Helene (moon)

Helene
	High-resolution view of leading hemisphere, showing gullies and apparent dust (regolith) flows (Cassini, June 2011)
Discovery [1]
Discovered by	P. Laques; J. Lecacheux
Discovery site	Pic du Midi Observatory
Discovery date	March 1, 1980
Designations
Designation	Saturn XII
Pronunciation	/ˈhɛlɪniː/[2]
Named after	Helen of Troy (Ἑλένη Helenē)
Alternative names	Dione B; S/1980 S 6;
Adjectives	Helenean /hɛlɪˈniːən/[3]
Orbital characteristics
Semi-major axis	377396 km
Eccentricity	0.0022
Orbital period	2.736915 d [4]
Inclination	0.199° (to Saturn's equator)
Satellite of	Saturn
Group	L4 Dione trojan
Physical characteristics
Dimensions	43.4 × 38.2 × 26 km [5]
Mean radius	17.6±0.4 km [5]
Albedo	1.67±0.20 (geometric) [6]

Helene /ˈhɛlɪniː/ 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.[7] In 1988 it was officially named after Helen of Troy, who was the granddaughter of Cronus (Saturn) in Greek mythology.[8] Helene is also designated Saturn XII (12), which it was given in 1982, and Dione B,[9] because it is co-orbital with Dione and located in its leading Lagrangian point (L₄). It is one of four known trojan moons.

Animation of Polydeuces's orbit relative to Saturn and Dione
Polydeuces · Helene · Dione · Saturn

Exploration

Helene was initially observed from Earth in 1980,[7] 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.

Selected observations

Mostly raw greyscale images with near infrared or ultraviolet channels.

Cassini image of Helene against the backdrop of Saturn's clouds (March 3, 2010)

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)

gollark: DNS over HTTPS (praise be) protects DNS queries now.

gollark: https://www.youtube.com/watch?v=WVDQEoe6ZWY

gollark: To be fair, HTTPS doesn't protect SNIs.

gollark: https://overengineer.dev/blog/2019/04/08/very-precarious-narrative.html

gollark: They can *misrepresent* it.

References

Lecacheux1980.
John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
also per "Helena". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)
Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.
NASA Celestia Archived March 9, 2005, at the Wayback Machine
Thomas 2010.
Verbiscer French et al. 2007.
IAUC 3496.
IAUC 4609.
Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983

Sources

Lecacheux, Jean. (July 1980). "A new satellite of Saturn: Dione B". Icarus. 43 (1): 111–115. Bibcode:1980Icar...43..111L. doi:10.1016/0019-1035(80)90093-7.
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.CS1 maint: ref=harv (link)
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. doi:10.1126/science.1134681. PMID 17289992. (supporting online material, table S1)

External links

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

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[FOOTNOTELecacheux1980-1] Lecacheux1980.

[2] John Walker (1839) A Critical Pronouncing Dictionary and Expositor of the English Language;
also per "Helena". Oxford English Dictionary (3rd ed.). Oxford University Press. September 2005. (Subscription or UK public library membership required.)

[3] Clarified as Helenéan in Earle (1841) Marathon: and other poems, p. 76.

[NASACelestia-4] NASA Celestia Archived March 9, 2005, at the Wayback Machine

[FOOTNOTEThomas2010-5] Thomas 2010.

[FOOTNOTEVerbiscer_French_et_al.2007-6] Verbiscer French et al. 2007.

[FOOTNOTEIAUC_3496-7] IAUC 3496.

[FOOTNOTEIAUC_4609-8] IAUC 4609.

[9] Transactions of the International Astronomical Union, Vol. XVIIIA, 1982 (mentioned in IAUC 3872: Satellites of Jupiter and Saturn, September 30, 1983

Moons of Saturn
Listed in approximately increasing distance from Saturn.
Ring shepherds	S/2009 S 1 Ring moonlets Pan Daphnis Atlas Prometheus Pandora
Co-orbitals	Epimetheus Janus
G Ring	Aegaeon
Alkyonides	Methone Anthe Pallene
Inner large (with trojans)	Mimas Enceladus Tethys Telesto Calypso Dione Helene Polydeuces
Outer large	Rhea Titan Hyperion Iapetus
Inuit group	Kiviuq Ijiraq Paaliaq S/2004 S 29 S/2004 S 31 Siarnaq Tarqeq
Norse group	Phoebe Skathi S/2004 S 37 S/2007 S 2 Skoll S/2004 S 13 (?) Greip Hyrrokkin Jarnsaxa Mundilfari S/2006 S 1 S/2004 S 17 (?) Bergelmir Narvi S/2004 S 20 Suttungr Hati S/2004 S 12 (?) S/2004 S 27 Farbauti Thrymr S/2004 S 30 Aegir S/2007 S 3 (?) Bestla S/2004 S 7 (?) S/2004 S 22 S/2004 S 23 S/2004 S 25 S/2004 S 32 S/2006 S 3 S/2004 S 38 Fenrir S/2004 S 28 Surtur Kari S/2004 S 35 Ymir S/2004 S 21 Loge S/2004 S 36 S/2004 S 39 S/2004 S 33 S/2004 S 34 Fornjot S/2004 S 26
Gallic group	Albiorix Bebhionn Erriapus Tarvos
Prograde outer satellites	S/2004 S 24
Rings of Saturn Cassini–Huygens Themis Chiron S/2004 S 6 S/2004 S 4 S/2004 S 3 In fiction

Saturn
Outline of Saturn
Geography	Dragon Storm Great White Spot Hexagon Magnetosphere Rings
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 S/2004 S 37 Albiorix S/2007 S 2 S/2004 S 29 Bebhionn Erriapus Skoll S/2004 S 31 Siarnaq Tarqeq S/2004 S 13 Greip Hyrrokkin Jarnsaxa Tarvos Mundilfari S/2006 S 1 S/2004 S 17 Bergelmir Narvi S/2004 S 20 Suttungr Hati S/2004 S 12 S/2004 S 27 Farbauti Thrymr S/2004 S 30 Aegir S/2007 S 3 Bestla S/2004 S 7 S/2004 S 22 S/2004 S 23 S/2004 S 25 S/2004 S 32 S/2006 S 3 S/2004 S 38 Fenrir S/2004 S 28 Surtur Kari S/2004 S 35 Ymir S/2004 S 21 S/2004 S 24 Loge S/2004 S 36 S/2004 S 39 S/2004 S 33 S/2004 S 34 Fornjot S/2004 S 26
Astronomy	Delta Octantis Saturn-crossing minor planets
Exploration	Cassini–Huygens (Huygens) timeline retirement Pioneer 11 Voyager program Voyager 1 Voyager 2
Related	Fiction Moons The Day the Earth Smiled In Saturn's Rings (2018 documentary)