21900 Orus
21900 Orus is a Jupiter trojan from the Greek camp, approximately 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". Orbit and classification. "Orus" is a dark Jovian asteroid orbiting in the leading Greek camp at Jupiter's 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. Section::::"Lucy" mission  target. "Orus" is planned to be visited by the "Lucy" spacecraft which was launched 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. Physical characteristics. "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. Lightcurve. 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 hours with a brightness variation of 0.18 magnitude (). In 2016, Mottola published a revised rotation period of , 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. 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, 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. Naming. 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 (). Satellite. Orus has a candidate satellite, detected while searching through Hubble images taken on August 7 and 8, 2018. Further observations are needed to determine physical characteristics of the satellite, which can help measure the mass of the primary.