Moon of Uranus is demoted

Posted: December 31, 2001

In an unusual move, the International Astronomical Union has recently stripped the title of "moon" from a body orbiting Uranus discovered in images taken by the Voyager 2 spacecraft.

The IAU concluded earlier this month that there is not as yet enough information to confirm that the object, designated S/1986 U 10, is in fact a natural satellite of Uranus. The decision was largely unnoticed until The Planetary Society, a space advocacy and education group, publicized the news on Friday.

Erich Karkoschka, a University of Arizona researcher, discovered the object in May 1999, more than 13 years after Voyager 2 flew by Uranus on its grand tour of the outer solar system. Karkoschka found the body in the public archive of Voyager 2 images as part of a project to compare those images with more recent ones taken by the Hubble Space Telescope. The object was given the temporary designation S/1986 U 10, indicating it was the 10th moon of Uranus discovered in images taken in 1986.

"This discovery is very unusual," Karkoschka said in 1999. "Typically, satellites are found within days after the discovery image has been taken. In this case, the discovery image is more than 13 years old."

Karkoschka found S/1986 U 10 in seven Voyager 2 images. The object appeared to be in nearly the same orbit as another moon, Belinda, about 75,000 km from Uranus. The object appeared to be about 40 km in diameter, based on its brightness in the Voyager images.

S/1986 U 10 has not been seen since the Voyager 2 discovery, despite recent observing efforts that have discovered several other small moons orbiting the planet. Based on this limited amount of data, the IAU concluded it is premature to consider the object a moon. That decision is based on criteria that the IAU has made stricter in recent years. Ironically, if S/1986 U 10 has been discovered at the time the Voyager 2 images were taken in 1986, it would likely have been confirmed as a moon at that time based on the looser criteria then in place.

The IAU indicated they would reevaluate S/1986 U 10's status as a potential moon of Uranus after the Hubble Space Telescope attempts to obtain an image of the moon. That image could come in the next couple years, depending on observing schedules for the orbiting observatory.

The demotion of S/1986 U 10 leaves Uranus, at least temporarily, with 20 known moons. This is third in the solar system behind Saturn, with 30 moons, and Jupiter, with 28. The discovery of S/1986 U 10 in 1999 made it at the time the 18th moon of Uranus, tying it with Saturn with the most in the solar system. However, a flurry of moon discoveries around Jupiter and Saturn within the last two years has put them well ahead of Uranus.