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For the moon of Jupiter, see Metis (moon).

9 Metis  

Discovery
Discovered by:	A. Graham
Discovery date:	April 25, 1848
Orbital characteristics
Epoch July 14, 2004 (JD 2453200.5)
Aphelion	400.548 Gm (2.678 AU)
Perihelion:	313.556 Gm (2.096 AU)
Semi-major axis:	357.052 Gm (2.387 AU)
Eccentricity:	0.122
Orbital period:	1346.815 d (3.69 a)
Avg. orbital speed:	19.21 km/s
Mean anomaly:	274.183°
Inclination:	5.576°
Longitude of ascending node:	68.982°
Argument of perihelion:	5.489°
Physical characteristics
Dimensions:	235×195×140 km[1][2]
Mass:	~9×1018 kg
Mean density:	~2.7 g/cm³[3]
Equatorial surface gravity:	~0.070 m/s²
Escape velocity:	~0.11 km/s
Rotation period:	0.2116 d (5.078 h)[4]
Albedo:	0.243 (geometric)[7]
Temperature:	~173 K max: 282 K (+9° C)[8]
Spectral type:	S-type[5]
Apparent magnitude:	8.1[6] to 11.83
Absolute magnitude:	6.28
Angular diameter:	0.23" to 0.071"

9 Metis (pronounced^[help] /ˈmiːtɪs/, Greek: Μήτις) is one of the largest 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.^[9]

The first 10 asteroids profiled against Earth's Moon. 9 Metis is second from right.

Metis was discovered by Andrew Graham on April 25, 1848; it was his only asteroid discovery.^[10] It is also the only asteroid to have been discovered as a result of observations from Ireland. Its name comes from the mythological Metis, a Titaness and Oceanid, daughter of Tethys and Oceanus.^[11] The name Thetis was also considered and rejected (it would later devolve to 17 Thetis).

Metis' direction of rotation is unknown at present, due to ambiguous data. Lightcurve analysis indicates that Metis' pole points towards either ecliptic coordinates (β, λ) = (23°, 181°) or (9°, 359°) with a 10° uncertainty.^[1] 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.^{[citation needed]}

Hubble space telescope images^[2][12] and lightcurve analyses^[1] are in agreement that Metis has an irregular elongated shape with one pointed and one broad end.^[1][12] Radar observations suggest the presence of a significant flat area,^[13] in agreement with the shape model from lightcurves.

The surface composition has been estimated as 30-40% metal-bearing olivine and 60-70% Ni-Fe metal.^[9]

Light curve data on Metis led to an assumption that it could have a satellite. However, subsequent observations failed to confirm this.^[14][15] Later searches with the Hubble Space Telescope in 1993 found no satellites.^[12]

Metis was once considered to be a member of an asteroid family known as the Metis family,^[16] 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.^[9] Coincidentally, both Metis and Amalthea have namesakes among Jupiter's inner moons.

Metis has been observed to occult stars no less than 5 times.^[17]

• shape model deduced from lightcurve
• "Notice of discovery of Metis", MNRAS 8 (1848) 146
• Irish Astronomical History: Markree Castle Observatory and The Discovery of the Asteroid Metis
• Yeomans, Donald K.. Horizons system. NASA JPL. Retrieved on 2007-03-20. — Horizons can be used to obtain a current ephemeris.