Actaea, formal designation (120347) Salacia I, is the only known moon of the classical Kuiper belt object 120347 Salacia.
It was discovered on 21 July 2006 by Keith S. Noll, Harold Levison, Denise Stephens, and Will Grundy with the Hubble Space Telescope. On 18 February 2011, it was officially named Actaea after the Nereid nymph Actaea.
Actaea follows a nearly circular orbit around Salacia at a distance of every days, with a low orbital 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 0.0023, it is the tightest trans-Neptunian binary with a known orbit.
The mass of the system is , with Actaea constituting perhaps 4% of this. Actaea is magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Early estimates assumed equal albedos for them, that will correspond to a diameter of . Actaea has almost the same color as Salacia (VâÂÂI = and , respectively), supporting the assumption of equal albedos. However, the ALMA study that is done by Kiss et al. in 2025 measured an extremely low albedo of for Actaea, yielding a diameter of , which 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 km), Dysnomia (615 km), Vanth (443 km), and Ilmarë (403 km).
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. Salacia and Actaea will next occult each other in 2067.