Scientists hope to gain more information about the dark side of the moon, and why the side facing Earth appears flatter than the mountainous far side of the moon.
Mapping the moon's uneven gravity - about one-sixth Earth's pull - should provide a clue. The spacecraft will spend the next three months orbiting 35 miles above the lunar surface. Scientists will monitor the slight variations in distance between the two to map the moon's gravitational field. This in turn will give an idea of what lies below the surface whether it is mountain ranges, lava tubes or craters.
Scientists hope the measurements will support or debunk recent research that suggested Earth once had two moons that collided and formed the far side highlands.
The $496 million mission - officially called the Gravity Recovery And Interior Laboratory or Grail for short - was launched last September from Florida. Instead of a direct three-day flight, the probes took a roundabout journey to save money and took 30 times longer to reach their destination than the Apollo astronauts.
Beginning next week, students will also get in on the action. Select middle school students will be able to use cameras on board the probes to photograph their favorite lunar target as part of an outreach project headed by Sally Ride, the first American woman in space.
The Associated Press contributed to this report.
