Actaea (moon)

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.

Its diameter is estimated to be about in diameter, making it the fifth-biggest known moon of a trans-Neptunian object, being after Charon (), Dysnomia (), Vanth (), and Ilmarë (). Under the assumption that Actaea has a density of , which is thought to be more typical that of trans-Neptunian objects' moons, The mass ratio between Actaea and Salacia will be approximately 0.044:1. It has an extremely low albedo of , which is approximately half the albedo of Salacia ().

History Discovery

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.

Naming On 18 February 2011, the moon was officially named Actaea after the Nereid of the same name. In Greek mythology, Actaea (; , from or 'rocky shore') was one of the 50 Nereid sea-nymphs. These 50 sea-nymphs are daughters of the "Old Man of the Sea" Nereus and the Oceanid Doris.Homer, Iliad 18.41; Apollodorus, 1.2.7; Hesiod, Theogony 240 Actaea and her other sisters appeared to Thetis when she cried out in sympathy for the grief of Achilles for his slain friend Patroclus.Homer, Iliad 18.39-51

Orbit

Orbital characteristics Actaea follows a nearly circular orbit around Salacia at a distance of once every days, with an inclination of approximately 17.2° and a low eccentricity of . The low eccentricity of Actaea's orbit suggests that the system has undergone tidal evolution since its formation.

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.

Tidal evolution 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 to be tidally locked. Salacia and Actaea will next occult each other in 2067.

Physical characteristics Size

+ History of diameter estimates for Actaea Year of Publication Diameter (km) References 2012 2017 2019 2019 2025 Actaea is approximately magnitudes fainter than Salacia, indicating a diameter ratio of 2.98 assuming if Salacia and Actaea has the exact same albedos. Early estimates assumed equal albedos for them, which have corresponded to a diameter of for Actaea. Since Actaea has almost the same neutral (grey) color as Salacia that strongly supports the assumption of equal albedos.

However, more accurate measurements in 2025 that was done by Kiss et al., measured an extremely low albedo of for Actaea, which is approximately half the albedo of Salacia (), yielding a larger diameter of , 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 (), Dysnomia (), Vanth (), and Ilmarë ().

Surface and spectrum The color of Actaea's surface is dark grey, with an extremely low geometric albedo of , which is only half of Salacia's, and a measured V–I color index of , which is considered a neutral color.

Mass and density The mass of the system is , assuming equal densities of around ≈, the satellite-to-primary mass ratio is would be 0:10:1. However, this would be an unrealistically high density for the satellite. Under the assumption that Actaea has a density of , which is thought to be more typical that of trans-Neptunian objects' moons, The mass ratio between Actaea and Salacia will be approximately 0.044:1.

Actaea (moon)

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