View source for Nereid (moon)

{{Short description|Moon of Neptune}}
{{Infobox planet
| name = Nereid
| image = Nereid-Voyager2.jpg
| image_scale = 
| caption = Nereid imaged by ''[[Voyager 2]]'' in 1989
| apsis = apsis
| mpc_name = Neptune II
| named_after = [[Nereid|Νηρηΐδες]] ''Nērēḯdes''
| pronounced = {{IPAc-en|ˈ|n|ɪər|i|ə|d}} {{respell|NEER|ee|əd}}<ref>{{OED|Nereid}}</ref>
| adjectives = Nereidian or Nereidean (both {{IPAc-en|ˌ|n|ɛr|i|ˈ|ɪ|d|i|ə|n}} {{respell|NERR|ee|ID|ee|ən}})<ref>{{OED|nereidian, nereidean}}</ref>
| discovery_ref = <ref name="Kuiper 1949" />
| discoverer = [[Gerard P. Kuiper]]
| discovered = 1 May 1949
| orbit_ref = <ref name="JPL-SSD-Neptune"/>
| epoch = 1 January 2020 ([[Julian day|JD]] 2458849.5)
| observation_arc = {{cvt|{{Days since|1 May 1949}}|days|years}}
| semimajor = {{cvt|5504000|km|AU|}}
| eccentricity = 0.749
| period = {{cvt|360.14|days|years|abbr=on}}
| mean_anomaly = 318.0[[Degree (angle)|°]]
| mean_motion = {{Deg2DMS|0.9996103408|sup=ms}} / day
| inclination = 5.8° {{small|(to the [[ecliptic]])}}<br/>{{nowrap|7.090° {{small|(to local [[Laplace plane]])}}}}<ref name="JPL-SSD-Neptune" />
| asc_node = 326.0°
| arg_peri = 290.3°
| satellite_of = [[Neptune]]
| group = 
| mean_diameter = {{val|357|13|u=km}}<ref name="Kiss2019"/>
| flattening = ≤0.23{{efn|1=Flattening derived from the maximum aspect ratio (a/c): <math>f\leq1-\frac{c}{a}</math>, where (a/c) ≤ 1.3.<ref name="Kiss2019"/>}}
| mass = {{val|3.57|e=19|u=kg}}<br/>({{val|5.98|e=-6|ul=Earth mass}})
| density = 1.5 g/cm<sup>3</sup> (assumed)<ref name="JPL-SSD-sat_phys"/>
| rotation = {{val|11.594|0.017}} h<ref name="Kiss2019"/>
| surface_grav = ≈{{Gr|0.0357|178.5}} m/s<sup>2</sup> ({{val|0.0076|u=[[G-force|''g'']]}})
| escape_velocity = 
| albedo = 0.24<ref name="Kiss2019"/>
| magnitude = 19.2{{citation needed|date=February 2023}}
| abs_magnitude = 4.4<ref name="MPC115892"/>
| single_temperature = ≈50 [[Kelvin|K]] {{small|(mean estimate)}}
}}

'''Nereid''', or '''Neptune II''', is the third-largest [[natural satellite|moon]] of [[Neptune]]. It has the most eccentric orbit of all known moons in the [[Solar System]].<ref name="JPL-SSD-Neptune"/> It was the second [[Moons of Neptune|moon of Neptune]] to be discovered, by [[Gerard Kuiper]] in 1949.

== Discovery and naming ==
[[File:Kuiper_colored.jpg|left|thumb|upright|Gerard P. Kuiper, discoverer of Nereid]]
Nereid was discovered on 1 May 1949 by [[Gerard P. Kuiper]] using photographic plates taken with the 82-inch telescope at the [[McDonald Observatory]]. He proposed the name in the report of his discovery. It is named after the [[Nereid]]s, sea-nymphs of [[Greek mythology]] and attendants of the god [[Neptune (mythology)|Neptune]].<ref name="Kuiper 1949" /> It was the second moon of Neptune to be discovered, and the last before the arrival of ''[[Voyager 2]]'' (not counting a single observation of an occultation by [[Larissa (moon)|Larissa]] in 1981).<ref name="Smith Soderblom et al. 1989" />

