9 Metis

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 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).

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 (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 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 and 60–70% Ni-Fe metal.

Light curve data on Metis led to an assumption that it could have a satellite. However, subsequent observations failed to confirm this. Later searches with the Hubble Space Telescope in 1993 found no satellites.

Mass

In 2007, Baer and Chesley estimated Metis to have a mass of 1.6-to-2.5×10¹⁹ kg. This would give this stony asteroid 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)×10¹⁹ kg. Metis appears to be more dense than most other asteroids with a diameter close to 200 km. 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, or 5,000,000 kilometres (3,100,000 mi), of Vesta on 19 August 2004.

Family relationships

Metis was once considered to be a member of an asteroid family 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 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. 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 that Kristensen used to derive an ellipsoidal profile of 210x170km. On 6 August 1989, Metis occulted a magnitude 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.

On March 7, 2014, Metis occulted the star HIP 78193 (magnitude 7.9) over parts of Europe and the Middle East.