| Discovery | |
|---|---|
| Discovered by | Annibale de Gasparis |
| Discovery date | July 29, 1851 |
| Designations | |
| Alternate name | none |
| Minor planet category | Main belt, (Eunomia family) |
| Epoch June 14, 2006 (JD 2453900.5) | |
| Aphelion | 469.429 Gm (3.138 AU) |
| Perihelion | 321.429 Gm (2.149 AU) |
| Semi-major axis | 395.429 Gm (2.643 AU) |
| Eccentricity | 0.187 |
| Orbital period | 1569.687 d (4.30 a) |
| Average orbital speed | 18.16 km/s |
| Mean anomaly | 286.102° |
| Inclination | 11.738° |
| Longitude of ascending node | 293.273° |
| Argument of perihelion | 97.909° |
| Physical characteristics | |
| Dimensions | 357×255×212 km[2] 268 km (mean) 330×245×205[3][4][5] 255.33 km IRAS[1] |
| Mass | 3.12 × 1019 kg[2] |
| Mean density | 3.09 ± 0.52 g/cm³[2] 3.8±0.7 g/cm³[6] (based on IRAS diameter of 255km) |
| Equatorial surface gravity | 0.08 m/s² |
| Escape velocity | 0.16 km/s |
| Rotation period | 0.2535 d (6.083 h)[1][7] |
| Albedo | 0.209 (geometric)[1] |
| Temperature | ~166 K max: 260 K (-13 °C) |
| Spectral type | S-type asteroid[1] |
| Apparent magnitude | 7.9[8] to 11.24 |
| Absolute magnitude (H) | 5.28[1] |
| Angular diameter | 0.29" to 0.085" |
| | |
15 Eunomia (pronounced /jʊˈnoʊmiə/ ew-NOH-mee-ə, Greek: Ευνομία) is a very large asteroid in the inner main asteroid belt. It is the largest of the stony (S-type) asteroids, and somewhere between the 8th to 12th largest Main Belt asteroid overall (uncertainty in diameters causes uncertainty in its ranking). It is the largest member of the Eunomia family of asteroids, and is estimated to contain 1% of the total mass of the asteroid belt.[6][9]
Eunomia was discovered by Annibale de Gasparis on July 29, 1851 and named after Eunomia, one of the Horae (Hours), a personification of order and law in Greek mythology.
Characteristics
As the largest S-type asteroid (with 3 Juno being a very close second), Eunomia has attracted a moderate amount of scientific attention. It contains slightly over one percent of the mass of the entire main belt.
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.[4] Its elongation led to the suggestion that Eunomia may be a binary object. However, this has been refuted.[5] It is a retrograde rotator with its pole pointing towards ecliptic coordinates (β, λ) = (-65°, 2°) with a 10° uncertainty.[4][5] This gives an axial tilt of about 165°.
Like other true members of the family, its surface is composed of silicates and some nickel-iron, and is quite bright. Calcium-rich pyroxenes and olivine, along with nickel-iron metal have been detected on Eunomia's surface. Spectroscopic studies suggest that Eunomia has regions with differing composition. 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,[4] and has a generally basaltic composition.[10]
This composition indicates that the parent body was likely subject to magmatic processes, and became at least partially differentiated under the influence of internal heating in the early period of the Solar System. The range of compositions of the remaining Eunomia family members, formed by a collision of the common parent body, is large enough to encompass all the surface variations on Eunomia itself. Interestingly, the majority of smaller family members 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 that was stripped of most of its crustal material by the family-forming impact, but perhaps not disrupted. However, there is uncertainty over Eunomia's internal structure and relationship to the family parent body. Computer simulations of the collision[11] are more consistent with Eunomia being a re-accumulation of most of the fragments of a completely shattered parent body. Coversely again, Eunomia's quite high density would indicate that it is not a rubble pile after all. Whetever 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 of asteroids. These indicate that the largest remaining fragment (that is, Eunomia) should have about 70% of the mass of the parent body,[12] which is consistent with Eunomia being a central remnant, with the crust and a part of the mantle stripped off.
These indications are also in accord with fresh mass determinations which indicate that Eunomia has a typical density for mostly intact stony asteroids, and not the anomalously low "rubble pile" density of ~1 g/cm³ that had been obtained earlier.
Eunomia has been observed occulting stars three times. It has a mean opposition magnitude of +8.5,[13] about equal to the mean brightness of Titan and can reach +7.9 at a near perihelion opposition.
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