View source for Irregular moon

{{short description|Captured satellite following an irregular orbit}}
{{See also|Asteroid capture}}
{{Multiple image 
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|align=right 
|width=230
|image1=Phoebe_cassini_full.jpg
|image2=Triton.usgs23.png
|footer = [[Phoebe (moon)|Phoebe]] (''top'') and [[Triton (moon)|Triton]] (''bottom''), two irregular satellites that have been visited by spacecraft
}}
In [[astronomy]], an '''irregular moon''', '''irregular satellite''', or '''irregular natural satellite''' is a [[natural satellite]] following an orbit that is irregular in some of the following ways: Distant; [[orbital inclination|inclined]]; [[orbital eccentricity|highly elliptical]]; [[Retrograde and prograde motion|retrograde]]. They have often been captured by their parent planet, unlike [[regular satellite]]s formed in orbit around them. Irregular moons have a stable orbit, unlike [[temporary satellite]]s which often have similarly irregular orbits but will eventually depart. The term does not refer to shape; [[Triton (moon)|Triton]], for example, is a [[planetary-mass moon|round moon]] but is considered irregular due to its orbit and origins.

{{As of|February 2024}}, 228 irregular moons are known, orbiting all four of the [[outer planet]]s ([[Jupiter]], [[Saturn]], [[Uranus]], and [[Neptune]]). The largest of each planet are [[Himalia (moon)|Himalia]] of Jupiter, [[Phoebe (moon)|Phoebe]] of Saturn, [[Sycorax (moon)|Sycorax]] of Uranus, and [[Triton (moon)|Triton]] of Neptune. Triton is rather unusual for an irregular moon; if it is excluded, then [[Nereid (moon)|Nereid]] is the largest irregular moon around Neptune. It is currently thought that the irregular satellites were once independent objects orbiting the Sun before being captured by a nearby planet, early in the history of the Solar System. An alternative suggests that they originated further out in the [[Kuiper belt]]<ref>{{Cite journal |last1=Pfalzner |first1=Susanne |last2=Govind |first2=Amith |last3=Wagner |first3=Frank W. |date=2024-09-01 |title=Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby |journal=The Astrophysical Journal Letters |volume=972 |issue=2 |pages=L21 |doi=10.3847/2041-8213/ad63a6 |doi-access=free |arxiv=2409.03529 |bibcode=2024ApJ...972L..21P |issn=2041-8205}}</ref> and were captured after the close flyby of another star<ref>{{Cite journal |last1=Pfalzner |first1=Susanne |last2=Govind |first2=Amith |last3=Portegies Zwart |first3=Simon |date=2024-09-04 |title=Trajectory of the stellar flyby that shaped the outer Solar System |url=https://www.nature.com/articles/s41550-024-02349-x |journal=Nature Astronomy |volume=8 |issue=11 |pages=1380–1386 |doi=10.1038/s41550-024-02349-x |arxiv=2409.03342 |bibcode=2024NatAs...8.1380P |issn=2397-3366}}</ref>

== Definition ==
{| align="center" class="wikitable" style="margin-left: 1em; margin-right: 0;"
|-
! Planet
! Hill radius<br/>r<sub>H</sub>&nbsp;([[Gigametre|10<sup>6</sup>&nbsp;km]])<ref name="Sheppard2006">{{Cite journal | last1 = Sheppard | first1 = S. S. | doi = 10.1017/S1743921305006824 | title = Outer irregular satellites of the planets and their relationship with asteroids, comets and Kuiper Belt objects | journal = Proceedings of the International Astronomical Union | volume = 1 | pages = 319–334 | year = 2006 |arxiv = astro-ph/0605041 | bibcode = 2006IAUS..229..319S | s2cid = 2077114 }}</ref>
! r<sub>H</sub>&nbsp;([[Degree (angle)|°]])<ref name="Sheppard2006" />
! Number known
! Farthest known<br/>satellite&nbsp;(10<sup>6</sup>&nbsp;km)
|-
| Jupiter
| 51
| 4.7
| 87
| 24.2 (0.47r<sub>H</sub>)
|-
| Saturn
| 69
| 3.0
| 122
| 26.4 (0.38r<sub>H</sub>)
|-
| Uranus
| 73
| 1.5
| 10
| 20.4 (0.28r<sub>H</sub>)
|-
| Neptune
| 116
| 1.5
| 9 (including Triton)
| 50.6 (0.44r<sub>H</sub>)
|}
{{Clear}}
There is no widely accepted precise definition of an irregular satellite. Informally, satellites are considered irregular if they are far enough from the planet that the [[Precession#Precession of planetary orbits|precession]] of their [[orbital plane (astronomy)|orbital plane]] is primarily controlled by the Sun, other planets, or other moons.<ref name="jpl-satelem">{{cite web
  |title      = Planetary Satellite Mean Orbital Parameters
  |url        = https://ssd.jpl.nasa.gov/sats/elem/sep.html
  |publisher  = [[Jet Propulsion Laboratory]]
  |access-date = 15 January 2024
  |archive-date = 6 October 2021
  |archive-url = https://web.archive.org/web/20211006095717/https://ssd.jpl.nasa.gov/sats/elem/sep.html
  |url-status = live}}</ref>

