Actaea (moon)

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.^[4]

Naming

On 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]

Orbital characteristics

Actaea follows a nearly circular orbit around Salacia at a distance of 5700+30
−29 km 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 0.0023, it is the tightest trans-Neptunian binary with a known orbit.^[3]

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 be tidally locked.^[3] Salacia and Actaea will next occult each other in 2067.^[3]

Size

Actaea is approximately 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 assuming equal albedos.^[3] Early estimates assumed equal albedos for them, which have corresponded to a diameter of 284±10 km for Actaea.^[12] 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 is 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

The surface of Actaea's color is dark grey,^[2][3] with an extremely low geometric albedo of 0.021±0.004,^[2] which is only half of Salacia's, and a measured V–I color index of 0.89±0.02, which is considered a neutral color.^[3]

Mass and density

The 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]