This artist's impression illustrates the collision theory. At the dawn of the Solar System a heavenly body the size of Mars probably hit the young Earth. As a result, material from the rocky mantle of the heavenly body and of the Earth was hurled into space. This collected in the shape of a ring on a path closely orbiting the Earth, where it then gradually 'clumped together' to form the Moon.
Credit: NASA/Joe Tucciarone.