Scientists have a new explanation for weird movements
of two small moons that shepherd one of Saturn’s rings:
Pandora, which keeps the narrow F ring from spreading
outward, and Prometheus, which rides herd along the same
ring’s inner edge.

Observations of the pair in recent years found them
far from where they should have been based on orbital
movements calculated from NASA’s Voyager spacecraft
observations during Saturn flybys in 1980 and 1981.
Pandora is about 20 degrees farther around in its orbit
than it would be if it had followed standard physics for
the past two decades. Prometheus lags behind its predicted
position by about the same amount. At the size of these
moons’ orbits, 20 degrees is more than 160,000 kilometers
(100,000 miles).

“Chaotic gravitational interactions between them can
fully account for these discrepancies,” said Dr. Nicole
Rappaport of NASA’s Jet Propulsion Laboratory, Pasadena,
Calif. She and Prof. Peter Goldreich of the California
Institute of Technology, Pasadena, reported the new
explanation this week at the annual meeting of the American
Astronomical Society’s Division of Planetary Sciences in
Birmingham, Ala.

With chaotic interactions, a barely perceptible
difference in starting conditions can make such a great
difference in later positions that the movements are not
fully predictable over time. The two moons give each other
a gravitational kick each time Pandora passes inside
Prometheus, about every 28 days. Because neither’s orbit is
quite circular, the distance between them on those
occasions — hence the strength of the kick — varies. The
perturbations lead to changes in motion that are not
periodic or predictable, say Rappaport and Goldreich.

They had predicted 20 years ago that Pandora’s motion
might be chaotic. “This is like a dream come true,”
Goldreich said of the observations that fit the prediction.
For him, it’s a recurring dream, since he and Dr. Scott
Tremaine, then a post-doctoral fellow at Caltech, also
predicted the very existence of shepherd moons — confirmed
later by Voyager — as an explanation for the narrowness of
the F ring.

By the principle of action and reaction, the same
transfer of momentum by which Pandora pushes F ring
particles inward toward Saturn and Prometheus pushes them
outward should also gradually push the two moons away from
the ring. The effects of Saturn’s wider A ring, which is
just inside the F ring, are even more important in pushing
the moons outward. The F ring would slowly widen. So,
although the discovery of the shepherd moons fulfilled a
scientific prediction explaining the current narrowness of
the F ring, it led to more questions about the longer-term
history and fate of the ring and shepherds.

One theory among several has been that the ring must
be quite young to still be so narrow, perhaps about 10
million-years-old. But in proportion to the age of Saturn
itself, that would be as if the ring were created in the
last three minutes of a 24-hour day, and some scientists
believe it unlikely that we would happen to witness such a
rare event. Another theory, proposed by Rappaport,
Goldreich and Tremaine in 1982, is that Pandora’s movement
includes an element of chaos.

The story took a further twist in the mid-1990s, while
Saturn’s rings were edge-on toward Earth so the timing was
good for observing Prometheus and Pandora with NASA’s
Hubble Space Telescope. Analysis of those and subsequent
observations by several researchers in New York and
Massachusetts revealed that each moon was about 20 degrees
from where normal orbital mechanics would have put it. This
was an amazing discovery. For the first time ever,
astronomers found that they could not predict the orbits of
objects in the sky.

Goldreich and Rappaport have demonstrated that chaotic
orbits of the F ring shepherds could produce changes in
location very similar to those observed with the Hubble
Space Telescope. “The chaos is due to the gravitational
interactions between the two moons,” Rappaport said. “This
is the first observation ever of chaotic orbital motions in
the solar system.” A larger moon of Saturn, Hyperion, is
already known to have chaotic rotation around its axis.

Goldreich and Rappaport hope they have found the piece
of the puzzle that will help resolve the problem of the age
of the rings.

JPL, NASA’s lead center for robotic exploration of the
solar system, is managed for NASA by the California
Institute of Technology, Pasadena.