“When Apollo 11 landed on the moon, NASA’s plan was to continue manned lunar missions through Apollo 20. But history turned out differently. The last three missions, still in planning stages, were canceled. Hardware that would have flown to the moon ended up as museum exhibits. And scientists and space enthusiasts were left to contemplate what Apollos 18 through 20 might have accomplished.

“On January 4, 1970, less than six months after Neil Armstrong and Buzz Aldrin left humanity’s first lunar footprints, NASA announced Apollo 20’s cancellation. Eight months later, the agency announced the scrapping of Apollo 19 as well as the original mission slated for Apollo 15 (Apollo 16 was renumbered 15, thereby giving the remaining two missions numbers 16 and 17).

“The three missions were canceled two to three years before they would have flown, so plans were still fluid as to their landing sites, crew assignments and other features. Similar to Apollos 15 through 17, but aimed at more scientifically rewarding, albeit riskier, landing sites, they likely would have been what NASA called “J” missions, involving three-day stays on the moon and the use of rovers to expand the scope of exploration. Such missions allowed broader sampling than the earlier “H” missions. (Apollo 11 alone was a “G” mission, focused primarily on landing and return.)

“Various possible landing sites were discussed in early planning. Among these were Copernicus, Gassendi and Tycho, large impact craters containing central peaks that were thrust upward at the time of impact, bringing material from deep within the lunar crust to the surface. Such craters provide a record of the solar system’s early history; a similar record on Earth has long since been obscured by plate tectonics, erosion and other processes. “The moon,” Schmitt says, “is where we’re going to get the information ultimately on what kind of environment existed on Earth at a time when the precursors to life were actually forming.”

“For both Apollo 17 and the canceled missions, Schmitt pressed NASA officials to consider a particularly ambitious objective: the Tsiolkovsky crater, located on the moon’s far side. “None of the Apollo missions were planned to land on the far side, and that is an awfully large area to leave unexplored,” Schmitt says. His proposal, perceived as too costly and risky, made little headway. Among its requirements would have been placing a communication satellite beyond the moon to maintain a radio link with Earth…”

Kenneth Sibler, Down to Earth: The Apollo Moon Missions That Never Were, Scientific American, July 16th, 2009.