GRAIL’s two probes, named Ebb and Flow by schoolchildren in a NASA competition, were launched in September 2011 (see ‘Twins to Probe Moon’s Heart’). The first probe began orbiting the Moon on 31 December 2011, with the second joining the next day. By March, they had begun detailed mapping. The two spacecraft exchange radio signals, recording fluctuations in their relative positions that are then used to reveal tiny accelerations and decelerations caused by variations in the Moon’s gravitational field. The average altitude of the primary mission was 55 kilometres — much lower than the orbit used by the Gravity Recovery and Climate Experiment (GRACE), a similar gravity-mapping mission for Earth that has to fly higher to avoid atmospheric friction. Occasionally, the GRAIL operations team brought the craft lower than 20 kilometres to further improve the resolution of the data. “Nothing beats flying low,” says Zuber.