According to preliminary orbits determined from the discovery observations of September and October, the moons were in retorograde orbits very distant from Uranus (at least 5 million, to perhaps 15 million km, from the planet). The objects are on orbits that are highly inclined with respect to the equator of the planet. Each of the other giant planets in the outer solar system has been known for some time to possess similar moons, called irregular satellites, but these are the first ones in this class to be discovered about Uranus. Such moons are believed to have been captured from interplanetary orbits early in the solar system's life.
During the remainder of 1997, the moons were observed several additional times by telescopes around the world (see IAU telegram 6780) leading to improved estimates of their future motion. However, in December 1997 the Earth moved to the opposite side of the Sun from Uranus, and thus our view of the planet and its moons was blocked until spring 1998.
During this winter period, we were able to find several photographic plates taken of the planet in 1984 at the Canada-France-Hawaii telescope, and a search of those plates by us (see IAU telegram 6833) revealed what were believed to be the moons, 14 years ago. Based on those positions, the orbits of the moons were extrapolated to the spring of 1998 for the time when Uranus would emerge from behind the Sun (see IAU telegram 6834).
In March 1998, P. Nicholson and B. Gladman arranged for recovery observations from 4-meter diameter telescopes in Australia and Chile; since the planet is in the southern sky it was first most easily observable from telescopes in the southern hemisphere in the early morning twilight this spring. The first recovery observations (of both moons) were obtained at the Anglo-Australian Observatory on March 27 1998 by R. Smith, J.B. Jones, and D. Windridge; they communicated the data to B. Gladman, who found both moons in images taken pointed at the predicted positions. Confirming observations (which were necessary to eliminate the possibility of confusion with much closer asteroids) were obtained at the Cerro Tololo Interamerican Observatory in Chile by P. Hall and D. Graham on March 31 1998, and sent to JJ Kavelaars who again measured both moons moving as expected with respect to the stellar background. These observations and the more accurate revised orbits are reported in IAU Telegrams 6869. and 6870.
According to the new orbits, the fainter moon is on a nearly circular
orbit 7 million kilometers (4.5 million miles) from the planet, in
a retrograde orbit inclined 40 degrees from the planet's orbital plane
around the Sun; that is, the moon goes around the planet in the opposite
sense that Uranus and the rest of the planets orbit the Sun.
The brighter moon is also retograde, but is on a highly eccentric
(non-circular) orbit with an average distance of 12 million kilometers
from the planet, getting as far away as 18 million kilometers at its
greatest separation from the planet.
These orbits continue to support the idea that these are captured moons,
although theories of the process by which this capture occurs are not
well developed.
Prior to these discoveries, Uranus had been thought to have 15 moons, five of them identified by ground-based telescopes (the last in 1948) and ten found by the Voyager spacecraft during its 1986 flight through the system. These known moons travel along nearly circular, equatorial orbits that lie close to the planet, between 2 and 23 planetary radii.
The most recent discoveries of irregular satellites were those of the Jovian satellites Leda in 1974 (also at Palomar Observatory) and Ananke in 1951, and the Neptunian moon Nereid in 1949. The new Uranian satellites are the faintest ever imaged from the ground, and their discoveries were made possible by the use of a large telescope and a very efficient detector.
The diameters of the new satellites are estimated to be 120 km and 60 km, assuming that they reflect about 7% of the sunlight striking them. In this case, the larger of the two moons would be comparable to the largest of the irregular satellites found about each of the other giant planets (Jupiter, Saturn and Neptune). The two new moons appear quite red, like some other outer solar system bodies; the color of the latter objects has been taken to indicate the presence of organic molecules formed in their icy surfaces by the bombardement of cosmic-rays over billions of years.
Moons of Uranus are traditionally named after characters in the poetry of
Shakespeare and Pope.
The discovery team intends to suggest the names Caliban and Sycorax to the
International Astronomical Union committee which officially approves
the names of planetary moons.
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.
Ariel is already the name of one of the large regular uranian satellites,
discovered in 1851.
One of the other large Uranian satellites is named Miranda, after
Prospero's daughter, which was discovered in 1948.
In The Tempest, Prospero frees Ariel from Sycorax's spell and enslaves
Caliban.