Himalia group

Background

The Himalia group was originally conceived as one of two collisional families among the eight 'classical' irregular satellites of Jupiter, the other being a retrograde group. While the retrograde moons were later found to belong to multiple families, the prograde cluster remained intact and developed into the Himalia group. The International Astronomical Union originally reserved names ending in '-a' for prograde moons, a convention later restricted to members of this group; other prograde moons now receive names ending in '-o'. Two possible satellites sighted in 2017 were later confirmed as S/2011 J 3 and S/2018 J 2 in 2023 and identified as part of the group.

Physically, the group is homogeneous, with neutral colors (B−V = 0.66, V−R = 0.36) similar to C-type asteroids. The spectral characteristics of Himalia, Elara, and Lysithea suggest different levels of aqueous alteration, implying the progenitor asteroid may have been 300 km in diameter with a differentiated interior. The group's size distribution indicates formation from a moon–moon collision rather than an impact with a passing planetesimal, requiring a relatively energetic impactor of about 13 km diameter.

Orbitally, the group has semi-major axes around 11.6 million km, inclinations of about 28°, and eccentricities between 0.11 and 0.24. Their orbits are closer to Jupiter and have lower eccentricities than other irregular moons. The group is thought to cause a dearth of irregular moons at distances 0.05–0.12 AU from Jupiter, as objects in that range are likely to collide with Himalia group members. The dispersion of orbital elements is larger than collisional dispersion can conventionally explain, possibly due to post-formation scattering, secondary collisions, or gravitational interactions during planetary migration.