16 Psyche (pronounced /ˈsaɪkiː/ SYE-kee, or as in Greek: Ψυχή) is one of the ten most massive main-belt asteroids. It is over 200 kilometers in diameter and contains a little less than 1% of the mass of the entire main asteroid belt. It is the most massive of the metallic M-type asteroids.
Psyche was discovered by Annibale de Gasparis on March 17, 1852 from Naples and named after the Greek mythological figure Psyche. The first fifteen asteroids to be discovered were given symbols by astronomers as a type of short-hand notation. In 1851, however, J. F. Encke suggested using a circled number. 16 Psyche was the first new asteroid to be discovered that was designated with this scheme (in 1852 by J. Ferguson).
Radar observations indicate a fairly pure iron-nickel composition. Psyche appears to be a genuine case of an exposed metallic core from a larger differentiated parent body. Unlike some other M-type asteroids, Psyche shows no sign of the presence of water or water-bearing minerals on its surface, consistent with its interpretation as a metallic body. Small amounts of pyroxene appear to be present.
If Psyche is the core remnant of a larger parent body, we might expect other asteroids on similar orbits. Psyche does not belong to any asteroid family. One hypothesis is that the collision occurred very early in the solar system's history, and all the other remnants have since been ground into fragments by subsequent collisions or had their orbits perturbed beyond recognition.
Psyche is massive enough that its perturbations on other asteroids can be measured, which enables a mass measurement. IRAS data shows Psyche to have a diameter of 253 km. Observations of an occultation using five chords suggest an outline of 214×181 km. Recent estimates of Psyche's smaller size has resulted in an increase in the estimated density which is more appropriate for a metallic asteroid. Psyche appears to have a fairly regular surface and is approximately ellipsoidal in shape. Recent lightcurve analysis indicates that its pole points towards either ecliptic coordinates (β, λ) = (-9°, 35°) or (β, λ) = (-2°, 215°) with a 10° uncertainty. This gives an axial tilt of 95°.
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