Neptune's Hasty Moon

Ever since Newton, astronomers have been calculating the orbits of planets and moons and getting them exactly right. But Galatea, a small satellite of Neptune, is ahead of schedule, observers reported this week in a circular of the International Astronomical Union--and no one knows why. To explain this puzzling haste, astronomers are blaming everything from the gravitational tug of Neptune's mysterious Adams ring to the pull of other moons to an error in the original orbital calculations.

A team led by Claude Roddier of the Institute for Astronomy of the University of Hawaii, Manoa, learned that the 160-kilometer moon was straying from its orbital timetable on 6 July, when they tracked it down with the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea. The observations--the first in the 9 years since Galatea was discovered by the Voyager 2 spacecraft--showed that Galatea was 5 +/-1 degrees ahead of its predicted position, or 8.6 minutes ahead of schedule. The difference, they said, is "possibly due to [Galatea's] interaction with Neptune's Adams ring."

The Adams ring, lying a mere 1000 kilometers outside Galatea's orbit, has a strange, arclike appearance, indicating that its dust particles are spread unevenly around its full circumference. Galatea's gravity is presumably sweeping the particles into clumps. But for the ring to pull back strongly enough to affect the satellite's orbit, says Carolyn Porco of the Lunar and Planetary Laboratory of the University of Arizona, it "would have to have substantial mass." She speculates "that there are bigger bodies [hidden in the ring], which are the source of the dust that we actually see."

Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, isn't so sure that there's a deviation to explain. For Galatea's orbit to accumulate five degrees of drift in 9 years, its period would have to differ from its predicted value by a mere 0.07 seconds. "My own inclination is that the prediction is off simply because the observations used for it were only [a few images] from Voyager," he says. Even if it isn't, Marsden thinks that subtle perturbations from other moons, or from the oblate shape of Neptune itself, could have skewed Galatea's orbit.