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FOR IMMEDIATE
RELEASE CONTACT: Mary
Ann Hill In 1986, cameras aboard the
Voyager spacecraft 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. The orbits of both moons were determined,
but as the observations spanned less than two weeks, it
was impossible to predict with useful accuracy where in
their orbits the moons would be a decade later. These two
moons are often referred to as "shepherds" because they
are believed to prevent the gradual radial spreading and
eventual dissolution of the ring. Further images of Uranus
were taken by Hubble between 1994 and 1998, but none
seemed to show Cordelia or Ophelia. Recently Erich Karkoschka of
the University of Arizona 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
French, who had refined the motions of both satellites in
1995. French and Nicholson 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, and the 16 years of occultation
data permitted these periods to be calculated to much
higher precision than that afforded by the Voyager
images. Despite the apparently good match to the theory,
however, the ripple amplitudes were so small (0.3 mile or
0.5 km on each edge) that the reality of the waves was in
some doubt. However, Karkoschka's
measurement of the position of Ophelia turned out to be
very close to the position predicted using the
wave-derived period. With this success in hand, French
suggested that Karkoschka search for Cordelia, the
fainter of the two satellites, and provided him with a
prediction of its position based on the studies of the
rippled edge of the epsilon ring. Karkoschka accepted the
challenge, examined the images closely and found Cordelia
exactly at the expected location. "What is particularly
exciting about these findings is that Erich's 'discovery'
of Ophelia was completely independent of our earlier
prediction based on a different scientific method," notes
French. "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 adds. "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," the
researchers note. "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." Contact Info: Erich
Karkoschka Phillip
Nicholson Richard
French Lunar and Planetary
Lab Astronomy
Department Whitin
Observatory University of
Arizona Cornell
University Wellesley
College Tucson, AZ
85721-0092 Ithaca NY
14853-6801 Wellesley, MA
02481 (520)
621-3994 (607)
255-8543 (781)
283-3747 erich@lpl.arizona.edu pdn2@cornell.edu rfrench@wellesley.edu ###
March 2, 2000
(781) 283-2376
TWO SMALL MOONS OF URANUSWELLESLEY,
Mass. -- Using images taken with the Hubble Space
Telescope, a University of Arizona scientist has found
two small moons of Uranus which have not been seen for 14
years. This recovery also marks another triumph for
celestial mechanics, as their positions were predicted by
astronomers Richard French of Wellesley College and
Philip Nicholson of Cornell University based on the
moons' gravitational interaction with the rings of
Uranus.
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