On December 17th 2012 two small spacecraft called Ebb and Flow punched into the lunar surface at over 3,700 miles an hour.

This ended the year long mission of NASA's Gravity Recovery and Interior Laboratory (GRAIL). The twin spacecraft spent most of this time orbiting the Moon's surface at a scarily low altitude of about 31 miles, sweeping in tandem above the dusty terrain never more than 140 miles apart from each other.

Gravity map of the Moon (Credit: NASA/JPL-Caltech/MIT/GSFC)

Microwave telemetry between the spacecraft, the Earth, and the application of basic geometry let GRAIL monitor the distance between Ebb and Flow to a precision of about a tenth of a micron - half the width of a human hair.

As with any planet or satellite the Moon's gravitational field is not perfectly symmetrical. Variations in the density and height of material produce tiny variations in the gravitational acceleration felt by other objects. By sensing Ebb and Flow's varying movement in orbit a detailed map of the lunar gravity field was constructed. With a knowledge of the topographic features on the surface this can be turned into the equivalent of a medical tomographic reconstruction of the lunar interior - and it's lumps and bumps.

The data is amazing, but GRAIL had one last gift to give. In the days leading up to their crash on the lunar surface the spacecraft returned imagery from their ever lowering orbits.

This is the quite surreal and beautiful timelapse footage taken by Ebb as it skimmed across part of the northern terrain of the Moon's far side at an altitude of only 6 miles on December 14th 2012. Enjoy.