View source for Galatea (moon)

{{Short description|Moon of Neptune}}
{{for|the asteroid|74 Galatea}}
{{Infobox planet
| name             = Galatea
| image            = Galatea moon.jpg
| discoverer       = [[Stephen P. Synnott]]<ref name="Willman-Neptune" /> and Voyager Imaging Team
| discovered       = July 1989
| caption          = Galatea as seen by ''[[Voyager 2]]'' (elongation is due to [[Smear (optics)|smearing]])
| pronounced       = {{IPAc-en|ɡ|æ|l|ə|ˈ|t|iː|ə}}<ref>{{OED|galatea}}</ref>
| mpc_name         = Neptune VI
| named_after      = [[Galatea (Greek myth)|Γαλάτεια]] ''Galateia''
| adjectives       = Galatean<ref>{{citation|author=AMIA|year=1999|title=Transforming health care through informatics}}</ref>
| orbit_ref        = <ref name="Jacobson2004" /><ref name="Showalter2019" />
| epoch            = 18 August 1989
| semimajor        = {{val|61952.57|u=km}}
| eccentricity     = 0.00022 ± 0.00008
| period           = 0.42874431 ± 0.00000001 d
| avg_speed        = 
| inclination      = {{ubl | 0.052 ± 0.011° (to Neptune equator) | 0.06° (to local [[Laplace plane]]) }}
| satellite_of     = [[Neptune]]
| group            = [[ring shepherd]]
| dimensions       = ({{val|204|10}})&thinsp;×&thinsp;({{val|184|16}})&thinsp;×&thinsp;({{val|144|8}}) km<ref name="Karkoschka2003" />
| mean_radius      = {{val|88|4|u=km}}<ref name="Karkoschka2003" />
| volume           = ~{{val|2.8|e=6|u=km3}}<ref name="Volume" group="lower-alpha" />
| mass             = {{val|1.94|e=18|u=kg}}{{#tag:ref|Giuliatti Winter et al. (2020), as cited in Madeira & Giuliatti Winter (2022).<ref name="Madeira & GiuliattiWinter 2022"/>}}
| density          = ~{{val|0.69|u=g/cm3}}<ref name="Density" group="lower-alpha" />
| surface_grav     = ~{{Gr|0.00194|102}}–{{Gr|0.00194|72}} [[Acceleration|m/s<sup>2</sup>]]<ref name="Surface gravity" group="lower-alpha" />
| escape_velocity  = ~{{V2|0.00194|102}}–{{V2|0.00194|72}} km/s<ref name="Escape velocity" group="lower-alpha" />
| rotation         = [[synchronous rotation|synchronous]]
| axial_tilt       = zero
| albedo           = 0.08<ref name="Karkoschka2003" /><ref name="jplssd" />
| magnitude        = 21.9<ref name="jplssd" />
| single_temperature = ~51 [[Kelvin|K]] mean (estimate)
}}

'''Galatea''' {{IPAc-en|ɡ|æ|l|ə|ˈ|t|iː|ə}}, also known as '''Neptune&nbsp;VI''', is the fourth-closest [[inner moon]] of [[Neptune]], and fifth-largest [[moon of Neptune]]. It is named after [[Galatea (mythology)|Galatea]], one of the fifty [[Nereids]] of [[Greek Mythology|Greek legend]], with whom [[Cyclopes|Cyclops]] [[Polyphemus]] was vainly in love.

== Discovery ==
[[File:Galatea feat.jpg|thumb|left|250px|'''Galatea''' inside of a faint [[Rings of Neptune#Adams ring|ring arc]] near [[Neptune]]]]
Galatea was discovered in late July 1989 from the images taken by the ''[[Voyager 2]]'' [[space probe|probe]]. It was given the temporary designation '''S/1989&nbsp;N&nbsp;4'''.<ref name="IAUC 4824" /> The discovery was announced (IAUC 4824) on 2 August 1989, and mentions "10 frames taken over 5 days", implying a discovery date of sometime before July 28. The name was given on 16 September 1991.<ref name="IAUC 5347" />

