120347 Salacia

Salacia (minor-planet designation: 120347 Salacia) is a large trans-Neptunian object (TNO) and possible dwarf planet in the Kuiper belt, approximately Template:Cvt 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.

Orbit

File:Salacia orbit 2018.png

The orbit of Salacia is similar to Pluto's, except for a near opposite longitude of ascending node. Its current position is near its most northern position above the ecliptic.

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 , with an average system density of ; 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 with the old mass estimate discussed below).

William Grundy et al. describe Salacia as a 'dwarf planet–sized TNO'. They had earlier rejected it as a dwarf planet due to its low estimated density, but backed off that position when they later found it to have the relatively high density of , though Salacia's extremely low albedo of 4% remains consistent with a lack of the kind of geological activity that would be expected of a dwarf planet.

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 at a distance of and with an eccentricity of . It was discovered on 21 July 2006 by Keith Noll, Harold Levison, Denise Stephens and William Grundy with the Hubble Space Telescope.

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 According to the estimate from 2017 based on an improved modelling, the size of Actaea is slightly larger at .

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. 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 (Template:IPAc-en), 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 Template:IPAc-en, was assigned on the same date. Actaea is a nereid or sea nymph.

Planetary symbols are no longer used much 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 (File:Salacia symbol (fixed width).svg, formerly File:Salacia symbol (fixed width, Greek).svg) as the symbol for Salacia; this symbol is not widely used.

See also

• 307261 Máni, a dwarf planet candidate in the Kuiper belt that is similar to Salacia in size and composition
• List of Solar System objects by size

External links

• (120347) Salacia at Johnston's Archive
• Salacia: As big as Ceres, but much farther away (Emily Lakdawalla – 2012/06/26)
• Template:PAGENAMEBASE at the JPL Small-Body DatabaseTemplate:EditAtWikidataTemplate:WikidataCheck