Mission to solve Moon’s mysteries

NASA’s GRAIL mission is set to begin its journey to the Moon to map the lunar gravity and reveal some of its secrets of formation.

How GRAIL spaceprobes will work
The two GRAIL spacecraft can use gravity fluctuations to map the lunar interior. Changes in the distance of the craft are measured with radio signals, which are caused by different concentrations of mass beneath them. (Credit: NASA/JPL)

The Gravity Recovery And Interior Laboratory (GRAIL) was due to launch on Thursday, September 8, but was postponed due to high upper-level winds at Cape Canaveral. It is currently due to launch at either 12:29 or 13:08 UT today. Update: GRAIL launched successfully at its second opportunity today.

GRAIL consists of twin spacecraft that should arrive at the Moon at the end of the year. The journey takes much longer than the three days of the Apollo missions. This is so that it will arrive at a much more leisurely pace than the Apollo missions and it thus needs less propellant to slow down and enter orbit around the Moon. GRAIL-A will arrive about 25 hours before GRAIL-B before they enter into the same orbit. GRAIL-A will trail GRAIL-B as they circle around our nearest terrestrial neighbour. They will spend 82 days collecting data in the science phase of the mission and then end their lunar adventure with a bang as they crash into the surface of the Moon.

GRACE map of Earth
A gravity map of the Earth, taken by the satellite GRACE. GRAIL will generate a similar map of the Moon. (Credit: NASA/University of Texas Center for Space Research)

GRAIL was inspired by the GRACE satellite, which was launched into orbit around the Earth in 2002 to obtain gravity maps of our planet. The GRAIL duo will do what a single satellite cannot easily do. The distance between the two probes will change slightly depending on the mass beneath them. This distance can be measured and transformed into a map of the gravitational field of the Moon. When they are flying close together and close to the Moon, they can map features on the surface such as mountains and craters. When flying further apart and at a greater distance from the Moon, they are sensitive to gravitational influences from the core and mantle.

GRAIL will dismantle some of the mysteries surrounding the Moon’s formation by collecting data on the thermal evolution. Terrestrial bodies like the Earth and Moon begin life as molten ball of rock which gradually cools over time. The core, mantle and crust are formed as the body cools. The thickness of the crust is dependent on the thermal evolution and GRAIL will provide important information about the thickness and strength of the crust.

GRAIL will investigate the Moon’s core by measuring tides. The Earth creates tides on the Moon, which cause 9 centimetre bulges in the rock. These deformations go right to the core as tides are raised. A varying gravitational field due to the Earth’s pull will allow GRAIL to map the interior of the Moon. This will enable scientists to find out if the Moon has a solid core, and if so how big it is.

Impacts on the Moon from the past 3.2 billion years have resulted in craters which have less gravity than the surrounding area. However older craters have the same gravity as the surrounding plane. Why is there such a difference? It is hoped that GRAIL will obtain some clues to help answer this question. Another mystery to be solved is why the lunar crust is thinner on the side of the Moon that is facing us than the side we can’t see. And a further lunar oddity is the increased concentrations of mass over some impact basins, which will be investigated by GRAIL.

By the time GRAIL ends its mission in May 2012 it will hopefully have made some important contributions to our understanding of the Moon.

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By Amanda Doyle

I am an astrophysics postdoctoral research assistant at the University of Warwick. I obtained my PhD from Keele University in 2014 and my thesis title was "Spectral analyses of solar-like stars". My research involves refining stellar parameters with the aim of improving our understanding of both stars and planets. I completed my masters in astronomy at Swinburne University of Technology via the Swinburne Astronomy Online programme in 2010, and I obtained my degree in physics with astronomy from Dublin City University in 2008. When I'm not doing research, I like to write about all aspects of astronomy. I am a freelance science writer and I contribute to Astronomy Now, NASA's Astrobiology Magazine, BBC Sky at Night magazine, Skymania News, and Sen. I am also the editor of Popular Astronomy magazine.

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