Margaret (moon)

Margaret, designated Uranus XXIII and originally called S/2003 U 3, is an irregular moon orbiting Uranus. It stands out as the only known irregular moon of Uranus with a prograde orbit—meaning it travels around the planet in the same direction that Uranus orbits the Sun. The moon was officially discovered by Scott S. Sheppard and David C. Jewitt on August 29, 2003, using the Mauna Kea Observatory, though it had been briefly spotted earlier by Matthew J. Holman and JJ Kavelaars on August 13, 2001.

Its name comes from Margaret, the servant of Hero in William Shakespeare’s Much Ado About Nothing, and also nods to the discoverers’ own family members. Margaret orbits Uranus at a great distance—about 14.4 million km (8.9 million mi) on average—taking roughly 4.53 years (1,655 days) to complete one revolution. Its path is highly eccentric and steeply inclined, giving it one of the most stretched-out orbits of any known moon in the Solar System. The Sun’s gravitational pull puts the moon in a Kozai resonance, causing large, synchronized swings in its orbital eccentricity and inclination over a 500-year cycle.

Margaret is estimated to be about 20 km (12 mi) in diameter, but most of its physical traits remain unknown. Margaret was first detected on August 13, 2001, by astronomers Matthew J. Holman and JJ Kavelaars at Cerro Tololo Inter-American Observatory while they searched for distant Uranian moons. They planned to reobserve it by September, but poor observing conditions prevented that. With only a single night’s data, its orbit could not be determined, and the moon was lost.

It wasn’t seen again until August 29, 2003, when Scott S. Sheppard and David C. Jewitt began a Uranian moon hunt using the 8.3-meter Subaru Telescope at Mauna Kea. After they reobserved Margaret with the Gemini North telescope on September 20, Minor Planet Center head Brian G. Marsden realized it matched the 2001 sighting. Kavelaars confirmed this by identifying a single Canada–France–Hawaii Telescope image from August 25, 2001. Follow-up observations ended on September 30, 2003, and the Minor Planet Center announced the discovery on October 7.

NASA and the International Astronomical Union credit Sheppard and Jewitt as the official discoverers. For years after its discovery, Margaret remained poorly tracked, with an uncertain orbit that risked losing it again.