== Physical properties ==

Galatea is irregularly shaped and shows no sign of any geological modification. It is likely that it is a rubble pile re-accreted from fragments of Neptune's original satellites, which were smashed up by perturbations from [[Triton (moon)|Triton]] soon after that moon's capture into a very eccentric initial orbit.<ref name="BanfieldMurray1992" />

Compositionally, Galatea appears to be similar to other small inner Neptunian satellites, with a deep 3.0 micron feature attributed to water ice or hydrated silicate minerals. It has a 0.08 albedo at 1.4 and 2.0 microns, dropping to 0.04 at 3.0 microns, and increasing to 0.12 at 4.6 microns.<ref>{{Cite journal |last1=Belyakov |first1=Matthew |last2=Davis |first2=M. Ryleigh |last3=Milby |first3=Zachariah |last4=Wong |first4=Ian |last5=Brown |first5=Michael E. |date=2024-05-01 |title=JWST Spectrophotometry of the Small Satellites of Uranus and Neptune |journal=The Planetary Science Journal |volume=5 |issue=5 |pages=119 |doi=10.3847/PSJ/ad3d55 |doi-access=free |arxiv=2404.06660 |bibcode=2024PSJ.....5..119B |issn=2632-3338}}</ref>

== Orbit ==
Galatea's orbit lies below Neptune's [[synchronous orbit]] radius, so it is slowly spiralling inward due to [[tidal acceleration#Tidal deceleration|tidal deceleration]] and may eventually impact the planet or break up into a new [[planetary ring]] system upon passing its [[Roche limit]] due to [[tidal forces|tidal stretching]].

Galatea appears to be a [[Planetary ring|shepherd moon]] for the [[Rings of Neptune|Adams ring]] that is {{convert|1000|km}} outside its orbit. [[Orbital resonance|Resonances]] with Galatea in the ratio 42:43 are also considered the most likely mechanism for confining the unique ring arcs that exist in this ring.<ref name="NamouniPorco" /> Galatea's mass has been estimated based on the radial perturbations it induces on the ring.<ref name="Porco1991" /><ref name="Madeira & GiuliattiWinter 2022"/>

== Notes ==

{{reflist
| group = lower-alpha
| refs =

<ref name="Volume">
Volume derived from the long axis ''A'', the medium axis ''B'' and the short axis ''C'': <math>\frac{\pi}{6} ABC</math>
</ref>

<ref name="Density">
Density derived from mass ''m'' and the volume ''V'': <math>\frac{m}{V}</math>
</ref>

<ref name="Surface gravity">
Surface gravity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\frac{Gm}{r^2}</math>
</ref>

<ref name="Escape velocity">
Escape velocity derived from the mass ''m'', the [[gravitational constant]] ''G'' and the radius ''r'': <math>\sqrt{\frac{2Gm}{r}}</math>
</ref>

}}

Since Galatea is irregularly shaped, the actual surface gravity and escape velocity will vary significantly between different positions on the surface.

== References ==

{{reflist
| refs =

<ref name="Willman-Neptune">
''Planet Neptune Data'' http://www.princeton.edu/~willman/planetary_systems/Sol/Neptune/
</ref>

<ref name="Jacobson2004">
{{cite journal| doi = 10.1086/423037| last1 = Jacobson | first1 = R. A.| last2 = Owen | first2 = W. M. Jr.| year = 2004| pages = 1412–1417| title = The orbits of the inner Neptunian satellites from Voyager, Earthbased, and Hubble Space Telescope observations| journal = Astronomical Journal| volume = 128| issue = 3| bibcode = 2004AJ....128.1412J| doi-access = free}}
</ref>

<ref name="Karkoschka2003">
{{cite journal| doi = 10.1016/S0019-1035(03)00002-2| last = Karkoschka| first = Erich| year = 2003| title = Sizes, shapes, and albedos of the inner satellites of Neptune| journal = Icarus| volume = 162| issue = 2| pages = 400–407| bibcode = 2003Icar..162..400K}}
</ref>