In practice, the satellite's [[semi-major axis]] is compared with the radius of the planet's [[Hill sphere]] (that is, the sphere of its gravitational influence), <math>r_H</math>. Irregular satellites have semi-major axes greater than 0.05 <math>r_H</math> with [[apoapsis|apoapses]] extending as far as to 0.65 <math>r_H</math>.<ref name="Sheppard2006" /> The radius of the Hill sphere is given in the adjacent table: Uranus and Neptune have larger Hill sphere radii than Jupiter and Saturn, despite being less massive, because they are farther from the Sun. However, no known irregular satellite has a semi-major axis exceeding 0.47 <math>r_H</math>.<ref name=denk/>

Earth's [[Moon]] seems to be an exception: it is not usually listed as an irregular satellite even though its precession is primarily controlled by the Sun{{citation needed|date=February 2020}} and its semi-major axis is greater than 0.05 of the radius of Earth's Hill sphere. On the other hand, Neptune's [[Triton (moon)|Triton]], which is probably a captured object, is usually listed as irregular despite being within 0.05 of the radius of Neptune's Hill sphere, so that Triton's precession is primarily controlled by Neptune's oblateness instead of by the Sun.<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= |archive-date=24 February 2024 |archive-url=https://web.archive.org/web/20240224094854/https://tilmanndenk.de/outersaturnianmoons/ |url-status=live }}</ref> Neptune's [[Nereid (moon)|Nereid]] and Saturn's [[Iapetus (moon)|Iapetus]] have semi-major axes close to 0.05 of the radius of their parent planets' Hill spheres: Nereid (with a very eccentric orbit) is usually listed as irregular, but not Iapetus.
{{clear}}

== Orbits ==

=== Current distribution ===
[[File:Irregular moon orbits a vs. i.jpg|thumb|300px|Irregular satellites of Jupiter (red), Saturn (green), Uranus (magenta) and Neptune (blue) (including Triton). The horizontal axis shows their distance from the planet ([[semi-major axis]]) expressed as a fraction of the planet's [[Hill sphere]]'s radius. The vertical axis shows their [[orbital inclination]]. Points or circles represent their relative sizes. Data as of February 2024.]]
The orbits of the known irregular satellites are extremely diverse, but there are certain patterns. [[Retrograde and direct motion|Retrograde orbit]]s are far more common (83%) than prograde orbits. No satellites are known with orbital inclinations higher than 60° (or smaller than 130° for retrograde satellites); moreover, apart from Nereid, no irregular moon has inclination less than 26°, and inclinations greater than 170° are only found in Saturn's system. In addition, some groupings can be identified, in which one large satellite shares a similar orbit with a few smaller ones.<ref name=denk/>

Given their distance from the planet, the orbits of the outer satellites are highly perturbed by the Sun and their orbital elements change widely over short intervals. The semi-major axis of [[Pasiphae (moon)|Pasiphae]], for example, changes as much as 1.5 Gm in two years (single orbit), the inclination around 10°, and the eccentricity as much as 0.4 in 24 years (twice Jupiter's orbit period).<ref name="Carruba2000">{{cite journal|doi=10.1006/icar.2002.6896|bibcode=2002Icar..158..434C|url=http://astrosun2.astro.cornell.edu/~valerio/val_c.pdf|title=On the Inclination Distribution of the Jovian Irregular Satellites|year=2002|last1=Carruba|first1=V.|last2=Burns|first2=Joseph A.|last3=Nicholson|first3=Philip D.|last4=Gladman|first4=Brett J.|journal=Icarus|volume=158|issue=2|pages=434–449|access-date=2006-09-09|archive-date=2009-02-27|archive-url=https://web.archive.org/web/20090227201039/http://astrosun2.astro.cornell.edu/~valerio/val_c.pdf|url-status=live}}</ref> Consequently, ''mean'' orbital elements (averaged over time) are used to identify the groupings rather than [[osculating orbit|osculating elements]] at the given date. (Similarly, the [[proper orbital elements]] are used to determine the [[Asteroid family|families of asteroids]].)

=== Origin ===
{{see also|Triton (moon)#Capture}}
Irregular satellites may have been captured from heliocentric orbits. (Indeed, it appears that the irregular moons of the giant planets, the [[Trojan asteroid|Jovian]] and [[Neptune Trojan|Neptunian trojans]], and grey [[Kuiper belt]] objects have a similar origin.<ref name=Nep>{{cite journal | last1 = Sheppard | first1 = S. S. | author-link2 = Chad Trujillo | last2 = Trujillo | first2 = C. A. | date = 2006 | title = A Thick Cloud of Neptune Trojans and Their Colors | journal = Science | volume = 313 | issue = 5786 | pages = 511–514 | doi = 10.1126/science.1127173 | pmid = 16778021 | bibcode = 2006Sci...313..511S | s2cid = 35721399 }}</ref>). Alternatively, [[Trans-Neptunian object|trans-neptunian objects]] may have been injected due to the close [[Passing star hypothesis|passing star]] and a fraction of these injected TNOs captured by the giant planets.<ref>{{Cite journal |last1=Pfalzner |first1=Susanne |last2=Govind |first2=Amith |last3=Wagner |first3=Frank W. |date=September 2024 |title=Irregular Moons Possibly Injected from the Outer Solar System by a Stellar Flyby |journal=The Astrophysical Journal Letters |volume=972 |issue=2 |pages=L21 |doi=10.3847/2041-8213/ad63a6 |doi-access=free |arxiv=2409.03529 |bibcode=2024ApJ...972L..21P |issn=2041-8205}}</ref> For this to occur, at least one of three things needs to have happened:
* energy dissipation (e.g. in interaction with the primordial gas cloud)
* a substantial (40%) extension of the planet's [[Hill sphere]] in a brief period of time (thousands of years)
* a transfer of energy in a [[three-body interaction]]. This could involve:
**  a collision (or close encounter) of an incoming body and a satellite, resulting in the incoming body losing energy and being captured.
**  a close encounter between an incoming binary object and the planet (or possibly an existing moon), resulting in one component of the binary being captured. Such a route has been suggested as most likely for [[Triton (moon)|Triton]].<ref name="Agnor06">{{cite journal |author=[[Craig B. Agnor|Agnor, C. B.]] and [[Douglas P. Hamilton|Hamilton, D. P.]] |title=Neptune's capture of its moon Triton in a binary-planet gravitational encounter |journal=Nature |date=2006 |volume=441 |pages=192–4 |bibcode=2006Natur.441..192A | doi=10.1038/nature04792 |pmid=16688170 |issue=7090|s2cid=4420518 }}</ref>

