A tiny second moon may once have orbited Earth before catastrophically slamming into the other one, a titanic clash that could explain why the two sides of the surviving lunar satellite are so different from each other, a new study suggests.
The second moon around Earth would have been about 750 miles (1,200 kilometers) wide and could have formed from the same collision between the planet and a Mars-sized object that scientists suspect helped create the moon we see in the sky today, astronomers said.
The gravitational tug of war between the Earth and moon slowed the rate at which it whirls, such that it now always shows just one side to Earth. The far side of the moon remained a mystery for centuries until 1959, when the Soviet Luna 3 spacecraft first snapped photos of it. (The far side is sometimes erroneously called the dark side, even though it has days and nights just like the near side.)