The GRAIL-B spacecraft (Gravity Recovery and Interior Laboratory), named "Flow", was one of two spacecraft launched as part of a NASA Discovery mission on 10 September 2011. The goal of the GRAIL mission was to map the Moon's gravity field in unprecedented detail in order to understand the structure of the lunar interior, place limits on the size of the Moon's core, and determine the subsurface structure of the lunar crust. Following a primary science mission and a subsequent extended mission, Flow was intentionally guided to an impact at the Sally K. Ride Impact Site on 17 December 2012. The crater resulting from the GRAIL B impact is 6 meters in diameter.
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