The Fanes legend has it that once every hundred years, a princess emerges from the Sass dla Porta to row around the lake beneath the full moon.

BBC: Legends of the Italian Dolomites

During that time, the GRAIL satellites will be tracking the distance between each other as the Moon passes beneath them.

FORBES: NASA's GRAIL Mission Will Unlock Secrets Of The Moon's Interior

Estimates that it could smack into the Moon caused great excitement, since previous planned impacts of rocket parts allowed Nasa to measure the shockwaves passing through the Moon and find out more about what lies beneath the surface.

BBC: NEWS | UK | Magazine | One giant (scrap)heap for mankind?

That phase will last about three months, as the Moon completes three rotations beneath the orbits of the two satellites.

FORBES: NASA's GRAIL Mission Will Unlock Secrets Of The Moon's Interior

The moon is of great interest because of hints that it may have a huge ocean of water beneath its surface.

CNN: Galileo moves on to Europa