9 Metis is one of the larger main-belt

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 ...

s. It is composed of

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

s and

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

, and may be the core remnant of a large asteroid that was destroyed by an ancient collision. Metis is estimated to contain just under half a percent of the total mass of the asteroid belt.

Discovery and naming

Metis was discovered by Andrew Graham on 25 April 1848, at Markree Observatory in Ireland; it was his only asteroid discovery. It also has been the only asteroid to have been discovered as a result of observations from Ireland until 7 October 2008, when, 160 years later, Dave McDonald from observatory J65 discovered (281507) 2008 TM₉. Its name comes from the mythological Metis, a Titaness and Oceanid, daughter of Tethys and

Oceanus In Greek mythology, Oceanus ( ; , also , , or ) was a Titans, Titan son of Uranus (mythology), Uranus and Gaia, the husband of his sister the Titan Tethys (mythology), Tethys, and the father of the River gods (Greek mythology), river gods ...

. The name ''Thetis'' was also considered and rejected (it would later devolve to

17 Thetis 17 Thetis is a stony asteroid from the inner regions of the asteroid belt, approximately 90 kilometers in diameter. It was discovered on 17 April 1852, by German astronomer Robert Luther at Bilk Observatory in Düsseldorf, Germany who deferred ...

). The historical symbol for Metis was an eye with a star above it. It is in the pipeline for

Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...

17.0 as U+1CEC3 𜻃 ().

Characteristics

Lightcurve-based 3D-model of Metis Metis's direction of rotation is unknown at present, due to ambiguous data. Lightcurve analysis indicates that the Metidian pole points towards either

ecliptic coordinates In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small So ...

(β, λ) = (23°, 181°) or (9°, 359°) with a 10° uncertainty.J. Torppa et al.
''Shapes and rotational properties of thirty asteroids from photometric data''
Icarus Vol. 164, p. 346 (2003). The equivalent equatorial coordinates are (α, δ) = (12.7 h, 21°) or (23.7 h, 8°). This gives an

axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...

of 72° or 76°, respectively.

Hubble Space Telescope The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the Orbiting Solar Observatory, first space telescope, but it is one of the largest and most ...

imagesA. D. Storrs et al., ''A closer look at main-belt asteroids 1: WF/PC images'', Icarus Vol. 173, p. 409 (2005).Hubble Space Telescope observations
and lightcurve analyses are in agreement that Metis has an irregular elongated shape with one pointed and one broad end. Radar observations suggest the presence of a significant flat area,D. L. Mitchell et al., ''Radar Observations of Asteroids 7 Iris, 9 Metis, 12 Victoria, 216 Kleopatra, and 654 Zelinda'', Icarus Vol. 118, p. 105 (1995). in agreement with the shape model from lightcurves. The Metidian surface composition has been estimated as 30–40% metal-bearing

olivine The mineral olivine () is a magnesium iron Silicate minerals, silicate with the chemical formula . It is a type of Nesosilicates, nesosilicate or orthosilicate. The primary component of the Earth's upper mantle (Earth), upper mantle, it is a com ...

and 60–70% Ni- Fe metal.

Light curve 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 ...

data on Metis led to an assumption that it could have a

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

. However, subsequent observations failed to confirm this. Later searches with the

in 1993 found no satellites.

Family relationships

Metis was once considered to be a member of an

asteroid family An asteroid family is a population of asteroids that share similar proper orbital elements, such as semimajor axis, eccentricity, and orbital inclination. The members of the families are thought to be fragments of past asteroid collisions. An ...

known as the Metis family,J. G. Williams, ''Asteroid Families – An Initial Search'', Icarus Vol. 96, p. 251 (1992). but more recent searches for prominent families did not recognize any such group, nor is a clump evident in the vicinity of Metis by visual inspection of proper orbital element diagrams. However, a spectroscopic analysis found strong spectral similarities between Metis and

113 Amalthea 113 Amalthea () is a stony Florian asteroid from the inner regions of the asteroid belt, approximately in diameter. It was discovered on 12 March 1871, by German astronomer Robert Luther at the Bilk Observatory in Düsseldorf, Germany. The elo ...

, and it is suggested that these asteroids may be remnants of a very old (at least ~1 Ga) dynamical family whose smaller members have been pulverised by collisions or perturbed away from the vicinity. The putative parent body is estimated to have been 300 to 600 km in diameter ( Vesta-sized) and differentiated. Metis would be the relatively intact core remnant (though smaller than

16 Psyche 16 Psyche ( ) is a large M-type asteroid, which was discovered by the Italian astronomer Annibale de Gasparis, on 17 March 1852 and named after the Greek goddess Psyche. The prefix "16" signifies that it was the sixteenth minor plane ...

), and Amalthea a fragment of the mantle, with 90% of the original body unaccounted for. Coincidentally, both Metis and Amalthea have namesakes among Jupiter's inner moons.

Occultations

In 1984 an occultation of a star produced seven

chords Chord or chords may refer to: Art and music * Chord (music), an aggregate of musical pitches sounded simultaneously ** Guitar chord, a chord played on a guitar, which has a particular tuning * The Chords (British band), 1970s British mod ...

that Kristensen used to derive an ellipsoidal profile of 210×170 km. On 6 August 1989, Metis occulted a

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 ...

8.7 star producing five chords suggesting a diameter of 173.5 km. Observations of an occultation on 11 February 2006, produced only two chords indicating a minimum diameter 156 km.(Chords)
All three of these occultations fit the ellipsoid 222×182×130 km suggested by Baer. On 7 March 2014, Metis occulted the star HIP 78193 (

7.9) over parts of Europe and the Middle East.Map

Notes

References

External links

shape model deduced from lightcurve

* ttp://ssd.jpl.nasa.gov/horizons.cgi?find_body=1&body_group=sb&sstr=9 JPL Ephemeris* (displays Elong from Sun and V mag for 2011)
Globe of 9 Metis
* * {{DEFAULTSORT:000009 Flora asteroids Metis Metis S-type asteroids (Tholen) 18480425