An artist’s depiction of the twin spacecraft that comprise NASA’s Gravity Recovery And Interior Laboratory (GRAIL) mission. During the GRAIL mission’s science phase, spacecraft (Ebb and Flow) transmit radio signals precisely defining the distance between them as they orbit the moon in formation. As the two fly over areas of greater and lesser gravity caused by both visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly. Mission scientists use this information to create a high-resolution map of the moon’s gravitational field. The data will allow scientists to understand what goes on below the lunar surface.
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