Cameras aboard the Voyager spacecraft in 1986 discovered Cordelia and Ophelia, two small moons less than 40 miles (60 km) in diameter which orbit just inside and outside the epsilon ring, the main ring of Uranus.

Researchers determined the orbits of both moons, but as the observations spanned less than two weeks, it was impossible for them to accurately predict where the moons would be in their orbits a decade later. These two moons are often referred to as "shepherds," as they are believed to prevent the gradual radial spreading and eventual dissolution of the ring.

Further images of Uranus were taken by HST between 1994 and 1998, but none seemed to show Cordelia or Ophelia.

Recently Erich Karkoschka of the UA Lunar and Planetary Lab, a researcher who last year discovered the 18th moon of Uranus, took a closer look at a series of images of Uranus taken with the HST in 1997.

To search for Cordelia and Ophelia, he stacked dozens of images on top of each other after first rotating them to allow for the orbital motions of the moons. Ophelia popped clearly into view.

He e-mailed his findings to Richard French of Wellesley College, who had refined the motions of both satellites in 1995. French and Philip Nicholson of Cornell University had analyzed precise measurements of the radii of both edges of the ring obtained from stellar occultation data going back to 1977, in a search for wavelike distortions which might provide direct evidence of gravitational interactions between the shepherd moons and the ring.

A telltale pattern of ripples was indeed found on each edge, with amplitudes and wavelengths which matched the predictions of the shepherding theory.

These patterns revolve around the ring at rates which match the orbital motions of Cordelia and Ophelia. With 16 years of occultation data, the scientists could calculate the orbital periods with much higher precision than could be calculated from the Voyager images.

Despite the apparently good match to the theory, however, the ripple amplitudes were so small (three tenths of a mile or a half kilometer on each edge) that the scientists were not entirely sure that the waves were real.

However, Karkoschka's measurement of the Ophelia's position turned out to be very close to the position predicted using the wave-derived period. Given this success, French suggested that Karkoschka search for Cordelia, the fainter of the two satellites.

French provided Karkoschka with a prediction of where Cordelia should be found based on the studies of the rippled edge of the epsilon ring. Karkoschka studied the images closely and found Cordelia exactly at the expected location.

"Based on these findings, the positions of both moons can now be predicted for many decades into the future," Nicholson says. "More importantly, the recovery of the two bodies at their predicted positions lends support to the wave detections and thus to the underlying shepherding theory, which was originally proposed by Peter Goldreich and Scott Tremaine of Caltech in 1978."

"Successful searches for planets or moons at positions predicted by their gravitational interactions with other objects have occurred in the past," Karkoschka says.

"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, he adds.

"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.'"

Karkoschka says that he, Nicholson and French agree that "these discoveries illustrate well the fundamental workings of science."

"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."