5145 Pholus is a centaur in an eccentric orbit

Orbit

In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...

, with a perihelion less than Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

's and aphelion greater than Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

's. Pholus has not come within one astronomical unit

Astronomical unit

An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....

 of a planet since 764 BC, and will not until 5290. It is believed that Pholus originated as a Kuiper belt

Kuiper belt

The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...

 object.

It was discovered by David L. Rabinowitz

David L. Rabinowitz

David Lincoln Rabinowitz is a researcher at Yale University. He has built CCD cameras and software for the detection of near-Earth asteroids and Kuiper Belt Objects, and his research has helped reduce the assumed number of near-Earth asteroids by half, from 1,000-2,000 to 500-1,000 He has also...

, then of the University of Arizona

University of Arizona

The University of Arizona is a land-grant and space-grant public institution of higher education and research located in Tucson, Arizona, United States. The University of Arizona was the first university in the state of Arizona, founded in 1885...

's Spacewatch Project, and named after Pholus

Pholus (mythology)

In Greek mythology, Pholus was a wise centaur and friend of Heracles who lived in a cave on or near Mount Pelion.The differing accounts vary in details, but each story contains the following elements: Herakles visited his cave sometime before or after the completion of his fourth Labor, the capture...

, the brother of the mythological Chiron

Chiron

In Greek mythology, Chiron was held to be the superlative centaur among his brethren.-History:Like the satyrs, centaurs were notorious for being wild and lusty, overly indulgent drinkers and carousers, given to violence when intoxicated, and generally uncultured delinquents...

, after which 2060 Chiron

2060 Chiron

2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...

 was named, in order to follow the tradition of naming this class of outer planet-crossing objects after centaur

Centaur

In Greek mythology, a centaur or hippocentaur is a member of a composite race of creatures, part human and part horse...

s.

Pholus was the second centaur to be discovered and was quickly found to be quite red in color, for which it has been occasionally nicknamed "Big Red". The color has been speculated to be due to organic compounds on its surface.

The surface composition of Pholus has been estimated from its reflectance spectrum using two spatially segregated components: dark amorphous carbon

Amorphous carbon

Amorphous carbon or free, reactive carbon, is an allotrope of carbon that does not have any crystalline structure. As with all glassy materials, some short-range order can be observed...

 and an intimate mixture of water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 ice, methanol

Methanol

Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

 ice, olivine

Olivine

The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 grains, and complex organic compounds (tholin

Tholin

Tholin [after the ancient Greek word meaning "not clear"] is a heteropolymer molecule formed by solar ultraviolet irradiation of simple organic compounds such as methane or ethane. Tholins do not form naturally on modern-day Earth, but are found in great abundance on the surface of icy bodies in...

s). The carbon black

Carbon black

Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...

 component was used to match the low albedo

Albedo

Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

of the object.

Unlike the first centaur, 2060 Chiron, Pholus has shown no signs of cometary activity.

The diameter of Pholus is estimated to be 185±16 km http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1996ciss.conf...97D&db_key=AST&high=3d6ea7529520743.

External links

• Orbital simulation from JPL (Java) / Ephemeris