(21601) 1998 XO89

' (provisional designation ') is a Jupiter trojan from the Greek camp, approximately 55 km in diameter. It was discovered on 15 December 1998, by astronomers with the Lincoln Near-Earth Asteroid Research at the Lincoln Lab's ETS near Socorro, New Mexico. The dark Jovian asteroid has a rotation period of 12.7 hours and belongs to the 80 largest Jupiter trojans. It has not been named since its numbering in February 2001.

Orbit and classification
is a dark Jovian asteroid orbiting in the leading Greek camp at Jupiter's Lagrangian point, 60° ahead of the Gas Giant's orbit in a 1:1 resonance. It is also a non-family asteroid in the Jovian background population.

It orbits the Sun at a distance of 5.0–5.4 AU once every 11 years and 11 months (4,356 days; semi-major axis of 5.22 AU). Its orbit has an eccentricity of 0.04 and an inclination of 19° with respect to the ecliptic. The body's observation arc begins with a precovery published by the Digitized Sky Survey and taken at the Siding Spring Observatory in March 1991, more than 7 years prior to its official discovery observation at Socorro.

Numbering and naming
This minor planet was numbered on 8 February 2001 (M.P.C. 22480). , it has not been named.

Physical characteristics
is an assumed C-type asteroid. Its V–I color index of 0.97 is typical for that of D-type asteroids, the dominant spectral type among the Jupiter trojans.

Rotation period
In April 2013, a rotational lightcurve of was obtained from photometric observations by Robert Stephens at the Center for Solar System Studies (CS3) in California. Lightcurve analysis gave a rotation period of $54.91 km$ hours and a brightness variation of 0.30 magnitude (U=2+). Observations by his college Brian Warner at CS3 in July 2017, gave a similar period of 12.530 hours with an amplitude of 0.25 magnitude (U=2+).

Diameter and albedo
According to the surveys carried out by the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, measures 54.91 and 56.08 kilometers in diameter and its surface has an albedo of 0.064 and 0.100, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for a carbonaceous asteroid of 0.057 and calculates a diameter of 55.67 kilometers based on an absolute magnitude of 10.0.