Lunar orbiters reveal a world shaped by impacts

Ars Technica » Scientific Method 2012-12-05

If you're spending the money to put something in orbit around the Moon, you'll likely stuff it full of as many scientific instruments as it can fit. But now NASA has sent not one, but two spacecraft to the Moon with only a single scientific instrument. Despite the light load, the GRAIL mission turns out to have been a phenomenal success, as it's providing the clearest map of the composition of our closest neighbor. The results show a world shaped by a combination of impacts (some of which probably penetrated to the Moon's mantle) and the stretch marks of an early expansion in its size.

The GRAIL mission was modeled on NASA's earlier GRACE satellites, which were used to study the Earth. The satellite's lone instrument keeps track of the distance between the orbiting pair, which changes due to the gravitational attraction caused the terrain below. Thanks to the Moon's lack of significant atmosphere and small gravitational draw, the twin GRAIL probes were able to orbit at an average of 55km above the surface, providing details nearly three times better than any previous mapping effort.

The initial mapping mission started in March 2011 and ran for a bit over a year. It was able to map differences with a resolution of about 13km, and it obtained more than 99.99 percent of all the data it possibly could, given the resolution of its instruments. This is likely to be the clearest picture we have of the Moon for a while.

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