|Discovered by||T. Kobayashi|
|Discovery site||Ōizumi Obs.|
|Discovery date||9 November 1999|
|MPC designation||(21900) Orus|
|Pronunciation||// · OHR-ə-s|
|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 (// OHR-əs), provisional designation 1999 VQ10, 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 after the Achaean warrior Orus from Greek mythology.
Orbit and classification
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.
Lucy mission target
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:
- 52246 Donaldjohanson — 20 April 2025: 4 km diameter C-type asteroid in the inner main-belt, member of ~130Myr old Erigone family;
- 3548 Eurybates — 12 August 2027: 64 km diameter C-type Jupiter Trojan in the Greek camp at L4, largest member of the only confirmed disruptive collisional family in the Trojans;
- 15094 Polymele — 15 September 2027: 21 km diameter P-type Trojan at L4, likely collisional fragment;
- 11351 Leucus — 18 April 2028: 34 km diameter D-type slow rotator Trojan at L4;
- 21900 Orus — 11 November 2028: 51 km diameter D-type Trojan at L4;
- 617 Patroclus — 2 March 2033: P-type binary Trojan. The primary, Patroclus, has a mean diameter of 113 km and its companion, Menoetius, has a diameter of 104 km. The pair orbit at a separation of 680 km. The binary resides in the Trojan camp at L5.
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.
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.
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).
- Noll, K. S. (2018), HST Proposal 15622, Space Telescope Science Institute, retrieved 7 June 2019
- "21900 Orus (1999 VQ10)". Minor Planet Center. Retrieved 17 June 2018.
- "JPL Small-Body Database Browser: 21900 Orus (1999 VQ10)" (2017-05-08 last obs.). Jet Propulsion Laboratory. Retrieved 17 June 2018.
- "List of Jupiter Trojans". Minor Planet Center. 1 June 2018. Retrieved 17 June 2018.
- "Asteroid (21900) Orus – Proper Elements". AstDyS-2, Asteroids – Dynamic Site. Retrieved 17 June 2018.
- Grav, T.; Mainzer, A. K.; Bauer, J. M.; Masiero, J. R.; Nugent, C. R. (November 2012). "WISE/NEOWISE Observations of the Jovian Trojan Population: Taxonomy". The Astrophysical Journal. 759 (1): 10. arXiv:1209.1549. Bibcode:2012ApJ...759...49G. doi:10.1088/0004-637X/759/1/49. (online catalog)
- Usui, Fumihiko; Kuroda, Daisuke; Müller, Thomas G.; Hasegawa, Sunao; Ishiguro, Masateru; Ootsubo, Takafumi; et al. (October 2011). "Asteroid Catalog Using Akari: AKARI/IRC Mid-Infrared Asteroid Survey". Publications of the Astronomical Society of Japan. 63 (5): 1117–1138. Bibcode:2011PASJ...63.1117U. doi:10.1093/pasj/63.5.1117. Retrieved 17 June 2018. (online, AcuA catalog p. 153)
- "LCDB Data for (21900) Orus". Asteroid Lightcurve Database (LCDB). Retrieved 17 June 2018.
- Mottola, Stefano; Di Martino, Mario; Erikson, Anders; Gonano-Beurer, Maria; Carbognani, Albino; Carsenty, Uri; et al. (May 2011). "Rotational Properties of Jupiter Trojans. I. Light Curves of 80 Objects". The Astronomical Journal. 141 (5): 32. Bibcode:2011AJ....141..170M. doi:10.1088/0004-6256/141/5/170.
- Mottola, Stefano; Marchi, Simone; Buie, Marc W.; Hellmich, Stephan; Di Martino, Mario; Proffe, Gerrit; Levison, Harold F.; Zangari, Amanda Marie (2016). "Ground-based characterization of Eurybates and Orus, two fly-by targets of the Lucy Discovery mission". DPS meeting #48. Bibcode:2016DPS....4820804M. Cite journal requires
- Veres, Peter; Jedicke, Robert; Fitzsimmons, Alan; Denneau, Larry; Granvik, Mikael; Bolin, Bryce; et al. (November 2015). "Absolute magnitudes and slope parameters for 250,000 asteroids observed by Pan-STARRS PS1 - Preliminary results". Icarus. 261: 34–47. arXiv:1506.00762. Bibcode:2015Icar..261...34V. doi:10.1016/j.icarus.2015.08.007.
- Levison, H. F.; Olkin, C.; Noll, K. S.; Marchi, S.; Lucy Team (March 2017). "Lucy: Surveying the Diversity of the Trojan Asteroids: The Fossils of Planet Formation" (PDF). 48th Lunar and Planetary Science Conference. Bibcode:2017LPI....48.2025L. Retrieved 13 April 2017.
- Dreier, Casey; Lakdawalla, Emily (30 September 2015). "NASA announces five Discovery proposals selected for further study". The Planetary Society. Retrieved 1 February 2016.
- "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 17 June 2018.
- Asteroid Lightcurve Database (LCDB), query form (info)
- 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
- 21900 Orus at the JPL Small-Body Database