120347 Salacia

Salacia (minor-planet designation: 120347 Salacia) is a large trans-Neptunian object in the Kuiper belt, approximately 850 km in diameter. It was discovered on 22 September 2004, by American astronomers Henry Roe, Michael Brown and Kristina Barkume at the Palomar Observatory in California, United States. Salacia orbits the Sun at an average distance that is slightly greater than that of Pluto. It was named after the Roman goddess Salacia and has a single known moon, Actaea.

Brown estimated that Salacia is nearly certainly a dwarf planet. However, William Grundy et al. argue that objects in the size range of 400–1,000 km, with densities of ≈ 1.2 g/cm3 or less and albedos less than ≈ 0.2, have likely never compressed into fully solid bodies or been resurfaced, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets. Salacia is at the upper end of this size range and has a very low albedo, though Grundy et al. later found it to have the relatively high density of $846 km$.

Orbit


Salacia is a non-resonant object with a moderate eccentricity (0.11) and large inclination (23.9°), making it a scattered–extended object in the classification of the Deep Ecliptic Survey and a hot classical Kuiper belt object in the classification system of Gladman et al., which may be the same thing if they are part of a single population that formed during the outward migration of Neptune. Salacia's orbit is within the parameter space of the Haumea collisional family, but Salacia is not part of it, as evidenced by its lack of the strong water-ice absorption bands.

Physical characteristics
As of 2019, the total mass of the Salacia–Actaea system is estimated at $854 km$, with an average system density of $866 km$; Salacia itself is estimated to be around 846 km in diameter. Salacia has the lowest albedo of any known large trans-Neptunian object. According to the estimate from 2017 based on an improved thermophysical modelling, the size of Salacia is slightly larger at 866 km and its density therefore slightly lower (calculated at $4.922 kg$ with the old mass estimate discussed below).

Salacia was previously believed to have a mass of around $4.38 kg$, in which case it would also have had the lowest density (around $1.5 g/cm3$) of any known large TNO; William Grundy and colleagues proposed that this low density would imply that Salacia never collapsed into a solid body, in which case it would not be in hydrostatic equilibrium. Salacia's infrared spectrum is almost featureless, indicating an abundance of water ice of less than 5% on the surface. Near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 revealed the presence of water ice in Salacia's surface. No signs of volatile ices such as methane were detected in JWST's spectrum of Salacia. Its light-curve amplitude is only 3%.

Satellite
Salacia has one known natural satellite, Actaea, that orbits its primary every $1.29 g/cm3$ at a distance of $1.26 g/cm3$ and with an eccentricity of $0.66$. It was discovered on 21 July 2006 by Keith Noll, Harold Levison, Denise Stephens and William Grundy with the Hubble Space Telescope.

Actaea is $0.4$ magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Hence, assuming equal albedos, it has a diameter of $0.83$ According to the estimate from 2017 based on an improved modelling, the size of Actaea is slightly larger at $0.044$.

Actaea has the same color as Salacia (V−I = $0.042$ and $4.36$, 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. 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. Salacia and Actaea will next occult each other in 2067.

Name
This minor planet was named after Salacia, the goddess of salt water and the wife of Neptune. The naming citation was published on 18 February 2011 (M.P.C. 73984).

The moon's name, Actaea, was assigned on the same date. Actaea is a nereid or sea nymph.

Planetary symbols are no longer used in astronomy, so Salacia never received a symbol in the astronomical literature. Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a stylised hippocamp (, formerly ) as the symbol for Salacia; this symbol is not widely used.