Actaea, formal designation (120347) Salacia I, is the only known moon of the large classical Kuiper belt object 120347 Salacia. It was discovered by Keith S. Noll, Harold F. Levison, Denise C. Stephen and William M. Grundy using discovery images taken by the Hubble Space Telescope on 21 July 2006.[4]
 Salacia and its moon Actaea, imaged by the Keck telescope on 3 August 2010. Actaea is the fainter object to the left of Salacia. | |
| Discovery | |
|---|---|
| Discovered by | Keith S. Noll, Harold F. Levison, Denise C. Stephen, William M. Grundy |
| Discovery date | 21 July 2006 |
| Designations | |
Designation | Salacia I |
| Pronunciation | /ækˈtiːə/ |
| S/2006 (120347) 1 | |
| Adjectives | Actaean /ækˈtiːən/ |
| Orbital characteristics[1] | |
| 5700+30 −29 km | |
| Eccentricity | 0.008±0.003 |
| 5.49389±0.00001 days | |
| Inclination | 17.2±0.5° |
| 108.9±1.6° | |
| 41+33 −22° | |
| Satellite of | Salacia |
| Physical characteristics | |
| 393±33 km[2] | |
| Mass | ~4.4% of Salacia[2][a] |
Mean density | ~0.7 g/cm3 (assumed)[2] |
| 5.49389±0.00001 d (synchronous)[1] | |
| Albedo | 0.021±0.004[2] |
Spectral type | V–I = 0.89±0.02[3] |
| 6.850±0.053[3] | |
Its diameter is estimated to be about 393±33 km in diameter,[2] making it the fifth-biggest known moon of a trans-Neptunian object, being after Charon (1212±1 km),[5] Dysnomia (615+60
−50 km),[6]: 7 Vanth (442.5±10.2 km),[7] and Ilmarë (403±40 km).[2] Under the assumption that Actaea has a density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects' moons,[2] The mass ratio between Actaea and Salacia will be approximately 0.044:1.[2][b] It has an extremely low albedo of 0.021±0.004, which is approximately half the albedo of Salacia (0.041±0.004).[2]
History
editDiscovery
editThis section needs expansion. You can help by adding missing information. (April 2026) |
Actaea was discovered on 21 July 2006 by Keith S. Noll, Harold F. Levison, Denise C. Stephen and William M. Grundy with the Hubble Space Telescope.[4]
Naming
editOn 18 February 2011, the moon was officially named Actaea after the Nereid of the same name. In Greek mythology, Actaea (/ækˈtiːə/; Ancient Greek: Ἀκταία, romanized: Aktaía, lit. 'the dweller on coasts',[8] from Ancient Greek: ἀκτή, romanized: akté, lit. 'seashore' or 'rocky shore') was one of the 50 Nereid sea-nymphs.[clarification needed] These 50 sea-nymphs are daughters of the "Old Man of the Sea" Nereus and the Oceanid Doris.[9][10] Actaea and her other sisters appeared to Thetis when she cried out in sympathy for the grief of Achilles for his slain friend Patroclus.[11]
Orbit
edit
−29 km away from Salacia.[1]
Orbital characteristics
editActaea follows a nearly circular orbit around Salacia at a distance of 5700+30
−29 km once every 5.49389±0.00001 days, with an inclination of approximately 17.2° and a low eccentricity of 0.008±0.003.[1] The low eccentricity of Actaea's orbit suggests that the system has undergone tidal evolution since its formation.[3]
The ratio of its semi-major axis to its primary's Hill radius is only about 0.0023, it is the tightest trans-Neptunian binary with a known orbit.[3]
Tidal evolution
editIt has been calculated that the Salacia system should have undergone enough tidal evolution to circularize their orbits, which is consistent with the low measured eccentricity, but that the primary need not to be tidally locked.[3] Salacia and Actaea will next occult each other in 2067.[3]
Physical characteristics
editSize
edit
| Year of Publication | Diameter (km) | References |
|---|---|---|
| 2012 | 303±35 | [12] |
| 2017 | 290±21 | [13] |
| 2019 | 284±10 | [14][c] |
| 2019 | 286±24 | [14] |
| 2025 | 393±33 | [2] |
Actaea is approximately 2.372±0.060 magnitudes fainter than Salacia, indicating a diameter ratio of 2.98 assuming if Salacia and Actaea has the exact same albedos.[3] Early estimates assumed equal albedos for them, which have corresponded to a diameter of 284±10 km for Actaea.[14] Since Actaea has almost the same neutral (grey) color as Salacia that strongly supports the assumption of equal albedos.[3]
However, more accurate measurements in 2025 that was done by Kiss et al., measured an extremely low albedo of 0.021±0.004 for Actaea, which is approximately half the albedo of Salacia (0.041±0.004),[2] yielding a larger diameter of 393±33 km,[2] that is approximately one-half the diameter of Salacia; thus, Salacia and Actaea are viewed by William Grundy et al. to be a binary system.
