The idea of a moon-forming collision is not new: The Giant Impact Theory put forth in the 1970s suggested that the moon resulted from a collision with a protoplanet approximately half the size of ancient Earth. But the physics underlying such a collision implied that the moon should be made up of debris mostly from the protoplanet. Since then we've discovered the moon is instead very chemically similar to Earth. Now, scientists have come up with two new models that explain how an impact could have resulted in a moon formed from Earth material.