Irregular moon

The term does not refer to shape; Triton, for example, is a round moon but is considered irregular due to its orbit and origins.As of February 2024[update], 228 irregular moons are known, orbiting all four of the outer planets (Jupiter, Saturn, Uranus, and 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[1] and were captured after the close flyby of another star[2] There is no widely accepted precise definition of an irregular satellite.(Indeed, it appears that the irregular moons of the giant planets, the Jovian and Neptunian trojans, and grey Kuiper belt objects have a similar origin.[7]).[8] For this to occur, at least one of three things needs to have happened: After the capture, some of the satellites could break up leading to groupings of smaller moons following similar orbits.The satellites enter the zone of the regular (larger) moons and are lost or ejected via collision and close encounters.[10] The asymmetry between the prograde and retrograde satellites can be explained very intuitively by the Coriolis acceleration in the frame rotating with the planet.[15][16][17] 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.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 m, is relatively shallow, at q ≃ 2.5.[citation needed] Regular satellites are usually tidally locked (that is, their orbit is synchronous with their rotation so that they only show one face toward their parent planet).Dynamical groupings of irregular satellites can be identified using these criteria and the likelihood of the common origin from a break-up evaluated.[22] When the dispersion of the orbits is too wide (i.e. it would require Δv in the order of hundreds of m/s) 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.The table at right shows the minimum radius (rmin) 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.[25] However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey.Throughout the Cassini mission, many Saturnian irregulars were observed from a distance: Albiorix, Bebhionn, Bergelmir, Bestla, Erriapus, Fornjot, Greip, Hati, Hyrrokkin, Ijiraq, Kari, Kiviuq, Loge, Mundilfari, Narvi, Paaliaq, Siarnaq, Skathi, Skoll, Suttungr, Tarqeq, Tarvos, Thrymr, and Ymir.[5] 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.
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 power law for the size distribution of objects in the Kuiper belt, where q ≈ 4 and thus N ~ D −3 . 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.
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 Pholus and three classical Kuiper belt objects are also plotted (grey labels, size not to scale). For comparison, see also colours of centaurs and KBOs .
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.
Animation of Himalia's orbit.
Jupiter · Himalia · Callisto
Irregular satellites of Saturn, showing how they cluster into groups. Data as of 2006. For explanation, see Jupiter diagram
Irregular satellites of Uranus (green) and Neptune (blue) (excluding Triton). Data as of 2006. For explanation, see Jupiter diagram
Distant Cassini image of Himalia
Asteroid capturePhoebeTritonastronomynatural satelliteinclinedhighly ellipticalretrograderegular satellitestemporary satellitesround moonouter planetsJupiterSaturnUranusNeptuneHimaliaSycoraxNereidKuiper belt106 kmorbital planesemi-major axisHill sphereapoapsesIapetusorbital inclinationRetrograde orbitsPasiphaeosculating elementsproper orbital elementsfamilies of asteroidsJovianNeptunian trojanstrans-neptunian objectspassing starthree-body interactionResonancesapocentersecularKozai resonanceCoriolis accelerationframe rotating2006 RH1202020 CD3power lawCanada-France-Hawaii TelescopecentaurPholusclassical Kuiper belt objectscolour indicesapparent magnitudefiltersalbedoD-type asteroidsCalibanProsperoSetebosHalimedesynchronousasteroidscollisional familiesorbital eccentricityProgradeHimalia groupC-type asteroidsinfraredThemistoValetudoCarme groupD-type asteroidAnanke groupAnankePasiphae groupCallirrhoeMegacliteSinopesecular resonancesGallic groupInuit groupNorse groupSkathiKiviuqIjiraqPaaliaqSiarnaqTarqeqradiusPsamatheVoyager 2CassiniNew HorizonsAlbiorixBebhionnBergelmirBestlaErriapusFornjotHyrrokkinMundilfariSuttungrTarvosThrymrTianwen-4BibcodeJet Propulsion LaboratoryTrujillo, C. A.Minor Planet CenterGladman, BrettHolman, Matthew J.The Astrophysical JournalSheppard, Scott S.Jewitt, David C.Kleyna, JanHolman, M. J.Kavelaars, J. J.Natural satellitesSolar SystemPlanetarysatellitesDwarf planetsatellitesHaumeaQuaoarMakemakeGonggongMinor-planetmoonsFlorenceDidymosDimorphosMoshup1994 CC2001 SN263KalliopeEuphrosyneDaphneEugeniaPetit-PrinceSylviaRomulusMinervaCamillaElektraKleopatraDactylRoxaneOlympiasPulcovaDinkineshPatroclusMenoetiusHektorSkamandriosEurybates2002 UX25Sila–NunamSalaciaActaeaIlmarëGǃkúnǁʼhòmdímàGǃòʼé ǃHú2013 FY27GanymedeCallistoEuropaTitaniaOberonCharonUmbrielTethysDysnomiaEnceladusMirandaProteusHiʻiakaHyperionDiscovery timelineInner moonsPlanetary-mass moonsNamingSubsatelliteRegular moons