Actaea (moon)

Actaea, formal designation (120347) Salacia I, is the only known moon of the classical Kuiper belt object 120347 Salacia. Its diameter is estimated to be 393 km (244 mi), 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),^[4] Vanth (443 km),^[5] and Ilmarë (403 km).^[2]

Actaea

Salacia and its moon Actaea, imaged by the Keck telescope on 3 August 2010. Actaea is the fainter object to the left of Salacia.
Discovery
Discovered by	Keith S. Noll, Harold F. Levison, Denise C. Stephen, William M. Grundy
Discovery date	21 July 2006
Designations
Designation	Salacia I
Pronunciation	/ækˈtiːə/
Alternative names	S/2006 (120347) 1
Adjectives	Actaean /ækˈtiːən/
Orbital characteristics[1]
Semi-major axis	5700+30 −29 km[1]
Eccentricity	0.008+0.003 −0.003[1]
Orbital period (sidereal)	5.49389±0.00001 days[1]
Inclination	17.2±0.5°
Longitude of ascending node	108.9±1.6°
Argument of perihelion	41+33 −22°
Satellite of	Salacia
Physical characteristics
Mean diameter	393±33 km[2]
Mass	≈ 20×1018 kg
Sidereal rotation period	5.49389±0.00001 d (synchronous)[1]
Albedo	0.021±0.004[2]
Spectral type	V–I = 0.89±0.02[3]
Absolute magnitude (H)	6.850±0.053[3]

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.^[6] On 18 February 2011, it was officially named Actaea after the Nereid nymph Actaea.

Orbit

Schematic of Actaea's orbit relative to Salacia

Actaea orbits its primary every 5.493 d at a distance of 5700+30
−29 km and with an eccentricity of 0.0084±0.0076.^[1] 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.^[3]

Physical characteristics

The mass of the system is 4.861+0.076
−0.074×10²⁰ kg,^[1] with Actaea constituting perhaps 4% of this.^[7] Actaea is 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos.^[3] Hence, assuming equal albedos, it has a diameter of 393±33 km.^[2] 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 be tidally locked.^[3] Salacia and Actaea will next occult each other in 2067.^[3]

See also

• Ilmarë, a similarly sized moon orbiting 174567 Varda
• Hiʻiaka, a similarly sized moon orbiting the dwarf planet Haumea