WASHINGTON (AFP) - Nasa on Saturday launched a pair of unmanned spacecraft on a journey to study the core of the Moon and hopefully reveal how it formed some 4.5 billion years ago. The twin GRAIL satellites, short for Gravity Recovery And Interior Laboratory, blasted off from Cape Canaveral Air Force Station in Florida aboard a Delta II heavy rocket at 9:08 am (1308 GMT). Liftoff of the Delta II with GRAIL, on a journey to the centre of the moon, NASA commentator George Diller said. The $500-million washing-machine-sized satellites will travel to the Moon for more than three months, arriving into a polar lunar orbit one after the other around New Years Day. With one spacecraft trailing the other, the plan is 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.