Discovered in 1852, the main-belt asteroid Psyche is unique in that it is believed to be the exposed core of an ancient protoplanet that never completely formed, or whose rocky outer layers were destroyed. It is the most massive known M-type (metallic) asteroid, with a composition of 90-percent iron and nickel, and 10-percent silicate rock — similar to Earth's core.
The Earth's nickel-iron core is a dynamo that creates a massive magnetic field around the planet. Scientists believe Psyche also generates a magnetic field, perhaps nearly as strong as that of Earth.
Psyche is an irregularly-shaped asteroid measuring about 150 miles (240 kilometers) along its largest side. This makes it the 10th or 11th largest known asteroid (sizes are not known precisely, preventing exact rankings). The largest asteroids, including Ceres and Vesta, are protoplanets – survivors from the early formation of the solar system.
Psyche orbits the sun at a distance of between 3.3 and 2.5 astronomical units (AU). One AU equals the distance from the Earth to the sun, about 93 million miles (150 million km).