16 Psyche (// SY-kee) is one of the ten most massive asteroids in the asteroid belt. It is over 200 km (120 mi) in diameter and contains a little less than 1% of the mass of the entire asteroid belt. It is thought to be the exposed iron core of a protoplanet. It is the most massive metallic M-type asteroid. Psyche was discovered by the Italian astronomer Annibale de Gasparis on 17 March 1852 from Naples and named after the Greek mythological figure Psyche.
The orbit of Psyche is near circular, between Mars and Jupiter
|Discovered by||Annibale de Gasparis|
|Discovery date||17 March 1852|
|MPC designation||(16) Psyche|
|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||279 × 232 × 189 km (±10%)|
|±11.5 km 113|
Equatorial escape velocity
±0.09 (radar) 0.37
|Tholen = M 
SMASS = X 
|9.22 to 12.19|
The first fifteen asteroids to be discovered were given symbols by astronomers as a type of shorthand notation. Psyche was given an iconic symbol, as were several other asteroids discovered over the next few years. This 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. In 1851, German astronomer J. F. Encke suggested using a circled number, and 16 Psyche was the first new asteroid to be discovered that was designated using this scheme when American astronomer James Ferguson published his observations in 1852.
Mass, size and shapeEdit
The first size estimate of Psyche came from IRAS thermal infrared emission observations. They showed that it had a diameter of about 253 km, although it was likely an overestimate as Psyche was viewed pole-on at that time. Observations of more than 100 chords in multiple stellar occultations together with radar observation as well as adaptive optics observations and light curve inversions allowed scientists to get more reliable estimates of Psyche's size and shape. According to them Psyche is likely to be an ellipsoid with dimensions 279×232×189 km (±10%) and a mean diameter of ±23 km. 226 The density of Psyche that follows from size and mass estimates mentioned above – ±1400 kg/m³ – is consistent of that of metallic object with a porosity of about 40%. 4500 This is higher than most previous estimates.
Psyche appears somewhat irregular in shape. There is a pronounced mass deficit near the equator at about 90° longitude comparable to Rheasilvia basin on Vesta. There are also two additional smaller (50–70 km in diameter) crater-like depressions near the south pole.
Composition and originEdit
Radar observations indicate that Psyche has a fairly pure iron–nickel composition, consistent with it having one of the highest radar albedos in the asteroid belt (±0.09). 0.37 Psyche seems to have a surface that is 90% metallic (iron), with small amounts – ±1% – of 6orthopyroxene.
The NASA Infrared Telescope Facility at the Mauna Kea Observatories released evidence (~3 μm absorption feature) of the presence of water or hydroxyl ions on the asteroid in October 2016. The water is believed to have reached Psyche via past impacts from smaller asteroids containing volatile substances such as carbon, hydrogen and water.
Based on its composition 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 Psyche's mantle may have been stripped away not by a single collision but by multiple (>3) relatively slow hit-and-run collisions with bodies of comparable or larger size. What is left is a metallic core covered by thin layer of silicates, which reveal 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 six 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 September 30, 2015, the Psyche orbiter mission was one of five Discovery Program semifinalist proposals.
The mission was approved by NASA on January 4, 2017 and was originally targeted to launch in October 2023, arriving at the asteroid in 2030, following an Earth gravity assist spacecraft maneuver in 2024 and a Mars flyby in 2025. In May 2017 the launch date was moved up to target a more efficient trajectory, launching in 2022 and arriving in 2026 with a Mars gravity assist in 2023.
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