Actaea, formal designation (120347) Salacia I, 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 colloquially referred to as moons, a deriv ...

of the classical Kuiper belt object

120347 Salacia Salacia (minor-planet designation: 120347 Salacia) is a large trans-Neptunian object (TNO) and possible dwarf planet in the Kuiper belt, approximately in diameter. It was discovered on 22 September 2004, by American astronomers Henry G. Roe, Hen ...

. Its diameter is estimated to be approximately , 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 of the same kind that are comparable in size. Definitions vary, but typically require the center of mass to be located outside of either object. (See animated examples.) The most common ki ...

. 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 ( ; ) is a psychopomp, the ferryman of the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the worlds of the living and ...

(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. Background Vanth is a female entity in the Etruscan underworld that is often accompanied either by additio ...

(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 ent ...

(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 (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

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

Nereid In Greek mythology, the Nereids or Nereides ( ; ; , also Νημερτές) are sea nymphs (female spirits of sea waters), the 50 daughters of the ' Old Man of the Sea' Nereus and the Oceanid Doris, sisters to their brother Nerites. They ofte ...

nymph Actaea.

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 Actaea constituting perhaps 4% of this. 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 Moons of dwarf planets Trans-Neptunian satellites