Is the moon made out of green cheese? What's more, how can we possibly figure out what the moon looks like inside? We have plenty of pretty pictures of the outside of the moon. We also know roughly what the Earth is like inside. It has a central core of metal -- a combination consisting mostly of iron (~80%), and small amounts of nickel. The very central core is solid because the pressure is so high, and surrounding this is molten magma. Above that is rock made mostly out of silicon, magnesium and oxygen. Is the internal structure of the moon similar to this?
On New Year's Eve and New Year's Day, two NASA satellites (called GRAIL), designed to answer this exact question, reached the moon. Launched last September, these twin satellites are roughly the size of washing machines and weigh slightly less than 300 pounds each. They have since very slowly made their way toward the moon, circling closer and closer. Now they will begin to orbit about it separated by roughly 100 miles, making one complete pass nearly every two hours.
Both satellites are equipped with a system which can tell them how far apart they are from each other and how far from the earth. Electromagnetic waves are shot between the ships and the planet. This method is accurate to 100th the width of a human hair (one micrometer)!
Since Newton's work, we know that the pull of gravity between two objects is proportional to the mass of each. That is, the gravity pulling them together is stronger when they weigh more. If one satellite is pulled slightly closer to the surface at a certain place, you know that the moon below weighs more. By watching how strongly it is deflected and using this to figure out how heavy the moon is beneath, you can make a good guess of what it is made of.
The result of this work will be an extremely accurate map of the internal structure of the moon. It can also tell us about areas that are particularly heavy in certain elements and give a glimpse at the history of our natural satellite. This is something that you can never see by eye!
A final cool thing about this mission: middle school students will be able to watch live videos of the moon from the 'MoonKAM' located aboard each craft. This will mark the first time that NASA has included instruments on a mission solely to broadcast educational information back.
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