Discovering Polydeuces

An image of Iapetus lit by 'Saturnshine' on 22 October 2004, showing the 'halo' to the east of Cassini Regio.

Six narrow-angle camera frames taken over a 3-hour period on 21 October 2004 led to the discovery of S/2004S5.180'181 A search of earlier Cassini images traced it back six months.182 Subsequently named Polydeuces, it is about 5 kilometres in size and is a Trojan of Dione. Trojans occupy the gravitationally stable Lagrange points 60 degrees ahead of, or in trail of, a body. Three Trojan moons had already been found in the Saturnian system:

Telesto and Calypso with Tethys, and Helene with Dione. Helene's position oscillates, but it remains within 15 degrees of the leading point. Polydeuces can evidently stray as much as 30 degrees from the trailing point.183,184 Cassini would pass Polydeuces at a range of only 150 kilometres in February 2005, but this was not realised in time to arrange an inspection, since encounter sequences had to be specified well in advance.

An image of Iapetus lit by 'Saturnshine' on 22 October 2004, showing the 'halo' to the east of Cassini Regio.

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