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.
(Image: © by Karl Tate, Infographics Artist)
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