Earth was smashed by a rock the size of Mars to make the moon

4.5 billion years ago, a rock called Theia crashed into Earth and formed the moon. Now we know that it was probably only about a tenth the mass of our planet

A little rock can pack a mighty punch. The object that smashed into Earth 4.5 billion years ago to create the moon was relatively small – roughly one-tenth the mass of Earth, according to the latest modelling.

Since the 1970s, astronomers have suspected that the moon was created when a giant protoplanet called Theia struck the newly formed Earth. The collision created a cloud of debris, which quickly coalesced into our planet’s partner. There’s just one nagging problem: this idea, known as the giant-impact hypothesis, can’t explain why the moon and Earth are chemically identical.

Later, two different hypotheses arose that could explain why the moon is Earth’s chemical clone, but they predict radically different masses for Theia. In one scenario, two half-Earths merged to form the Earth-moon system. But the second hypothesis suggests Theia was a small, high-velocity projectile that