<ref name="Showalter2019">{{cite journal|last1= Showalter|first1=M. R.|last2=de Pater|first2= I.|last3= Lissauer|first3=J. J.|last4= French|first4=R. S.|url=https://www.spacetelescope.org/static/archives/releases/science_papers/heic1904/heic1904a.pdf|title= The seventh inner moon of Neptune|journal= Nature|volume= 566|issue= 7744|year= 2019|pages= 350–353|doi= 10.1038/s41586-019-0909-9|pmid=30787452|pmc=6424524|bibcode= 2019Natur.566..350S}}</ref>

<ref name="jplssd">{{cite web
| title = Planetary Satellite Physical Parameters
| date = 2008-10-24
| publisher = [[JPL]] (Solar System Dynamics)
| url = http://ssd.jpl.nasa.gov/?sat_phys_par
| access-date = 2008-12-13
| url-status = dead
| archive-date = 2013-11-01
| archive-url = https://web.archive.org/web/20131101144111/http://ssd.jpl.nasa.gov/?sat_phys_par
}}</ref>

<ref name="Porco1991">
{{cite journal
| doi = 10.1126/science.253.5023.995
| last = Porco
| first = C.C.
| year = 1991
| title = An Explanation for Neptune's Ring Arcs
| journal = Science
| volume = 253
| issue = 5023
| pages = 995–1001
| pmid = 17775342
| bibcode = 1991Sci...253..995P
| s2cid = 742763
}}
</ref>

<ref name="IAUC 4824">
{{cite journal
| last = Marsden
| first = Brian G.
| title = Satellites of Neptune
| date = August 2, 1989
| journal = IAU Circular
| volume = 4824
| url = http://www.cbat.eps.harvard.edu/iauc/04800/04824.html
| access-date = 2011-10-26
}}
</ref>

<ref name="IAUC 5347">
{{cite journal
| last = Marsden
| first = Brian G.
| title = Satellites of Saturn and Neptune
| date = September 16, 1991
| journal = IAU Circular
| volume = 5347
| url = http://www.cbat.eps.harvard.edu/iauc/05300/05347.html
| access-date = 2011-10-26
}}
</ref>

<ref name="BanfieldMurray1992">
{{cite journal| doi = 10.1016/0019-1035(92)90155-Z| last1 = Banfield| first1 = Don| last2 = Murray| first2 = Norm| date=October 1992 | title = A dynamical history of the inner Neptunian satellites| journal = Icarus| volume = 99| issue = 2| pages = 390–401| bibcode = 1992Icar...99..390B}}
</ref>

<ref name="NamouniPorco">
{{cite journal
| doi = 10.1038/417045a
| last1 = Namouni
| first1 = F.
| last2 = Porco
| first2 = C.
| year = 2002
| title = The confinement of Neptune's ring arcs by the moon Galatea
| journal = Nature
| volume = 417
| issue = 6884
| pages = 45–7
| url = http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v417/n6884/abs/417045a_fs.html
| pmid = 11986660
|bibcode = 2002Natur.417...45N | s2cid = 4430322
}}
</ref>

<ref name="Madeira & GiuliattiWinter 2022">
{{Cite journal |last1=Madeira |first1=Gustavo |last2=Winter |first2=Silvia Maria Giuliatti |date=2022-04-21 |title=Numerical analysis of processes for the formation of moonlets confining the arcs of Neptune |journal=Monthly Notices of the Royal Astronomical Society |volume=513 |issue=1 |pages=297–309 |arxiv=2204.01063 |doi=10.1093/mnras/stac944 |issn=0035-8711 |doi-access=free}}
</ref>

}}

== External links ==

{{Commons|Galatea (moon)}}
* [https://web.archive.org/web/20021203185919/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Nep_Galatea Galatea Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.dtm.ciw.edu/users/sheppard/satellites/nepsatdata.html Neptune's Known Satellites] (by [[Scott S. Sheppard]])

{{Moons of Neptune}}
{{Solar System moons (compact)}}
{{Neptune}}

{{DEFAULTSORT:Galatea (Moon)}}
[[Category:Moons of Neptune]]
[[Category:Astronomical objects discovered in 1989]]
[[Category:Moons with a prograde orbit]]