- Press Release
- Oct 31, 2023
For the first time in 14 years, astronomers from Arizona, Cornell and Wellesley spot two ‘shepherd’ Uranian moons
Contact: Blaine P. Friedlander Jr.
Office: (607) 255-3290
ITHACA, N.Y. — One day before the space shuttle Challenger tragedy, Voyager II 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
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 II, 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 II 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.
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
The web version of this release may be found at:
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