This List of Artificial Objects on the Moon lists 21 objects as "intentionally crashed". Because of the moon's lumpy gravity due to sub-surface mass concentrations most low lunar orbits will be fairly unstable and sooner or later intersect the surface (i.e. crash).
It looks like the detailed gravity distribuion, and identification of "safe" orbits wasn't worked out until 2001. Further, before the Apollo landings, I think there were no seismometers on the moon. Crashes are potential sources of seismic signals.
So were all 21 of these objects suddenly maneuvered intentionally out of orbit in a way that targets the surface, or were some intentionally put into orbits that basically didn't stand a chance of lasting and the idea was to let them hit sooner or later?
For Group A (below) why were those intentionally crashed? What was the point or motivation behind the intent?
- Group A: Before lunar seismometers
- Broup B: After seismometers, before full mascon-awareness
- Broup C: After full mascon-awareness