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15 Eunomia

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15 Eunomia
Discovery[1]
Discovered byAnnibale de Gasparis
Discovery siteNaples Obs.
Discovery date29 July 1851
Designations
Pronunciation/juːˈnoʊmiə/[2]
Named after
Eunomia
AdjectivesEunomian /juːˈnoʊmiən/[3]
Symbol15 Eunomia symbol (historical)
Orbital characteristics[4]
Epoch 21 November 2025 (JD 2461000.5)
Uncertainty parameter 0
Observation arc174.18 yr
Aphelion3.13834 AU
Perihelion2.14603 AU
2.64219 AU
Eccentricity0.187781
4.29490 yr (1568.71 d)
113.726°
Inclination11.7614°
292.881°
98.5072°
Jupiter MOID2.4255 AU
TJupiter3.340
Proper orbital elements[5]
2.64357 AU AU
0.14887
13.109°
Precession of perihelion long.
42.698 arcsec / yr
Precession of asc. node
-52.044 arcsec / yr
Physical characteristics
Dimensions340 km × 248 km × 229 km (± 14 km × 13 km × 14 km)[6]: 18 
270±3 km[6]: 6 
Flattening0.47[a]
Mass3.026+0.093
−0.101×1019 kg[7]: 8 [b]
Mean density
2.96±0.21 g/cm3[6]: 6 
6.083 h (0.2535 d)[4]
−11° to orbit (retrograde)[6]: 18 
355°±2°[6]: 18 
−70°±2°[6]: 18 
0.187[6]: 6 
0.248±0.042[4]
0.163±0.030[5]
S-type asteroid[4]
U−B=0.451±0.019[4]
B−V=0.839±0.015[4]
7.9[8] to 11.24
5.41[4]
0.29″ to 0.085″

15 Eunomia is a large asteroid in the middle asteroid belt. It is the largest of the stony (S-type) asteroids, with a mean diameter of about 270 kilometres (170 mi). It is between the eight and twelfth largest main belt asteroid, containing about 1% of its mass. It was discovered on 29 July 1851 by astronomer Annibale de Gasparis, who chose to name the asteroid after the Greek goddess Eunomia. It is elongated and irregular in shape, and rotates once every six hours and five minutes.

Eunomia is the largest member of the Eunomia family, one of the largest known asteroid families in the main belt. Asteroids of the Eunomia family share orbital and compositional similarities to Eunomia itself.

History

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Discovery

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Eunomia was discovered by Italian astronomer Annibale de Gasparis on the night of 29 July 1851 at Naples Observatory in Naples, Italy. There, while observing around the 18th hour of right ascension just below the ecliptic, he spotted a starlike object with a magnitude of 9.[9]: 130  Though the object's low position in the sky prevented de Gasparis from observing it as thoroughly,[9]: 130  he concluded that it was a "new planet" and reported his discovery in a letter to A.C. Petersen in Altona, Germany. Petersen in turn announced de Gasparis's discovery in the journal Astronomische Nachrichten on 22 August 1851.[10]

Naming and symbol

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De Gasparis selected the name Eunomia after Eunomia of Greek mythology, who was one of the Horae and the personification of law and order.[11]: 14  He described its symbol as a star above a heart: 15 Eunomia symbol (U+1CEC8 𜻈 in Unicode 17.0).[12]: 3, 8 

Orbit

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Eunomia orbits the Sun at an average distance—its semi-major axis—of 2.64 astronomical units (AU), taking 4.29 Earth years to complete one revolution.[4] It is located in the middle main asteroid belt,[13] and its orbit is inclined by 11.8° with respect to the ecliptic. Due to its orbital eccentricity of 0.19, its distance from the Sun ranges from 2.15 AU at perihelion to 3.14 AU at aphelion.[4]

Eunomia family

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Main article: Eunomia family

Eunomia is the largest member of the Eunomia family, comprising an estimated ~70% of its mass.[14]: 73  The Eunomia family is one of the largest asteroid families known, with 13099[15] identified members.[13]: 452  It occupies the middle main belt, with an outer boundary at the 8:3 mean-motion resonance with Jupiter and an inner boundary at a series of secular resonances.[16]: 1153  Its members orbit with relatively high eccentricities and inclinations.[17]: 446  Most members are stony S-type asteroids,[13]: 452  though A-type asteroids are more common than average in the family.[18]: 1 

