21900 Orus is a

Jupiter trojan The Jupiter trojans, commonly called trojan asteroids or simply trojans, are a large group of asteroids that share the planet Jupiter's orbit around the Sun. Relative to Jupiter, each Trojan (celestial body), trojan Libration point orbit, librat ...

asteroid An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

from the Greek camp, approximately in diameter, and a target of the ''Lucy'' mission to be visited in November 2028. It is among the 100 largest Jupiter trojans and has a

rotation period In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...

of 13.5 hours. It was discovered on 9 November 1999, by Japanese amateur astronomer

Takao Kobayashi is a Japanese amateur astronomer and a discoverer of minor planets who currently works at the Ōizumi Observatory. The asteroid 3500 Kobayashi is named after him. Career Kobayashi has discovered more than 2000 asteroids using CCD technology ...

at his private

Ōizumi Observatory Ōizumi Observatory (List of observatory codes, obs. code: IAU code#411, 411) is a private astronomy, astronomical observatory in Ōizumi, Gunma, Ōizumi, Gunma Prefecture, Japan. Takao Kobayashi has made discoveries of numerous minor planets at ...

in Gunma Prefecture, Japan, and later named Orus after a slain Achaean warrior from the ''

Iliad The ''Iliad'' (; , ; ) is one of two major Ancient Greek epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by modern audiences. As with the ''Odyssey'', the poem is divided into 24 books and ...

''.

Orbit and classification

Orus is a dark

orbiting in the leading Greek camp at Jupiter's

Lagrangian point In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium (mechanics), equilibrium for small-mass objects under the gravity, gravitational influence of two massive orbit, orbiting b ...

, 60 ° ahead of its orbit in a 1:1 resonance . It is also a non-

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

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 In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...

of 5.13 AU). Its orbit has an

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-Centre (geometry), center, in geometry * Eccentricity (g ...

of 0.04 and an

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Eart ...

of 8 ° with respect to the

ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...

. The body's

observation arc In observational astronomy, the observation arc (or arc length) of a Solar System body is the time period between its earliest and latest observations, used for tracing the body's path. It is usually given in days or years. The term is mostly use ...

begins with a

precovery In astronomy, precovery (short for pre-discovery recovery) is the process of finding the image of a celestial object in images or photographic plates predating its discovery, typically for the purpose of calculating a more accurate orbit. This ha ...

, published by the

Digitized Sky Survey The Digitized Sky Survey (DSS) is a digital data, digitized version of several photography, photographic astronomical surveys of the night sky, produced by the Space Telescope Science Institute between 1983 and 2006. Versions and source materia ...

and taken at

Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...

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 was launched in 2021. The flyby is scheduled for 11 November 2028, and will approach the asteroid to a distance of at a relative velocity of .

Physical characteristics

Orus is characterized as a D-type and

C-type asteroid C-type (carbonaceous ) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...

by the ''Lucy'' mission team and by

Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical ...

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 In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

survey of 80 Jupiter trojans, using the 1.2-meter telescope at

Calar Alto Observatory The Calar Alto Observatory (Centro Astronómico Hispano en Andalucía or "Spanish Astronomical Centre in Andalusia") is an astronomical observatory located in Almería province in Spain on Calar Alto, a mountain in the Sierra de Los Filabre ...

. The obtained rotational lightcurve rendered a period of hours with a brightness variation of 0.18

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

(). 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 AKARI (ASTRO-F) was an infrared astronomy satellite developed by Japan Aerospace Exploration Agency, in cooperation with institutes of Europe and Korea. It was launched on 21 February 2006, at 21:28 UTC (06:28, 22 February JST) by M-V rocket ...

satellite and the

NEOWISE Wide-field Infrared Survey Explorer (WISE, observatory code C51, Explorer 92 and MIDEX-6) was a NASA infrared astronomy space telescope in the Explorers Program launched in December 2009.. . WISE discovered thousands of minor planets and nu ...

mission of NASA's

Wide-field Infrared Survey Explorer Wide-field Infrared Survey Explorer (WISE, List of observatory codes, observatory code C51, Explorer 92 and MIDEX-6) was a NASA infrared astronomy Space observatory, space telescope in the Explorers Program launched in December 2009.. . WISE L ...

, the body has an

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

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 According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...

was named from

Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of ancient Greek folklore, today absorbed alongside Roman mythology into the broader designation of classical mythology. These stories conc ...

after Orus, an Achaean warrior in Homer's ''

''. He was killed in the

Trojan War The Trojan War was a legendary conflict in Greek mythology that took place around the twelfth or thirteenth century BC. The war was waged by the Achaeans (Homer), Achaeans (Ancient Greece, Greeks) against the city of Troy after Paris (mytho ...

by the Trojan prince

Hector In Greek mythology, Hector (; , ) was a Trojan prince, a hero and the greatest warrior for Troy during the Trojan War. He is a major character in Homer's ''Iliad'', where he leads the Trojans and their allies in the defense of Troy, killing c ...

, after whom the largest Jupiter trojan

624 Hektor 624 Hektor is the largest Jupiter trojan and the namesake of the Hektor family, with a highly elongated shape equivalent in volume to a sphere of approximately 225 to 250 kilometers diameter. It was discovered on 10 February 1907, by astronom ...

is named. The approved naming citation was published by the

Minor Planet Center The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Funct ...

on 22 February 2016 ().

Possible satellite

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