By Dr David Whitehouse
Science Editor, BBC News website
The Cassini spacecraft has found a new moon of Saturn in a gap between the planet's rings, and scientists say it is making ripples in the ring system.
The new moon was found by the Cassini spacecraft (Image: Nasa/JPL/SSI)
It is only the second moon found inside Saturn's ring system and it has been provisionally named S/2005 S1.
Cassini reached Saturn in July 2004, and is now starting to concentrate on observing the planet's ring system.
The ripples in the rings are caused by the moon's gravity, something that can be used to calculate its mass.
It was first seen in a time-lapse sequence of images taken on 1 May as Cassini's path took it to higher inclinations in its orbit around Saturn.
A day later, an even closer view was obtained, which has allowed a measure of the moon's size and brightness.
The images show the tiny object in the centre of the so-called Keeler gap in Saturn's A ring. The wavy patterns of the gap edges are generated by the moon's gravitational influence.
The Keeler gap is located about 250km (155 miles) inside the outer edge of the A ring, which is also the outer edge of the bright main ring system.
The new object is about 7km (4 miles) across and reflects about half the light falling on it - a similar brightness to that found in particles of the nearby rings.
"It's too early to make out the shape of the orbit, but what we've seen so far of its motion suggests that it is very near the exact centre of the gap, just as we had surmised," said Dr Joseph Spitale, planetary scientist at the US Space Science Institute in Boulder, Colorado.
The new moonlet orbits approximately 136,505km (84,820 miles) from the centre of Saturn. More Cassini observations will be needed to determine whether the moon's orbit around the planet is circular or elliptical.
The other moon that moves inside the ring system is Pan, 25km (16 miles) across, which orbits in the Encke gap. Atlas and other moons exist outside the main ring system, as do the two F ring shepherd moons, Prometheus and Pandora.
Scientists had predicted the new moon's presence after last July's sighting of a set of peculiar spiky and wispy features in the Keeler gap's outer edge.
The similarities of the Keeler gap features to those noted in Saturn's F ring and the Encke gap led imaging scientists to conclude that a small body, a few kilometres across, was awaiting discovery.
"The obvious effect of this moon on the surrounding ring material will allow us to determine its mass and test our understanding of how rings and moons affect one another," said Dr Carl Murray, an imaging team member from Queen Mary college, University of London, UK.
Dr Carolyn Porco, imaging team leader at the Space Science Institute, added: "Some of the most illuminating dynamical systems we might hope to study with Cassini are those involving moons embedded in gaps.
"By examining how such a body interacts with its companion ring material, we can learn something about how the planets in our Solar System might have formed out of the nebula of material that surrounded the Sun long ago.
"We anticipate that many of the gaps in Saturn's rings have embedded moons, and we'll be in search of them from here on."