|Discovered by||T. Kobayashi|
|Discovery site||Ōizumi Obs.|
|Discovery date||9 November 1999|
|Orus (Greek mythology)|
|1999 VQ10 · 1998 VD18|
|Jupiter trojan |
Greek  · background 
|Orbital characteristics |
|Epoch 23 March 2018 (JD 2458200.5)|
|Uncertainty parameter 0|
|Observation arc||18.49 yr (6,754 d)|
|11.61 yr (4,240 d)|
|0° 5m 5.64s / day|
|Known satellites||none known|
|Jupiter MOID||0.0181 AU|
55.67 km (calculated)
|C  · D |
V–I = 0.950±0.040
21900 Orus // is a Jupiter trojan from the Greek camp, approximately 53 kilometers (33 miles) in diameter, and a target of the Lucy mission to be visited in November 2028. The dark Jovian asteroid belongs to the 100 largest Jupiter trojans and has a rotation period of 13.5 hours. It was discovered on 9 November 1999, by Japanese amateur astronomer Takao Kobayashi at his private Ōizumi Observatory in Gunma Prefecture, Japan, and later named Orus after a slain Achaean warrior from the Iliad.
Orus is a dark Jovian asteroid orbiting in the leading Greek camp at Jupiter's L4 Lagrangian point, 60° ahead of its 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 4.9–5.3 AU once every 11 years and 7 months (4,240 days; semi-major axis of 5.13 AU). Its orbit has an eccentricity of 0.04 and an inclination of 8° with respect to the ecliptic. The body's observation arc begins with a precovery, published by the Digitized Sky Survey and taken at Palomar Observatory in November 1951, or 48 years prior to its official discovery observation.
Orus is planned to be visited by the Lucy spacecraft which will launch in 2021. The fly by is scheduled for 20 November 2028, and will approach the asteroid to a distance of 1000 kilometers at a velocity of 7.1 kilometers per second. The mission's targets with their flyby dates are:
Orus is characterized as a D-type and C-type asteroid by the Lucy mission team and by PanSTARRS photometric survey, respectively. It has a V–I color index of 0.95, seem among most larger D-type Jupiter trojans.
The first photometric observations of Orus have been made in October 2009, by astronomer Stefano Mottola in a photometric lightcurve survey of 80 Jupiter trojans, using the 1.2-meter telescope at Calar Alto Observatory. The obtained rotational lightcurve rendered a period of 13.45±0.08 hours with a brightness variation of 0.18 magnitude (U=2).
In 2016, Mottola published a revised rotation period of 13.48617±0.00007 h, from ground-based observations taken over five apparitions in support of the Lucy mission. He finds that Orus is a retrograde rotator. The lightcurve suggests the presence of a large crater in the proximity of its north pole.
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 53.87 and 50.81 kilometers, respectively. The Collaborative Asteroid Lightcurve Link assumes a standard albedo for carbonaceous C-type asteroids of 0.057 and calculates a diameter of 55.67 kilometers with an absolute magnitude of 10.0.
This 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. The approved naming citation was published by the Minor Planet Center on 22 February 2016 (M.P.C. 98711).