Around 20% of asteroids located within the Eunomia family region are spectrally dissimilar interlopers. Most of these interlopers belong to the overlapping Adeona family, whose namesake is the C-type asteroid 145 Adeona.[19]: 584 

Rotation

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Eunomia has a sidereal day of 6 hours and 5 minutes, which was determined from analyzing its lightcurve, or variations in its observed brightness.[4] It has retrograde spin (clockwise when viewed from north of the ecliptic), rotating in the opposite direction compared to Earth.[13]: 453  Its north pole is pointed towards the ecliptic south, with an axial tilt of 20° relative to its orbit (11° to its orbital plane).[6]: 18 

Physical characteristics

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Eunomia is the largest and most massive S-type asteroid,[18]: 1 [7]: 8  with a volume-equivalent mean diameter of 270 kilometres (170 mi) and an estimated mass of about 3×1019 kilograms (6.6×1019 lb).[6]: 6 [7]: 8  Eunomia is between the eighth and twelfth largest asteroid in the main belt[20] and contains about 1% of its mass.[7]: 8 [21] It has an estimated density of about 3 g/cm3.[6]: 6  It is elongated, irregular,[22]: 737  and egg-shaped, with a pointed end and a blunted end.[13]: 453 

Spectrum and composition

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Eunomia is classified as a stony S-type asteroid,[18]: 1  the second-most common asteroid spectral type.[23] In Michael Gaffey's classification scheme, Eunomia is an S(III)-type, implying the presence of calcium-rich pyroxenes on its surface. Mineralogical studies using astronomical spectroscopy identified the presence of olivine, troilite, and iron–nickel alloy.[13]: 453  Its surface is compositionally uneven: the pointed end has olivine and a mineralogy similar to stony-iron meteorites, while the blunt end has iron-rich pyroxenes and basalt.[24]: 446 

The variability of Eunomia's surface mineralogy may be a sign that it underwent partial planetary differentiation in its past. The overabundance of A-type asteroids in its family additionally support partial or complete differentiation.[18]: 1 

See also

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Notes

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  1. ^ Flattening derived from the maximum aspect ratio (c/a): , where (c/a) = 0.53±0.02.[6]: 6 
  2. ^ Siltala and Granvik reported Eunomia's mass as 1.522+0.047
    −0.051    ×10−11     Solar masses (M☉), where 1 M☉ is 1.988×1030 kg

