TheEunomia orEunomian family (FIN:502) is a largeasteroid family ofS-type asteroids named after the asteroid15 Eunomia. It is the most prominent family in the intermediateasteroid belt and the 6th-largest family with nearly six thousand known members, or approximately 1.4% of all asteroids in theasteroid belt.[1][2]: 23
By far the largest member is15 Eunomia, the largest of all the "stony"S-type asteroid, It is about 300 km across along its longest axis, has a 250 km mean radius, and lies close to thebarycenter of the family. Eunomia has been estimated to contain about 70–75% of the matter from the original parent body. This had amean diameter of about 280 km and was disrupted by the catastrophic impact that created the family.[3] It is likely that the parent body was at least partlydifferentiated, because the surface of Eunomia and spectra of the smaller family members show some variation.[4][5][6] This notwithstanding, other studies have indicated that the body that was definitively shattered by the impact that created the family was probably already somewhat fragmented by previous smaller collisions.[7] The impactor was probably a smaller, yet still very substantial asteroid of 50 km diameter (or so) that hit at a speed of about 22,000 km/h.[8]
The other Eunomian asteroids are quite regularly distributed in orbital space around Eunomia. The next largest member identified by the[9] analysis was258 Tyche of 65 km diameter. However, its orbit lies at the very margin of what can be considered the family region, and it may well be an interloper. The largest clear family members are about 30 km diameter, with several asteroids in this size range.
Spectroscopic studies have shown that the family members span a noticeable range of compositions, although all remain within theS spectral class. As such they are of generally stony (rather than icy) surface composition that includessilicates and somenickel-iron, and are quite bright for their size.
The family contains relatively large numbers of small objects. Since most of these smaller objects are "eroded" away over time due to secondary collisions, gravitational perturbations, and theYarkovsky effect, this indicates that the Eunomia family was created relatively recently (on an astronomical timescale).[8][10]
TheCassini-Huygens spacecraft flew by2685 Masursky, a small family member, in 2000. However, the encounter distance of about one million kilometers was too large for surface features to be resolved.
The Eunomia family is located between the 3:1 and 8:3resonances withJupiter, at relatively high inclinations.
A HCM numerical analysis by Zappalà, et al.[9] determined a large group of 'core' family members, whoseproper orbital elements lie in the approximate ranges
| ap | ep | ip | |
|---|---|---|---|
| min | 2.54 AU | 0.121 | 11.6° |
| max | 2.72 AU | 0.180 | 14.8° |
At the presentepoch, the range ofosculatingorbital elements of these core members is
| a | e | i | |
|---|---|---|---|
| min | 2.53 AU | 0.078 | 11.1° |
| max | 2.72 AU | 0.218 | 15.8° |
The Zappalà 1995 analysis found 439 core members, while a search of a recent proper element database[11] for 96944minor planets in 2005 yielded 4649 objects lying within the rectangular-shaped region defined by the first table above. By 2014, Nesvorný identified at total of 5,670, or approximately 1.4% of all asteroids, using theHierarchical Clustering Method.[1][2]: 23
A number ofinterlopers have been identified, which share the same orbital elements as the true family members, but can not have come from the same breakup because of spectral (hence, compositional) differences. The following have been identified in a spectral survey:85 Io,141 Lumen,546 Herodias.[5][12]: 646
{{cite book}}:|journal= ignored (help)