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Polydeuces, one of Dione's trojan moons
A Trojan moon is a natural satellite of a planet occupying the L4 or L5 equilateral Lagrangian points of a primary-moon system. They are named by analogy with the Trojan asteroids, which occupy the L4 and L5 points of the Sun-Jupiter system.
Four examples are known, all in Saturn's system: Telesto and Calypso bracket Tethys, whilst Helene and the newly discovered Polydeuces bracket Dione. The Earth-Moon points have been repeatedly searched for Trojans, but nothing else than a possible slight overabundance of dust (see Kordylewski clouds) has ever been found.
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