Inastronomy, anirregular moon,irregular satellite, orirregular natural satellite is anatural satellite following an orbit that is irregular in some of the following ways: distant;inclined;highly elliptical;retrograde. They have often beencaptured by their parent planet, unlikeregular satellites that formed in orbit around them. Irregular moons have a stable orbit, unliketemporary satellites which often have similarly irregular orbits but will eventually depart. The term does not refer to shape;Triton, for example, is around moon but is considered irregular due to its orbit and origins.
As of April 2025[update], 358 irregular moons are known, orbiting all four of theouter planets (Jupiter,Saturn,Uranus, andNeptune). The largest of each planet areHimalia of Jupiter,Phoebe of Saturn,Sycorax of Uranus, andTriton of Neptune. Triton is rather unusual for an irregular moon; if it is excluded, thenNereid 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 theKuiper belt[1] and were captured after the close flyby of another star.[2]
| Planet | Hill radius rH (106 km)[3] | rH (°)[3] | Number known | Farthest known satellite (106 km) |
|---|---|---|---|---|
| Jupiter | 51 | 4.7 | 89 | 24.2 (0.47rH) |
| Saturn | 69 | 3.0 | 250 | 28.0 (0.41rH) |
| Uranus | 73 | 1.5 | 10 | 20.4 (0.28rH) |
| Neptune | 116 | 1.5 | 9 (including Triton) | 50.7 (0.44rH) |
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 theprecession of theirorbital plane is primarily controlled by the Sun, other planets, or other moons.[4]
In practice, the satellite'ssemi-major axis is compared with the radius of the planet'sHill sphere (that is, the sphere of its gravitational influence),. Irregular satellites have semi-major axes greater than 0.05 withapoapses extending as far as to 0.65.[3] 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.[5]
Earth'sMoon 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] and its semi-major axis is greater than 0.05 of the radius of Earth's Hill sphere. On the other hand, Neptune'sTriton, 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.[5] Neptune'sNereid and Saturn'sIapetus 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.
The orbits of the known irregular satellites are extremely diverse, but there are certain patterns.Retrograde orbits 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.[5]
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 ofPasiphae, for example, changes as much as 1.5 million km 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).[6] Consequently,mean orbital elements (averaged over time) are used to identify the groupings rather thanosculating elements at the given date. (Similarly, theproper orbital elements are used to determine thefamilies of asteroids.)
Irregular satellites may have been captured from heliocentric orbits. (Indeed, it appears that the irregular moons of the giant planets, theJovian andNeptunian trojans, and greyKuiper belt objects have a similar origin.[7]). Alternatively,trans-Neptunian objects may have been injected due to the closepassing star and a fraction of these injected TNOs captured by the giant planets.[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 togroupings of smaller moons following similar orbits.Resonances could further modify the orbits making these groupings less recognizable.
The current orbits of the irregular moons are stable, in spite of substantial perturbations near theapocenter.[10]The cause of this stability in a number of irregulars is the fact that they orbit with asecular orKozai resonance.[11]
In addition, simulations indicate the following conclusions:
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 rH (Hill sphere radius) can be stable, whereas for retrograde orbits stability can extend out to 0.67 rH.
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 rH would leave Jupiter in as little as forty years. The effect can be explained by so-calledevection 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.[10]
The asymmetry between the prograde and retrograde satellites can be explained very intuitively by theCoriolis acceleration in theframe rotating with the planet. For the prograde satellites the acceleration points outward and for the retrograde it points inward, stabilising the satellite.[12]
The capture of an asteroid from a heliocentric orbit is not always permanent. According to simulations,temporary satellites should be a common phenomenon.[13][14] The only observed examples are2006 RH120 and2020 CD3, which were temporary satellites ofEarth discovered in 2006 and 2020, respectively.[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.
The size distribution of asteroids and many similar populations can be expressed as apower 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,, with a diameter smaller than a particular size,, is approximated as:
The value ofq is determined through observation.
