abstract:Several planned missions of the Apollo manned Moon landing program of the 1960s and 1970s were canceled for a variety of reasons, including changes in technical direction, the Apollo 1 fire, hardware delays, and budget limitations. Immediately after the program achieved its primary objective in July 1969 with Apollo 11 to land men on the Moon and return them safely, nine more landing missions were planned, but the last three, Apollo 18, 19 and 20, were cancelled.