NASA's twin spacecraft orbit the moon to study its gravitational field and internal structure.
Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's gravity field, as depicted in this artist's rendering. (NASA)
A NASA handout image shows an artist's concept of the GRAIL mission's twin spacecraft in orbit around the moon. (Reuters/NASA)
Artist's concept of GRAIL-B performing its lunar orbit insertion burn. (NASA)
The September 10 launch of the United Launch Alliance Delta II Heavy rocket carrying NASA's twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft. (NASA)