Actaea, officially (120347) Salacia I Actaea, is the single known natural satellite of the classical Kuiper belt object 120347 Salacia. Its diameter is estimated 300 km, which is ⅓ of the diameter of Salacia; thus, Salacia and Actaea are viewed by some to be a binary object. Given that the following size estimates are correct, Actaea is about the sixthbiggest known moon of a transNeptunian object, after Charon (1212 km), Dysnomia (685 km), Vanth (378 km), Ilmarë (361 km) and Hiiaka (320 km).
Actaea  

Salacia and Actaea orbit


Discovery  
Discovered by  Keith S. Noll, Harold F. Levison, Denise C. Stephen, William M. Grundy 
Discovery date  21 Jul 2006 
Designations  
S/2006 (120347) 1, Salacia I 

Adjectives  Actaean 
Orbital characteristics  
5619 ± 87 km  
Eccentricity  0.0084 ± 0.0076 
5.49380 ± 0.00016 days  
Satellite of  Salacia 
Physical characteristics  
Dimensions  303 ± 35 km 
Mass  1.86 × 10^{19} kg 
Mean density

1.16 +0.59/0.36 g/cm³ 
≈ 0.05 km/s  
Albedo  ≈ 0.035 +0.010/−0.007 
1.9 mag  
It was discovered on 21 July 2006 by Keith S. Noll, Harold Levison, Denise Stephens and Will Grundy with the Hubble Space Telescope.^{[1]} On 18 February 2011, it was officially named Actaea after the nereid Aktaia.
Actaea orbits its primary every 5.49380±0.00016 d at a distance of 5619±87 km and with an eccentricity of 0.0084±0.0076.^{[2]} The ratio of its semimajor axis to its primary's Hill radius is 0.0023, the tightest transNeptunian binary with a known orbit.^{[3]}
Actaea is 2.372±0.060 magnitudes fainter than Salacia,^{[4]} implying a diameter ratio of 2.98 for equal albedos.^{[3]} Hence, assuming equal albedos, it has a diameter of 303±35 km^{[4]} 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.^{[3]} 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.^{[3]} The low density calculated for the system (1.16 g/cm^{3}) implies that both Salacia and Actaea consist chiefly of water ice. Salacia and Actaea will next occult each other in 2067.^{[3]} The mass of the system is 4.66 ± 0.22 × 10^{20} kg, with about 4% of this being in Actaea.^{[3]}
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