[en] | 21900 Orus

Orus is a dark Jupiter trojan asteroid orbiting in the leading Greek camp at Jupiter’s L₄ Lagrangian point, 60° ahead of its orbit in a 1:1 resonance(see Trojans in astronomy). It is also a non-family asteroid in the Jovian background population.^[5]

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.^[3] 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.^[1]

Orus is planned to be visited by the Lucy spacecraft which was launched in 2021. The flyby is scheduled for 20 November 2028, and will approach the asteroid to a distance of 1,000 km (620 mi) at a relative velocity of 7.1 km/s (16,000 mph).^[9]

Orus is characterized as a D-type and C-type asteroid by the Lucy mission team and by Pan-STARRS photometric survey, respectively.^[9][8] It has a V–I color index of 0.95, seem among most larger D-type Jupiter trojans.^[7]

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).^[7][10]

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.^[11]

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.^[12][13] 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.^[7]

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.^[1] The approved naming citation was published by the Minor Planet Center on 22 February 2016 (M.P.C. 98711).^[14]

Orus has a candidate satellite, detected while searching through Hubble images taken on 7–8 August 2018.^[16] Further observations are needed to determine physical characteristics of the satellite, which can help measure the mass of the primary.

• Discovery program

• Asteroid Lightcurve Database (LCDB), query form (info Archived 16 December 2017 at the Wayback Machine)
• Dictionary of Minor Planet Names, Google books
• Discovery Circumstances: Numbered Minor Planets (20001)–(25000) – Minor Planet Center
• Asteroid 21900 Orus at the Small Bodies Data Ferret
• 21900 Orus at AstDyS-2, Asteroids—Dynamic Site
  • Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info
• 21900 Orus at the JPL Small-Body Database
  • Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters