Douglas Isbell
Headquarters, Washington, DC          October 5, 1995
(Phone:  202/358-1753)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4514)


RELEASE:  95-172

SATURN MOON MYSTERY CONTINUES:
COULD HUBBLE HAVE DISCOVERED SHATTERED SATELLITES?

     NASA's Hubble Space Telescope has discovered several
orbiting clumps of icy rubble that could be the remnants of
recently shattered moonlets orbiting near the outer edge of
Saturn's ring system.

     Astronomers say this could represent the discovery of
a new class of ephemeral, transitional object in the solar
system which provides new clues to the origin and evolution
of Saturn's spectacular rings.

     This startling conclusion is based upon Hubble's
observation of Saturn made as Earth crossed the plane of
the ring system on August 10, which provided a rare
opportunity to seek out faint satellites in and near the
ring plane.

     "Ring plane crossing" refers to the brief interval
when the Earth crosses the plane of Saturn's rings,
allowing them to be seen edge on. At such times, the
usually bright rings are seen only as a faint, thin line,
and Saturn's smaller satellites become visible.  These
events are rare, occurring in groups of two or four at
intervals of about 14.5 years.  The previous series of
crossings occurred in 1980.

     The latest Hubble pictures gave astronomers an
opportunity to confirm the presence of two new satellite
first discovered by the Space Telescope in images taken
during the May 22 ring plane crossing. Rather than solving
the moon question, however, the August observations
presented astronomers with a new mystery.

     "We realized these moons are too bright to have gone
undetected when the Voyager spacecraft flew by Saturn in
1980 and 1981," said Philip Nicholson of Cornell
University, Ithaca, NY.

     A further complication is that the August pictures
seem to show at least three new objects, and in different
orbits from the two May objects.

      "They also appear to be very elongated or arc-like,
unlike a satellite should be," Nicholson said. "One
possibility is that they are large clouds of debris from
small satellites shattered by impacts with chunks of space
debris, possibly comets, sometime during the 14 years since
the Voyager 2 flyby."

     Just as a small handful of chalk dust can make a large
dust cloud if tossed in the air, a shattered moonlet would
be much brighter and more visible than when all of its mass
is compressed into a single solid body.

     The discovery of objects in this transitional phase is
not totally unexpected, Nicholson said, because one
scenario for the origin of Saturn's ring system is that it
is made up of countless fragments from several pulverized
moons. This idea is reinforced by the fact the new objects
orbit Saturn near the narrow F ring, which is a dynamic
transition zone between the main rings and the larger
satellites. Moonlets in this region can easily be disrupted
by Saturn's tidal pull if they are fractured by an impact,
forming a cloud of debris.  Eventually such a cloud would
spread around the moon's orbit to form a new ring.

     The dynamics of this zone also are evident in Hubble's
observations of the satellite Prometheus. Although a third
object seen in the May images was first suspected to be
another new satellite because its location did not match
the predicted position for any of the known satellites
charted by Voyager, it now appears that this body is in
fact Prometheus, which has slipped in its orbit by 20
degrees from the predicted position. Nicholson suggests
that this may be a consequence of a "collision" of
Prometheus with the F ring, which is believed to have
occurred in early 1993.  The moon may have passed close
enough to one of the denser, lumpy regions of the F ring to
have its orbit changed.

     The researchers plan to obtain further observations of
Saturn's moons and rings during the third ring plane
crossing, which occurs on November 21.

                    -end-