Mapping the gravitational field of the moon is no small feat. Both crafts had to approach the moon at speeds that were only a fraction of that used in the Apollo missions. Grail A and B cruised above the lunar surface at a distance of 50 km while maintaining a tightly controlled 175-225 km spacing between one another. Over the course of several passes the two craft were able to detect gravitational variations on the lunar surface as they drifted incrementally closer to or further from one another.
Although mapping the gravitational variance across the lunar surface was one of the GRAIL mission’s chief responsibilities, it also had other goals. According to Wikipedia these other goals were:
· Map the structure of the lunar crust and lithosphere
· Understand the asymmetric thermal evolution of the Moon
· Determine the subsurface structure of impact basins and the origin of lunar mascons
· Ascertain the temporal evolution of crustal brecciation and magmatism
· Place limits on the size of the Moon's inner core
While the two spacecraft have performed admirably, today will mark the end of their mission… But oh what an end it will be.
But why would NASA want to crash these twin spacecraft?
Aside from the fact that they were running out of fuel, NASA has a vested interest in preserving lunar heritage sites (Apollo landers, etc.) that dot the moon’s surface. Scientists also think that the impact could tell them a little bit more about what lies just beneath the lunar crust. However, due to the crafts size, scientists are doubtful they’ll discover much.
Watch a Video of the Grail Mission’s Crash Trajectory
Images and Video Courtesy of NASA and Wikipedia