After the capture, some of the satellites could break up leading to [[#Families with a common origin|groupings]] of smaller moons following similar orbits. [[Orbital resonance|Resonances]] could further modify the orbits making these groupings less recognizable.

=== Long-term stability ===
The current orbits of the irregular moons are stable, in spite of substantial perturbations near the [[apocenter]].<ref name="Nesvorny2003">{{cite journal|bibcode=2003AJ....126..398N|url=http://www.journals.uchicago.edu/AJ/journal/issues/v126n1/202528/202528.web.pdf|title=Orbital and Collisional Evolution of the Irregular Satellites|last1=Nesvorný|first1=David|last2=Alvarellos|first2=Jose L. A.|last3=Dones|first3=Luke|last4=Levison|first4=Harold F.|journal=The Astronomical Journal|volume=126|issue=1|pages=398|year=2003|doi=10.1086/375461|s2cid=8502734|access-date=2006-07-29|archive-date=2020-04-15|archive-url=https://web.archive.org/web/20200415190214/https://www.journals.uchicago.edu/AJ/journal/issues/v126n1/202528/202528.web.pdf|url-status=dead}}</ref>
The cause of this stability in a number of irregulars is the fact that they orbit with a [[Secular resonance|secular]] or [[Kozai mechanism|Kozai resonance]].<ref name="Burns2004">{{Cite journal |arxiv = astro-ph/0408119|doi = 10.1086/424937|bibcode = 2004AJ....128.2518C|title = On the Secular Behavior of Irregular Satellites|year = 2004|last1 = Ćuk|first1 = Matija|last2 = Burns|first2 = Joseph A.|journal = The Astronomical Journal|volume = 128|issue = 5|pages = 2518–2541|s2cid = 18564122}}</ref>

In addition, simulations indicate the following conclusions:
* Orbits with inclinations between 50° and 130° are very unstable: their eccentricity increases quickly resulting in the satellite being lost<ref name="Carruba2000"/>
* Retrograde orbits are more stable than prograde (stable retrograde orbits can be found further from the planet)
Increasing eccentricity results in smaller pericenters and large apocenters. The satellites enter the zone of the regular (larger) moons and are lost or ejected via collision and close encounters. Alternatively, the increasing perturbations by the Sun at the growing apocenters push them beyond the Hill sphere.

Retrograde satellites can be found further from the planet than prograde ones. Detailed numerical integrations have shown this asymmetry. The limits are a complicated function of the inclination and eccentricity, but in general, prograde orbits with semi-major axes up to 0.47 r<sub>H</sub> (Hill sphere radius) can be stable, whereas for retrograde orbits stability can extend out to 0.67 r<sub>H</sub>.

The boundary for the semimajor axis is surprisingly sharp for the prograde satellites. A satellite on a prograde, circular orbit (inclination=0°) placed at 0.5 r<sub>H</sub> would leave Jupiter in as little as forty years. The effect can be explained by so-called ''evection resonance''. The apocenter of the satellite, where the planet's grip on the moon is at its weakest, gets locked in resonance with the position of the Sun. The effects of the perturbation accumulate at each passage pushing the satellite even further outwards.<ref name="Nesvorny2003" />

The asymmetry between the prograde and retrograde satellites can be explained very intuitively by the [[Coriolis acceleration]] in the [[Rotating frame|frame rotating]] with the planet. For the prograde satellites the acceleration points outward and for the retrograde it points inward, stabilising the satellite.<ref name="HamBurns91">{{Cite journal |doi = 10.1016/0019-1035(91)90039-V|bibcode = 1991Icar...92..118H|title = Orbital stability zones about asteroids|year = 1991|last1 = Hamilton|first1 = Douglas P.|last2 = Burns|first2 = Joseph A.|journal = Icarus|volume = 92|issue = 1|pages = 118–131|doi-access = free}}</ref>

