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Main belt asteroid

Metis (minor planet designation: 9 Metis) is one of the larger main belt asteroids. It is composed of silicates and metallic nickel iron, 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 passed within 0.034 AU, or , of Vesta on 19 August 2004.

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. The name Thetis was also considered and rejected (it would later devolve to 17 Thetis). The historical symbol for Metis was an eye with a star above it. It is in the pipeline for Unicode 17.0 as U+1CEC3 𜻃 .

Metis' direction of rotation is unknown at present, due to ambiguous data. Lightcurve analysis indicates that the Metidian pole points towards either ecliptic coordinates (β, λ) = (23°, 181°) or (9°, 359°) with a 10° uncertainty. The equivalent equatorial coordinates are (α, δ) = (12.7 h, 21°) or (23.7 h, 8°). This gives an axial tilt of 72° or 76°, respectively. Hubble Space Telescope images and lightcurve analyses in agreement with the shape model from lightcurves. The Metidian surface composition has been estimated as 30–40% metal bearing olivine and 60–70% Ni Fe metal. Later searches with the Hubble Space Telescope in 1993 found no satellites. 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, 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. On 6 August 1989, Metis occulted a magnitude 8.7 star producing five chords suggesting a diameter of 173.5 km. All three of these occultations fit the ellipsoid 222×182×130 km suggested by Baer.