History records that the last manned expedition to the Moon took place in December of 1972. The crew of Apollo 17, Gene Cernan, Ron Evens, and Harrison Schmitt, blasted off on December 7th. Cernan and Schmitt landed at the Taurus Littrow region of the Moon and explored it for three days. Apollo 17 returned to the Earth when it splashed down in the Pacific on December 19th. Since then, no one has been back to the Moon.
It is not generally known that three further Apollo expeditions to the Moon had been planned, with the space craft and launch vehicles built, and crews named for at least one of the missions. Partly because of budget cuts mandated by the US government at the time, these missions were cancelled.
The first mission to be cancelled was the flight of Apollo 20, on January 4th, 1970. Since Saturn V launch vehicles were no longer being manufactured, also due to budget cuts, the decision was made to use Apollo 20’s Saturn V for the planned Skylab mission.
The other two Apollo missions, Apollo 18 and Apollo 19 were cancelled on September 2nd, 1970. This decision was taken purely for budget reasons, saving perhaps a total of just over forty million dollars (circi 1970.) The hardware that was originally planned for these missions has, for the most part, become museum displays.
What if these last three missions had not been cancelled? What would they have entailed?
The original crew of Apollo 18 consisted of Richard Gordon, who had been command module pilot for Apollo 12, Vance Brand, and Harrison Schmitt. When Apollo 18 was cancelled Schmitt, the only trained geologist to fly on an Apollo mission to the Moon, was transferred to the crew of Apollo 17, bumping Joe Engel.
Apollo 18 had been planned to land at Schroter’s Valley, which resembled a river-like channel. The flight, based on the way Apollo flights were spaced out after Apollo 13, would have likely taken place in July, 1973.
Apollo 19 would have likely consisted of Fred Haise, who had flown on Apollo 13, Bill Pogue, and Gerald Carr. Pogue and Carr would eventually fly on the final Skylab mission. Apollo 19 would have landed at the Hyginus Rill region, probably in December, 1973. Copernicus Crater had also been suggested for Apollo 19.
The crew of Apollo 20 is based on speculation. In the normal course of crew rotation it would have consisted of Pete Conrad, who had already commanded Apollo 12, Paul Weitz, and Jack Lousma. Since Conrad had already walked on the Moon and his crew had in any case been transferred to the Skylab program, the crew of Apollo 20 would have consisted of Stuart Roosa, who had been command module pilot on Apollo 14, Jack Lousma, and Don Lind. Apollo 20 would have taken place in July, 1974 and would have landed either in the Marius Hills or, had some of the operational constraints been lifted, Tycho Crater.
Apollo missions beyond Apollo 19 would have depended on the Saturn V assembly line being kept open. NASA documents mention an Apollo 21 mission for planning purposes, but no crew or destination is mentioned.
NASA established an Apollo Applications Projects, which would have used Apollo and Saturn hardware for various science based missions. One plan would have used a Saturn V to land a “lunar shelter” on the Moon and a second Saturn V to land a crew to stay in that shelter for stays of up to 200 days. A modified lunar lander, designated the “lunar taxi”, would have landed all three crew members of an Apollo. Other proposals ranged from a space station launched by a number of Saturn Vs to a Venus flyby. The only actual Apollo Applications project that flew was Skylab, which had three crews between May 1973 and early 1974.
Apollo missions that never flew have been depicted in fiction. James Michener’s novel Space features an Apollo 18 mission to the lunar farside. Shane Johnson’s novel Ice depicts Apollo 19 landing on the lunar South Pole. Mark R. Whittington’s Children of Apollo relates a whole series of Apollo expeditions beyond Apollo 17, exploring such regions of the Moon as Copernicus Crater, Tycho Crater, the lunar farside, and the lunar South Pole.