5145 Pholus is an eccentric

centaur A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse. Centaurs are thought of in many Greek myths as being ...

in the

outer Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, approximately in diameter, that crosses the orbit of both

Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It has only one-eighth the average density of Earth; h ...

and

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 ...

. It was discovered on 9 January 1992 by American astronomer

David Rabinowitz David Lincoln Rabinowitz (born 1960) is an American astronomer, discoverer of minor planets and researcher at Yale University. Career David Rabinowitz has built CCD cameras and software for the detection of near-Earth and Kuiper belt obje ...

(uncredited) of UA's

Spacewatch The Spacewatch Project is an astronomical survey that specializes in the study of minor planets, including various types of asteroids and comets at University of Arizona telescopes on Kitt Peak near Tucson, Arizona, in the United States. The Sp ...

survey at the

Kitt Peak National Observatory The Kitt Peak National Observatory (KPNO) is a United States astronomy, astronomical observatory located on Kitt Peak of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono Oʼodham Nation, west-southwest of Tucson, Arizona. With ...

in Arizona, United States. The very reddish object has an elongated shape and a

rotation period The rotation period of a celestial object (e.g., star, gas giant, planet, moon, asteroid) may refer to its sidereal rotation period, i.e. the time that the object takes to complete a single revolution around its axis of rotation relative to the ...

of 9.98 hours. It was named after the centaur Pholus from Greek mythology.

Orbit and classification

''Pholus'' was the second

to be discovered. Centaurs are objects in between the

asteroid An asteroid is a minor planet of the inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic or icy bodies with no atmosphere. ...

and

trans-Neptunian 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, ...

populations of the Solar System that is, beyond Jupiter's and within Neptune's orbit which behave with characteristics of both asteroids and

comet A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena ar ...

s. It orbits the Sun at a distance of 8.8–31.9 AU once every 91 years and 9 months (33,526 days;

semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...

of 20.35 AU). Its orbit has an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...

of 0.57 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

of 25 ° with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of the Earth around the Sun. From the perspective of an observer on Earth, the Sun's movement around the celestial sphere over the course of a year traces out a path along the ecliptic again ...

. It is a Saturn-, Uranus- and Neptune-crosser, crossing the orbits of these

giant planet The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. Ther ...

s at a mean-distance of 9.6, 11.9 and 30.1 AU from the Sun, respectively. ''Pholus'' has not come within one

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 ...

of a planet since 764 BC, and will not until 5290. It is believed that it originated 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 ...

Discovery and naming

''Pholus'' was discovered by

(not officially credited), working with the

program, at

on 9 January 1992. Rabinowitz'es discovery was confirmed by Eugene and

Carolyn Shoemaker Carolyn Jean Spellmann Shoemaker (June 24, 1929 – August 13, 2021) was an American astronomer and a co-discoverer of Comet Shoemaker–Levy 9, Comet Shoemaker–Levy 9. She discovered 32 comets (then a record for the most by an individual) ...

who identified the object on images they previously took on 1 January 1992. The discovery was announced by James Scotti on 23 January 1992 in an ''

IAU Circular The International Astronomical Union Circulars (IAUCs) are notices that give information about astronomical phenomena. IAUCs are issued by the International Astronomical Union's Central Bureau for Astronomical Telegrams (CBAT) at irregular interval ...

'' (IAUC 5434) of the

Central Bureau for Astronomical Telegrams The Central Bureau for Astronomical Telegrams (CBAT) is the official international clearing house for information relating to transient astronomical events. The CBAT collects and distributes information on comets, natural satellites, novae, supern ...

. A first

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 ...

was taken at the Australian

Siding Spring Observatory Siding Spring Observatory near Coonabarabran, New South Wales, Australia, part of the Research School of Astronomy & Astrophysics (RSAA) at the Australian National University (ANU), incorporates the Anglo-Australian Telescope along with a coll ...

in 1977, extending the centaur's

observation arc In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...

by 15 years prior to its discovery. It was the second discovery of a centaur after

2060 Chiron 2060 Chiron is a small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs—bodies orb ...

discovered by

Charles Kowal Charles Thomas Kowal (November 8, 1940 – November 28, 2011) was an American astronomer known for his observations and discoveries in the Solar System. As a staff astronomer at Caltech's Mount Wilson and Palomar Mountain observatories between ...

in 1977. In 1993, while with the Spacewatch program, David Rabinowitz went on to discover another centaur,

7066 Nessus 7066 Nessus is a very red centaur on an eccentric orbit, located beyond Saturn in the outer Solar System. It was discovered on 26 April 1993, by astronomers of the Spacewatch program at the Kitt Peak National Observatory in Tucson, Arizona. The ...

. 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 by the ''Minor Planet Names Committee'' for Pholus, a

from

Greek mythology A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...

. As with 2060 Chiron, named after his brother

Chiron In Greek mythology, Chiron ( ; also Cheiron or Kheiron; ) was held to be the superlative centaur amongst his brethren since he was called the "wisest and justest of all the centaurs". Biography Chiron was notable throughout Greek mythology ...

