NASA has put two new satellites into orbit around the moon this weekend to map its gravity.
The first Grail-A craft fired its main engine late on Saturday to slow itself sufficiently to take up an elliptical path around the lunar body.
The BBC reports the Grail-B will attempt the same manoeuvre on Sunday.
Scientists expect the data collected to reveal what exactly lies beneath the moon's surface, shedding new light on how the lunar body formed and how it has changed over time.
Probes the size of washing machines will measure tiny differences in gravity, caused by the uneven way the massive rock is distributed.
The BBC reports the pair were launched from Cape Canaveral, Florida, last September and took a long spiral route to their destination.
This weekend's approach to the Moon was designed to bring them in over the south pole.