Two spacecraft from Nasa have begun mapping the moon's gravity to study the moon's interior and thermal history.

The near-twins, Gravity Recovery and Interior Laboratory (GRAIL) spacecraft, GRAIL-A and GRAIL-B, began their probe on Tuesday to find out why the moon's side that always faces Earth appears flatter than the mountainous far side, reported the Associated Press.

One specific spacecraft will follow the other as they circle the moon and scientists will monitor the slight variations in distance between the two to map the moon's gravitational field.

"Literally and figuratively, I'm over the moon," said Maria Zuber, mission chief scientist at the Massachusetts Institute of Technology in an email to the AP.

The two GRAIL spacecraft, which were renamed as Ebb and Flow, were launched together on Sept. 10, 2011.

The first Nasa Grail spacecraft entered the moon's orbit on 31 December, 2011 and the second spacecraft reunited with the first one in the lunar orbit on 1 January, 2012.

The GRAIL mission will be conducting six science investigations.

The mission will also help to understand the development of the other planetary bodies in the inner solar system including Mercury, Venus, Earth, and Mars as the moon offers a unique look into the distant past of planetary evolution, according to Nasa

Data from the spacecraft are expected once they move their positions just 35 miles from the moon's surface.