GRAIL mission will map Moon's gravitational field

The Gravity Recovery and Interior Laboratory better known as GRAIL is the next lunar exploration mission. The objective of this mission is to obtain a very precise map of the Moon's gravitational field, and based on this information determine the Moon's interior structure, composition, and understand its thermal evolution. To achieve this goal two spacecraft will be placed in the same orbit around the Moon. The probes will sense the relative distance between them, while they fly over the satellite's surface. The variations in measurements will be caused by the different levels of gravity. This data will be transmitted to Earth were scientists will translate all this information into a “high-resolution map”.

The two GRAIL spacecraft will be launched together on September 8, 2001 (In case of any issue, the launch period has been established between September 8 and October 19, 2011). Once into the space the probes will be separated and take different trajectories. When the probes reach the Moon, they will be pulled gradually by the gravitational forces, and after a series of maneuvers both will be placed in the same low-altitude (55 Km), near-circular, polar lunar orbit. This procedure will take about three to four months, and the reasons for taking so long are associated with fuel saving, stability of the internal payload and better control of both spacecraft in their insertion into the lunar orbit. It's planned that the first probe (GRAIL-A) arrives on December 31, 2011 and the second one (GRAIL-B) 25 hours later on  January 1, 2011.

Once placed into the right orbit, the probes will be ready to start working. This phase is known as the science phase, and the operations will begin in March 8, 2012 lasting about 82 days. During this period of time the two spacecraft will be able to keep simultaneously both Ka-band antennas pointed to each other (used to measure the distance between them) and their solar panels facing the sun.

In the science phase, both probes will use their Lunar Gravity Ranging System (LGRS) to measure the distance between each other with a precision of down to a few microns. This information will be acquired by the Deep Space Network facilities located in GoldStone California; Madrid, Spain; and Canberra Australia. The data processing will be done (at different levels) at JPL, the Goddard Space Flight Center (GSFC) and MIT.

Additionally, each probe will carry a MoonKAM (Moon Knowledge Acquired by Middle School Students). This camera will be used for educational purposes allowing students of middle school chose which specific features of the Moon's surface they want to be photographed for future study and research. The program is led by Dr. Sally Ride, America's first woman in space, and the University of California San Diego. The MoonKAM website will show where each spacecraft will be at any given date and time, so the students will be able to choose the location they want to be photographed and submit their request. All the pictures collected will be available in the website along with the name of the school and class that request it.

Sources:

• NASA's Mission website
• MIT GRAIL project web page 
• Lockheed Martin: Gravity Recovery and Interior Laboratory 

All images in this article are property of NASA. If you want to see more visit:

• GRAIL Mission website Multimedia