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NASA latest Moon mission GRAIL (The Gravity Recovery and Interior Laboratory) has successfully entered lunar orbit. The mission to map the Moons gravity in unprecedented detail consists of two small identical space probes each weighing only 132kg. After a journey to the Moon lasting nearly four months, the US space agency announced on the 1st January 2012 that after a series of braking manoeuvers both spacecraft were successfully placed in preliminary orbits around the Earths only natural satellite.

Artist concept of GRAIL mission (NASA/JPL-Caltech)

Grail-A the first of the two spacecraft entered lunar orbit on the 31st December 2011, followed the next day by Grail-B. The insertion manoeuvers placed the spacecraft into a near-polar, highly elliptical orbit of 90 x 8,363 kilometres (56 x 5,197 miles), with a period of 11.5 hours. The mission schedule is to reduce the orbits of both spacecraft over the next few months until they are in a near circular orbit only 50 kilometres (31 miles) above the lunar surface. The orbital period will then be less than 2 hours with the spacecraft separation by only 200 kilometres.

At this stage the science part of the mission can begin. As Grail-A orbits the moon it will experience small accelerations and decelerations as it flies through the uneven gravity field. With Grail-B following close behind, the changes in velocity of Grail-A can be determined by accurately measuring the distance between the two probes. Scientists can then translate this information into a high-resolution map of the moon's gravitational field from which they hope to better understand what goes on below the lunar surface.

NASA Administrator Charles Bolden said, "NASA greets the new year with a new mission of exploration. The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet. We begin this year reminding people around the world that NASA does big, bold things in order to reach for new heights and reveal the unknown."

Further reading

NASA launches GRAIL Moon mission twin probes