Himalia group

File:TheIrregulars JUPITER Himalia.svg

This diagram compares the orbital elements and relative sizes of the largest four members of the Himalia group. The horizontal axis illustrates their average distance from Jupiter, the vertical axis their orbital inclination, and the circles their relative sizes.

File:TheIrregulars JUPITER GROUPS.svg

This diagram illustrates all the irregular satellites of Jupiter. The Himalia group is bunched together near the top of the diagram. An object's position on the horizontal axis indicates its distance from Jupiter. The vertical axis indicates its inclination. Eccentricity is indicated by yellow bars illustrating the object's maximum and minimum distances from Jupiter. Circles illustrate an object's size in comparison to the others.

The Himalia group is a group of prograde irregular satellites of Jupiter that follow similar orbits to Himalia and are thought to have a common origin.<ref name="SJ2003">Scott S. Sheppard, David C. Jewitt An abundant population of small irregular satellites around Jupiter, Nature, 423 (May 2003), pp.261-263 (pdf) Template:Webarchive</ref>

The known members of the group are (in order of increasing distance from Jupiter):

Name	Diameter (km)	Period (days)	Notes
Leda	21.5	240.93
Ersa	3	249.23
Himalia	139.6 Template:Small	250.56	largest member and group prototype
S/2018 J 2	3	250.88
Pandia	3	251.91
Lysithea	42.2	259.20
Elara	79.9	259.64
S/2011 J 3	3	261.77
Dia	4	278.21

Two additional possible satellites discovered by Sheppard in 2017 have been identified to be likely part of the Himalia group, but were too faint (mag >24) to be tracked and confirmed as satellites.<ref name="Sheppard2018">Template:Cite journal</ref>

The International Astronomical Union (IAU) reserves names for moons of Jupiter ending in -a (Leda, Himalia and so on) for the moons in this group to indicate prograde motions of these bodies relative to Jupiter, their gravitationally central object.<ref>Template:Cite book</ref>

Characteristics and origin

The objects in the Himalia group have semi-major axes (distances from Jupiter) in the range of 11.10 and 12.30 Gm, inclinations between 27.2° and 29.1°, and eccentricities between 0.11 and 0.24. All orbit in a prograde direction. In physical appearance, the group is very homogeneous, all satellites displaying neutral colours (colour indices B−V = 0.66 and V−R = 0.36) similar to those of C-type asteroids. Given the limited dispersion of the orbital parameters and the spectral homogeneity, it has been suggested that the group could be a remnant of the break-up of an asteroid from the main asteroid belt.<ref name="Grav2003">Template:Cite journal</ref> The radius of the parent asteroid was probably about 89 km, only slightly larger than that of Himalia, which retains approximately 87% of the mass of the original body. This indicates the asteroid was not heavily disturbed.<ref name="SJ2003"/>

Numerical integrations show a high probability of collisions among the members of the prograde group during the lifespan of the Solar System (e.g. on average 1.5 collisions between Himalia and Elara). In addition, the same simulations have shown fairly high probabilities of collisions between prograde and retrograde satellites (e.g. Pasiphae and Himalia have a 27% probability of collision within 4.5 gigayears). Consequently, it has been suggested that the current group could be a result of a more recent, rich collisional history among the prograde and retrograde satellites as opposed to the single break-up shortly after the planet formation that has been inferred for the Carme and Ananke groups.<ref name="Nesvorny2004"> David Nesvorný, Cristian Beaugé, and Luke Dones Collisional Origin of Families of Irregular Satellites, The Astronomical Journal, 127 (2004), pp. 1768–1783 (pdf).</ref>

References

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