|Discovered by||Annibale de Gasparis|
|Discovery date||17 March 1852|
|Epoch JD 2453300.5 (22 October 2004)|
|Aphelion||3.328 AU (497.884 Gm)|
|Perihelion||2.513 AU (375.958 Gm)|
|2.921 AU (436.921 Gm)|
|4.99 yr (1823.115 d)|
|Dimensions||(278±5 × 232±6 × 164±4) km|
[best ellipsoid fit = 277 km × 238 km × 168 km]
279 × 232 × 189 km ( ± 10% )
|Volume||5.8×106 km3 (best fit)|
|~180 m/s (~600 ft/s)|
|Tholen = M |
SMASS = X 
Bus-DeMeo = Xk 
|9.22 to 12.19|
16 Psyche (//) is a large asteroid discovered by the Italian astronomer Annibale de Gasparis, working in Naples, on 17 March 1852 and named after the Greek mythological figure Psyche. It is one of the dozen most massive asteroids, containing about one percent of the mass of the asteroid belt, and is over 200 kilometres (120 mi) in diameter. Psyche is thought to be the exposed core of a protoplanet, and is the most massive of the metal-rich M-type asteroids. Its composition and density match mesosiderite meteorites and it is likely their parent body. Psyche is scheduled for space exploration, targeted for January 2026. The prefix "16" signifies that it was the sixteenth minor planet in order of discovery.
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 , a semicircle topped by a star, represents a butterfly's wing, symbol of the soul (psyche is the Greek word for "soul"), and a star.
However the iconic symbols for all asteroids were superseded and Psyche's symbol never came into use. With more than a dozen asteroids discovered, remembering all their individual emblems became increasingly unwieldy, and in 1851, German astronomer J.F. Encke suggested using a circled number instead: ⑯. The first new asteroid that was designated in 1852 using this new scheme was 16 Psyche, when American astronomer James Ferguson published his observations.
Psyche is massive enough that its gravitational perturbations on other asteroids can be observed, which enables a mass measurement. The values for the mass of (3.38±0.28)×10−11 M☉ and the density of 6.98±0.58 g/cm3 obtained from a 2002 analysis by Kuzmanoski and Kovačević, of a close encounter with asteroid (13206) 1997 GC22. The new, high density estimate suggests that 16 Psyche must be composed mostly of metals. As of 2019, the best mass estimate is (2.41±0.32)×1019 kg, with a derived bulk density of 3.99±0.26 g/cm3.
The first size estimate of Psyche came from IRAS thermal infrared emission observations. They showed that it had a diameter of about 253 kilometres (157 mi), although it was likely an overestimate as Psyche was viewed pole-on at that time.
Light curve analysis indicates Psyche appears somewhat irregular in shape. There is a pronounced mass deficit near the equator at about 90° longitude comparable to Rheasilvia basin on 4 Vesta. There are also two additional smaller (50–70 km in diameter) crater-like depressions near the south pole. Psyche's north pole points towards the ecliptic coordinates β = 28°, λ = −6°, with a 4° uncertainty. This gives an axial tilt of 95°.
Observations of two multi-chord stellar occultations of 2010 and 2014 allow the matching of light curve inversions DAMIT model 1806 that give an equivalent-volume mean diameter of 216±12 km, and an equivalent surface mean diameter of 227±13 km. The density of Psyche derived from these estimates, 3.7±0.6 g/cm3, is consistent with that of other metallic asteroids.
Observations of Psyche with Very Large Telescope's adaptive optics SPHERE imager revealed two large craters, on the order of 90 km across, which were provisionally named Meroe // and Panthia //, after the twin witches in the Roman novel Metamorphoses by Apuleius.
Observations indicate that Psyche has a metal-pyroxene composition, consistent with it having one of the brightest radar albedos in the asteroid belt (0.37±0.09). Its density, 4.0±0.3 g/cm3, is compatible with mesosiderite meteorites (≈ 4.25 g/cm3) and the Steinbach meteorite (≈ 4.1 g/cm3).
The NASA Infrared Telescope Facility at the Mauna Kea Observatories reported evidence (~3 μm absorption feature) of hydroxyl ions on the asteroid in October 2016 that may suggest water ice. Since Psyche is thought to have formed under dry conditions without the presence of water, the hydroxyl may have reached Psyche via past impacts from smaller carbonaceous asteroids.
Psyche appears to be an exposed metallic core or a fragment of a metallic core from a larger differentiated parent body some 500 kilometers in diameter. If Psyche is indeed one, there could be other asteroids on similar orbits. However, Psyche is not part of any identified asteroid family. One hypothesis is that the collision that formed Psyche 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. However, this scenario is considered to have a probability of just 1%. An alternative is that Psyche was broken by impacts, but not catastrophically torn apart. In this case, it may be a candidate for the parent body of the mesosiderites, a class of stony–iron meteorites.
Another possibility is that Psyche may be an endmember of diverse relic bodies left by the inner planet formation. The asteroid's mantle may have been stripped away not by a single collision but by multiple (more than three) relatively slow sideswipe collisions with bodies of comparable or larger size. What is left is a metallic core covered by a thin layer of silicates, which reveals itself spectrally. In such a case, Psyche would be analogous to Mercury but much less massive.
No spacecraft has visited Psyche, but in 2014 a mission to Psyche was proposed to NASA. A team led by Lindy Elkins-Tanton, the director of the School for Earth and Space Exploration at Arizona State University, presented a concept for a robotic Psyche orbiter. This team argued that 16 Psyche would be a valuable object for study because it is the only metallic core-like body discovered so far.
The spacecraft would orbit Psyche for 20 months, studying its topography, surface features, gravity, magnetism, and other characteristics and would be based on current technology, avoiding high cost and the necessity to develop new technologies. On 30 September 2015, the Psyche orbiter mission was one of five Discovery Program semifinalist proposals.
The mission was approved by NASA on 4 January 2017 and was originally targeted to launch in October 2023, with an Earth gravity assist maneuver in 2024, a Mars flyby in 2025, and arriving at the asteroid in 2030. In May 2017, the launch date was moved up to target a more efficient trajectory, launching in 2022, with a Mars gravity assist in 2023 and arriving in 2026.
[in a footnote] Herr Professor de Gasparis schreibt mir, in Bezug auf den von ihm März 17 entdeckten neuen Planeten: J'ai proposé, avec l'approbation de Mr. Hind, le nom de Psyché pour la nouvelle planète, ayant pour symbole une aile de papillon surmontée d'une étoile.
the first successful attempt based on a dynamical method