The most popular theory is that the Moon was torn from the Earth in a huge planetary collision about 4.5 billion years ago. An object at least as large as Mars, it is proposed, crashed into Earth, throwing vast clouds of debris into orbit. This then cooled and coalesced under the influence of gravity to form the Moon.

So far, the hypothesis is supported by data: it fits with the evidence of lunar rocks and observations from spacecraft orbiting the Moon. As the material that formed the Moon would have come from the Earth’s crust and mantle, not its iron-rich core, the relative lack of iron on the Moon is potentially explained. The data collected by Smart-1 could now supply conclusive evidence.

The site of the crash was chosen to give scientists the best possible chance of getting useful information from the spacecraft’s last moments. The Lake of Excellence is a volcanic plain. It lies just on the dark side of the Moon, where it is not illuminated by the Sun, but where some dim light is provided by “Earthshine” — light reflected on to the Moon from Earth. This gave astronomers a good chance of watching the impact and examining the light from it to provide chemical information about the debris. Had it occurred against too bright a background, the flash would have been impossible to see.

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