120347 Salacia

Last updated
120347 Salacia
Salacia Hubble.png
Salacia and its moon Actaea, imaged by the Hubble Space Telescope on 21 July 2006.
Discovery [1] [2]
Discovered by H. G. Roe
M. E. Brown
K. M. Barkume
Discovery site Palomar Obs.
Discovery date22 September 2004
(120347) Salacia
Pronunciation /səˈlʃə/ (sə-LAY-shə)
Named after
Salacia (Roman mythology) [2]
2004 SB60
TNO [1]  · Cubewano [3]
Extended [4]
Adjectives Salacian
Orbital characteristics [1]
Epoch 31 May 2020 (JD 2459000.5)
Uncertainty parameter 3
Observation arc 37.16 yr (13,572 days)
Earliest precovery date25 July 1982
Aphelion 46.670 AU
Perihelion 37.697 AU
42.184 AU
Eccentricity 0.10636
273.98 yr (100,073 days)
0° 0m 12.951s / day
Inclination 23.921°
Known satellites 1 (Actaea)
Physical characteristics
Mean diameter
846±21 km [5] [lower-alpha 1]
854±45 km(equal albedos) [6]
866±37 km [7]
Mass (4.922±0.071)×1020 kg (system) [5]
(4.38±0.16)×1020 kg (system mass) [6] [8]
Mean density
1.5±0.12 g/cm3 [5]
(system) [6]

1.26±0.16 g/cm3 [7]
6.09  h (0.254  d)
Sidereal rotation period
6.09 h [1]
0.044±0.004 [6]
0.042±0.004 [7]
BB [9]
B−V=0.66±0.06 [9]
V−R=0.40±0.04 [9]
V−I=0.83±0.04 [9]
4.360±0.011 (system) [8]
4.476±0.013 (Salacia) [8]
6.850±0.053 (Actaea) [8]
3.9 [1]

    120347 Salacia, provisional designation 2004 SB60, is a large planetoid in the Kuiper belt, approximately 850 kilometers in diameter. As of 2018, it is located 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. [2] 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. [10] However, William Grundy et al. argue that objects in the size range of 400–1000 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. [11] Salacia is in this size range and has a very low albedo, though Grundy et al. later found it to have the relatively high density of 1.5±0.1 g/cm3. [5]


    The orbit of Salacia is similar to Pluto, except for a near opposite longitude of ascending node. Its current position is near its most northern position above the ecliptic. Salacia orbit 2018.png
    The orbit of Salacia is similar to Pluto, 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 in the classification system of Gladman et al., [12] which may be a non-distinction if they are part of a single population that formed during the outward migration of Neptune. [8] 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. [8]

    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. [5] Salacia has the lowest albedo of any known large trans-Neptunian object. [8] 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). [7]

    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; [6] 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. [11] Salacia's infrared spectrum is almost featureless, indicating an abundance of water ice of less than 5% on the surface. [6] [13] Its light-curve amplitude is only 3%. [8]

    Mike Brown's website lists Salacia as nearly certainly a dwarf planet, [10] but the IAU has not formally recognized it as such. [14] [15]


    Artistic model of Salacia and Actaea's near circular orbit viewed at an angle Salacia Actaea orbit simulation.png
    Artistic model of Salacia and Actaea's near circular orbit viewed at an angle

    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. [16]

    Actaea is 2.372±0.060 magnitudes fainter than Salacia, [6] implying a diameter ratio of 2.98 for equal albedos. [8] Hence, assuming equal albedos, it has a diameter of 286±24 km [6] According to the estimate from 2017 based on an improved modelling the size of Actaea is slightly larger being 290±21 km. [7]

    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. [8]

    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. [8] 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. [8] Salacia and Actaea will next occult each other in 2067. [8]


    This minor planet was named after Salacia ( /səˈlʃə/ ), the goddess of salt water and the wife of Neptune. [2] Naming citation was published on 18 February 2011 ( M.P.C. 73984). [17]

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

    See also


    1. approximation if Salacia and Actaea were both spherical and had the same albedo

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    Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS4, and Earth along with the Moon
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e EightTNOs.pngCharonCharonNixNixKerberosStyxHydraHydraDysnomiaDysnomiaErisErisNamakaNamakaHi'iakaHi'iakaMK2MK2XiangliuXiangliuGonggongGonggongWeywotWeywotQuaoarQuaoarSednaSednaVanthVanthOrcusOrcusActaeaActaeaSalaciaSalacia2002 MS42002 MS4
    Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS4 , and Earth along with the Moon
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