Actaea (moon)

Actaea, officially (120347) Salacia I Actaea, is a natural satellite of the classical Kuiper belt object 120347 Salacia. Its diameter is estimated to be approximately 300 km (190 mi), which is approximately one-third 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 is about the sixth-biggest known moon of a trans-Neptunian object, after Charon (1212 km), Dysnomia (700 km),^[2] Vanth (443 km),^[3] Ilmarë (326 km),^[4] and Hiʻiaka (320 km), but possibly also Hiisi (250 km).

Actaea

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/
Alternative names	S/2006 (120347) 1
Adjectives	Actaean /ækˈtiːən/
Orbital characteristics[1]
Semi-major axis	5724±27 km
Eccentricity	0.0098±0.0038
Orbital period (sidereal)	5.493882±0.000023 days
Inclination	23.59±0.36°
Longitude of ascending node	45.2±1.6°
Argument of perihelion	134±23°
Satellite of	Salacia
Physical characteristics
Dimensions	284±10 km[1]
Mass	≈ 20×1018 kg
Albedo	≈ 0.035 +0.010/−0.007[citation needed]
Spectral type	V–I = 0.89±0.02 (Actaea)
Absolute magnitude (H)	1.9 mag[citation needed]

Discovery and name

It was discovered on 21 July 2006 by Keith S. Noll, Harold Levison, Denise Stephens, and Will Grundy with the Hubble Space Telescope.^[5] On 18 February 2011, it was officially named Actaea after the Nereid nymph named Actaea.

Orbit

Schematic of Actaea's orbit relative to Salacia

Actaea orbits its primary every 5.493 d at a distance of 5619±87 km and with an eccentricity of 0.0084±0.0076.^[6] The ratio of its semi-major axis to its primary's Hill radius is 0.0023, the tightest trans-Neptunian binary with a known orbit.^[7]

Physical characteristics

The mass of the system is (4.92±0.07)×10²⁰ kg, with Actaea constituting perhaps 4% of this.^[1] Actaea is 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos.^[7] Hence, assuming equal albedos, it has a diameter of 284±10 km.^[1] Actaea has the same color as Salacia (V−I = 0.89±0.02 and 0.87±0.01, respectively), supporting the assumption of equal albedos.^[7] It 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 be tidally locked.^[7] Salacia and Actaea will next occult each other in 2067.^[7]

References

1. Grundy, W. M.; Noll, K. S.; Roe, H. G.; Buie, M. W.; Porter, S. B.; Parker, A. H.; Nesvorný, D.; Benecchi, S. D.; Stephens, D. C.; Trujillo, C. A. (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.
2. Brown, Michael E.; Butler, Bryan J. (2018-09-18). "Medium-sized satellites of large Kuiper belt objects". The Astronomical Journal. 156 (4): 164. arXiv:1801.07221. Bibcode:2018AJ....156..164B. doi:10.3847/1538-3881/aad9f2. S2CID 119343798.
3. 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.
4. Grundy, W.M.; Porter, S.B.; Benecchi, S.D.; Roe, H.G.; Noll, K.S.; Trujillo, C.A.; Thirouin, A.; Stansberry, J.A.; Barker, E.; Levison, H.F. (September 2015). "The mutual orbit, mass, and density of the large transneptunian binary system Varda and Ilmarë". Icarus. 257: 130–138. arXiv:1505.00510. Bibcode:2015Icar..257..130G. doi:10.1016/j.icarus.2015.04.036. S2CID 44546400.
5. "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.
6. Johnston Archive: (120347) Salacia and Actaea
7. 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.