Actaea, officially (120347) Salacia I Actaea, is a natural satellite of the

classical Kuiper belt A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major a ...

planetoid

120347 Salacia 120347 Salacia, provisional designation , is a large planetoid in the Kuiper belt, approximately 850 kilometers in diameter. As of 2018, it is located 44.8 astronomical units from the Sun, and reaches apparent magnitude 20.7 at opposition. Salac ...

. Its diameter is estimated , 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 In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...

(1212 km), Dysnomia (700 km), Vanth (443 km),

Ilmarë Ilmarë,Stressed on the first syllable or Varda I, full designation , is the single known natural satellite of the Kuiper Belt planetoid 174567 Varda. It was discovered by Keith Noll et al. in 2009, at a separation of about 0.12 arcsec, using d ...

(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 Harold F. (Hal) Levison (born 1959) is an American planetary scientist specializing in planetary dynamics. He currently works at the Southwest Research Institute, Boulder, CO, Boulder, Colorado, and studies planetary orbits and their evolution th ...

, 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 at a distance of and with an eccentricity of .Johnston Archive: (120347) Salacia and Actaea
/ref> 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 , with perhaps 4% of this being in Actaea. Actaea is magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Hence, assuming equal albedos, it has a diameter of . Actaea has the same color as Salacia (V−I = and , 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.

References

{{Moons of dwarf planets 120347 Salacia 20060721 Discoveries by Denise C. Stephens