15810 Arawn, provisional designation , is a trans-Neptunian object (TNO) from the inner regions of the Kuiper belt, approximately in diameter. It belongs to the plutinos, the largest class of resonant TNOs. It was named after Arawn, 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 at Roque de los Muchachos Observatory 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 spacecraft, which imaged it from a distance of in April 2016; this and its other observations have allowed its rotation period to be determined. Orbit and physical properties Arawn is moving in a relatively eccentric orbit entirely beyond the orbit of Neptune. With a semi-major axis of 39.4 AU, it orbits the Sun once every 247 years and 6 months (90,409 days). Its orbit has perihelion (closest approach to the Sun) of 34.7 AU, an aphelion (farthest distance from the Sun) of 44.1 AU, an eccentricity of 0.12 and an inclination of 4° with respect to the ecliptic. It is a plutino, being trapped in a 2:3 mean motion resonance with Neptune, similarly to dwarf planet Pluto, the largest known plutino. It measures approximately in diameter, based on an absolute magnitude of 7.6, and estimated albedo of 0.1. Observations by the Hubble Space Telescope show that Arawn has a very red surface. In April 2016, its rotation period of 5.47 hours was determined. Quasi-satellite dynamical state and orbital evolution In 2012 Arawn was hypothesized to be in a quasi-satellite 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 New Horizons 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