, the tradition is to name this class of outer planet-crossing objects after the half-human, half-horse mythological creatures. In the mythological account, Pholus died by a self-inflicted wound from a poisoned arrow used by

Heracles Heracles ( ; grc-gre, Ἡρακλῆς, , glory/fame of Hera), born Alcaeus (, ''Alkaios'') or Alcides (, ''Alkeidēs''), was a divine hero in Greek mythology, the son of Zeus and Alcmene, and the foster son of Amphitryon.By his adoptive ...

''(see

5143 Heracles 5143 Heracles, provisional designation , is a highly eccentric, rare-type asteroid and synchronous Binary asteroid, binary system, classified as near-Earth object of the Apollo asteroid, Apollo group, approximately 4.8 kilometers in diameter. The ...

)'', who buried Pholus on the mountain Pholoe. The official was published by the

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Function ...

on 14 July 1992 (). A symbol derived from that for

, ( x14px), was devised in the late 1990s by German astrologer Robert von Heeren. It replaces Chiron's K with a P for Pholus.

Physical characteristics

Spectral type and color

After its discovery, ''Pholus'' was quickly found to be very red in color. The color has been speculated to be due to organic compounds on its surface. It is classified as a Z class object on the

Tholen Tholen () is a 25,000 people municipality in the southwest of the Netherlands. The municipality of Tholen takes its name from the town of Tholen, which is the largest population center in the municipality. The municipality consists of two peninsu ...

taxonomic scheme. The object has been classified by astronomers as RR and RR-U type, respectively. Polarimetric observations with ESO's

Very Large Telescope The Very Large Telescope (VLT) is a telescope facility operated by the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each with a primary mirror 8.2 m across, ...

in 2007 and 2008, revealed noticeable negative polarization at certain phase angles, distinctly different from that of trans-Neptunian objects. ''Pholus'' appears to have a rather homogeneous surface with small amount of water frost on its darker parts. The surface composition of ''Pholus'' has been estimated from its reflectance spectrum using two spatially segregated components: dark

amorphous carbon Amorphous carbon is free, reactive carbon that has no crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amorp ...

and an intimate mixture of

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

ice,

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...

ice,

olivine The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...

grains, and complex organic compounds (

tholin Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic ray irradiation of simple carbon-containing compounds such as carbon ...

s). The

carbon black Carbon black (subtypes are acetylene black, channel black, furnace black, lamp black and thermal black) is a material produced by the incomplete combustion of coal and coal tar, vegetable matter, or petroleum products, including fuel oil, fluid ...

component was used to match the low

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 the object. Unlike

, ''Pholus'' has shown no signs of

ary activity.

Diameter and albedo

Diameter calculations range from 99 to 190 kilometers with a corresponding

between 0.155 and 0.04. According to the

Herschel Space Observatory The Herschel Space Observatory was a space observatory built and operated by the European Space Agency (ESA). It was active from 2009 to 2013, and was the largest infrared telescope ever launched until the launch of the James Webb Space Telesc ...

with its PACS instrument, ''Pholus'' measures 99 kilometers in diameter and its surface has an

of 0.155, while a study from 1996 derived a diameter of 185 km. During 2003–2004, observations with the 1.8-meter

Vatican Advanced Technology Telescope The 1.8 meter Alice P. Lennon Telescope and its Thomas J. Bannan Astrophysics Facility, known together as the Vatican Advanced Technology Telescope (VATT), is a Gregorian telescope observing in the optical and infrared situated on Mount Graham i ...

(VATT) on Mount Graham Observatory, Arizona, determined an elongated shape, , with a mean-diameter of 190 kilometers, based on a low albedo of 0.04. Johnston's archive lists a diameter of 107 km with an albedo of 0.126, and ''Collaborative Asteroid Lightcurve Link'' assumes a standard albedo of a

carbonaceous Carbon () is a chemical element with the chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—its atom making four electrons available to form covalent bond, covalent chemical bonds. It belongs to gro ...

body of 0.057 and derives a diameter of 165 km based on an

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse Logarithmic scale, logarithmic Magnitude (astronomy), astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent mag ...

of 7.64.

Rotation period

In March 2005, a rotational

lightcurve 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 frequ ...

of ''Pholus'' was obtained from photometric observations by Tegler using the VATT at Mount Graham. Lightcurve analysis gave a

of 9.980 hours with a brightness amplitude of 0.60

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

(). Alternative period determinations were also conducted by Hoffmann, Franham and Buie with concurring results of 9.977, 9.982 and 9.983 hours, respectively ().

References

External links

Kuiper Belt Object Magnitudes and Surface Colors
Stephen Tegler
SOLEX

Asteroid Lightcurve Database (LCDB)
query form

)
Dictionary of Minor Planet Names
Google books

Minor Planet Center * * {{DEFAULTSORT:Pholus Centaurs (small Solar System bodies) 005145 Named minor planets 005145 19920109