We're going back several billion years here. The planet Earth had already started to form, when it was hit by a gigantic protoplanet roughly three times the mass of present-day Mars. As a result, debris was blown into Earth orbit. Some of the debris started to rain back down on the planet, but much of it coalesced in orbit to form the Moon. This image shows the Moon just after it had formed as viewed from the surface of the newly reconstructed Earth. Fire still rains from the sky – the early Solar System was a much more violent place than it is now.