For irregular moons, a shallow power law (q ≃ 2) is observed for sizes of 10 to 100 km,† but a steeper law (q ≃ 3.5) is observed for objects smaller than 10 km. An analysis of images taken by theCanada–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, atq ≃ 2.5. Thus it can be extrapolated that Jupiter should have600+600
−300 moons 400 m in diameter or greater.[18]
For comparison, the distribution of largeKuiper belt objects is much steeper (q ≈ 4). That is, for every object of 1000 km there are a thousand objects with a diameter of 100 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.
†For every object of 100 km, ten objects of 10 km can be found.
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'sHimalia (about 75%), Saturn'sPhoebe (about 98%), Uranus'sSycorax (about 90%), and Neptune'sNereid (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.)[5]
The colours of irregular satellites can be studied viacolour indices: simple measures of differences of theapparent magnitude of an object throughblue (B), visiblei.e. green-yellow (V), andred (R)filters. 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).
| albedo[19] | neutral | reddish | red |
|---|---|---|---|
| low | C3–8% | P2–6% | D2–5% |
| medium | M10–18% | A13–35% | |
| high | E25–60% |
Each planet's system displays slightly different characteristics. Jupiter's irregulars are grey to slightly red, consistent withC,P andD-type asteroids.[20] 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 andCaliban) are light red, whereas the smallerProspero andSetebos are grey, as are the Neptunian satellitesNereid andHalimede.[21]
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]
Regular satellites are usually tidally locked (that is, their orbit issynchronous 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 moonsHimalia,Phoebe,Sycorax, andNereid (to compare with their orbital periods of hundreds of days). Such rotation rates are in the same range that is typical forasteroids.[citation needed]Triton, being much larger and closer to its parent planet, is tidally locked.
Some irregular satellites appear to orbit in 'groups', in which several satellites share similar orbits. The leading hypothesis is that these objects constitutecollisional families, parts of a larger body that broke up.
Simple collision models can be used to estimate the possible dispersion of the orbital parameters given a velocity impulseΔv. Applying these models to the known orbital parameters makes it possible to estimate the Δv necessary to create the observed dispersion. A Δv of tens of meters per seconds (5–50 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.[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. 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.
Typically, the following groupings are listed (dynamically tight groups displaying homogenous colours are listed inbold)
Sinope, sometimes included into the Pasiphae group, is red and given the difference in inclination, it could be captured independently.[20][24]Pasiphae and Sinope are also trapped insecular resonances with Jupiter.[10][22]
The following groupings are commonly listed for Saturn's satellites:
| Planet | rmin[3] |
|---|---|
| Jupiter | 1.5 km |
| Saturn | 3 km |
| Uranus | 7 km |
| Neptune | 16 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 minimumradius (rmin) of satellites that can be detected with current technology, assuming analbedo 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 (Δv ≈ 300 km), implying a large diameter of the impactor (395 km), which is incompatible in turn with the size distribution of the fragments. Instead, the existence of two groupings has been speculated:[20]
These two groups are distinct (with 3σ confidence) in their distance from Uranus and in their eccentricity.[25]However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red, whereas the smaller moons are grey.[21]
For Neptune, a possible common origin ofPsamathe andNeso has been noted.[26] Given the similar (grey) colours, it was also suggested thatHalimede could be a fragment ofNereid.[21] The two satellites have had a very high probability (41%) of collision over the age of the solar system.[27]
To date, the only irregular satellites to have been visited close-up by a spacecraft areTriton andPhoebe, the largest of Neptune's and Saturn's irregulars respectively. Triton was imaged byVoyager 2 in 1989 and Phoebe by theCassini probe in 2004.Voyager 2 also captured a distant image of Neptune'sNereid in 1989, andCassini captured a distant, low-resolution image of Jupiter'sHimalia in 2000.New Horizons captured low-resolution images of Jupiter's Himalia,Elara, andCallirrhoe in 2007. Throughout theCassini 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, andYmir.[5]
TheTianwen-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.[28]
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