GRAIL starts mapping lunar gravity
Sen— NASA’s dual Gravity Recovery And Interior Laboratory (GRAIL) spacecraft began their scientific investigation of the Moon at 01:15 GMT on 7 March.
This phase of the mission will last 84 days, and after mapping the Moon three times a high resolution image of the Moon’s gravitational field will be produced. This will allow scientists to gain a better understanding of the internal structure of our natural satellite.
The science phase will see the two craft, named Ebb and Flow, transmit radio signals to each other to monitor the distance between the two. This distance changes slightly with varying amounts of mass on the Moon below, such as when the craft pass over mountains or craters.
This allows a picture of the interior to be built up, along with the strength of the lunar gravity. GRAIL will perform other interesting measurements such as using the nine centimetre tidal bulges in the lunar rocks, caused by the Earth, to peer into the lunar interior and see if the Moon has a solid core.
GRAIL will also be able to measure the thickness of the Moon’s crust, which will give insight into the thermal history of the Moon, as the crust thickness depends on exactly how the Moon cooled after it formed.
"The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do," said GRAIL principal investigator Maria Zuber, "but it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."
GRAIL’s height above the lunar surface will vary between 16 and 51 kilometres, depending on the exact type of gravity mapping being performed.
GRAIL was launched on 10 September 2011 and the two spacecraft, Ebb and Flow, reached lunar orbit on 31 December 2011 and 1 January 2012. The lengthy journey allowed the craft to save fuel as they decelerated gently on arrival.
Since entering the Moon's orbit, Ebb and Flow have gradually changed from an elliptical orbit to a circular one that is more useful for the scientific investigations.
You can read Sen’s report about GRAIL’s arrival at the Moon here.