Authors: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The Moon is a ball of left-over debris from a cosmic collision that took place more than four billion years ago. A Mars-sized asteroid - one of the countless planetesimals that were frantically churning our solar system into existence - hit the infant Earth, bequeathing it a very large natural satellite.

Seth Shostak

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