Actaea, officially (120347) Salacia I Actaea, is a

natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

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 A binary system is a system of two astronomical bodies which are close enough that their gravitational attraction causes them to orbit each other around a barycenter ''(also see animated examples)''. More restrictive definitions require that thi ...

. 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 Vanth is a chthonic figure in Etruscan mythology shown in a variety of forms of funerary art, such as in tomb paintings and on sarcophagi. Vanth is a female demon in the Etruscan underworld that is often accompanied either by additional Vanth fi ...

(443 km), Ilmarë (326 km) and Hiiaka (320 km), but possibly also

Hiisi Hiisi (; plural ''hiidet'' ) is a term in Finnic mythologies, originally denoting sacred localities and later on various types of mythological entities. In later, Christian-influenced folklore, they are depicted as demonic or trickster-like entit ...

(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 The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

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

nereid In Greek mythology, the Nereids or Nereides ( ; grc, Νηρηΐδες, Nērēḯdes; , also Νημερτές) are sea nymphs (female spirits of sea waters), the 50 daughters of the 'Old Man of the Sea' Nereus and the Oceanids, Oceanid Doris ...

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 Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

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