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The next launch opportunities at Florida's Cape Canaveral will open Friday at 8:33 am (1233 GMT) or 9:12 am (1312 GMT), the US space agency said.
Scientists hope the satellites will help solve mysteries about how the Moon formed, how the unexplored far side differs from the near side which humans have walked on, and whether there was once another Moon that melded with ours.
Known as GRAIL (Gravity Recovery And Interior Laboratory), the twin satellites would arrive in the Moon's polar orbit around the start of the New Year and circle it for about three months, using gravity mapping to reveal what lies inside.
Copyright AFP (Agence France-Presse), 2011