== Physical characteristics ==
Nereid is third-largest of [[Moons of Neptune|Neptune's satellites]], and has a [[mean radius]] of about {{convert|180|km|mi|-1}}.<ref name="Kiss2019"/> It is by far the largest normal [[irregular moon|irregular satellite]] known,<ref name="GravHolmanKavelaars2003" /> having about two-thirds the mass of all irregular moons combined.<ref name=denk>{{cite web |url=https://tilmanndenk.de/outersaturnianmoons/ |title=Outer Moons of Saturn |last=Denk |first=Tilmann |date=2024 |website=tilmanndenk.de |publisher=Tilmann Denk |access-date=25 February 2024 |quote=}}</ref> ([[Triton (moon)|Triton]] is much larger, but is very unusual for an irregular satellite.) Nereid also accounts for about 98% of the mass of Neptune's entire irregular satellite system altogether (excluding Triton), which is similar to the situation of [[Phoebe (moon)|Phoebe]] at Saturn (the second-largest normal irregular moon in the Solar System).<ref name=denk/>

The shape of Nereid is unknown.<ref name="Shaefer2008" /> Nereid is similar in size to [[Saturn]]'s moon [[Mimas (moon)|Mimas]], though Nereid is denser. Since 1987 some photometric observations of Nereid have detected large (by ~1 of magnitude) variations of its brightness, which can happen over years and months, but sometimes even over a few days. They persist even after a correction for distance and phase effects. On the other hand, not all astronomers who have observed Nereid have noticed such variations. This means that they may be quite chaotic. To date there is no credible explanation of the variations, but, if they exist, they are likely related to the rotation of Nereid. Nereid's rotation could be either in the state of forced [[precession]] or even [[chaotic rotation]] (like [[Hyperion (moon)|Hyperion]]) due to its highly elliptical orbit.

In 2016, extended observations with the Kepler space telescope showed only low-amplitude variations (0.033 magnitudes). Thermal modeling based on infrared observations from the [[Spitzer Space Telescope|Spitzer]] and [[Herschel Space Observatory|Herschel]] space telescopes suggest that Nereid is only moderately elongated with a maximum [[aspect ratio]] of 1.3:1, which disfavors forced precession of the rotation.<ref name="Kiss2019" /> The thermal model also indicates that the surface roughness of Nereid is very high, likely similar to the Saturnian moon [[Hyperion (moon)|Hyperion]].<ref name="Kiss2019" />

[[Electromagnetic spectrum|Spectrally]], Nereid appears neutral in colour<ref name="GravHolmanFraser2004" /> and [[Ice|water ice]] has been detected on its surface.<ref name="BrownKoreskoBlake" /> Its spectrum appears to be intermediate between [[Uranus]]'s moons [[Titania (moon)|Titania]] and [[Umbriel]], which suggests that Nereid's surface is composed of a mixture of water ice and some spectrally neutral material.<ref name="BrownKoreskoBlake" /> The spectrum is markedly different from [[minor planet]]s of the outer solar system, [[centaur (minor planet)|centaur]]s [[5145 Pholus|Pholus]], [[2060 Chiron|Chiron]] and [[10199 Chariklo|Chariklo]], suggesting that Nereid formed around Neptune rather than being a captured body.<ref name="BrownKoreskoBlake" />

[[Halimede (moon)|Halimede]], which displays a similar gray neutral colour, may be a fragment of Nereid that was broken off during a collision.<ref name="GravHolmanFraser2004" />

== Orbit and rotation ==
Nereid [[orbit]]s [[Neptune]] in the [[Retrograde motion|prograde]] direction at an [[semimajor axis|average distance]] of {{convert|5504000|km|mi|abbr=on|sigfig=5}}, but its high [[eccentricity (orbit)|eccentricity]] of 0.749 takes it as close as {{convert|1381500|km|mi|abbr=on}} and as far as {{convert|9626500|km|mi|abbr=on}}.<ref name="JPL-SSD-Neptune"/>

