Gravity Recovery and Interior Laboratory (GRAIL) consist of two small spacecrafts (GRAIL A [Ebb] and B [Flow]). It was launched on September 10, 2011. It used high-quality gravitational field mapping of the Moon to determine its interior structure. On August 31, 2012, the two spacecrafts started collecting data at a lower altitude.
urn:nasa:pds:context:investigation:mission.gravity_recovery_and_interior_laboratory
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