21900 Orus

21900 Orus (/ˈɔərəs/), provisional designation 1999 VQ₁₀, is a carbonaceous Jupiter trojan of the Greek camp, approximately 53 kilometers in diameter. It was discovered on 9 November 1999, by Japanese amateur astronomer Takao Kobayashi at his private Ōizumi Observatory in Gunma Prefecture, Japan. It may be visited by the spacecraft Lucy, a proposed mission concept by NASA as of 2016.

The dark C-type body orbits the Sun in Jupiter's leading L₄ point at a distance of 4.9–5.3 AU once every 11 years and 7 months (4,241 days). Its orbit has an eccentricity of 0.04 and an inclination of 8° with respect the plane of the ecliptic. The first precovery was taken by the Digitized Sky Survey at Palomar Observatory in 1951, extending the body's observation arc by 48 years prior to its discovery.

In October 2009, Orus was observed by astronomer Stefano Mottola in a photometric light-curve survey of 80 Jupiter trojans, using the 1.2-meter telescope at Calar Alto Observatory. The obtained rotational light-curve rendered a period of 7001134500000000000♠13.45±0.08 hours with a brightness variation of 0.18 magnitude (U=2).

According to the surveys carried out by the Japanese Akari satellite and the NEOWISE mission of NASA's Wide-field Infrared Survey Explorer, the body has an albedo of 0.083 and 0.075, with a diameter of 53.9 and 50.8 kilometers, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for carbonaceous asteroids of 0.057 and calculates a diameter of 55.7 kilometers.

The minor planet was named from Greek mythology after Orus, an Achaean warrior in Homer's Iliad. He was killed in the Trojan War by the Trojan prince Hector, after whom the largest Jupiter trojan, 624 Hektor, is named. Naming citation was published on 22 February 2016 (M.P.C. 98711).