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Last Updated: Thursday, 6 April 2006, 13:24 GMT 14:24 UK
Cassini's super scenes on Saturn
By Jonathan Amos
BBC News science reporter, in Leicester

The images are used to refine details of the moons' orbits (Image: Nasa/JPL/SSI)

Cassini's mission to Saturn was always going to provide majestic picture opportunities, but the images returned to Earth have been even better than expected.

The ringed planet has the looks to put all the other Solar System bodies in the shade.

The picture featured on this page is one of the very latest to be released by the Cassini Imaging Team.

It gives a stunning view across the top of the rings to the large moon Titan in the background. In the foreground, almost lost in the shadows, is one of Titan's little siblings, Janus.

The picture was presented here at the UK National Astronomy Meeting by Professor Carl Murray, of Queen Mary, University of London.

A member of the Cassini Imaging Team, he was reviewing the progress of the mission which has been in orbit since July 2004.

"They are fantastic images and we could spend a lot of time just planning what would be a great picture opportunity, but, actually, all of these images are done for science," he told the BBC News website.

"The reason for doing 'mutual event' observations like this is to improve our understanding of the orbits of these moons."

That positional knowledge is needed by the mission controllers who "drive" Cassini through the Saturnian system, using the gravitational attraction from different objects to manoeuvre the spacecraft on a path to its next observational target. But the purpose goes deeper, still.

Saturn's ring system is wonderfully complex. Having very precise orbital characteristics for the many satellites that run among the rings enables the scientists to make sense of the bands and the very subtle features that are imprinted on them.

"The orbital evolution of the small satellites in particular is determined by their interaction with the rings - they exchange angular momentum. The rings, we believe, are collapsing and the moons should be moving out," said Professor Murray.

"One of our goals is to detect that motion outwards over the course of the mission. If we do that, it will confirm our suspicions that the ring lifetime is short, of the order of a few hundred million years."

Little Janus is not quite circular; the largest dimension on its irregular shape is just under 200km (124 miles).

It is famous for the gravitational dance it performs with another moon, Epimetheus. Separated by 50km (30 miles) or so, the satellites share the same orbit in a "dual carriageway" arrangement. Every four years they swap places: the inner moon becomes the outer and vice versa.

The suggestion is that they may once have been part of the same body.

The Cassini-Huygens mission is a joint venture between the US space agency (Nasa), the European Space Agency (Esa) and the Italian Space Agency (ASI).


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