now known to orbit the planet.

URANUS SATELLITE TABLE


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                  Distance Period    Size                       
No.   Satellite    (km)    (days)    (km)     Discoverer   Date
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V     Miranda      129 800  1.413  240x234x233 Kuiper       1948
I     Ariel        191 000  2.520  581x578x579 Lassell      1851
II    Umbriel      266 200  4.144      585     Lassell      1851
III   Titania      435 800  8.706      789     Herschel     1787
VI    Oberon       583 600 13.463      761     Herschel     1787
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VI    Cordelia      49 752  0.335       13     Voyager 2    1986
VII   Ophelia       53 764  0.376       16     Voyager 2    1986
VIII  Bianca        59 165  0.435       27     Voyager 2    1986
IX    Cressida      61 767  0.464       41     Voyager 2    1986
X     Desdemona     62 659  0.474       35     Voyager 2    1986

XI    Juliet        64 358  0.494       83     Voyager 2    1986
XII   Portia        66 097  0.511       70     Voyager 2    1986
XIII  Rosalind      69 927  0.558       36     Voyager 2    1986
XXVII Cupid         74 392  0.613        8.9   Showalter    2003
XIV   Belinda       75 256  0.624       45     Voyager 2    1986

XXV   Perdita       76 417  0.638       13     Karkoschka   1999
XV    Puck          86 004  0.762       77     Voyager 2    1985
XXVI  Mab           97 736  0.923       12.4   Showalter    2003
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XXII  Francisco  4 276 000  266.6        6     Kavelaars    2003
XVI   Caliban    7 231 000  579.7       40     Gladman      1997
XX    Stephano   7 948 000  677.3       15     Gladman      1999
XXI   Trinculo   8 578 000  759.0        5     Holman       2001
XVII  Sycorax   12 213 000  1288d       95     Nicholson    1997

XXIII Margaret  14 345 000  1687d        5.5   Sheppard     2003
XVIII Prospero  16 243 000  1977d       15     Petit        1999
XIX   Setebos   17 501 000  2235d       15     Kavelaars    1999
XXIV  Ferdinand 20 901 000  2887d        6     Kavelaars    2001
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The Uranian Rings

Many of the ring were first discovered in 1977 during an star occultation of the planet. Another was discovered by Voyager 2, followed by two outer rings in 2003. Presently there are twelve rings in total. There is strong evidence of interactions with the other moons, causing peaks or clumps, and even the so-called λ ring does not fully encircle the planet and appears in smaller clumps of orbiting material. Several are asociateed with so-called shepherd moons as seen with Sauturn. I.e. The ε ring seems to be shepherded by Cordelia and Ophelia. In 2003, based on the HST images the R2003 U2 ring was revealed to be controlled by the moons Portia and Rosalind. The newest, Mab, influences the R2003 U1 ring showing wave-like oscillations.

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        Distance  Width
Ring      (km)     (km)
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1986U2R   38 000   2 500
6         41 840     1.5
5         42 230     c.2
4         42 580     c.2
α         44 720    4-10
β         45 670    5-11
η         47 190     1.6
γ         47 627     1-4
δ         48 290     3-8
λ         50 024     c.2
ε         51 140   20-96
R2003 U2  67 300   3 800
R2003 U1  97 700  17 000
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Last Update : 25th September 2012

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