Salacia (minor-planet designation: 120347 Salacia) is a large trans-Neptunian object (TNO) and possible dwarf planet in the Kuiper belt that is probably between 800 km (500 mi) and 875 km (544 mi) 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. Salacia and Actaea form a binary system where both bodies are tidally locked to the other, similar to Pluto and Charon.
Keck Telescope image of Salacia (bright, center) and its moon Actaea (faint, at left) | |
| Discovery | |
|---|---|
| Discovered by | H. G. Roe M. E. Brown K. M. Barkume |
| Discovery site | Palomar Obs. |
| Discovery date | 22 September 2004 |
| Designations | |
| (120347) Salacia | |
| Pronunciation | /səˈleɪʃə/ (sə-LAY-shə) |
Named after | Salacia (Roman mythology) |
| 2004 SB60 (provisional designation) | |
Minor planet category | TNO · classical (hot) extended |
| Adjectives | Salacian |
| Symbol | or (rare) |
| Orbital characteristics | |
| Epoch 31 May 2020 (JD 2459000.5) | |
| Uncertainty parameter 3 | |
| Observation arc | 37.16 yr (13,572 days) |
| Earliest precovery date | 25 July 1982 |
| Aphelion | 46.670 AU |
| Perihelion | 37.697 AU |
| 42.184 AU | |
| Eccentricity | 0.10636 |
| 273.98 yr (100,073 days) | |
Mean anomaly | 123.138° |
| 0° 0m 12.951s / day | |
| Inclination | 23.921° |
| 279.880° | |
| 312.294° | |
| Known satellites | 1 (Actaea) |
| Physical characteristics | |
Mean diameter |
|
| Mass | 4.861+0.076 −0.074×1020 kg (System mass) |
Mean density | 1.50±0.12 g/cm3 1.26±0.16 g/cm3 |
Sidereal rotation period | 5.49403±0.00016 d (synchronous) |
| 0.041±0.004 (2025) 0.042±0.004 (2017) | |
| |
| 20.7 | |
Absolute magnitude (H) | 4.360±0.011 (Salacia+Actaea) 4.476±0.013 (Salacia) 6.850±0.053 (Actaea) 4.15 |
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 4.861+0.076
−0.074×1020 kg, with an average system density of 1.5±0.1 g/cm3; Salacia itself is estimated to be around 838 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 1.26 g/cm3 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 1.5 g/cm3. Salacia's extremely low albedo of 4% remains consistent with a lack of the kind of geological activity that is thought to be typical of dwarf planets.
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 5.49380±0.00016 d at a distance of 5700+30
−29 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 probably between 360 km (220 mi) and 425 km (264 mi) in diameter. It is 2.372±0.060 magnitudes fainter than Salacia, implying a diameter ratio of 3 for equal albedos. 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. 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 (/səˈleɪʃə/), 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 /ækˈtiːə/, 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 (, formerly ) 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
- 174567 Varda and its moon Ilmarë, a system with similar characteristics
- List of Solar System objects by size
Notes
- approximation if Salacia and Actaea were both spherical and had the same albedo
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