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, 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), Vanth (443 km), Ilmarë (326 km), and Hiiaka (320 km), but possibly also Hiisi (250 km).

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. On 18 February 2011, it was officially named Actaea after the Nereid nymph named Actaea.

Orbit
Actaea orbits its primary every $5,724$ at a distance of $0.01$ and with an eccentricity of $5.494$. 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.

Physical characteristics
The mass of the system is $23.59$, with Actaea constituting perhaps 4% of this. Actaea is $45.2$ magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Hence, assuming equal albedos, it has a diameter of $134$. Actaea has the same color as Salacia (V−I = $284 km$ and $20 kg$, respectively), supporting the assumption of equal albedos. 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.