Feature Planetary Science

Competing ideas abound for how Earth got its moon

Earth's satellite may have formed from one giant impact or from about 20 small ones

the moon

CRASH TEST   There is some debate about just what kind of collision formed the moon. 

NASA

By Thomas Sumner

The moon’s origin story does not add up. Most scientists think that the moon formed in the earliest days of the solar system, around 4.5 billion years ago, when a Mars-sized protoplanet called Theia whacked into the young Earth. The collision sent debris from both worlds hurling into orbit, where the rubble eventually mingled and combined to form the moon.