The animation below shows the final flight path for NASA's twin Gravity Recovery and Interior Laboratory (GRAIL) spacecraft, which will make a planned impact on the north pole area of the moon today, at about 2:28 Pacific Standard Time.  

The two probes, named Ebb and Flo by schoolchildren in Montana, are making this planned crash because their residual fuel levels and low orbit preclude further scientific experiments.

This inclination by geeky NASA engineers to crash their soon to be useless probes into the lunar ground must derive out of some universal tendency to use something worn out to "go out in style," as it were.

I recall in high school there was a guy who was going to junk his junker car, but decided to crash it first, to see what it was like.  The result, after driving the car into a cut bank, was decidedly dramatic, and never to be forgotten. Same with little old Ebb and Flow.  I'm sure they will have the onboard cameras (if there are any) filming all the way to impact.  The geeks back at mission control will be talking about it for years.