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Montage
of Uranus and its moons, from Voyager imagery.
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| THE MOONS OF URANUS | |||
| Uranus has 21 moons. The satellites and their surface features are named after characters and places from Shakespeare's plays. Several of the smaller moons are named after characters from Alexander Pope's "Rape of the Lock". Of the satellites, 5 can be considered large - the other 16 are small. | |||
| Small satellites | |||
| Most of Uranus' moons have orbits of low eccentricity (nearly circular) and orbit close to Uranus' equatorial plane. There are a number of exceptions though- the 6 irregular satellites, which include Caliban and Sycorax have more elliptical and inclined orbits. | |||
 The orbits of Uranus' small moons. |
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| The small satellites are all less than 155 kilometres in diameter, and are dark, reflecting less than 10% of the sunlight they receive. Their small sizes and dark surfaces meant the discovery of 11 of them had to await the visit of a spacecraft, Voyager 2 in 1986. However new techniques and better Earth-based telescopes have enabled astronomers to find additional small moons over the last few years. | |||
| Caliban, Sycorax (discovered in 1997), Prospero, Stephano, Setebos, and S/2001 U1 (discovered in 1999 and 2001) are the outermost moons of Uranus, orbiting far beyond Oberon and the inner moons. With orbit's ranging from 720,000 kilometres to 1.7 million kilometres from Uranus, they are thought to be captured asteroids. | |||
 The orbit of Caliban. |
 The orbit of Sycorax. |
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| Little is known about the surfaces of the small satellites, only Puck was imaged. The picture of Puck's surface reveals an irregular and cratered surface. The ten small satellites within Puck's orbit are probably very similar. | |||
| Shepherds | |||
| Cordelia and Ophelia are shepherd satellites - that is, they orbit either side of one of Uranus' rings, and maintain the ring's orbital position. | |||
 The orbits of Cordelia, Ophelia, and the Epsilon ring. |
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| More about the rings of Uranus | |||
 Cordelia. |
 Ophelia. |
 Bianca. |
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| More about Cordelia | |||
| More about Ophelia | |||
| More about Bianca | |||
 Cressida. |
 Desdemona. |
 Juliet. |
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| More about Cressida | |||
| More about Desdemona | |||
| More about Juliet | |||
 Portia. |
 Rosalind. |
 Belinda. |
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| More about Portia | |||
| More about Rosalind | |||
| More about Belinda | |||
 Sycorax. |
 Puck. |
 Caliban. |
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| More about Sycorax | |||
| More about Puck | |||
| More about Caliban | |||
| Large satellites | |||
| The large satellites all orbit Uranus with very circular orbits in the plane of Uranus' equator, low eccentricities and small inclinations | |||
 The orbits of Uranus' larger moons. |
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| The larger satellites, with the exception of Miranda, were found in the 18th and 19th centuries by two British astronomers, William Lassell and William Herschel. Miranda was discovered in 1948 by Gerard Kuiper. | |||
| Of the large satellites, Miranda is the smallest with a diameter of 472 kilometres, and Titania the largest, at 1578 kilometres across. | |||
| The density of the large satellites is known. These range from 1200 kg m-3 (Miranda) to 1670 kg m-3 (Ariel) . This suggests that they consist of a large amount of water ice, as well as rock, in the ratio of 40:60 or 50:50. Despite the presence of a large quantity of ice, the large moons are not particularly reflective. They have albedo values of about 0.25. Ariel is the brightest object in the Uranian system with an albedo of 0.34. | |||
| Nevertheless, the larger moons of Uranus are all brighter than the smaller moons.Two of the large moons are not absolutely spherical. They are slightly irregular in form. Miranda's dimensions are 480 x 468 x 466 kilometres. Ariel is slightly oblate (flattened) by 6 kilometres. In such a large body this difference is not noticeable. | |||
| Ariel, Umbriel, Titania and Oberon | |||
| The moons Ariel and Umbriel form a pair in terms of size (1162 kilometres and 1170 kilometres) and orbit (about 200,000 kilometres). Titania and Oberon form another pair (1578 kilometres and 1522 kilometres) and orbit at a comparable distance (about 500,000 kilometres). | |||
| What is strange is that the surfaces of Titania and Ariel are alike and the surfaces of Oberon and Umbriel are alike. Titania and Ariel have heavily cratered surfaces but show evidence of a great deal of crustal movement. Oberon and Umbriel however, show little sign of surface movement and have simple but heavily pocked surfaces. | |||
| Why should each moon have a twin with a surface like that of one of the other pair, yet be of a similar size and orbit? Could their orbits have changed after they had evolved? | |||
 Ariel. |
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| More about Ariel | |||
| More about Umbriel | |||
 Titania |
 Oberon |
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| More about Titania | |||
| More about Oberon | |||
| Miranda | |||
| The biggest surprise of Voyager's visit to Uranus was Miranda's weird surface. It was necessity though, that brought Voyager just 3000 kilometres from Miranda, enabling it to return high resolution images of its surface. Miranda would otherwise not have been the geologists' first choice of target. Of the Uranian satellites, however, Miranda has the most varied array of landscapes. It possesses one of the most perplexing surfaces discovered in our Solar System. Miranda looks very much as if it has been broken apart and glued back together again. | |||
 Miranda. |
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| More about Miranda | |||
| Uranus' other moons | |||
| Uranus' 21st moon S/2001 U1 was discovered in a series of images collected using the Canada France Hawaii Telescope on Mauna Kea and the Blanco telescope at the Cerro-Tololo Inter-American Observatory in Chile. | |||
| It is a small irregular satellite, orbiting counter to Uranus' rotation and tracing an elliptical path. Like Uranus' other irregular satellites (Caliban, Stephano, Sycorax, Prospero and Setebos) it orbits 8.5 million kilometres from the planet in an orbit inclined with respect to the plane of Uranus' equator. | |||
| Apart from a rough estimate of size, nothing is known of the moon's surface or its physical characteristics- its mass, density, or composition. In the discovery image, S/2001 U1 is just a small blob of light. Its brightness suggests that it is about 10 kilometres across, similar in size to Stephano, and one of the smallest Uranian moons found so far. | |||
| S/2001 U1 could be a captured solar orbiting asteroid, caught by Uranus' gravitational pull; or the result of a collision between a comet and larger moon already orbiting Uranus. | |||
| Astronomers think that Uranus may have many more moons, but finding them will be very difficult. When a suspected moon is found careful observations taking several months are required to determine the object's orbital properties, and so that it is not lost and has to be rediscovered. | |||
| The moons of Uranus are named after characters from Shakespearean plays. The name eventually chosen for S/2001 U1 will follow this convention. | |||
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