Assuming that the following size estimates are correct, Actaea would be about the fifth-biggest known moon of a trans-Neptunian object, after Charon (1212±1 km),[5] Dysnomia (615+60
−50 km),[6]: 7 Vanth (442.5±10.2 km),[7] and Ilmarë (403±40 km).[2]
Surface and spectrum
editMass and density
editThe mass of the system is 4.861+0.076
−0.074×1020 kg,[1] assuming equal densities of around ≈1.45 g/cm3, the satellite-to-primary mass ratio is would be 0:10:1.[2] However, this would be an unrealistically high density
for the satellite.[2] Under the assumption that Actaea has a density of 0.7 g/cm3, which is thought to be more typical that of trans-Neptunian objects' moons,[2] The mass ratio between Actaea and Salacia will be approximately 0.044:1.[2]
See also
edit- Ilmarë, a similarly sized moon orbiting 174567 Varda
Notes
editReferences
edit- 1 2 3 4 5 Collyer, Cameron; Fernandez-Valenzuela, Estela; Jose Luis Ortiz; Holler, Bryan J.; Proudfoot, Benjamin; Morales, Nicolas; Morales, Rafael; Benecchi, Susan; Rommel, Flavia L.; Grundy, Will; Ragozzine, Darin (2025). "Synchronous Rotation in the (120347) Salacia-Actaea System". The Planetary Science Journal. 6 (11): 270. arXiv:2509.02734. Bibcode:2025PSJ.....6..270C. doi:10.3847/PSJ/ae0b6a.
- 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Kiss, Csaba; Gabányi, Krisztina; Moór, Attila; Müller, Thomas; Fernandez-Valenzuela, Estela; Moullet, Arielle; et al. (September 2025). ALMA submm measurements of the trans-Neptunian binary system satellites Ilmarë, Actaea, Hi’iaka and Namaka. EPSC-DPS Joint Meeting 2025. Vol. 18. doi:10.5194/epsc-dps2025-905.
- 1 2 3 4 5 6 7 8 9 10 Stansberry, J.A.; Grundy, W.M.; Mueller, M.; et al. (2012). "Physical Properties of Trans-Neptunian Binaries (120347) Salacia–Actaea and (42355) Typhon–Echidna". Icarus. 219 (2): 676–688. Bibcode:2012Icar..219..676S. CiteSeerX 10.1.1.398.6675. doi:10.1016/j.icarus.2012.03.029.
- 1 2 "IAUC 8751: (120347) 2004 SB_60; 2006gi, 2006gj; V733 Cep". Cbat.eps.harvard.edu. Archived from the original on 2013-12-03. Retrieved 2014-06-14.
- 1 2 Stern, S.A.; Bagenal, F.; Ennico, K.; Gladstone, G.R.; Grundy, W.M.; McKinnon, W.B.; Moore, J.M.; Olkin, C.B.; Spencer, J.R. (2015-10-16). "The Pluto system: Initial results from its exploration by New Horizons". Science. 350 (6258) aad1815. arXiv:1510.07704. Bibcode:2015Sci...350.1815S. doi:10.1126/science.aad1815. PMID 26472913. S2CID 1220226.
- 1 2 Brown, Michael E.; Butler, Bryan J. (2023-10-01). "Masses and Densities of Dwarf Planet Satellites Measured with ALMA". The Planetary Science Journal. 4 (10): 193. arXiv:2307.04848. Bibcode:2023PSJ.....4..193B. doi:10.3847/PSJ/ace52a.
- 1 2 Sickafoose, A. A.; Bosh, A. S.; Levine, S. E.; Zuluaga, C. A.; Genade, A.; Schindler, K.; Lister, T. A.; Person, M. J. (2019-02-01). "A stellar occultation by Vanth, a satellite of (90482) Orcus". Icarus. 319: 657–668. arXiv:1810.08977. Bibcode:2019Icar..319..657S. doi:10.1016/j.icarus.2018.10.016. S2CID 119099266.
- ↑ Kerényi, Carl (1951). The Gods of the Greeks. London: Thames and Hudson. p. 64.
- ↑ Homer, Iliad 18.41; Apollodorus, 1.2.7; Hesiod, Theogony 240
- ↑ Bane, Theresa (2013). Encyclopedia of Fairies in World Folklore and Mythology. McFarland, Incorporated, Publishers. p. 11, 12. ISBN 978-0-7864-7111-9.
- ↑ Homer, Iliad 18.39-51
- ↑ J.A. Stansberry (2012). "Physical Properties of Trans-Neptunian Binaries (120347) Salacia–Actaea and (42355) Typhon–Echidna". Elsevier. Retrieved 2012-04-27.
- ↑ Brown, M. E.; Butler, B. J. (2017). "The density of mid-sized Kuiper belt objects". EPSC-DPS Joint Meeting 2017. Pasadena, CA. EPSC-DPS2017-159.
- 1 2 3 Grundy, W. M.; Noll, K. S.; Roe, H. G.; Buie, M. W.; Porter, S. B.; Parker, A. H.; et al. (2019). "Mutual Orbit Orientations of Transneptunian Binaries" (PDF). Icarus. 334: 62–78. Bibcode:2019Icar..334...62G. doi:10.1016/j.icarus.2019.03.035. ISSN 0019-1035. S2CID 133585837. Retrieved 2019-10-26.