=== Temporary captures ===

The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations, [[temporary satellite]]s should be a common phenomenon.<ref name="Carlisle">{{cite news |author=Camille M. Carlisle |title=Pseudo-moons Orbit Earth |work=Sky & Telescope |date=December 30, 2011 }}</ref><ref name="Fedorets">{{cite journal |last1=Fedorets |first1=Grigori |last2=Granvik |first2=Mikael |last3=Jedicke |first3=Robert |title=Orbit and size distributions for asteroids temporarily captured by the Earth-Moon system |journal=Icarus |volume=285 |pages=83–94 |date=March 15, 2017 |doi=10.1016/j.icarus.2016.12.022 |bibcode=2017Icar..285...83F }}</ref>  The only observed examples are {{mpl|2006 RH|120}} and {{mpl|2020 CD|3}}, which were temporary satellites of [[Earth]] discovered in 2006 and 2020, respectively.<ref name="Shefford">{{cite web |url=http://www.birtwhistle.org/Gallery6R10DB9.htm |title=2006 RH120 ( = 6R10DB9) (A second moon for the Earth?) |publisher=Great Shefford Observatory |date=September 14, 2017 |access-date=2017-11-13 |url-status=live |archive-url=https://web.archive.org/web/20150206154817/http://www.birtwhistle.org/Gallery6R10DB9.htm |archive-date=2015-02-06 }}</ref><ref name="Sinott">{{cite news |title=Earth's "Other Moon" |work=Sky & Telescope |author=Roger W. Sinnott |date=April 17, 2007 |url=http://www.skyandtelescope.com/news/7067527.html |access-date=2017-11-13 |url-status=dead |archive-url=https://web.archive.org/web/20120402120646/http://www.skyandtelescope.com/news/7067527.html |archive-date=2012-04-02 }}</ref><ref name="MPEC-2020-D104">{{cite web
 |title        = MPEC 2020-D104 : 2020 CD3: Temporarily Captured Object
 |url          = https://minorplanetcenter.net/mpec/K20/K20DA4.html
 |work         = Minor Planet Electronic Circular
 |publisher    = [[Minor Planet Center]]
 |date         = 25 February 2020
 |access-date  = 25 February 2020
 |archive-date = 22 October 2020
 |archive-url  = https://web.archive.org/web/20201022054826/https://minorplanetcenter.net/mpec/K20/K20DA4.html
 |url-status   = live
}}</ref>

== Physical characteristics ==
{{image frame
|content={{Graph:Chart
   | width=300
   | height=150
   | type=rect
   | x= Amalthea,Himalia,Phoebe,Sycorax,Nereid
   | y1=2.08,4.2,8.29,2.3,27.0
   | showValues=format:.1f
}}
|width=400
|caption=Comparative masses of the largest irregular moons and Jupiter's largest inner moon Amalthea (for comparison). Values are ×10<sup>18</sup>&nbsp;kg. One at each outer planet is > 1{{e|18}}&nbsp;kg. Sycorax and Nereid are estimated, not measured; Nereid may not be a captured body. Mars's moons Phobos and Deimos would not be visible at this scale while Triton would dominate.
|border=no
}}

=== Size ===
[[File:TheKuiperBelt PowerLaw2.svg|thumb|220px|The power law for the size distribution of objects in the Kuiper belt, where ''q'' ≈ 4 and thus N ~ D<sup>−3</sup>. That is, for every Kuiper belt object of a particular size, there are approximately 8 times as many objects half that size and a thousands times as many objects one-tenth that size.]]
Because objects of a given size are more difficult to see the greater their distance from Earth, the known irregular satellites of Uranus and Neptune are larger than those of Jupiter and Saturn; smaller ones probably exist but have not yet been observed. Bearing this observational bias in mind, the size distribution of irregular satellites appears to be similar for all four giant planets.

The size distribution of asteroids and many similar populations can be expressed as a [[power law]]: there are many more small objects than large ones, and the smaller the size, the more numerous the object. The mathematical relation expressing the number of objects, <math>N\,\! </math>, with a diameter smaller than a particular size, <math>D\,\! </math>, is approximated as:
: <math> \frac{d N}{d D} \sim D^{-q}</math> with ''q'' defining the slope. 
The value of ''q'' is determined through observation.

For irregular moons, a shallow power law (''q'' ≃ 2<!--that is, N ~ D<sup>−1</sup>-->) is observed for sizes of 10 to 100&nbsp;km,<sup>†</sup> but a steeper law (''q'' ≃ 3.5) is observed for objects smaller than 10&nbsp;km. An analysis of images taken by the [[Canada-France-Hawaii Telescope]] in 2010 shows that the power law for Jupiter's population of small retrograde satellites, down to a detection limit of ≈ 400&nbsp;m, is relatively shallow, at ''q'' ≃ 2.5. Thus it can be extrapolated that Jupiter should have {{val|600|600|300}} moons 400&nbsp;m in diameter or greater.<ref name="Ashton2020">{{cite journal
 |title = The Population of Kilometer-scale Retrograde Jovian Irregular Moons
 |first1 = Edward |last1 = Ashton
 |first2 = Matthew |last2 = Beaudoin
 |first3 = Brett |last3 = Gladman
 |journal = The Planetary Science Journal |author-link3 = Brett J. Gladman
 |date = September 2020
 |volume = 1 |issue = 2 |page = 52 |doi = 10.3847/PSJ/abad95 |arxiv = 2009.03382
 |bibcode = 2020PSJ.....1...52A |s2cid = 221534456 |doi-access = free }}</ref>

For comparison, the distribution of large [[Kuiper belt]] objects is much steeper (''q'' ≈ 4). That is, for every object of 1000&nbsp;km there are a thousand objects with a diameter of 100&nbsp;km, though it's unknown how far this distribution extends. The size distribution of a population may provide insights into its origin, whether through capture, collision and break-up, or accretion.

