Nobody knows exactly how the Moon formed. The most common theory is that shortly after Earth formed; it was hit by an object the size of Mars. The impact was so powerful that it sent billions of tonnes of molten material into space. This debris was held in orbit around Earth, and eventually solidified to form the Moon.
Analysis of samples brought back from the NASA Apollo missions suggest that the Earth and Moon are a result of a giant impact between an early proto-planet and an astronomical body called Theia.
‘There used to be a number of theories about how the Moon was made and it was one of the aims of the Apollo program to figure out how we got to have our Moon,’ says Sara. Prior to the Apollo mission research there were three theories about how the Moon formed. Capture theory suggests that the Moon was a wandering body (like an asteroid) that formed elsewhere in the solar system and was captured by Earth’s gravity as it passed nearby. In contrast, accretion theory suggested that the Moon was created along with Earth at its formation. Finally, according to the fission scenario, Earth had been spinning so fast that some material broke away and began to orbit the planet.
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.