On December 17, Ebb and Flow, the two spacecraft of NASA’s GRAIL (Gravity Recovery and Interior Laboratory) mission impacted the Moon at a speed of 3,760 miles per hour. This ended a nine month mission to study the Moon’s gravitational field.
When spacecraft first orbited the Moon, it was discovered that they would dip slightly when passing over certain areas. Scientists concluded that there was significantly more mass concentrated in these areas than in the rest of the Moon’s crust. They were given the name “mascons,” – short for mass concentrations. The five largest mascons coincide with dark lava filled craters or lunar “maria” on the Moon’s near side. Mascons have not been found on Earth.
What is the nature and origin of the mascons? The GRAIL mission was designed to help answer these questions by making a very precise gravity map of the Moon that will help scientists probe its underground structure. As the two washing-machine-sized spacecraft flew over regions of greater and lesser gravity, each spacecraft’s speed and the distance between them would change slightly. Instruments on board the spacecraft were able to detect changes in distance of a few thousandths of a millimeter. Now, scientists will transform this data collected over many orbits into a high-resolution map of the Moon’s gravitational field.
Also on board each spacecraft was a set of cameras dubbed MoonKAM (Moon Knowledge Acquired by Middle School Students). MoonKAM acquired more than 115,000 images of the lunar surface that were proposed by middle school students. The camera project was led by Sally Ride Science, founded by Dr. Sally Ride, America’s first woman in space, who died in July. NASA has named the site where twin gravity probes hit the Moon in honor of Sally Ride.