What is most widely accepted today is the giant-impact theory. It proposes that the Moon formed during a collision between the Earth and another small planet, about the size of Mars. The debris from this impact collected in an orbit around Earth to form the Moon.
The Apollo missions brought back over a third of a tonne of rock and soil from the Moon.
'When the Apollo rocks came back, they showed that the Earth and the Moon have some remarkable chemical and isotopic similarities, suggesting that they have a linked history,' says Sara.
'If the Moon had been created elsewhere and was captured by the Earth's gravity we would expect its composition to be very different from the Earth's.
'If the Moon was created at the same time, or broke off the Earth, then we would expect the type and proportion of minerals on the Moon to be the same as on Earth. But they are slightly different.'
The minerals on the Moon contain less water than similar terrestrial rocks. The Moon is rich in material that forms quickly at a high temperature.
'In the seventies and eighties there was a lot of debate which led to an almost universal acceptance of the giant impact model.'
Lunar meteorites are also an important source of data for studying the origins of the Moon.
'In some ways meteorites can tell us more about the Moon than Apollo samples because meteorites come from all over the surface of the Moon,' adds Sara, 'while Apollo samples come from just one place near the equator on the near side of the Moon.'