is a non-resonant trans-Neptunian object and binary system from the Kuiper belt located in the outermost region of the Solar System, approximately in diameter. It was first observed on 18 November 1998, by American astronomer Marc Buie and Robert Millis at the Kitt Peak National Observatory in Arizona, United States. In December 2000, a minor-planet moon, designated S/2000 () 1 with a diameter of , was discovered in its orbit. After Charon in 1978, it was the first of many satellites since discovered in the outer Solar System.
Orbit and classification
Located beyond the orbit of Neptune, is a non-resonant classical Kuiper belt object (cubewano) of the so-called hot population, which have higher inclinations than those of the cold population. It orbits the Sun at a distance of 40.4–48.5 AU once every 297 years (108,345 days; semi-major axis of 44.48 AU). Its orbit has an eccentricity of 0.09 and an inclination of 7° with respect to the ecliptic. The body's observation arc begins at Kitt Peak with its first observation on 18 November 1998.
Numbering and naming
As of 2025, this minor planet has not been numbered nor named by the Minor Planet Center.
Physical characteristics
is expected to have a low albedo due to its blue (neutral) color. Other sources assume a higher albedo of 0.10 and 0.16, respectively (see below). It has a V–I color index of 0.91, notably lower than the mean-color index for cubewanos, and in between that of comets and Jupiter trojans.
Satellite
On 22 December 2000, French astronomers Christian Veillet and Alain Doressoundiram in collaboration with J. Shapiro discovered that the observed body was a binary, using the Canada–France–Hawaii Telescope at Mauna Kea on the Big Island of Hawaii, United States. The discovery was announced on 16 April 2001, and the smaller body received the provisional designation .
It was the first trans-Neptunian binary discovered after Charon in 1978, the largest satellite in the Pluto–Charon system. Since then, many trans-Neptunian binaries have been discovered. is also one of the most symmetrical binaries known in the Solar System.
The satellite has a highly eccentric orbit with an eccentricity of with an exceptionally long orbital period of and a semi-major axis of .
The discovery team concluded that, if the primary and secondary have the same albedo, their size ratio is 1.2, and if they also have the same densities, their mass ratio is 1.74 – that is, that the secondary has 60% the mass of the primary, or a bit over a third of the system mass. If they have equal densities of 1 g/cm3, that would mean nominal diameters of 148 km and 123 km, respectively, and albedos of 0.05~0.06.
Diameter and albedo
Assuming a density of 1 g/cm3, the primary measures , the satellite has a diameter of (a ratio of 0.883) with a combined system diameter of . The Collaborative Asteroid Lightcurve Link assumes an albedo of 0.10 and calculates a system diameter of based on an absolute magnitude of 6.7, while Mike Brown finds a diameter of with a lower albedo of 0.04.
Rotation period
As of 2020, no rotational lightcurve of has been obtained from photometric observations. The body's rotation period, pole and shape remain unknown.
References
External links
The binary Kuiper-belt object , publication in Nature, 18 April 2002 The binary Kuiper Belt Object 1998 WW31, Christian Veillet - CFHT - 17 April 2002 IAUC 7610: S/2000 (1998 WW_31) 1, satellite discovery – Central Bureau for Astronomical Telegrams, 16 April 2001 Orbit Fit and Astrometric record for 1998 WW31, Marc W. Buie, SwRI (Space Science Department) and S/2000 () 1 MPEC 2008-O05 : Distant Minor Planets (2008 AUG. 2.0 TT)