BBC - Saturn probe sights mystery moons

Cassini-Huygens, Nasa http://saturn.jpl.nasa.gov/index.cfm

The Cassini probe en route to Saturn has spotted two "shepherd" moons which
keep one of the planet's rings in check through their gravitational influence.
Prometheus and Pandora stabilise Saturn's narrow, ribbon-like F-ring.

But the moons may have other, chaotic effects on the F-ring - their orbits
become unpredictable when they approach each other very closely.

Cassini will begin a four-year mission of exploration when it enters orbit
around Saturn on 1 July 2004.

It will release its piggybacked Huygens probe about six months later for
descent through the thick atmosphere of Saturn's moon Titan.

Clumping effect

The two small, irregularly shaped shepherd moons exert a gravitational
influence on particles that make up the F-ring.

The outermost satellite, Pandora, which is about 84km (52 miles) across, keeps
the F-ring from spreading away from Saturn.

The innermost moon, Prometheus, which is about 102km (63 miles) across,
prevents the F-ring from spreading towards the giant planet.

However, the moons may also be responsible for features that sometimes form in
the ring known as clumps and strands.

Better data

The moons were discovered in images returned by the Voyager 1 spacecraft in
1980.

One of the goals for the Cassini-Huygens mission is to get more accurate data
on the moons' orbits. Monitoring how their orbits change over the course of
the probe's mission will help Cassini determine the moons' masses.

This will help scientists learn more about Prometheus and Pandora and give a
more complete understanding of their effects on the planet's rings.

The image was taken when Cassini was 55.5 million km (34.5 million miles) from
Saturn.

Cassini-Huygens is a cooperative project of the US space agency (Nasa), the
European Space Agency (Esa) and the Italian Space Agency.

Nick wrote:

Cassini-Huygens, Nasa http://saturn.jpl.nasa.gov/index.cfm

The Cassini probe en route to Saturn has spotted two "shepherd" moons which
keep one of the planet's rings in check through their gravitational influence.
Prometheus and Pandora stabilise Saturn's narrow, ribbon-like F-ring.

But the moons may have other, chaotic effects on the F-ring - their orbits
become unpredictable when they approach each other very closely.

I dimly remember these moons being recovered (by Hubble) a couple of
years ago when they were found to be about 30 or 60 degrees away from
the positions predicted by the 1980 data, the first definitely
unpredicatable, possibly chaotic, orbits in the solar system, a cause
for much rejoicing among the cognoscenti.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47