15 Eunomia is a very largeasteroid in the middleasteroid belt. It is the largest of the stony (S-type) asteroids, with3 Juno as a close second. It is quite amassive asteroid, in 6th to 8th place (to within measurement uncertainties). It is the largestEunomian asteroid, and is estimated to contain 1% of the mass of the asteroid belt.[7][8]
Eunomia was discovered byAnnibale de Gasparis on July 29, 1851, and named afterEunomia, one of theHorae (Hours), a personification of order andlaw inGreek mythology. Its historical symbol is a heart with a star on top; it was encoded inUnicode 17.0 as U+1CEC8 ().[9][10]
As the largest S-type asteroid (with3 Juno being a very close second), Eunomia has attracted a moderate amount of scientific attention.
Eunomia appears to be an elongated but fairly regularly shaped body, with what appear to be four sides of differing curvature and noticeably different average compositions.[11] Its elongation led to the suggestion that Eunomia may be abinary object, but this has been refuted.[12] It is aretrograde rotator with its pole pointing towardsecliptic coordinates (β, λ) = (−65°, 2°) with a 10° uncertainty.[11][12] This gives anaxial tilt of about 165°.
Like other true members of the family, its surface is composed ofsilicates and somenickel-iron, and is quite bright.Calcium-richpyroxenes andolivine, along with nickel-iron metal, have been detected on Eunomia's surface.Spectroscopic studies suggest that Eunomia has regions with differing compositions: A larger region dominated by olivine, which is pyroxene-poor and metal-rich, and another somewhat smaller region on one hemisphere (the less pointed end) that is noticeably richer in pyroxene,[11] and has a generallybasaltic composition.[13]
This composition indicates that the parent body was likely subject tomagmatic processes, and became at least partially differentiated under the influence ofinternal heating in the early period of the Solar System. The range of compositions of the remainingEunomian asteroids, formed by a collision of the common parent body, is large enough to encompass all the surface variations on Eunomia itself. The majority of smaller Eunomian asteroids are more pyroxene rich than Eunomia's surface, and contain very few metallic (M-type) bodies.
Altogether, these lines of evidence suggest that Eunomia is the central remnant of the parent body of the Eunomia family, which was stripped of most of its crustal material by the disrupting impact, but was perhaps not disrupted itself. However, there is uncertainty over Eunomia's internal structure and relationship to the parent body. Computer simulations of the collision[14] are more consistent with Eunomia being a re-accumulation of most of the fragments of a completely shattered parent body, yet Eunomia's quite high density would indicate that it is not a rubble pile after all. Whatever the case in this respect, it appears that any metallic core region, if present, has not been exposed.
An older explanation of the compositional differences, that Eunomia is a mantle fragment of a far larger parent body (with a bit of crust on one end, and a bit of core on the other), appears to be ruled out by studies of the mass distribution of the entire Eunomia family. These indicate that the largest fragment (that is, Eunomia) has about 70% of the mass of the parent body,[15] which is consistent with Eunomia being acentral remnant, with the crust and part of the mantle stripped off.
These indications are also in accord with recent mass determinations which indicate that Eunomia's density is typical of mostly intact stony asteroids, and not the anomalously low "rubble pile" density of ~1 g/cm3 that had been reported earlier.
^abcdeP. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis.Astronomy & Astrophysics 54, A56
^Šidlichovský, M. (1999), Svoren, J.; Pittich, E. M.; Rickman, H. (eds.), "Resonances and chaos in the asteroid belt",Evolution and source regions of asteroids and comets : proceedings of the 173rd colloquium of the International Astronomical Union, held in Tatranska Lomnica, Slovak Republic, August 24–28, 1998, pp. 297–308,Bibcode:1999esra.conf..297S.
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