120347 Salacia, provisional designation , is a large planetoid in the

Kuiper belt The Kuiper belt () is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...

, approximately 850 kilometers in diameter. As of 2018, it is located 44.8

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...

s from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, and reaches

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

20.7 at

opposition Opposition may refer to: Arts and media * ''Opposition'' (Altars EP), 2011 EP by Christian metalcore band Altars * The Opposition (band), a London post-punk band * '' The Opposition with Jordan Klepper'', a late-night television series on Com ...

. Salacia was discovered on 22 September 2004, by American astronomers Henry Roe, Michael Brown and Kristina Barkume at the

Palomar Observatory Palomar Observatory is an astronomical research observatory in San Diego County, California, United States, in the Palomar Mountain Range. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...

in California, United States. It has been observed 124 times, with

precovery In astronomy, precovery (short for pre-discovery recovery) is the process of finding the image of an object in images or photographic plates predating its discovery, typically for the purpose of calculating a more accurate orbit. This happens mos ...

images back to 25 July 1982. Salacia orbits the Sun at an average distance that is slightly greater than that of

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

. It was named after the Roman goddess

Salacia In ancient Roman mythology, Salacia ( , ) was the female divinity of the sea, worshipped as the goddess of salt water who presided over the depths of the ocean. Neptune was her consort. That Salacia was the consort of Neptune is implied by Va ...

and has a single known moon, Actaea. Brown estimated that Salacia is nearly certainly a

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to p ...

. 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.W.M. Grundy, K.S. Noll, M.W. Buie, S.D. Benecchi, D. Ragozzine & H.G. Roe, 'The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ()', ''Icarus'
(forthcoming, available online 30 March 2019)
DOI: 10.1016/j.icarus.2018.12.037, 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 .

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 The Deep Ecliptic Survey (DES) is a project to find Kuiper belt objects (KBOs), using the facilities of the National Optical Astronomy Observatory (NOAO). The principal investigator is Robert L. Millis. Since 1998 through the end of 2003, the sur ...

and a

hot classical A classical Kuiper belt object, also called a cubewano ( "QB1-o"), is a low-eccentricity Kuiper belt object (KBO) that orbits beyond Neptune and is not controlled by an orbital resonance with Neptune. Cubewanos have orbits with semi-major axe ...

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 The parameter space is the space of possible parameter values that define a particular mathematical model, often a subset of finite-dimensional Euclidean space. Often the parameters are inputs of a function, in which case the technical term for the ...

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 , with an average system density of ; Salacia itself is estimated to be around 846 km in diameter. Salacia has the lowest

albedo Albedo (; ) is the measure of the diffuse reflection of sunlight, solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body ...

of any known large

trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (au). Typically, ...

. 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 (based on old mass). Salacia was previously believed to have a mass of around , in which case it would also have had the lowest density (around ) 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 In fluid mechanics, hydrostatic equilibrium (hydrostatic balance, hydrostasy) is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. In the planetary ...

. Salacia's

infrared spectrum Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

is almost featureless, indicating an abundance of water ice of less than 5% on the surface. Its

light-curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular freq ...

amplitude is only 3%.

Satellite

Salacia has one

natural satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are often colloquially referred to as ''moons'' ...

, 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 Denise C. Nuttall Stephens is an associate professor of astronomy in the College of Physical and Mathematical Sciences in the Department of Physics and Astronomy at Brigham Young University. Education and research experience Stephens graduated ...

and William Grundy with the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

. 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 being . 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 Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

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 The Hill sphere of an astronomical body is the region in which it dominates the attraction of satellites. To be retained by a planet, a moon must have an orbit that lies within the planet's Hill sphere. That moon would, in turn, have a Hill sp ...

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 According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

was named after

(), the goddess of salt water and the wife of

Neptune Neptune is the eighth planet from the Sun and the farthest known planet in the Solar System. It is the fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times ...

. Naming citation was published on 18 February 2011 (). The moon's name, ''Actaea'' , was assigned on the same date. Actaea is a

nereid In Greek mythology, the Nereids or Nereides ( ; grc, Νηρηΐδες, Nērēḯdes; , also Νημερτές) are sea nymphs (female spirits of sea waters), the 50 daughters of the 'Old Man of the Sea' Nereus and the Oceanids, Oceanid Doris ...

or sea nymph.

Planetary symbol A planet symbol (or ''planetary symbol'') is a graphical symbol used in astrology and astronomy to represent a classical planet (including the Sun and the Moon) or one of the modern planets. The symbols were also used in alchemy to represent the me ...

s are no longer much used in astronomy, so Salacia never received a symbol in the astronomical literature. There is no standard symbol for Salacia used by astrologers either. Denis Moskowitz, a software engineer in Massachusetts who designed the symbols for most of the dwarf planets, proposed a stylised

hippocamp The hippocampus or hippocamp, also ''hippokampos'' (plural: hippocampi or hippocamps; grc, ἱππόκαμπος, from , "horse" and , "sea monster"

(

) as the symbol for Salacia, befitting Salacia as Neptune's consort.

Notes

References

External links

(120347) Salacia
at Johnston's Archive

(Emily Lakdawalla – 2012/06/26) * {{DEFAULTSORT:Salacia 120347 120347 Discoveries by Michael E. Brown Named minor planets Binary trans-Neptunian objects 20040922