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21900 Orus
	Shape model of Orus viewed from multiple orthogonal perspectives
Discovery
Discovered by	T. Kobayashi
Discovery site	Ōizumi Obs.
Discovery date	9 November 1999
Designations
MPC designation	(21900) Orus
Pronunciation	/ˈɔːrəs/
Named after	Orus (Greek mythology)
Alternative designations	1999 VQ10 · 1998 VD18
Minor planet category	Jupiter trojan ; Greek · background
Adjectives	Orian
Orbital characteristics
	Epoch 25 February 2023 (JD 2460000.5)
	Uncertainty parameter 0
Earliest precovery date	8 November 1951
Aphelion	5.124 AU
Perihelion	4.930 AU
Semi-major axis	5.318 AU
Eccentricity	0.0379
Orbital period (sidereal)	11.60 yr (4,237 d)
Mean anomaly	356.275°
Mean motion	0° 5m 5.892s / day
Inclination	8.469°
Longitude of ascending node	258.554°
Argument of perihelion	181.258°
Jupiter MOID	0.0218 AU
TJupiter	2.977
Physical characteristics
Dimensions	70.7 × 63.0 × 51.4 km
Mean diameter	60.5±0.9 km (area equivalent)
Synodic rotation period	13.486190±0.000017 h
Axial tilt	149° (wrt ecliptic); 154° (wrt orbit)
Pole ecliptic latitude	−59°
Pole ecliptic longitude	33°
Geometric albedo	0.040±0.002
Spectral type	C · D ; B–V = 0.799±0.031; V–R = 0.454±0.021
Absolute magnitude (H)	10.204±0.006

21900 Orus /ˈɔːrəs/ is a Jupiter trojan asteroid from the Greek camp, approximately 53 kilometers (33 miles) in diameter, and a target of the Lucy mission to be visited in November 2028.^[9] It is among the 100 largest Jupiter trojans and has a rotation period of 13.5 hours.^[7] 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.^[1]

Orbit and classification

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]

Lucy mission target

Orus is planned to be visited by the Lucy spacecraft which was launched in 2021. The flyby is scheduled for 11 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]

Physical characteristics

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]

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 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]

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.^[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]

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

Possible satellite

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.

References

^ ^a ^b ^c ^d ^e ^f "21900 Orus (1999 VQ10)". Minor Planet Center. Retrieved 17 June 2018.
^ Noah Webster (1884) A Practical Dictionary of the English Language
^ ^a ^b ^c "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.
^ ^a ^b "Asteroid (21900) Orus – Proper Elements". AstDyS-2, Asteroids – Dynamic Site. Retrieved 17 June 2018.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ ^j ^k Mottola, Stefano; Hellmich, Stephan; Buie, Marc W.; Zangari, Amanda M.; Stephens, Robert D.; Di Martino, Mario; et al. (January 2023). "Shape Models of Lucy Targets (3548) Eurybates and (21900) Orus from Disk-integrated Photometry". The Planetary Science Journal. 4 (1): 20. Bibcode:2023PSJ.....4...18M. doi:10.3847/PSJ/acaf79. 18.
^ ^a ^b ^c ^d ^e "LCDB Data for (21900) Orus". Asteroid Lightcurve Database (LCDB). Retrieved 17 June 2018.
^ ^a ^b 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. S2CID 53493339.
^ ^a ^b ^c ^d 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 (1964): 2025. Bibcode:2017LPI....48.2025L. Retrieved 13 April 2017.
^ 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". AAS/Division for Planetary Sciences Meeting Abstracts #48. 48. DPS meeting #48: 208.04. Bibcode:2016DPS....4820804M.
^ 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. S2CID 119101711. (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. (online, AcuA catalog p. 153)
^ "MPC/MPO/MPS Archive". Minor Planet Center. Retrieved 17 June 2018.
^ Noll, K. S. (2018), HST Proposal 15622, Space Telescope Science Institute, retrieved 7 June 2019
^ ^a ^b Noll, K. S.; Grundy, W. M.; Buie, M. W.; Levison, H. F.; Olkin, C.; Marchi, S.; Brown, M. E.; Mottola, S. (22 August 2018), 15622 – Confirmation of a Binary Companion to 21900 Orus (PDF), Space Telescope Science Institute