Saturn's 18th moon linked to dusty object.

Last year, Mark R. Showalter completed a painstaking computer search through some 30,000 images of Saturn, each obtained more than a decade ago by Voyager 2. His labor resulted in the discovery of Saturn's 18th moon, now known as 1981 S13 -- the only satellite known to lie within the planet's main system of three clearly visible rings (SN: 8/4/90, p.69). Further analysis of the photos has now enabled the astronomer to pinpoint the position of this satellite -- soon to be renamed Pan -- to within 1 kilometer.

The finding, says Showalter, an astronomer at Stanford University, makes Pan "the most accurately determined location of anything in Saturn's ring system." As a result, astronomers can now use Pan as "an anchor," or reference point, for locating other ring features with five to 10 times the previously existing accuracy. Showalter reports the findings in the June 27 NATURE.

Tracking Pan through photos taken during nine of its consecutive orbits, Showalter calculated that the moon lies 133,583 km from Saturn's center. Pinpointing that distance makes it possible to more accurately measure, for example, the position of spiral density waves -- local distortions in the main rings that orbit in sync with Saturn;s moons outside the rings. He also found that Pan's orbit -- at the center of a 320-km-wide region called Encke's gap, inside Saturn's outermost main ring -- coincides with that of a dusty ringlet. Showalter says the matching orbits indicate that the ringlet formed when micrometeoroids bombarded Pan, chipping off small bits of it.

Showalter says he has "bled dry" Pan studies with the Voyager images. But the possibility of pinpointing Pan with even better accuracy may lie just a few years away. Showalter holds out hope that the Hubble Space Telescope may glimpse the satellite during a rare alignment of Earth and Saturn in 1995.

In that year, Earth's orbit will permit a brief, edge-on view of Saturn's rings. Hubble, which by then should have received corrective optics to compensate for its flawed primary mirror, may be able to discern Pan (with a radius of about 10 km) sticking above or below the plane of the rings like a bug on a phonograph record, Showalter says. If that observation doesn't pan out, researchers will have to await high-resolution photos sent back from the planned spacecraft Cassini, scheduled to reach Saturn early next decade.