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Tiny new moon found in Saturn’s outer ring

NASA's Cassini spacecraft has found a tiny moon, or moonlet, orbiting Saturn. The moonlet is embedded within Saturn's sixth, or G ring, and is believed to be a main source of the G ring’s material.

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Saturn and its rings
Saturn and its rings

Cassini imaging scientists, including Professor Carl Murray from Queen Mary’s Astronomy Unit, first spotted the speck of light on 15 August 2008. It has since been captured by cameras on board the Cassini spacecraft on multiple occasions, most recently 20 February 2009. The finding is reported in today's edition of the International Astronomical Union circular.

The tiny moonlet, which is half a kilometre (about a third of a mile) wide, is embedded within Saturn's G ring. Previously, the G ring was the only dusty ring that was not clearly associated with a known moon, which was unusual.

Professor Murray explains: "The moon's discovery and the disturbance of its trajectory by the neighbouring moon Mimas highlights the close association between moons and rings that we see throughout the Saturn system. Hopefully, we will learn more in the future about how such arcs form and interact with their parent bodies."

Saturn's rings were named in the order they were discovered.  Working outward they are: D, C, B, A, F, G and E. The G ring is one of the outer diffuse rings. Within the faint G ring there is a relatively bright and narrow, 250-kilometre-wide (150-miles) arc of ring material, which extends 150,000 kilometres (90,000 miles), or one-sixth of the way around the ring's circumference. The moonlet moves within this ring arc.

Previous Cassini plasma and dust measurements indicated that this partial ring may be produced from relatively large, icy particles embedded within the arc, such as this moonlet.

The moonlet is too small to be resolved by Cassini's cameras, so its size cannot be measured directly. However, Cassini scientists estimated the moonlet's size by comparing its brightness to another small Saturnian moon, Pallene.

This brings the number of Saturnian ring arcs with embedded moonlets found by Cassini to three. 

Early next year, Cassini's camera will take a closer look at the arc and the moonlet.  The Cassini Equinox mission, an extension of the original four-year mission, is expected to continue until Autumn 2010.

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