The twin spacecraft took off from Cape Canaveral Air Force Station in Florida on a three-month journey to the Moon at 9:08 am (1308 GMT) aboard a Delta II rocket.
"Liftoff of the Delta II with GRAIL, on a journey to the center of the moon," NASA commentator George Diller said upon blast-off of the GRAIL mission, which stands for Gravity Recovery and Interior Laboratory.
High upper level winds delayed the first launch attempt on Thursday, and also briefly set back Saturday's launch.
The duo will travel to the Moon for more than three months, arriving into a polar lunar orbit one after the other around New Year's Day. With one spacecraft trailing the other, the plan is for the two to use gravity tools to map the terrain beneath, revealing the contents of the inner core of the Moon, about which little is known.
The mission should also shed light on the unexplored far side of the Moon, and perhaps tell scientists whether there was once a second Moon that fused with ours.
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