<sup>†</sup><small>For every object of 100&nbsp;km, ten objects of 10&nbsp;km can be found.</small>

Around each giant planet, there is one irregular satellite that dominates, by having over three-quarters the mass of the entire irregular satellite system: Jupiter's [[Himalia (moon)|Himalia]] (about 75%), Saturn's [[Phoebe (moon)|Phoebe]] (about 98%), Uranus' [[Sycorax (moon)|Sycorax]] (about 90%), and Neptune's [[Nereid (moon)|Nereid]] (about 98%). Nereid also dominates among irregular satellites taken altogether, having about two-thirds the mass of all irregular moons combined. Phoebe makes up about 17%, Sycorax about 7%, and Himalia about 5%: the remaining moons add up to about 4%. (In this discussion, Triton is not included.)<ref name=denk/>

=== Colours ===
[[File:TheIrregulars Colours.svg|thumb|300px|This diagram illustrates the differences of colour in the irregular satellites of Jupiter (red labels), Saturn (yellow) and Uranus (green). Only irregulars with known colour indices are shown. For reference, the [[Centaur (planetoid)|centaur]] [[5145 Pholus|Pholus]] and three [[classical Kuiper belt object]]s are also plotted (grey labels, size not to scale).
For comparison, see also [[Centaur (planetoid)#Physical characteristics|colours of centaurs]] and [[trans-Neptunian object#Physical characteristics|KBOs]].]]
The colours of irregular satellites can be studied via [[Color index|colour indices]]: simple measures of differences of the [[apparent magnitude]] of an object through [[blue]] (B), visible ''i.e.'' green-yellow (V), and [[red]] (R) [[filter (optics)|filter]]s. The observed colours of the irregular satellites vary from neutral (greyish) to reddish (but not as red as the colours of some Kuiper belt objects).

{| align="right" class="wikitable" style="margin-left: 1em; margin-right: 0;"
! [[albedo]]<ref>Based on the definitions from ''Oxford Dictionary of Astronomy'', {{ISBN|0-19-211596-0}}</ref>
! neutral
! reddish
! red
|-
| low
| '''[[C-type asteroid|C]]''' <sub>3–8%</sub>
| '''[[P-type asteroid|P]]''' <sub>2–6%</sub>
| '''[[D-type asteroid|D]]''' <sub>2–5%</sub>
|-
| medium
|
| '''[[M-type asteroid|M]]''' <sub>10–18%</sub>
| '''[[A-type asteroid|A]]''' <sub>13–35%</sub>
|-
| high
|
| '''[[E-type asteroid|E]]''' <sub>25–60%</sub>
|
|}

Each planet's system displays slightly different characteristics. Jupiter's irregulars are grey to slightly red, consistent with [[C-type asteroid|C]], [[P-type asteroid|P]] and [[D-type asteroid]]s.<ref name="Grav2003">{{Cite journal |arxiv = astro-ph/0301016|doi = 10.1016/j.icarus.2003.07.005|bibcode = 2003Icar..166...33G|title = Photometric survey of the irregular satellites|year = 2003|last1 = Grav|first1 = Tommy|last2 = Holman|first2 = Matthew J.|last3 = Gladman|first3 = Brett J.|last4 = Aksnes|first4 = Kaare|journal = Icarus|volume = 166|issue = 1|pages = 33–45|s2cid = 7793999}}</ref> Some groups of satellites are observed to display similar colours (see later sections). Saturn's irregulars are slightly redder than those of Jupiter.

The large Uranian irregular satellites ([[Sycorax (moon)|Sycorax]] and [[Caliban (moon)|Caliban]]) are light red, whereas the smaller [[Prospero (moon)|Prospero]] and [[Setebos (moon)|Setebos]] are grey, as are the Neptunian satellites [[Nereid (moon)|Nereid]] and [[Halimede (moon)|Halimede]].<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>

=== Spectra ===
With the current resolution, the visible and near-infrared spectra of most satellites appear featureless. So far, water ice has been inferred on Phoebe and Nereid and features attributed to aqueous alteration were found on Himalia.{{citation needed|date=February 2024}}

=== Rotation ===
Regular satellites are usually tidally locked (that is, their orbit is [[Synchronous orbit|synchronous]] with their rotation so that they only show one face toward their parent planet). In contrast, tidal forces on the irregular satellites are negligible given their distance from the planet, and rotation periods in the range of only ten hours have been measured for the biggest moons [[Himalia (moon)|Himalia]], [[Phoebe (moon)|Phoebe]], [[Sycorax (moon)|Sycorax]], and [[Nereid (moon)|Nereid]] (to compare with their orbital periods of hundreds of days). Such rotation rates are in the same range that is typical for [[asteroid]]s.{{citation needed|date=February 2024}} [[Triton (moon)|Triton]], being much larger and closer to its parent planet, is tidally locked.
{{clear}}

== Families with a common origin ==
Some irregular satellites appear to orbit in 'groups', in which several satellites share similar orbits. The leading hypothesis is that these objects constitute [[Collisional family|collisional families]], parts of a larger body that broke up.