[[File:Nereid's orbit around Neptune.svg|thumb|240px|Nereid's highly eccentric orbit around Neptune.]]The unusual orbit suggests that it may be either a captured [[asteroid]] or [[Kuiper belt]] object, or that it was an inner moon in the past and was perturbed during the capture of Neptune's largest moon [[Triton (moon)|Triton]].<ref name="BrownKoreskoBlake" /> If the latter is true, it may be the only survivor of Neptune's original (pre-[[Capture_of_Triton|Triton capture]]) set of [[regular satellite]]s.<ref name= "Brozovic2019">{{cite journal|title= Orbits and resonances of the regular moons of Neptune|first1=M. |last1=Brozović|first2=M. R. |last2=Showalter|first3=R. A. |last3=Jacobson|first4=R. S. |last4=French|first5=J. J. |last5=Lissauer|first6=I. |last6=de Pater|journal= Icarus|date= March 2020|volume=338|pages=113462|doi= 10.1016/j.icarus.2019.113462|arxiv= 1910.13612|bibcode=2020Icar..33813462B |s2cid=204960799 }}</ref>

In 1991, a rotation period of Nereid of about 13.6&nbsp;hours was determined by an analysis of its light curve.<ref name="Williams1991" /> In 2003, another rotation period of about {{nowrap|11.52 ± 0.14 hours}} was measured.<ref name="GravHolmanKavelaars2003" /> However, this determination was later disputed, and other researchers for a time failed to detect any periodic modulation in Nereid's light curve from ground-based observations.<ref name="Shaefer2008" /> In 2016, a clear rotation period of 11.594 ± 0.017&nbsp;hours was determined based on observations with the [[Kepler space telescope]].<ref name="Kiss2019"/>

== Exploration ==
The only spacecraft to visit Nereid was ''[[Voyager 2]]'', which passed it at a distance of {{convert|4700000|km|mi|abbr=on}}<ref name="JonesB1991" /> between 20 April and 19 August 1989.<ref name="Jacobson1991" /> ''Voyager 2'' obtained 83 images with observation accuracies of {{convert|70|km|mi|abbr=on}} to {{convert|800|km|mi|abbr=on}}.<ref name="Jacobson1991" /> Prior to ''Voyager 2'''s arrival, observations of Nereid had been limited to ground-based observations that could only establish its intrinsic brightness and [[orbital elements]].<ref name="NASA-Pic" /> Although the images obtained by ''Voyager 2'' do not have a high enough resolution to allow surface features to be distinguished, ''Voyager 2'' was able to measure the size of Nereid and found that it was grey in colour and had a higher [[albedo]] than Neptune's other small satellites.<ref name="Smith Soderblom et al. 1989" />

If selected, The Arcanum mission<ref>{{cite journal | doi=10.1017/aer.2023.114 | title=Concept of operations for the Neptune system mission Arcanum | date=2024 | last1=McKevitt | first1=J.E. | last2=Beegadhur | first2=S. | last3=Ayin-Walsh | first3=L. | last4=Dixon | first4=T. | last5=Criscola | first5=F. | last6=Patadia | first6=D. | last7=Bulla | first7=S. | last8=Galinzoga | first8=J. | last9=Wadsworth | first9=B. | last10=Bornberg | first10=C. | last11=Sharma | first11=R. | last12=Moore | first12=O. | last13=Kent | first13=J. | last14=Zaripova | first14=A. | last15=Parkinson-Swift | first15=J. | last16=Laad | first16=A. | journal=The Aeronautical Journal | volume=128 | issue=1321 | pages=469–488 | doi-access=free }}</ref> would do a flyby of Nereid before its primary purposes of orbiting Neptune and observing Triton.<ref>{{cite arXiv | eprint=2106.09409 | last1=McKevitt | first1=James | last2=Bulla | first2=Sophie | last3=Dixon | first3=Tom | last4=Criscola | first4=Franco | last5=Parkinson-Swift | first5=Jonathan | last6=Bornberg | first6=Christina | last7=Singh | first7=Jaspreet | last8=Patel | first8=Kuren | last9=Laad | first9=Aryan | last10=Forder | first10=Ethan | last11=Ayin-Walsh | first11=Louis | last12=Beegadhur | first12=Shayne | last13=Wedde | first13=Paul | author14=Bharath Simha Reddy Pappula | last15=McDougall | first15=Thomas | last16=Foghis | first16=Madalin | last17=Kent | first17=Jack | last18=Morgan | first18=James | last19=Raj | first19=Utkarsh | last20=Heinreichsberger | first20=Carina | title=An L-class Multirole Observatory and Science Platform for Neptune | date=2021 | class=astro-ph.IM }}</ref>

