Earth once had two moons, which merged in a slow-motion collision that took several hours to complete, researchers propose in Nature today.

Both satellites would have formed from debris that was ejected when a Mars-size protoplanet smacked into Earth late in its formation period. Whereas traditional theory states that the infant Moon rapidly swept up any rivals or gravitationally ejected them into interstellar space, the new theory suggests that one body survived, parked in a gravitationally stable point in the Earth–Moon system. Continue reading “Early Earth may have had two moons” →