Actaea

Actaea, officially (120347) Salacia I Actaea, is a natural satellite of the large classical Kuiper belt object 120347 Salacia. Its diameter is estimated 300 km (190 mi), which is approximately one-third the diameter of Salacia; thus, Salacia and Actaea are viewed by some astronomers to be a binary system.[by whom?] 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 Hiʻiaka (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 Aktaia.

Orbit
Actaea orbits its primary every 5.493 d at a distance of 5619±87 km and with an eccentricity of 0.0084±0.0076. 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
Actaea is 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Hence, assuming equal albedos, it has a diameter of 303±35 km 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. 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 have been tidally locked. The low density calculated for the system (1.16 g/cm3) implies that both Salacia and Actaea consist chiefly of water ice. Salacia and Actaea will next occult each other in 2067. The mass of the system is 4.66 ± 0.22  × 1020 kg, with about 4% of this being in Actaea.