15810 Arawn, provisional designation , is a

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 ...

(TNO) from the inner regions of 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 tim ...

, approximately in diameter. It belongs to the

plutino In astronomy, the plutinos are a dynamical group of trans-Neptunian objects that orbit in 2:3 mean-motion resonance with Neptune. This means that for every two orbits a plutino makes, Neptune orbits three times. The dwarf planet Pluto is the l ...

s, the largest class of resonant TNOs. It was named after

Arawn In Welsh mythology, Arawn (; ) was the king of the otherworld realm of Annwn who appears prominently in the first branch of the Mabinogi, and alluded to in the fourth. In later tradition, the role of the king of Annwn was largely attributed to ...

, the ruler of the Celtic underworld, and discovered on 12 May 1994, by astronomers Michael Irwin and Anna Żytkow with the 2.5-metre

Isaac Newton Telescope The Isaac Newton Telescope or INT is a 2.54 m (100 in) optical telescope run by the Isaac Newton Group of Telescopes at Roque de los Muchachos Observatory on La Palma in the Canary Islands since 1984. Originally the INT was situated at He ...

Roque de los Muchachos Observatory Roque de los Muchachos Observatory ( es, Observatorio del Roque de los Muchachos, ORM) is an astronomical observatory located in the municipality of Garafía on the island of La Palma in the Canary Islands, Spain. The observatory site is operated ...

in the Canary Islands, Spain. Arawn is unique in that it has been observed at a much closer distance than most Kuiper belt objects, by the ''

New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe that was launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research ...

'' spacecraft, which imaged it from a distance of in April 2016; this and its other observations have allowed its

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 ...

to be determined.

Orbit and physical properties

Arawn is moving in a relatively eccentric orbit entirely beyond the orbit 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 ...

. With a

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 39.4 AU, it orbits the Sun once every 247 years and 6 months (90,409 days). Its orbit has

perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

(closest approach to the Sun) of 34.7 AU, an

aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ell ...

(farthest distance from the Sun) of 44.1 AU, 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-Centre (geometry), center, in geometry * Eccentricity (g ...

of 0.12 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 4 ° 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

, being trapped in a 2:3

mean motion resonance In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relationsh ...

with Neptune, similarly to dwarf planet

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 ...

, the largest known plutino. It measures approximately in diameter, based on an absolute

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 ...

of 7.6, and estimated albedo of 0.1. Observations by 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 ...

show that Arawn has a very red surface. In April 2016, its

of 5.47 hours was determined.

Quasi-satellite dynamical state and orbital evolution

In 2012 Arawn was hypothesized to be in a

quasi-satellite A quasi-satellite is an object in a specific type of co-orbital configuration (1:1 orbital resonance) with a planet (or dwarf planet) where the object stays close to that planet over many orbital periods. A quasi-satellite's orbit around the Sun t ...

loop around Pluto, as part of a recurring pattern, becoming a Plutonian quasi-satellite every 2 Myr and remaining in that phase for nearly 350,000 years. Measurements made by the ''

'' probe in 2015 made it possible to calculate the motion of Arawn much more accurately. These calculations confirm the general dynamics described in the hypotheses. However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on this motion, since its orbit is primarily controlled by Neptune with only occasional smaller perturbations caused by Pluto.

Origin

Arawn is moving in a very stable orbit, likely as stable as Pluto's. This suggests that it might be a primordial plutino formed around the same time Pluto itself and

Charon In Greek mythology, Charon or Kharon (; grc, Χάρων) is a psychopomp, the ferryman of Hades, the Greek underworld. He carries the souls of those who have been given funeral rites across the rivers Acheron and Styx, which separate the wo ...

came into existence. It is unlikely to be relatively recent debris originated in collisions within Pluto's system or a captured object.

Observation

Arawn is currently relatively close to Pluto. In 2017 it was only 2.7 AU from Pluto. Before 486958 Arrokoth was discovered in 2014, Arawn was the best known target for a flyby by the ''

'' spacecraft after its Pluto flyby in 2015. Arawn was one of the first objects targeted for distant observations by ''New Horizons'', which were taken on 2 November 2015. More observations were made in April 2016. On 2 November 2015 Arawn was imaged by the LORRI instrument aboard ''New Horizons'', making it the closest observation of a Kuiper belt object other than the Pluto–Charon system by a factor of 15. Between 7–8 April 2016, ''New Horizons'' imaged Arawn from a new record distance of about 111 million kilometres, using the LORRI instrument. The new images allowed the science team at Southwest Research Institute (SwRI) in Boulder, Colorado, to further pinpoint the location of Arawn to within 1000 kilometers. The new data also made it possible for scientists to observe its rotation period, which was determined to be 5.47 hours. File:PIA205891994 JR1newhorizons.gif, April 2016 observations of Arawn by ''New Horizons'' File:New Horizons KEM Trajectory.png, Trajectory of ''New Horizons'' and other nearby Kuiper belt objects File:1994_JR1_orbit_2015.png, Orbit of Arawn near Pluto Arawn occulted a star on 25 August 2022.RECON: TNO occultation with 15810
- Marc W. Buie

References

Notes

External links

Asteroid Lightcurve Database (LCDB)
query form

)
Dictionary of Minor Planet Names
Google books

– Observatoire de Genève, Raoul Behrend

– Minor Planet Center * {{DEFAULTSORT:Arawn 015810 Discoveries by Anna N. Żytkow Discoveries by Michael J. Irwin Named minor planets 015810 New Horizons 19940512