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For nearly three months, the spacecraft will chase each other around the moon, meticulously flying in formation. The distance between the two probes will range from 40 miles to 140 miles. Radio signals bouncing between the twins will provide their exact locations, even on the far side of the moon.

Scientists will be able to measure even the slightest variations in the gap between orbiting Grail-A and Grail-B - every single second.