In one announcement, astronomers revealed the discoveries of two previously anonymous moons. One is part of a large group of satellites around Uranus, while the other orbits a member of the asteroid belt called 121 Hermione.
In the International Astronomical Union (IAU) Circular 7980, astronomers from the United States and Canada presented the orbital elements of an object originally spotted last year. Provisionally called S/2001 U1, the satellite is "irregular," having an orbit that's highly eccentric and inclined to the ecliptic. According to co-discoverer Matthew Holman of the Harvard-Smithsonian Center for Astrophysics (CfA), if the object has an albedo typical of irregular satellites (reflecting six percent of incoming sunlight), its faint magnitude translates into a diameter of 20 kilometers.
Holman and his colleagues used the 4-meter Blanco Telescope at Cerro Tololo Inter-American University in Chile to first image Uranus's newfound moon on August 13, 2001. During the following 13 months, astronomers used a number of powerful telescopes around the world to track the faint object. Not until all of this data was compiled could Brian Marsden of the CfA determine an accurate orbit for S/2001 U1.
"Tracking irregular satellites is an enormous undertaking involving the effort of many people," Holman explains. "Such objects are easily lost if they are not re-observed within 1 to 2 months after their discovery. If lost, they must simply be rediscovered."
Last fall, Uranus had 21 recognized moons. However, in December the IAU revoked the provisional status of one moon, designated 1986 U10, dropping the count to 20. The new discovery raises the planet's moon count back to 21.
The discoverers are also hunting down at least two other possible satellites of Uranus.
"We do have other uranian candidates," Holman says. "Unfortunately, poor weather and poor imaging conditions affected some of our critical follow-up observing opportunities last year, so we were not able to establish definitive orbits for the others. We will try in the future to 'rediscover' those satellite candidates."
In the same circular, another team from the U.S. and France announced its detection of a moon around a 209-kilometer-wide asteroid named Hermione. The group observed Hermione on September 28 from the 10-meter Keck II telescope in Hawaii, but only a half hour before the asteroid pair set behind the dome of Keck I. This wasn't enough time to see the satellite move relative to Hermione, so the team can't yet determine an orbit for it. "All we know for sure is that it was at least 630 kilometers separation at the time we observed it," reports Southwest Research Institute co-discoverer William Merline.
The researchers estimate that the satellite is about 13 km wide. They also verified that there was no other known background or foreground object that could have posed as a partner to Hermione during the observations.
The study of Hermione is part of a search for satellites around approximately 750 asteroids. According to Merline, the new asteroid pair resembles six of the other eight main-belt asteroid partners discovered so far. Like Hermione, the majority consist of a large primary asteroid orbited by a small moon. Of the two oddballs, one (90 Antiope) is a true "double" consisting of two nearly equal partners, and the other (3749 Balam) has two widely separated components.
"Our observations are telling us a great deal about how these binaries are made and about the collisional conditions in the main-belt and other asteroid populations," Merline says. "Similarities and differences among the detected binaries give us insight into the formation mechanisms.
"Further, the presence of a small satellite gives us a chance to measure the density of the primary asteroid. And density is one of the most fundamental quantities we need to know if we want to understand a body's composition and structure."