Buzzing like a beehive: Uranus now swarming with distant moons

Uranus: image produced by CalvinJ. Hamilton

French Mirror

Brief Summary

Three years ago, Uranus was thought to be unusual among the giant planets, being the only one without `irregular' satellites. Irregular satellites are distant moons whose orbits are either very non-circular, or very inclined with respect to the equator of the planet or both. These moons are thought to be bodies captured into orbit about the planet during the planet's formation some four and a half billion years ago; Jupiter has 8, Saturn 1, and Neptune 2. Then, in 1997, a team of astronomers using the Mount Palomar 5-meter telescope discovered the first two irregular moons of Uranus in a brief search around the planet using modern technology. Now three more probable satellites have been discovered over this last summer, bringing the planet's total to 5 irregular and 16 regular moons, making it the most populated planetary satellite system known.

 The newest moons were discovered by an international collaboration of astronomers, working in July at the Canada-France-Hawaii Telescope  (CFHT) on the lofty 4200m (14,000 feet) peak of Mauna Kea, Hawaii. The discoverers were Brett Gladman (team leader), Jean-Marc Petit, and Hans Scholl (all of the Observatoire de la Cote d'Azur in Nice France), JJ Kavelaars (McMaster University, Canada), and Matthew Holman (Harvard-Smithsonian Center for Astrophysics, USA). Both Gladman and Kavelaars were members of the team that had discovered the first two satellites in 1997. Now that those two moon are confirmed to be orbiting the planet, their previous temporary designations of S/1997 U 1 and S/1997 U 2 have become the discoverers' proposed names of Caliban and Sycorax, pending final approval of the International Astronomical Union in 2000.

Somewhat smaller than the moons discovered in 1997, these new candidate moons are about  20 times fainter and 4 or 5 times smaller,Discovery image of S/1999 U 1Discovery image of S/1999 U 2 measuring about 20 kilometers in diameter and orbiting  Uranus some 10 and 25 million kilometers from that planet's center.  These new satellites were discovered using the worlds largest electronic camera. The new camera, known as the CFH12k, images a patch of sky about the size of a full moon.  Using this large electronic detector  the team searched the region around the planet where faint moons were  likely  to be found.

These new objects have only been tracked, for a few weeks so their exact orbits about Uranus are very ill determined.  It is possible they may not even be moons but some other object passing near Uranus.  "Given how they are following the planet exactly it is unlikely that these are interlopers" says Gladman.

The time-line for this year's discoveries is as follows:

July 17-21/99
The discovery team searches the region around Uranus using the new CFH12k, camera at the Canada-France-Hawaii Telescope.  in Hawaii. This new camera system is a mosaic of digital detectors which takes a picture of a large region of the sky; no other system in the world images as large a patch of sky to the same dept. The new camera allowed the team to search over 90 percent of the region of sky around the planet where satellites might be found. The team immediately discovered 2 new satellite candidates, which were tracked on 3 of the 4 nights of observation.

 July 27/99
The two new moon candidates are announced, and designated S/1999 U 1 and S/1999 U 2 in IAU Circular #7230 from the Minor Planet Center (MPC) of the International Astronomical Union (IAU). In that annoucement, Brian Marsden (MPC, Cambridge) points out that although the objects could be comets passing by Uranus (known as Centaurs), "the absence of other slow-moving objects nearby with apparent motions remotely resembling that of Uranus suggests that the objects are indeed satellites".

July 30/99
The search for moving objects in the images conducted while at the telescope was accomplished via two different computer programs, one written by H. Scholl and the other by M. Holman. The use of semi-automatic methods is necessary due to the immense amount of data generated by the camera system (about 25 Gigabytes per night). These computer codes compare the 3 images taken at different times and identify `candidate' moving objects; these candidates are then shown to a human operator who confirms or rejects them interactively. This human interaction is necessary since the computer can be easily confused by chance alignments of various features in the images, especially when looking for very faint objects. Two codes are used because the two different techniques in their algorithms tend to make them sensitive to a slightly different set of objects, and thus one will sometimes find a candidate that the other misses. In the case of S/1999 U 1 and S/1999 U 2, each moon was found by only one of the computer codes during the first night of observing (July 17/99), allowing us to track these two objects during the subsequent 3 nights. However, the computer codes need to be 'tuned' to the data to achieve the maximum performance for finding the faintest possible objects. This was done by Holman in the final week of July, who then searched the entire set of images a second time. He was rewarded with the discovery of a third moon candidate (S/1999 U 3), which had been imaged on July 17th only. Due to having only this single night of observation, it was not even known which direction around the planet this third candidate was orbiting!  At this point the critical work was to track these objects so as to watch them move around the planet. The only way to prove their identity as moons of Uranus is to acquire enough observations to prove that the objects are orbiting about the planet rather than the Sun (and just happened to be passing close by the planet at the time of observation). The hunt was on!

S/1999 U 3 found by Holman in July 1999 dataset. Discovery image of S/1999 U 3

Aug 7-11/99
Before the discovery, August observing time at the famous Mount Palomar 5-meter telescope in California had been acquired by P. Nicholson (Cornell), Gladman, and J.A. Burns (Cornell) for the express purpose of tracking any candidate satellites discovered at the wide-field CFHT. During this period all of the candidates were tracked, proving that they were remaining in the vicinity of the planet, significantly increasing the chances that all 3 were in fact moons of the planet. S/1999 U 3 was shown to be moving almost radially away from the planet, as projected on the sky, indicating that we are seeing its orbit almost edge on.

Sept 1-2
Gladman, observing with D. Davis and C. Neese (Planetary Science Institue, Tucson) re-acquires all three candidates from the Kitt Peak 4-meter telescope during a portion of time otherwise unusable in an asteroid-survey project. The objects are still tracking the planet. Observations are reported to the Minor Planet Center Sept. 15. At this point orbital acceleration of the two closest moons around Uranus can be detected in the data, further increasing the certainty of their identification as moons of the planet.

 Sept 4/99
S/1999 U 3 is formally announced in IAU Circular 7248, at which time the identity of the objects as satellites is still not 100 percent secure.

Sept 15/99
Using the Kitt Peak obersvations of Sept 2, calculations by B. Marsden of the Minor Planet Center show that S/1999 U 2 is confirmed as a satellite.

USA Contact:

Matthew Holman: Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, 617-496-7775, email:

Canada Contact:

JJ Kavelaars: McMaster University, Hamilton, 905-525-9140x27106, email:, URL:

France Contacts:

 Brett Gladman: Observatoire de la Cote d'Azur, 011 33 4 9200 3191, email URL: (English and French Versions)
Jean-Marc Petit: Observatoire de la Cote d'Azur, 011 33 4 9200 3089, email URL:
Hans Scholl: Observatoire de la Cote d'Azur, 011 33 4 9200 3126, email