== See also ==
* [[Moons of Neptune]]

== Notes ==
<references group="lower-alpha" />

== References ==
{{reflist
| colwidth = 30em
| refs =

<ref name="Kuiper 1949">
{{cite journal| doi = 10.1086/126166| last = Kuiper | first = G. P.| date=August 1949 | title = The Second Satellite of Neptune| journal = Publications of the Astronomical Society of the Pacific| volume = 61| issue = 361 | pages = 175–176| bibcode = 1949PASP...61..175K| doi-access = free}}
</ref>

<ref name="JPL-SSD-Neptune">
{{cite web
 |author      = Jacobson, R. A. — AJ
 |date        = 2009-04-03
 |title       = Planetary Satellite Mean Orbital Parameters
 |work        = JPL satellite ephemeris
 |publisher   = [[JPL]] (Solar System Dynamics)
 |url         = http://ssd.jpl.nasa.gov/?sat_elem
 |access-date  = 2011-10-26
 |url-status     = deviated
 |archive-url  = https://web.archive.org/web/20111014234918/http://ssd.jpl.nasa.gov/?sat_elem
 |archive-date = October 14, 2011}}</ref>

<!-- <ref name="Jacobson 2009-AJ">
{{cite journal| doi = 10.1088/0004-6256/137/5/4322| last = Jacobson| first = R. A.| date = 3 April 2009| title = The Orbits of the Neptunian Satellites and the Orientation of the Pole of Neptune| journal = [[The Astronomical Journal]]| volume = 137| issue = 5| pages = 4322–4329| bibcode=2009AJ....137.4322J| doi-access = free}}
</ref> -->

<ref name="JPL-SSD-sat_phys">
{{cite web
 |title       = Planetary Satellite Physical Parameters
 |publisher   = [[JPL]] (Solar System Dynamics)
 |url         = http://ssd.jpl.nasa.gov/?sat_phys_par
 |access-date  = 2011-10-26
 |archive-url  = https://web.archive.org/web/20100527091333/http://ssd.jpl.nasa.gov/?sat_phys_par
 |archive-date = 2010-05-27
 |url-status     = live}}</ref>

<ref name="GravHolmanKavelaars2003">
{{cite journal
| first = T.
| last = Grav
|author2=M. Holman |author3=J. J. Kavelaars
 | title = The Short Rotation Period of Nereid
| journal = The Astrophysical Journal
| date = 2003
| volume = 591
| issue = 1
| pages = 71–74
| arxiv = astro-ph/0306001
| doi = 10.1086/377067
| bibcode = 2003ApJ...591L..71G| s2cid = 8869351
}}</ref>

<ref name="Smith Soderblom et al. 1989">
{{cite journal| doi = 10.1126/science.246.4936.1422| last1 = Smith| first1 = B. A.| last2 = Soderblom| first2 = L. A.| last3 = Banfield| first3 = D.| last4 = Barnet| first4 = C.| last5 = Basilevsky| first5 = A. T.| last6 = Beebe| first6 = R. F.| last7 = Bollinger| first7 = K.| last8 = Boyce| first8 = J. M.| last9 = Brahic| first9 = A.| year = 1989| title = Voyager 2 at Neptune: Imaging Science Results| journal = Science| volume = 246| issue = 4936| pages = 1422–1449| bibcode = 1989Sci...246.1422S| pmid = 17755997| s2cid = 45403579| ref = {{sfnRef|Smith Soderblom et al.|1989}}| url = https://zenodo.org/record/1230992}}</ref>

<ref name="BrownKoreskoBlake">
{{cite journal
| last = Brown
| first = Michael E.
| author2 = Koresko, Christopher D.
| author3 = Blake, Geoffrey A.
| title = Detection of Water Ice on Nereid
| journal = [[The Astrophysical Journal]]
| volume = 508
| issue = 2
| pages = L175–L176
| date = December 1998
| doi = 10.1086/311741
| pmid = 11542819
| bibcode=1998ApJ...508L.175B
| citeseerx = 10.1.1.24.1200| s2cid = 17451226
}}</ref>

