' Uranus' New Moons

Uranus' New Moons

Several tiny moons have been discovered recently, most from ground based imagery! With these discoveries, Uranus joins the jovian planets in having a number of small moons in irregular orbits.

Caliban and Sycorax

Uranus XVI and XVII

   Two new moons of Uranus, provisionally known as Uranus XVI (S/1997 U 1) and Uranus XVII (S/1997 U 2), have been discovered. The names Caliban and Sycorax proposed by the discoverers have now been offically accepted by the IAU. Caliban (S/1997 U 1) orbits about 7.2 million km from Uranus and is about 80 km in diameter. Sycorax (S/1997 U 2) orbits about 12.2 million km from Uranus and is about 160 km in diameter. These size estimates are based on their apparent brightness and assuming an albedo of about 7%. The orbits are retrograde and highly inclined.

   Caliban is a character from Shakespeare's play The Tempest; a savage and deformed slave of the magician Prospero, and is the son of the witch Sycorax who imprisoned the fairy Ariel for disobedience. In The Tempest, Prospero frees Ariel from Sycorax's spell and enslaves Caliban.

   Caliban was discovered by Brett Gladman, Phil Nicholson, Joseph Burns, and JJ Kavelaars using the 200 inch Hale telescope; the first images were taken 1997 Sept 6 and 7. Credit for Sycorax goes to Nicholson, Gladman, Burns, and Kavelaars.

   The image above is the discovery image of S/1997 U 1; the one to the right is S/1997 U 2.

   Prior to this discovery, Uranus was the only one of the gas giants not known to have "irregular" moons, that is, ones whose orbits are not direct and approximately in the plane of the planet's equator.

   Like the other irregular moons (eg. Jupiter's outer 8 moons, Phoebe and Nereid), these are probably captured asteroids. It is highly unlikely that they could have been formed initially in these orbits.

   The composition of the two moons, according Nicholson, "is probably a plum-pudding mixture of rocks and ice." Both moons are unusually red in color which may suggest a historical link with the Kuiper Belt.

   These are the dimmest moons ever to have been imaged with a ground-based telescope.

More about these moons

(Note: in some of the material below, Caliban (S/1997 U 1) is referred to as the "fainter" of the two and Sycorax (S/1997 U 2) as the "brighter" one.)

1986 U 10

   This tiny moon is as yet unnamed.

   Imaged by Voyager 2 in 1986; discovered in 1999 by Erich Karkoschka of the Lunar and Planetary Lab of The University of Arizona in Tucson.

   It's orbit is nearly identical to that of Belinda, about 75,000 km from Uranus.

   It is about 40 km in diameter.

More about 1986 U 10

Prospero, Setebos and Stephano

Uranus XVIII, Uranus XIX and Uranus XX

   These tiny moons are also named for characters in The Tempest: Prospero is a powerful magician who enslaves Ariel; Serving as a butler on Ariel's ship, Stephano and Caliban plot to murder Prospero; Setebos is the god of Sycorax.

   First imaged by Kavelaars, Gladman, Holman et al with the Canada-France-Hawaii Telescope on Mauna Kea in July 1999.

   Their orbits are uncertain at this point but are about 10 and 25 million km from Uranus.

   They are probably about 30 or 40 km in diameter (assuming an albedo of .07).

More about these moons

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Bill Arnett; last updated: 2000 Aug 23