Farewell to Peggy

Before the Cassini spacecraft makes its final plunge into Saturn's atmosphere in September this year, one of its last acts will be to take a photo of an intriguing object that Professor Carl Murray of the School of Physics and Astronomy has been studying for nearly four years.

Nicknamed "Peggy" after Prof. Murray's mother-in-law, since it was first noticed on her birthday in 2013, the object appears as a small smudge of light at the very edge of Saturn's main ring system, the A-ring. The smudge is actually a cloud of dust particles some 2000 km long that is being disturbed by the presence of Peggy itself, thought to be a solid object less than 5 km across. The dust may well be the result of a collision between Peggy and another object in the ring.

At that size, it has not been possible to obtain a direct image of Peggy proper and the studies so far have involved tracking Peggy's gravitational effect on the rings. But now, as Cassini nears the end of its life and moves closer to the rings, it will be possible to obtain a very high resolution image which Prof. Murray hopes will reveal Peggy.

The Cassini mission, with which Prof. Murray has been involved since 1990, has provided a unique opportunity to study not only one of our most spectacular neighbouring planets, but also the dynamical processes that shape the amazing ring system. From these detailed studies of Saturn's rings we can extrapolate to larger structures in the Universe, in particular the "protoplanetary discs" of material around forming stars from which systems of planets grow. An interesting development is that we now think that planets migrate within the disc as they form, a feature illustrated by Peggy, which has moved in and out by a few kilometres every so often. This is important for our understanding of how the Solar System as a whole formed and evolved.

These final months of the Cassini spacecraft bring to and end a very productive era in planetary astronomy. Even as Carl Murray prepares to say farewell to Peggy, he looks forward to its being revealed to view at last.

This BBC News report includes an interview with Carl Murray about Peggy.

Image credit:  NASA/JPL-Caltech/Space Science Institute