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Twin GRAIL Probes To Map Lunar Gravity Field
smitty777 writes "The two washing machine sized satellites from the GRAIL program (Gravity Recovery and Interior Laboratory) launched in September are set to enter lunar orbit this weekend. As can be seen from this nifty infographic, the probes will monitor the gravitational field from orbit via the precise distance measurements of microwaves passing between the two satellites. From the article: 'The twin spacecrafts are named Grail-A and Grail-B. Grail-A will enter the moon's orbit on New Year's Eve, Grail-B will follow on New Year's Day. "The purpose of the GRAIL mission is to obtain gravity data on the Moon. And with that data, the scientists are able to determine the structure of the lunar interior, from crust to core," said David Lehman of NASA's Jet Propulsion Laboratory.' This is similar to the earlier GRACE project, which not only helped map out the gravity field of the Earth, but also helped map drought conditions in the U.S."
The SMART-1 probe launched by ESA took about 13 months to reach Lunar orbit, propelled by a small ion engine and solar panels. Even then it started from a geostationary transfer orbit as a Getaway Special piggybacked on the commercial launch of two communications satellites via an Ariane 5.
Believe it or not, this long route (via the Earth-Sun L-1 (Lagrange-1) point is a lower energy trajectory (that is, it takes less energy to send the probes) than the more direct route followed by the Apollo missions. This allows for a smaller launcher (or conversely, a larger payload for the same sized launcher). See http://www.nasa.gov/pdf/582116main_GRAIL_launch_press_kit.pdf.