- Irregular moon
astronomy, an irregular moon is a natural satellitefollowing a distant, inclined, and often retrograde orbit. They are believed to have been captured by their parent planet, unlike regular satellites, which form " in situ".
Ninety-three irregular satellites have been discovered since 1997, orbiting all four of the
giant planets ( Jupiter, Saturn, Uranusand Neptune). Before 1997, only ten were known, including Phoebe, the largest irregular satellite of Saturn, and Himalia, the largest irregular satellite of Jupiter. Sycorax, the largest irregular moon of Uranus, was discovered in 1997.It is currently thought that the irregular satellites were captured from heliocentric orbits near their current locations, shortly after the formation of their parent planet. An alternative theory, that they originated further out in the Kuiper Belt, is not supported by current observations.
According to current knowledge, the number of irregular satellites orbiting Uranus and Neptune is smaller than that of Jupiter and Saturn. However, it is believed this is simply a result of observational difficulties due to the greater distance of Uranus and Neptune. The table at left shows the minimum
radius(rmin) of satellites that can be detected with current technology, assuming an albedoof 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:
*Sycorax group These two groups are distinct (with 3σ confidence) in their distance from Uranus and in their eccentricity.
Scott S. Sheppard, David C. Jewitt, and Jan Kleyna"An Ultradeep Survey for Irregular Satellites of Uranus: Limits to Completeness", The Astronomical Journal, 129 (2005), pages 518–525 ( [http://arxiv.org/abs/astro-ph/0410059 preprint] ).] .However, these groupings are not directly supported by the observed colours: Caliban and Sycorax appear light red while the smaller moons are grey.
For Neptune, a possible common origin of Psamathe and Neso has been noted
Scott S. Sheppard, David C. Jewitt, Jan Kleyna, "A Survey for "Normal" Irregular Satellites Around Neptune: Limits to Completeness"( [http://arxiv.org/abs/astro-ph/0604552 preprint] )] . Given the similar (grey) colours, it was also suggested that Halimede could be a fragment of Nereid. The two satellites have had a very high probability (41%) of collision over the age of the solar system.M.Holman, JJ Kavelaars, B.Gladman, T.Grav, W.Fraser, D.Milisavljevic, P.Nicholson, J.Burns, V.Carruba, J-M.Petit, P.Rousselot, O.Mousis, B.Marsden, R.Jacobson, "Discovery of five irregular moons of Neptune", Nature, 430 (2004), pp. 865-867 ( [http://cfa-www.harvard.edu/~mholman/nature_final.pdf Final preprint (pdf)] )]
To date, the only irregular satellite to have been visited by a spacecraft is Phoebe, the largest of Saturn's irregulars, which was photographed by the "Cassini" probe in
2005. "Cassini" also captured a distant, low resolution image of Jupiter's Himalia in 2000. There are no spacecraft planned to visit any irregular satellites in the future.
* [http://www.ifa.hawaii.edu/~jewitt/irregulars.html David Jewitt's pages]
* [http://www.ifa.hawaii.edu/~sheppard/satellites/ Scott Sheppard'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]
*Ephemeris [http://cfa-www.harvard.edu/iau/NatSats/NaturalSatellites.html from IAU]
Solar System moons (compact)
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