In 2012, NASA scientists embarked on a quest to uncover the moon’s internal structure and composition. Their holy grail? A dual spacecraft mission literally called GRAIL (Gravity Recovery and Interior Laboratory), designed to make the most accurate gravitational map of the moon so far.

Using the technique pioneered by NASA’s Grace mission orbiting Earth, GRAIL mapped the moon’s gravity by measuring the push and pull between the twin spacecraft, named ‘’Ebb’ and ‘Flow’ respectively by two contest-winning fourth graders.

Ebb and Flow gathered data on the moon’s far side, even communicating with each other when Earth’s signal was obscured, breaking new ground in interplanetary teamwork. With their mission complete, Ebb and Flow bid farewell to their quest, gracefully crashing into a mountain near the lunar north pole.