NASA's Twin Spacecraft Orbit the Moon

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)

Another view of GRAIL's launch. (NASA)

Technicians prepare to hoist one of the two GRAIL spacecraft upon completion of a thermal vacuum test at the Lockheed Martin Space Systems facility in Denver. (NASA)

Technicians lift one of two spacecraft for NASA's GRAIL to a test stand in the Astrotech payload processing facility in Titusville, Florida. (NASA)

GRAIL spacecraft logo emblazoned on the first stage of a United Launch Alliance Delta II launch vehicle. (NASA)