16 Psyche () is a large M-type asteroid discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 and named after the Greek goddess Psyche. The prefix "16" signifies that it was the sixteenth minor planet in order of discovery. It is the largest and most massive of the M-type asteroids, and one of the dozen most massive asteroids. It has a mean diameter of approximately and contains about one percent of the mass of the asteroid belt. Historically, it was hypothesized to be the exposed core of a protoplanet, but numerous recent studies have all but ruled that out. Psyche will be explored by the spacecraft of the same name, humanity's first visit to a metallic asteroid. Launch is planned in 2023 and arrival in 2029. Symbol Astronomers created icon-like symbols for the first fifteen asteroids to be discovered, as a type of shorthand notation consistent with older notation for the classical planets. Psyche was given an iconic symbol, as were a few other asteroids discovered after 16 Psyche. The symbol was a butterfly's wing topped by a star ( or ), as the butterfly was a Greek symbol of the soul (psyche being the Greek word for "soul"). However, the iconic symbols for all asteroids were superseded before Psyche's symbol ever came into use. With more than a dozen asteroids discovered, remembering all their individual emblems became increasingly cumbersome, and in 1851, German astronomer J.F. Encke suggested using a circled number instead: . The first asteroid designated with the new scheme was Psyche, when American astronomer J. Ferguson published his observations in 1852. Characteristics Size The first size estimate of Psyche was and came from IRAS thermal infrared emission observations. This is 15% larger than the currently accepted mean value, but was later found to be an accurate estimate for the IRAS viewing aspect because Psyche was viewed pole-on at the time of the measurement. Psyche has been observed to occult a star on nine occasions. Four of these, 2004, 2010, 2014, 2019 generated multi-chord data sets and have been used along with adaptive optics imaging and three-dimensional modeling to estimate Psyche's mean diameter, with recent models all converging to an equivalent-volume mean diameter of km. Mass and bulk density Psyche is massive enough that its gravitational perturbations on other asteroids can be observed, which enables a mass measurement. Historical values for its mass have ranged