References

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  1. ^ "(15) Eunomia = 1851 OA". Minor Planet Center. Archived from the original on 5 February 2026. Retrieved 18 May 2026. (5060 obs)
  2. ^ Noah Webster (1884) A Practical Dictionary of the English Language
  3. ^ "Eunomian". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  4. ^ a b c d e f g h i j "JPL Small-Body Database Lookup: 15 Eunomia (A851 OA)" (2025-11-07 last obs.). Jet Propulsion Laboratory. Archived from the original on 15 November 2020. Retrieved 18 May 2026.
  5. ^ a b "SsoCard". Institut de mécanique céleste et de calcul des éphémérides. Retrieved 22 May 2026.
  6. ^ a b c d e f g h i j k Vernazza, P.; Ferrais, M.; Jorda, L.; Hanuš, J.; Carry, B.; Marsset, M.; et al. (October 2021). "VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis". Astronomy & Astrophysics. 654: A56. Bibcode:2021A&A...654A..56V. doi:10.1051/0004-6361/202141781. hdl:10261/263281. S2CID 239104699. A56.
  7. ^ a b c d Siltala, L.; Granvik, M. (February 2022). "Masses, bulk densities, and macroporosities of asteroids (15) Eunomia, (29) Amphitrite, (52) Europa, and (445) Edna based on Gaia astrometry". Astronomy & Astrophysics. 658: A65. arXiv:2111.06700. Bibcode:2022A&A...658A..65S. doi:10.1051/0004-6361/202141459. ISSN 0004-6361.
  8. ^ Donald H. Menzel & Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets (2nd ed.). Boston, MA: Houghton Mifflin. pp. 391. ISBN 0-395-34835-8.
  9. ^ a b Hind, John Russell (1852). "Chapter VIII. The Minor or Ultra-Zodiacal Planets.". The Solar System: Descriptive Treatise Upon the Sun, Moon, and Planets, Including an Account of All the Recent Discoveries. New York, United States: G. P. Putnam's Sons. p. 126.
  10. ^ A. C., Petersen (22 August 1851). "Planeten-Circular". Astronomische Nachrichten (in German). 33 (2): 31–32. doi:10.1002/asna.18520330211.
  11. ^ Schmadel, Lutz D. (2003). Dictionary of minor planet names. Vol. 1 (5th ed.). Berlin Heidelberg New York: Springer-Verlag. p. 16. doi:10.1007/978-3-540-29925-7_13. ISBN 3-540-00238-3.
  12. ^ Bala, Gavin Jared; Miller, Kirk (18 September 2023). "Unicode request for historical asteroid symbols" (PDF). unicode.org. Unicode. Archived (PDF) from the original on 26 September 2023. Retrieved 26 September 2023.
  13. ^ a b c d e f Nathues, Andreas; Mottola, Stefano; Kaasalainen, Mikko; Neukum, Gerhard (June 2005). "Spectral study of the Eunomia asteroid family". Icarus. 175 (2): 452–463. doi:10.1016/j.icarus.2004.12.013. Archived from the original on 4 December 2024. Retrieved 22 May 2026.
  14. ^ Tanga, P.; Cellino, A.; Michel, P.; Zappalà, V.; Paolicchi, P.; Dell'Oro, A. (1999). "On the Size Distribution of Asteroid Families: The Role of Geometry". Icarus. 141 (1): 65–78. Bibcode:1999Icar..141...65T. doi:10.1006/icar.1999.6148. Archived from the original on 5 February 2021. Retrieved 6 September 2021.
  15. ^ Nesvorný, David (23 March 2026). "A Census before Rubin of Asteroid Families in the Main Belt". The Astrophysical Journal Supplement Series. 283 (2): 63. arXiv:2602.20382. Bibcode:2026ApJS..283...63N. doi:10.3847/1538-4365/ae4bd4. ISSN 0067-0049.
  16. ^ Carruba, V.; Michtchenko, T. A. (December 2007). "A frequency approach to identifying asteroid families". Astronomy & Astrophysics. 475 (3): 1145–1158. Bibcode:2007A&A...475.1145C. doi:10.1051/0004-6361:20077689. ISSN 0004-6361.
  17. ^ Lazzaro, D (December 1999). "The Eunomia Family: A Visible Spectroscopic Survey". Icarus. 142 (2): 445–453. Bibcode:1999Icar..142..445L. doi:10.1006/icar.1999.6213. Archived from the original on 25 July 2024. Retrieved 22 May 2026.
  18. ^ a b c d Phua, Yu Yu; de Kleer, Katherine; Butler, Bryan; Cambioni, Saverio; Shepard, Michael (February 2026). "Thermophysical Modeling of Asteroid (15) Eunomia from Spatially Resolved ALMA and VLA Data". The Planetary Science Journal. 7 (2): 47. Bibcode:2026PSJ.....7...47P. doi:10.3847/PSJ/ae3751. ISSN 2632-3338.
  19. ^ Radović, Viktor; Novaković, Bojan; Carruba, Valerio; Marčeta, Dušan (September 2017). "An automatic approach to exclude interlopers from asteroid families". Monthly Notices of the Royal Astronomical Society. 470 (1): 576–591. arXiv:1705.09226. doi:10.1093/mnras/stx1273. ISSN 0035-8711.
  20. ^ "Asteroid Eunomia at opposition, December 2024". BBC Sky at Night Magazine. December 2024. Archived from the original on 8 March 2026. Retrieved 22 May 2026.
  21. ^ Pitjeva, E. V. (2018). "Masses of the Main asteroid Belt and the Kuiper Belt from the Motions of Planets and Spacecraft". Solar System Research. 44 (8–9): 554–566. arXiv:1811.05191. Bibcode:2018AstL...44..554P. doi:10.1134/S1063773718090050. S2CID 119404378.
  22. ^ Tanga, Paolo (2003). "Asteroid observations with the Hubble Space Telescope FGS". Astronomy & Astrophysics. 401 (2): 733–741. Bibcode:2003A&A...401..733T. doi:10.1051/0004-6361:20030032. S2CID 8977642.
  23. ^ Howells, Kate (18 June 2024). "What are asteroids made of?". The Planetary Society. Archived from the original on 20 April 2026. Retrieved 22 May 2026.
  24. ^ Reed, Kevin L.; Gaffey, Michael J.; Lebofsky, Larry A. (1997). "Shape and Albedo Variations of Asteroid 15 Eunomia". Icarus. 125 (2): 446. Bibcode:1997Icar..125..446R. doi:10.1006/icar.1996.5627. Archived from the original on 18 April 2022. Retrieved 6 September 2021.
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