=== Dynamic groupings ===
Simple collision models can be used to estimate the possible dispersion of the orbital parameters given a velocity impulse [[delta-v|Δ''v'']]. Applying these models to the known orbital parameters makes it possible to estimate the Δ''v'' necessary to create the observed dispersion. A Δ''v'' of tens of meters per seconds (5–50&nbsp;m/s) could result from a break-up. Dynamical groupings of irregular satellites can be identified using these criteria and the likelihood of the common origin from a break-up evaluated.<ref name="Nesvorny2004">{{cite journal|doi=10.1086/382099|bibcode=2004AJ....127.1768N|url=http://www.boulder.swri.edu/~davidn/papers/irrbig.pdf|title=Collisional Origin of Families of Irregular Satellites|year=2004|last1=Nesvorn|first1=David|last2=Beaug|first2=Cristian|last3=Dones|first3=Luke|journal=The Astronomical Journal|volume=127|issue=3|pages=1768–1783|s2cid=27293848|access-date=2006-08-02|archive-date=2017-08-09|archive-url=https://web.archive.org/web/20170809124713/http://www1.boulder.swri.edu/~davidn/papers/irrbig.pdf|url-status=live}}</ref>

When the dispersion of the orbits is too wide (i.e. it would require Δ''v'' in the order of hundreds of m/s)
* either more than one collision must be assumed, i.e. the cluster should be further subdivided into groups
* or significant post-collision changes, for example resulting from resonances, must be postulated.

=== Colour groupings ===
When the colours and spectra of the satellites are known, the homogeneity of these data for all the members of a given grouping is a substantial argument for a common origin. However, lack of precision in the available data often makes it difficult to draw statistically significant conclusions. In addition, the observed colours are not necessarily representative of the bulk composition of the satellite.

=={{anchor|Group}} Observed groupings ==

=== Irregular satellites of Jupiter ===
[[File:TheIrregulars JUPITER.svg|thumb|300px|The orbits of Jupiter's irregular satellites, showing how they cluster into groups. Satellites are represented by circles that indicate their relative sizes. An object's position on the horizontal axis shows its distance from Jupiter. Its position on the vertical axis indicates its [[orbital inclination]]. The yellow lines indicate its [[orbital eccentricity]]  (i.e. the extent to which its distance from Jupiter varies during its orbit). Data as of 2006.]]
Typically, the following groupings are listed (dynamically tight groups displaying homogenous colours are listed in '''bold''')
* [[Retrograde and prograde motion|Prograde]] satellites
** The '''[[Himalia group]]''' shares an average inclination of 28°. They are confined dynamically (Δ''v'' ≈ 150&nbsp;m/s). They are homogenous at visible wavelengths (having neutral colours similar to those of [[C-type asteroid]]s) and at near [[infrared]] wavelengths<ref name="Grav2004">{{Cite journal |arxiv = astro-ph/0312571|doi = 10.1086/420881|bibcode = 2004ApJ...605L.141G|title = Near-Infrared Photometry of the Irregular Satellites of Jupiter and Saturn|year = 2004|last1 = Grav|first1 = Tommy|last2 = Holman|first2 = Matthew J.|journal = The Astrophysical Journal|volume = 605|issue = 2|pages = L141–L144|s2cid = 15665146}}</ref>
** The prograde satellites [[Themisto (moon)|Themisto]], [[Carpo (moon)|Carpo]], and [[Valetudo (moon)|Valetudo]] are not part of any known group.
[[File:Animation of Himalia orbit around Jupiter.gif|thumb|left|Animation of Himalia's orbit.<br>{{legend2|RoyalBlue| Jupiter}}{{·}}{{legend2| Magenta | Himalia}}{{·}}{{legend2|Cyan|Callisto}}]]
* [[Retrograde motion|Retrograde]] satellites
** The '''[[Carme group]]''' shares an average inclination of 165°. It is dynamically tight (5 < Δ''v'' < 50&nbsp;m/s). It is very homogenous in colour, each member displaying light red colouring consistent with a [[D-type asteroid]] progenitor.
** The '''[[Ananke group]]''' shares an average inclination of 148°. It shows little dispersion of orbital parameters (15 < Δ''v'' < 80&nbsp;m/s). [[Ananke (moon)|Ananke]] itself appears light red but the other group members are grey.
** The [[Pasiphae group]] is very dispersed. [[Pasiphae (moon)|Pasiphae]] itself appears to be grey, whereas other members ([[Callirrhoe (moon)|Callirrhoe]], [[Megaclite (moon)|Megaclite]]) are light red.
[[Sinope (moon)|Sinope]], sometimes included into the Pasiphae group, is red and given the difference in inclination, it could be captured independently.<ref name="Grav2003" /><ref name="SheppardJewitt2003">{{Cite journal | last1 = Sheppard | first1 = S. S. | last2 = Jewitt | first2 = D. C. | doi = 10.1038/nature01584 | title = An abundant population of small irregular satellites around Jupiter | journal = Nature | volume = 423 | issue = 6937 | pages = 261–263 | year = 2003 | pmid = 12748634 | url = http://www.dtm.ciw.edu/users/sheppard/sheppardjupiter.pdf | bibcode = 2003Natur.423..261S | s2cid = 4424447 | access-date = 2015-08-29 | archive-date = 2013-10-15 | archive-url = https://web.archive.org/web/20131015103409/http://www.dtm.ciw.edu/users/sheppard/sheppardjupiter.pdf | url-status = live }}</ref>
Pasiphae and Sinope are also trapped in [[secular resonance]]s with Jupiter.<ref name="Nesvorny2003" /><ref name="Nesvorny2004" />
{{clear}}

