9 Metis
Overview
 
9 Metis is one of the larger main-belt asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s. It is composed of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

lic nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, 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.
Metis was discovered by Andrew Graham on 25 April 1848, at Markree Observatory
Markree Observatory
-History:In 1830, Col. Edward Joshua Cooper MP eldest son of Edward Synge Cooper MP, and Ann, daughter of Henry Vansittart, Governor of Bengal, set up Markree Observatory on the grounds of Markree Castle....

 in Ireland; it was his only asteroid discovery.
Encyclopedia
9 Metis is one of the larger main-belt asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...

s. It is composed of silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...

s and metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

lic nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

, 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
Markree Observatory
-History:In 1830, Col. Edward Joshua Cooper MP eldest son of Edward Synge Cooper MP, and Ann, daughter of Henry Vansittart, Governor of Bengal, set up Markree Observatory on the grounds of Markree Castle....

 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 2008 TM9. Its name comes from the mythological Metis
Metis (mythology)
In Greek mythology, Metis was of the Titan generation and, like several primordial figures, an Oceanid, in the sense that Metis was born of Oceanus and Tethys, of an earlier age than Zeus and his siblings...

, a Titaness
Titan (mythology)
In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age....

 and Oceanid
Oceanid
In Greek mythology and, later, Roman mythology, the Oceanids were the three thousand daughters of the Titans Oceanus and Tethys. Each was the patroness of a particular spring, river, sea, lake, pond, pasture, flower or cloud...

, daughter of Tethys
Tethys (mythology)
In Greek mythology, Tethys , daughter of Uranus and Gaia was an archaic Titaness and aquatic sea goddess, invoked in classical Greek poetry but not venerated in cult. Tethys was both sister and wife of Oceanus...

 and Oceanus
Oceanus
Oceanus ; , Ōkeanós) was a pseudo-geographical feature in classical antiquity, believed by the ancient Greeks and Romans to be the world-ocean, an enormous river encircling the world....

. The name Thetis was also considered and rejected (it would later devolve to 17 Thetis
17 Thetis
17 Thetis is a large main-belt asteroid. It is an S-type asteroid, therefore giving it a relatively bright silicate surface.It was discovered by R. Luther on April 17, 1852. It was his first asteroid discovery. Its name comes from Thetis, the mother of Achilles in Greek mythology.One Thetidian...

).

Characteristics

Metis' direction of rotation is unknown at present, due to ambiguous data. Lightcurve analysis indicates that the Metidian pole points towards either ecliptic coordinates
Ecliptic coordinate system
The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the celestial sphere over the course of a year. It is also the intersection of the Earth's orbital plane and the celestial...

 (β, λ) = (23°, 181°) or (9°, 359°) with a 10° uncertainty. The equivalent equatorial coordinates
Equatorial coordinate system
The equatorial coordinate system is a widely-used method of mapping celestial objects. It functions by projecting the Earth's geographic poles and equator onto the celestial sphere. The projection of the Earth's equator onto the celestial sphere is called the celestial equator...

 are (α, δ) = (12.7 h, 21°) or (23.7 h, 8°). This gives an axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 of 72° or 76°, respectively.

Hubble space telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 images 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, in agreement with the shape model from lightcurves.

The Metidian surface composition has been estimated as 30-40% metal-bearing olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 and 60-70% Ni
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

-Fe
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 metal.

Light curve data on Metis led to an assumption that it could have a satellite
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....

. However, subsequent observations failed to confirm this. Later searches with the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...

 in 1993 found no satellites.

Mass

In 2007, Baer and Chesley estimated Metis to have a mass of 1.6-to-2.5 kg. This would give this stony asteroid
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...

 a density of about 6 (3.3 to 8.9) g/cm³. A more recent estimate by Baer suggests it has a mass of (1.47±0.20) kg. Metis appears to be more dense than most other asteroids with a diameter close to 200km. This may support the theory that Metis is the core remnant of a large evolved asteroid for which 90% of the original mass has been lost.

Metis passed within 0.034AU (5 million kilometres) of Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

 on 19 August 2004.

Family relationships

Metis was once considered to be a member of an asteroid family
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...

 known as the Metis family, 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
Proper orbital elements
The proper orbital elements of an orbit are constants of motion of an object in space that remain practically unchanged over an astronomically long timescale...

 diagrams.

However, a spectroscopic analysis found strong spectral similarities between Metis and 113 Amalthea
113 Amalthea
113 Amalthea is a fairly typical rocky main-belt asteroid orbiting in the inner regions of the belt. It was discovered by R. Luther on March 12, 1871....

, 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
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....

-sized) and differentiated
Planetary differentiation
In planetary science, planetary differentiation is the process of separating out different constituents of a planetary body as a consequence of their physical or chemical behaviour, where the body develops into compositionally distinct layers; the denser materials of a planet sink to the center,...

. Metis would be the relatively intact core remnant, and Amalthea a fragment of the mantle. Coincidentally, both Metis and Amalthea have namesakes among Jupiter's inner moons.

Occultations

In 1984 an occultation of a star produced seven chords
Chord (astronomy)
In the field of astronomy the term chord typically refers to a line crossing an object which is formed during an occultation event. By taking accurate measurements of the start and end times of the event, in conjunction with the known location of the observer and the object's orbit, the length of...

 that Kristensen used to derive an ellipsoidal profile of 210x170km. On 6 August 1989, Metis occulted a magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...

 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. All three of these occultations fit the ellipsoid 222×182×130 km suggested by Baer.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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