1969
NASA announced that the backup crew of Apollo 14 would be Gene Cernan, Ronald Evans, and Joe Engle—putting them in line, via the Apollo crew-rotation system, to become the prime crew for Apollo 17.
September 1970
After the planned launch of Apollo 18 was canceled, NASA faced pressure from the scientific community to fly a professional geologist on a lunar landing; this ultimately drove the decision to assign Harrison Schmitt to Apollo 17.