=== Irregular satellites of Saturn ===
[[File:TheIrregulars SATURN.svg|thumb|300px|Irregular satellites of Saturn, showing how they cluster into groups. Data as of 2006. For explanation, see Jupiter diagram]]
The following groupings are commonly listed for Saturn's satellites:
* Prograde satellites
** The '''[[Gallic group]]''' shares an average inclination of 34°. Their orbits are dynamically tight (Δ''v'' ≈ 50&nbsp;m/s), and they are light red in colour; the colouring is homogenous at both visible and near infra-red wavelengths.<ref name="Grav2004" />
** The [[Inuit group]] shares an average inclination of 46°. Their orbits are widely dispersed (Δ''v'' ≈ 350&nbsp;m/s) but they are physically homogenous, sharing a light red colouring.
* Retrograde satellites
** The [[Norse group]] is defined mostly for naming purposes; the orbital parameters are very widely dispersed. Sub-divisions have been investigated, including
*** The [[Phoebe (moon)|Phoebe]] group shares an average inclination of 174°; this sub-group too is widely dispersed, and may be further divided into at least two sub-sub-groups
*** The [[Skathi (moon)|Skathi]] group is a possible sub-group of the Norse group
{{clear}}
<gallery mode=packed heights=180px>
File:Animation of Saturn's Inuit group of satellites.gif|Animation of Saturn's Inuit group of satellites<br>{{legend2|RoyalBlue| [[Kiviuq (moon)|Kiviuq]]}}{{·}}{{legend2|Lime| [[Ijiraq (moon)|Ijiraq]]}}{{·}}{{legend2|Gold| [[Paaliaq]]}}{{·}}{{legend2|OrangeRed| [[Siarnaq]]}}{{·}}{{legend2|Cyan|[[Tarqeq]]}}
File:Animation of Phoebe orbit around Saturn.gif|Animation of Phoebe's orbit.<br>{{legend2|RoyalBlue| Saturn}}{{·}}{{legend2| Magenta | Phoebe}}{{·}}{{legend2|Cyan|Titan}}
</gallery>

=== Irregular satellites of Uranus and Neptune ===
[[File:TheIrregulars NEPTUNE URANUS.svg|thumb|300px|Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). Data as of 2006. For explanation, see Jupiter diagram]]
{| align="right" class="wikitable" style="margin-left: 1em; margin-right: 0;"
! Planet
! r<sub>min</sub><ref name="Sheppard2006" />
|-
| Jupiter
| align="right" |  1.5&nbsp;km
|-
| Saturn
| align="right" |  3&nbsp;km
|-
| Uranus
| align="right" |  7&nbsp;km
|-
| Neptune
| align="right" | 16&nbsp;km
|}
According to current knowledge, the number of irregular satellites orbiting Uranus and Neptune is smaller than that of Jupiter and Saturn. However, it is thought that this is simply a result of observational difficulties due to the greater distance of Uranus and Neptune. The table at right shows the minimum [[radius]] (r<sub>min</sub>) of satellites that can be detected with current technology, assuming an [[albedo]] of 0.04; thus, there are almost certainly small Uranian and Neptunian moons that cannot yet be seen.

Due to the smaller numbers, statistically significant conclusions about the groupings are difficult. A single origin for the retrograde irregulars of Uranus seems unlikely given a dispersion of the orbital parameters that would require high impulse (Δ''v'' ≈ 300&nbsp;km), implying a large diameter of the impactor (395&nbsp;km), which is incompatible in turn with the size distribution of the fragments. Instead, the existence of two groupings has been speculated:<ref name="Grav2003" />
* [[Caliban (moon)|Caliban]] group
* [[Sycorax (moon)|Sycorax]] group
These two groups are distinct (with 3σ confidence) in their distance from Uranus and in their eccentricity.<ref name="SheppardUranus2005">{{Cite journal | last1 = Sheppard | first1 = S. S. | last2 = Jewitt | first2 = D. | last3 = Kleyna | first3 = J. | title = An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness | doi = 10.1086/426329 | journal = The Astronomical Journal | volume = 129 | issue = 1 | pages = 518–525 | year = 2005 |arxiv = astro-ph/0410059 |bibcode = 2005AJ....129..518S | s2cid = 18688556 }}</ref>
However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey.<ref name="GravHolmanFraser2004" />

