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Astronomers find 'shepherd' Uranian moons after 14 years
Posted: March 4, 2000

The two moons as seen in Voyager 2 image. Photo: NASA/JPL
One day before the space shuttle Challenger tragedy, Voyager 2 scientists proudly announced the discovery of two small moons around the planet Uranus. Initially dubbed 1986 U7 and 1986 U8, the moons later officially received the Shakespearean names Cordelia and Ophelia. And in the 14 years since their detection, astronomers have been unable to find them. Until now.

Scientists from the University of Arizona, Cornell University and Wellesley College announced today the recovery of Cordelia and Ophelia. This recovery, accomplished by using physical theory and images from the Hubble Space Telescope, confirms, for the first time, the orbits of those Uranian moons, and it gives other astronomers the chance to monitor their travels. The astronomers believe these two moons keep Uranus' thin epsilon ring from gradual radial spreading and eventual dissolution, and thus they are referred to as "shepherd moons."

"Ever since these narrow rings were found around Uranus, it was realized that something was holding them together. What's holding the epsilon ring together are these two moons," says Philip Nicholson, Cornell professor of astronomy, and one of the researchers who contributed to this find. "The same phenomenon is probably happening in other ringed systems," he says.

The moons were discovered in January of 1986 through images sent back to Earth by Voyager 2, and the official announcement was made Jan. 27, 1986, the day before the Challenger accident.

Cordelia and Ophelia are so small -- estimated to be 20 to 30 miles in diameter -- and so far away -- more than 1.7 billion miles from our sun -- that even the powerful Hubble Space Telescope has had trouble seeing them. The orbits of both moons were determined in 1986. But because the observations from Voyager 2 spanned less than two weeks, it was impossible for astronomers to predict, with useful accuracy, where Ophelia and Cordelia would be in their orbits a decade later.

A few weeks ago, Erich Karkoschka, a researcher with the University of Arizona's Lunar and Planetary Lab, began re-examining Hubble Space Telescope images taken in 1997. He electronically stacked dozens of Hubble images on top of each other, matching them pixel for pixel and allowing for the orbital motions of the moons. By comparing the images in this manner, Ophelia popped clearly into view.

But Cordelia still was missing.

Hubble Space Telescope peers deep into Uranus' atmosphere to see clear and hazy layers created by a mixture of gases. Using infrared filters, Hubble captured detailed features of three layers of Uranus' atmosphere. Photo: Erich Karkoschka (University of Arizona Lunar & Planetary Lab) and NASA

Richard French, an astronomer from Wellesley College, and Nicholson had analyzed precise measurements of the radii of both ring edges obtained from stellar occultation data going back to 1977. They had been searching for wavelike distortions that might provide direct evidence of gravitational interactions between the shepherd moons and the ring. The astronomers found a telltale pattern of ripples at the ring's edge, with amplitudes and wavelengths that matched the predictions of the shepherding theory. These ripple patterns revolve around the ring at rates that match the orbital motions of Cordelia and Ophelia. French and Nicholson's years of data permitted these periods to be derived more precisely than calculations from the Voyager images.

Karkoschka's measurement of Ophelia's position turned out to be very close to the position predicted using French and Nicholson's wave-derived measurements. French, using wave-derived data, then provided Karkoschka with a prediction of Cordelia's orbital position. Armed with this information, Karkoschka went back to the Hubble Space Telescope images and, sure enough, found Cordelia exactly at the expected position.

Gravitational perturbations have tipped off astronomers to the existence of celestial bodies for a long time. In 1846, Johannes Galle of the Berlin Observatory discovered the planet Neptune near the position predicted by Urbain Le Verrier of Paris, based on irregularities in the orbit of Uranus. (John Adams of Cambridge had made a similar, but unpublished prediction.) And in 1991, Mark Showalter of Stanford University discovered Pan, the smallest known satellite of Saturn, following up a prediction based on wavelike distortions observed on either side of the Encke Gap in Saturn's A Ring.

"These discoveries illustrate well the fundamental workings of science," says Karkoschka. "First, observations reveal an unexpected object or phenomenon. Predictions are then made based on one or more theoretical models. Further observations support or disprove the models and thereby clarify our understanding of one aspect of the universe in which we live."

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