On Oct. 4, 1957, a primitive, 100-pound metal ball was sent hurtling into orbit around Earth, beeping back radio signals like taunts.

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The spacecraft's data indicates the asteroid has a very primitive surface - rather like the chondritic meteorites that are sometimes picked up on Earth and which are assumed to have come from bodies that experienced no melting or differentiation.

BBC: A fleeting encounter with an 'inbetweener'