For Neptune, a possible common origin of [[Psamathe (moon)|Psamathe]] and [[Neso (moon)|Neso]] has been noted.<ref name="SheppardJewittKleyna2006">{{cite journal| doi = 10.1086/504799| last1 = Sheppard| first1 = Scott S.| author-link1 = Scott S. Sheppard| last2 = Jewitt| first2 = David C.| author-link2 = David C. Jewitt| last3 = Kleyna| first3 = Jan| author-link3 = Jan Kleyna| year = 2006| title = A Survey for "Normal" Irregular Satellites around Neptune: Limits to Completeness| journal = The Astronomical Journal| volume = 132| issue = 1| pages = 171–176| bibcode = 2006AJ....132..171S| arxiv = astro-ph/0604552| s2cid = 154011}}</ref> Given the similar (grey) colours, it was also suggested that  [[Halimede (moon)|Halimede]] could be a fragment of  [[Nereid (moon)|Nereid]].<ref name="GravHolmanFraser2004" /> The two satellites have had a very high probability (41%) of collision over the age of the solar system.<ref name="HolmanKavelaarsGrav2004">{{cite journal| doi = 10.1038/nature02832| last1 = Holman| first1 = M. J.| author-link = Matthew J. Holman| last2 = Kavelaars| first2 = J. J.| author-link2 = John J. Kavelaars| last3 = Grav| first3 = T.| display-authors = 3| year = 2004| last4 = Gladman| first4 = B. J.| author-link4 = Brett J. Gladman| last5 = Fraser| first5 = W. C.| last6 = Milisavljevic| first6 = D.| last7 = Nicholson| first7 = P. D.| last8 = Burns| first8 = J. A.| last9 = Carruba| first9 = V.| title = Discovery of five irregular moons of Neptune| journal = Nature| volume = 430| issue = 7002| pages = 865–867| pmid = 15318214| bibcode = 2004Natur.430..865H| s2cid = 4412380| url = https://www.cfa.harvard.edu/~mholman/nature_final.pdf| access-date = 24 October 2011| archive-date = 2 November 2013| archive-url = https://web.archive.org/web/20131102023048/https://www.cfa.harvard.edu/~mholman/nature_final.pdf| url-status = live}}</ref>

== Exploration ==
[[File:Himalia.png|thumb|Distant ''Cassini'' image of Himalia]]
To date, the only irregular satellites to have been visited close-up by a spacecraft are [[Triton (moon)|Triton]] and [[Phoebe (moon)|Phoebe]], the largest of Neptune's and Saturn's irregulars respectively. Triton was imaged by ''[[Voyager 2]]'' in 1989 and Phoebe by the ''[[Cassini probe|Cassini]]'' probe in 2004. ''Voyager'' 2 also captured a distant image of Neptune's [[Nereid (moon)|Nereid]] in 1989, and ''Cassini'' captured a distant, low-resolution image of Jupiter's [[Himalia (moon)|Himalia]] in 2000. ''[[New Horizons]]'' captured low-resolution images of Jupiter's Himalia, [[Elara (moon)|Elara]], and [[Callirrhoe (moon)|Callirrhoe]] in 2007. Throughout the ''Cassini'' mission, many Saturnian irregulars were observed from a distance: [[Albiorix (moon)|Albiorix]], [[Bebhionn (moon)|Bebhionn]], [[Bergelmir (moon)|Bergelmir]], [[Bestla (moon)|Bestla]], [[Erriapus (moon)|Erriapus]], [[Fornjot (moon)|Fornjot]], [[Greip (moon)|Greip]], [[Hati (moon)|Hati]], [[Hyrrokkin (moon)|Hyrrokkin]], [[Ijiraq (moon)|Ijiraq]], [[Kari (moon)|Kari]], [[Kiviuq (moon)|Kiviuq]], [[Loge (moon)|Loge]], [[Mundilfari (moon)|Mundilfari]], [[Narvi (moon)|Narvi]], [[Paaliaq (moon)|Paaliaq]], [[Siarnaq (moon)|Siarnaq]], [[Skathi (moon)|Skathi]], [[Skoll (moon)|Skoll]], [[Suttungr (moon)|Suttungr]], [[Tarqeq (moon)|Tarqeq]], [[Tarvos (moon)|Tarvos]], [[Thrymr (moon)|Thrymr]], and [[Ymir (moon)|Ymir]].<ref name=denk/>

The [[Tianwen-4]] mission (to launch 2029) is planned to focus on the regular moon Callisto around Jupiter, but it may fly-by several irregular Jovian satellites before settling into Callistonian orbit.<ref name=AJ2023>{{Cite web |url=https://www.planetary.org/articles/chinas-plans-for-outer-solar-system-exploration|title=China's plans for outer Solar System exploration|author=Andrew Jones |date=2023-12-21 |publisher=
The Planetary Society|accessdate=2023-12-27}}</ref>
{{clear}}

== Gallery ==
<gallery mode=packed heights=200px style="text-align:left">
File:Jupiter irregular moon orbits Jan 2021.png|71 irregular moons of Jupiter (with Callisto for comparison; the other Galileans are also visible near the centre, though not labelled explicitly). Data as of 2021.
File:Saturn irregular moon orbits 2023.png|122 irregular moons of Saturn (with Titan, Hyperion, and Iapetus for comparison). Data as of 2023.
File:Uranus irregular moon orbits Jan 2021.png|9 irregular moons of Uranus. Data as of 2021.
File:Neptune irregular moon orbits Jan 2021.png|6 irregular moons of Neptune (excluding Triton). Data as of 2021.
</gallery>

== References ==
{{reflist|2}}

== External links ==
* [http://www2.ess.ucla.edu/~jewitt/irregulars.html David Jewitt's pages]
* Discovery circumstances [http://ssd.jpl.nasa.gov/?sat_discovery from JPL]
* Mean orbital elements [http://ssd.jpl.nasa.gov/?sat_elem from JPL]
* [http://www.minorplanetcenter.org/iau/NatSats/NaturalSatellites.html MPC: Natural Satellites Ephemeris Service]
* Tilmann Denk: Outer Moons of [https://tilmanndenk.de/outerjovianmoons/ Jupiter] and [https://tilmanndenk.de/outersaturnianmoons/ Saturn]
{{Solar System moons (compact)}}

{{DEFAULTSORT:Irregular Moon}}
[[Category:Irregular satellites| ]]
[[Category:Moons]]
[[Category:Orbits]]