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UA researcher rediscovers two key moons around Uranus

Since the discovery of moons around Uranus in 1986, scientists have been unable to confirm theories of gravitational interactions between two key moons and the dominant ring around the planet.

Now, a University of Arizona researcher has unraveled the mystery simply by looking at photographs.

UA Lunar and Planetary Laboratory researcher Erich Karkoschka rediscovered the moons - which had not been seen since they were originally found in 1986 - by examining 1997 photos taken by the Hubble Space Telescope.

According to scientists' theories, the two moons - Cordelia and Ophelia - are key to understanding why the main ring around Uranus is uniform and well-defined.

"There were many questions as to why the rings were staying in place and not separating," Karkoschka said.

Richard French, a researcher at Wellesley College in Wellesley, Mass., worked with Karkoschka to find Cordelia and Ophelia.

"When the Uranian rings were discovered in 1977, they were a real shock, since it was hard to see what could keep a narrow ring from spreading," French stated in an e-mail interview.

Karkoschka and other scientists have long theorized that the ring was held in place because of a gravitational influence between the two moons and the ring.

Scientists theorized that the moons keep the rocks and particles of the ring in place similar to the way a shepherd keeps his sheep in line.

The dominant ring - composed of dark rocks from a few inches to a meter in size - sits directly between the two moons' orbits.

The 1997 Hubble Space Telescope pictures were faint, making it difficult for scientists to locate the two moons. Researchers - including Karkoschka- nearly gave up hope of finding them.

"I remember talking to him a few years ago, when he told me that he had recovered all but two of the small Uranian satellites. I said, 'That's great, Erich, but what about Cordelia and Ophelia? Those are the two that are most interesting dynamically. Are you sure you can't find them in the images?'" French said.

Then Karkoschka tried a new approach in his last attempt to find the moons. Using a computer, he stacked images taken by the Hubble Space Telescope on top of each other, making light picked up by the camera brighter.

Astronomer Philip Nicholson, of Cornell University in Ithaca, N.Y., made up the final link of the team.

"It was kind of like stacking negatives on top of one another and making a print. The more you stack, the brighter the image," Nicholson said.

Karkoschka was successful using this method and rediscovered the moons last month.

"It was an unexpected bonus," Nicholson said.

With the rediscovery, scientists were able to confirm a relationship between Cordelia and Ophelia, and the dominant ring.

"The theory said that the inner edge of the ring would have a wavy edge, and that these patterns would move along the edge of the ring with exactly the same period as the (moons) that produced them," French said.

Using the photographs, their theories proved true.

French said the finding was crucial.

"If he (Kakoschka) had not found them, we would have been left in the position (of) having made a prediction that could not yet be observationally tested." French said. "What this rediscovery does is to place the shepherding theory on a stronger observational foundation."

Currently, there are no plans for another spacecraft to get close enough to the distant planet to take more pictures.

"It takes about 10 years for a spacecraft to get there," Kakoschka added.