Two almost identical probes are set to orbit the moon in order to measure the uneven gravity field and the composition of the natural satellite down to its core.

The Grail spacecrafts were launched from Florida coast in September and are set to reach their destination. The two spacecrafts will arrive 24 hours apart and they are found to be on their tracks.

Mission chief scientist Maria Zuber of the Massachusetts Institute of Technology said, "Both spacecraft have performed essentially flawlessly since launch, but one can never take anything for granted in this business." The probes are officially known as Grail-A and Grail-B.

The Gravity Recovery and Interior Laboratory (Grail) probes will ignite their engines to slow down so that they would enter the orbit of the moon. NASA engineers controlling the spacecraft will repeat this maneuver several times over the coming days.

Project manager David Lehman of NASA's Jet Propulsion Laboratory expressed that he would be worried as the orbits arrive on their destination on New Year's Eve. The spacecraft will will spend the next two months flying in formation until they reach about 56 kilometers above the surface with an average separation of 124 miles 200 kilometers.

The probes are the first to be fully dedicated to measure lunar gravity as several earlier crafts have only succeeded partially in the goal.