MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Stages in the formation of the Earth-Moon system. The Earth had recently formed (4600 million years ago) when it was struck by a large protoplanet, sometimes dubbed Theia, roughly three times the mass of Mars. Theia was completely obliterated by the oblique impact, which sent debris into orbit around Earth (2) while the cores of the two worlds merged. In time the debris coalesced to form the Moon, at a distance roughly one-tenth of where it is today. And (4), the fully-formed system.
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