Helene (moon)

Helene
Discovery
Cassini image of Helene against the backdrop of Saturn's clouds (March 3, 2010)
Discovered by	Laques and Lecacheux^[1]
Discovery site	Pic du Midi Observatory
Discovery date	March 1, 1980
Designations
Alternative names	Helene Dione B Saturn XII (12) S/1980 S 6
Orbital characteristics
Semi-major axis	7008377396000000000♠377396 km
Eccentricity	6997220000000000000♠0.0022
Orbital period	7005236469456000000♠2.736915 d ^[2]
Inclination	0.199° (to Saturn's equator)
Satellite of	Saturn Dione (L4 trojan)
Physical characteristics
Dimensions	43.4 × 38.2 × 26 km ^[3]
Mean radius	7004176000000000000♠17.6±0.4 km ^[3]
Albedo	7000167000000000000♠1.67±0.20 (geometric) ^[4]

Helene (/ˈhɛlɪniː/ HEL-ə-nee;^{[lower-alpha 1]} Greek: Ἑλένη) is a moon of Saturn. It was discovered by Pierre Laques and Jean Lecacheux in 1980 from ground-based observations at Pic du Midi Observatory,^[1] and was designated S/1980 S 6.^[5] In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus (Saturn) in Greek mythology.^[6] Helene is also designated Saturn XII (12), which it was given in 1982, and Dione B,^{[lower-alpha 2]} because it is co-orbital with Dione and located in its leading Lagrangian point (L₄). It is one of four known trojan moons.

Exploration

Helene was initially observed from Earth in 1980,^[5] and Voyager flybys of Saturn in the early 1980s allowed much closer views. The Cassini–Huygens mission, which went into orbit around Saturn in 2004, provided still better views, and allowed more in-depth analysis of Helene, including views of the surface under different lighting conditions. Some of the closest images of Helene to date are from the Cassini spacecraft's 1800 km flyby on March 3, 2010, and another very successful imaging sequence occurred in June 2011. There have been many other approaches over the course of the Cassini mission, and future flybys may yield additional data.

Selected observations

Mostly raw greyscale images with near infrared or ultraviolet channels.

High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011)

Flow-like features on Helene's leading hemisphere (Cassini, January 2011)

Helene's Saturn-facing side, lit by saturnshine (Cassini, March 2010)

Close-up of Helene with Saturn in the background (Cassini, March 2010)

Cassini image from March 3, 2010

Cassini orbiter image from November 2008

Cassini image taken July 2007

Voyager 2 image (August 1981)

References

Notes

↑ /ˈhɛlɪniː/ HEL-ə-nee is the regular pronunciation, as expected from the Greek etymology, but /hɪˈliːniː/ hə-LEE-nee and /hɪˈliːn/ hə-LEEN are also heard. The Modern Greek pronunciation is e-LAY-nee.
↑ Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983)

Citations

1 2 Lecacheux1980.
↑ NASA Celestia Archived March 9, 2005, at the Wayback Machine.
1 2 Thomas 2010.
↑ Verbiscer French et al. 2007.
1 2 IAUC 3496.
↑ IAUC 4609.

Sources

Lecacheux, Jean. (July 1980). "A new satellite of Saturn: Dione B" (discovery). Icarus. 43 (1). doi:10.1016/0019-1035(80)90093-7. Retrieved 2016-05-01.
Marsden, Brian G. (July 31, 1980). "Satellites of Saturn" (discovery). IAU Circular. 3496. Retrieved 2011-12-23.
Marsden, Brian G. (September 30, 1983). "Satellites of Jupiter and Saturn". IAU Circular. 3872. Retrieved 2011-12-23.
Marsden, Brian G. (June 8, 1988). "Satellites of Saturn and Uranus" (naming). IAU Circular. 4609. Retrieved 2011-12-23.
Thomas, P. C. (July 2010). "Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission" (PDF). Icarus. 208 (1): 395–401. Bibcode:2010Icar..208..395T. doi:10.1016/j.icarus.2010.01.025.
Verbiscer, A.; French, R.; Showalter, M.; Helfenstein, P. (9 February 2007). "Enceladus: Cosmic Graffiti Artist Caught in the Act". Science. 315 (5813): 815. Bibcode:2007Sci...315..815V. PMID 17289992. doi:10.1126/science.1134681. Retrieved 20 December 2011. (supporting online material, table S1)

External links

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Helene Profile by NASA's Solar System Exploration; see instead Cassini Solstice Mission: Helene
The Planetary Society: Helene
Helene has two faces—The Planetary Society : Helene Mini Atlas—Mar. 11, 2010
Cassini catches Helene—The Planetary Society : Video & Views—Jun. 20, 2011

Moons of Saturn
Listed in approximately increasing distance from Saturn. Temporary names in italics.
Ring shepherds	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus Pandora
Co-orbitals	Epimetheus Janus
G Ring	Aegaeon
Mimas and Alkyonides	Mimas Methone Anthe Pallene
Inner large (with trojans)	Enceladus Tethys Telesto Calypso Dione Helene Polydeuces
Outer large	Rhea Titan Hyperion Iapetus
Inuit group	Kiviuq Ijiraq Paaliaq Siarnaq Tarqeq
Norse group	Phoebe Skathi S/2007 S 2(?) Skoll S/2004 S 13(?) Greip Hyrrokkin Jarnsaxa Mundilfari S/2006 S 1(?) S/2004 S 17(?) Bergelmir Narvi Suttungr Hati S/2004 S 12(?) Farbauti Thrymr Aegir S/2007 S 3(?) Bestla S/2004 S 7(?) S/2006 S 3 Fenrir Surtur Kari Ymir Loge Fornjot
Gallic group	Albiorix Bebhionn Erriapus Tarvos
Rings of Saturn Cassini–Huygens Themis Chiron S/2004 S 6 S/2004 S 4 S/2004 S 3 In fiction

Saturn (outline)
Geography	Dragon Storm Great White Spot Hexagon Magnetosphere Rings Diamonds
Moons	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus ?(S/2004 S 6 S/2004 S 4 S/2004 S 3) Pandora Epimetheus Janus Aegaeon Mimas Methone Anthe Pallene Enceladus Tethys Telesto Calypso Dione Helene Polydeuces Rhea Titan Hyperion Iapetus Kiviuq Ijiraq Phoebe Paaliaq Skathi Albiorix S/2007 S 2 Bebhionn Erriapus Skoll Siarnaq Tarqeq S/2004 S 13 Greip Hyrrokkin Jarnsaxa Tarvos Mundilfari S/2006 S 1 S/2004 S 17 Bergelmir Narvi Suttungr Hati S/2004 S 12 Farbauti Thrymr Aegir S/2007 S 3 Bestla S/2004 S 7 S/2006 S 3 Fenrir Surtur Kari Ymir Loge Fornjot
Astronomy	Delta Octantis Saturn-crossing minor planets
Exploration	Cassini–Huygens Enceladus Explorer Pioneer 11 Voyager program
Related	Fiction Saturn Moons

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