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Polydeuces (moon)

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Saturn's moon Polydeuces
Saturn's moon Polydeuces
Discovered by Cassini Imaging Science Team
Discovery date October 24, 2004
Pronunciation /ˌpɒlɨˈdjsz/ POL-i-DEW-seez
Saturn XXXIV (34)
S/2004 S5
Orbital characteristics[1]
377,396 km [a]
Eccentricity 0.0192 [1]
2.736915 d [a]
Inclination 0.1774 ± 0.0015° [1]
Satellite of Saturn
Physical characteristics
Dimensions 3 × 2.5 × 2 km[2]
Mean radius
1.3 ± 0.4 km[2]
Mass 1–5 ×1013 kg[b]
assumed synchronous

Polydeuces, or Saturn XXXIV (34), is a small natural satellite of Saturn that is co-orbital with the moon Dione and librates around its trailing Lagrangian point (L5). Its diameter is estimated to be 2–3 km.[2] It is pronounced /ˌpɒlɨˈdjsz/ POL-i-DEW-seez; Greek: Πολυδεύκης. Dione's other co-orbital moon is Helene, which is bigger and located at the leading L4 point.[3]

Polydeuces was discovered by the Cassini Imaging Team on October 24, 2004,[4] in images taken on October 21, 2004,[5][6] and given the temporary designation S/2004 S 5. Subsequent searches of earlier Cassini imaging showed it in images as far back as April 9, 2004.[4] Of the four known Lagrangian co-orbitals in the Saturn system ('trojan moon'), Polydeuces wanders the farthest from its Lagrangian point: its distance behind Dione varies from 33.9° to 91.4° with a period of 790.931 days (for comparison, L5 trails Dione by 60°).[1] Polydeuces's libration is large enough that it takes on some qualities of a tadpole orbit, as evidenced by the clear asymmetry between excursions towards and away from Dione. In the course of one such cycle, Polydeuces's orbital radius also varies by about ± 7660 km with respect to Dione's.[4]

The name Polydeuces was approved by the IAU Working Group on Planetary System Nomenclature on January 21, 2005.[7] In Greek mythology, Polydeuces is another name for Pollux, twin brother of Castor, son of Zeus and Leda.


  1. ^ a b The mean semi-major axis and period must be identical to Dione's.
  2. ^ based on density 0.5–2 g/cm³

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