Roger Hanson: The scientific importance of the world's most valuable asteroid
In 1992 NASA decided that resources should be directed towards lower cost but high scientific value, unmanned missions exploring the Solar System.
To date several of these have been implemented including the Stardust mission which passed through the tail of a comet and Mars Pathfinder which landed a robotic vehicle on Mars.
Earlier this year, NASA approved a project proposed by Lindy Elkins-Tanton of the University of Arizona – it is a mission to visit an asteroid, 16 Psyche, (pronounced Sy-kee).
16 Psyche was discovered by Italian astronomer Annibale de Gasparis in 1852 and is located in the Asteroid Belt.
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The Asteroid Belt is a pancake shaped disc comprising millions of rocks orbiting the Sun between the orbits of Mars and Jupiter. Psyche was the 16th asteroid to be discovered and is named after a character in Greek mythology.
The difference between a planet and an asteroid is that a planet can be broadly defined as an object which orbits the Sun and has a sufficiently strong gravitational field to compress its atoms tight enough to form a sphere.
An asteroid also orbits the Sun but is a much smaller rocky, irregularly shaped, inactive body, not massive enough to form the distinctive sphere of a planet.
The Solar System formed about 4.6 billion years ago with the gravitational collapse of a giant cloud of gas and dust, most of which collapsed to form the Sun.
The balance of the material remained in a flattened disc with the Sun at its centre. The planets formed in this disc by a process called accretion in which the gas and dust particles slowly clumped, drawn together by their gravitational interaction.
Some of the rocks larger than about 200 metres across, collided to form progressively larger objects until, over many millions of years the planets gradually emerged.
Mars is only 10 per cent of the mass of the Earth but Jupiter which is 318 times more massive than Earth has a powerful gravitational field.
During the formation of the Solar System, the dust particles in the region between the orbits of Mars and Jupiter were tugged and pulled by the disruptive gravities of Jupiter and Mars.
These particles couldn't clump beyond the stage of large rocks because of these perturbation forces.
Of the millions of asteroids in the Asteroid Belt about 200 managed to grow larger than 100kms across. 16 Psyche is one of these.
It is the densest known asteroid and is about 200kms across.
Asteroids and planets, during their formation, were subjected to heat from three sources: the Sun's radiation, heat generated by their gravitational compression and most importantly, from heat due to the radioactive decay of some of the chemical elements present.
The ensuing high temperatures partially melted the contents, forming magma.
As a result, heavier, denser materials sank to the core of the embryo planet or asteroidand the less dense constituents rose to the surface.
16 Psyche is extremely unusual in that it is made almost entirely of dense material, specifically iron and nickel. It is thought this asteroid is the naked core of a previously larger object.
16 Psyche was probably about 500kms across but became involved in a high speed collision with another orbiting object.
The violent collision stripped the outer layers leaving only the core. The mission to 16 Psyche is due to be launched in 2022, arriving there in 2026 and will spend two years orbiting the asteroid.
The spacecraft will be crammed with instruments designed to investigate the detailed composition and the electromagnetic and gravitational fields of the asteroid.
It is hoped that this extremely rare view of the core of a rocky object will provide new insights and scientific information about the early Solar System and about the cores of asteroids and some planets, including the Earth.
Little is known about the Earth's inaccessible core other than it is probably made of iron and nickel.
Lucy Watkins-Tanton points out that being a giant lump of iron and nickel, 16 Psyche is worth at least 10,000 quadrillion US dollars – many times the value of the world economy – if only it could be hauled back to Earth.