1998 WW31

, is a non-resonant trans-Neptunian object and binary system from the Kuiper belt located in the outermost region of the Solar System, approximately 148 km 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 123 km, was discovered in its orbit. After Charon in 1978, it was the first of nearly 100 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 2018, 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
 is a binary minor planet with a minor-planet moon in its orbit. On 22 December 2000, French astronomers Christian Veillet and Alain Doressoundiram in collaboration with J. Shapiro discovered the satellite 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 received the provisional designation S/2000 1.

It was the first trans-Neptunian binary discovered after Charon in 1978, the largest satellite in the Pluto–Charon system. Since then nearly 100 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 eccentricity of $123 km$ with an exceptionally long orbital period of $148 km$ and a semi-major axis of $192.1 km$.

Diameter and albedo
While the primary measures 148 km, the satellite has a diameter of 123 km (a ratio of 0.883) with a combined system diameter of 192 km. The Collaborative Asteroid Lightcurve Link assumes an albedo of 0.10 and calculates a system diameter of 192 km based on an absolute magnitude of 6.7, while Mike Brown finds a diameter of 267 km 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.