<ref name="Williams1991">
{{cite journal
| last1 = Williams
| first1 = I.P.
| last2 = Jones | first2 = D.H.P.
| last3 = Taylor | first3 = D.B.
| title = The rotation period of Nereid
| journal = [[Monthly Notices of the Royal Astronomical Society]]
| volume = 250
| pages = 1P–2P
| date = 1991
| bibcode = 1991MNRAS.250P...1W
| doi=10.1093/mnras/250.1.1p| doi-access = free
}}</ref>

<ref name="Shaefer2008">
{{cite journal
| last1 = Schaefer
| first1 = Bradley E.
| author-link = Bradley Schaefer
| last2 = Tourtellotte | first2 = Suzanne W.
| last3 = Rabinowitz | first3 = David L.
| last4 = Schaefer | first4 = Martha W.
| title = Nereid: Light curve for 1999–2006 and a scenario for its variations
| journal = [[Icarus (journal)|Icarus]]
| volume = 196
| issue = 1
| pages = 225–240
| date = 2008
| bibcode = 2008Icar..196..225S
| doi = 10.1016/j.icarus.2008.02.025
| arxiv = 0804.2835| s2cid = 119267757
}}</ref>

<ref name="GravHolmanFraser2004">
{{cite journal| doi = 10.1086/424997| last1 = Grav| first1 = Tommy| last2 = Holman| first2 = Matthew J.| author-link2 = Matthew J. Holman| last3 = Fraser| first3 = Wesley C.| date = 2004-09-20| title = Photometry of Irregular Satellites of Uranus and Neptune| journal = [[The Astrophysical Journal]]| volume = 613| issue = 1| pages = L77–L80| arxiv = astro-ph/0405605| bibcode = 2004ApJ...613L..77G| s2cid = 15706906}}</ref>

<ref name="JonesB1991">
{{cite book
| last = Jones
| first = Brian
| title = Exploring the Planets
| publisher = W.H. Smith
| date = 1991
| location = Italy
| pages = [https://archive.org/details/exploringplanets0000jone/page/59 59]
| isbn = 978-0-8317-6975-8
| url = https://archive.org/details/exploringplanets0000jone/page/59
}}</ref>

<ref name="Jacobson1991">
{{cite journal
| last = Jacobson
| first = R.A.
| title = Triton and Nereid astrographic observations from Voyager 2
| journal = Astronomy and Astrophysics Supplement Series
| volume = 90
| issue = 3
| pages = 541–563
| date = 1991
| bibcode = 1991A&AS...90..541J}}</ref>

<ref name="NASA-Pic">
{{cite web
| title = PIA00054: Nereid
| publisher = [[NASA]]
| date = 1996-01-29
| url = http://photojournal.jpl.nasa.gov/target/Nereid
| access-date = 2009-11-08}}</ref>

<ref name="Kiss2019">{{cite journal
| title = Nereid from space: Rotation, size and shape analysis from K2, Herschel and Spitzer observations
| url = https://academic.oup.com/mnras/article-pdf/457/3/2908/8000447/stw081.pdf
| display-authors = etal
| first1 = C. |last1 = Kiss
| first2 = A. |last2 = Pál
| first3 = A. I. |last3 = Farkas-Takács
| first4 = G. M. |last4 = Szabó
| first5 = R. |last5 = Szabó
| first6 = L. L. |last6 = Kiss
| date = April 2016
| journal = Monthly Notices of the Royal Astronomical Society
| volume = 457
| issue = 3
| pages = 2908–2917
| doi = 10.1093/mnras/stw081
| doi-access = free
| arxiv = 1601.02395
| bibcode = 2016MNRAS.457.2908K}}</ref>

<ref name="MPC115892">{{cite web
  |title      = M.P.C. 115892
  |url        = https://minorplanetcenter.net/iau/ECS/MPCArchive/2019/MPC_20190827.pdf
  |work       = Minor Planet Circular
  |publisher  = Minor Planet Center
  |date       = 27 August 2019}}</ref>

}}

{{Moons of Neptune|state=uncollapsed}}
{{Solar System moons (compact)}}
{{Neptune}}
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{{DEFAULTSORT:Nereid (Moon)}}
[[Category:Moons of Neptune]]
[[Category:Irregular satellites]]
[[Category:Astronomical objects discovered in 1949|19490501]]
[[Category:Discoveries by Gerard Kuiper]]
[[Category:Moons with a prograde orbit]]