• Twin NASA spacecraft are set to blast into lunar orbit at the weekend on a mission to study the subterranean structure of the Moon in order to better understand the origins of planets.

  • The first Gravity Recovery and Interior Laboratory (GRAIL-A) will begin orbiting the Moon at 2121 GMT on December 31, followed by GRAIL-B on January 1 at around 2205 GMT, NASA said in a statement.

  • "This mission will rewrite the textbooks on the evolution of the Moon,

  • The $500 million pair of washing machine-sized satellites were launched on September 10 on a mission to map the Moon's inner core for the first time.

  • Beginning in March 2012, the two unmanned spacecraft will send radio signals that allow scientists to create a high-resolution map of the Moon's gravitational field, helping them to better understand its sub-surface features and the origins of other bodies in the solar system.

  • The mission should shed light on the unexplored far side of the Moon and test a hypothesis that there was once a second Moon that fused with Earth's Moon.

  • The two spacecraft have covered more than 2.5 million miles (4 million kilometers) since they were launched in September, according to NASA.

  • GRAIL-A was 65,860 miles (106,000 kilometers) from the Moon. GRAIL-B was 79,540 miles (128,000 kilometers) away.

  • Scientists believe that the Moon was formed when a planet-sized object crashed into the Earth, throwing off material that eventually became Earth's airless, desolate satellite.