120347 Salacia

120347 Salacia, provisional designation 2004 SB60, is a trans-Neptunian object in the Kuiper belt, approximately 850 kilometers in diameter. As of 2018, it is located about 44.8 astronomical units from the Sun, and reaches apparent magnitude 20.7 at opposition.

Salacia was discovered on 22 September 2004, by American astronomers Henry Roe, Michael Brown and Kristina Barkume at the Palomar Observatory in California, United States. It has been observed 124 times, with precovery images back to 25 July 1982. 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 very likely a dwarf planet. However, William Grundy et al. argue that objects such as Salacia, in the size range of 400–1000 km, with albedos less than ≈0.2 and densities of ≈1.2 g/cm3 or less, have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets.

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 in the classification system of Gladman et al., which may be a non-distinction 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, because it lacks the strong water-ice absorption bands typical of its members.

Physical characteristics
As of 2019, the total mass of the Salacia–Actaea system is estimated at (4.922±0.071)×1020 kg, with an average system density of 1.51 g/cm3; 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 different at 866 km and its density is therefore lower being 1.26 g/cm3 (based on old mass).

Salacia was previously believed to have a mass of around (4.38±0.16)×1020 kg, in which case it would also have had the lowest density (around 1.29 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. Its light-curve amplitude is only 3%.

Mike Brown's website lists Salacia as nearly certainly a dwarf planet, but the IAU has not formally recognized it as such.

Satellite
Salacia has one natural satellite, Actaea, that orbits its primary every 5.49380±0.00016 d at a distance of 5619±89 km and with an eccentricity of 0.0084±0.0076. It was discovered on 21 July 2006 by Keith Noll, Harold Levison, Denise Stephens and William Grundy with the Hubble Space Telescope.

Actaea is 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 2.98 for equal albedos. Hence, assuming equal albedos, it has a diameter of 286±24 km According to the estimate from 2017 based on an improved modelling the size of Actaea is slightly larger being 290±21 km.

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.

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 have been 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 (/sæˈleɪʃə/), the goddess of salt water and the wife of Neptune. Naming citation was published on 18 February 2011 (M.P.C. 73984).

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