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Theia
Theia spacepedia.png
Theia

Date of Discovery

wasn't discovered, was hypothesised

Theia is a hypothesized ancient planetary-mass object in the early Solar System that, according to the giant impact hypothesis, collided with another planetary-mass object, Gaia (the early Earth) around 4.5 billion years ago. According to the hypothesis, Theia was an Earth trojan about the size of Mars, with diameter of about 6,102 km (3,792 miles). Geologist Edward Young of the University of California, Los Angeles, drawing on an analysis of rocks collected by Apollo missions 12, 15, and 17, proposes that Theia collided head-on with Earth, in contrast to the previous theory that suggested a glancing impact. Models of the impact indicate that Theia's debris gathered around Earth to form the early Moon.

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