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 expeditions, which may have included landings in Copernicus or Tycho craters, were canceled for multiple reasons. Tighter budgets, imposed by Congress and the Nixon administration, were a major factor. NASA's spending had peaked in the mid-1960s, at which time its labor force of staffers and contractors totaled some 400,000. In January 1970 that workforce had shrunk to 190,000, and NASA was unveiling plans to eliminate another 50,000 jobs.

But the cancellations were "not just about money," says Roger Launius, senior curator at the Smithsonian National Air and Space Museum, noting that Saturn 5 rockets and other hardware already had been purchased. A key motive, in his view, was that NASA officials "weren't sure that it was money well spent." Public interest in seeing men on the moon had declined after Apollo 11, and the mechanical travails that threatened the lives of the crew of Apollo 13 in April 1970 heightened concerns about the risks of lunar missions.

NASA managers at that time were already quite conservative, says astronaut Harrison Schmitt, who was tentatively expected to be on one of the canceled missions. "I think they had much less confidence in the Saturn 5–Apollo systems than did the people who were flying in them and operating them." Schmitt, a geologist who later became a U.S. senator, flew on Apollo 17 instead, his assignment pushed up so at least one scientist would get to explore the moon before the program ended.

The cancellations also reflected competition among NASA priorities, as orbital projects vied with the moon program for money and hardware. One consequence of curbing Apollo was freeing a heavy-lift Saturn 5 to launch the Skylab orbital station in 1973. The prospective development of a space shuttle—endorsed by a presidential task force in 1969—had begun diverting attention at the agency, as well.

Apollo's benefits to science did not ensure political and bureaucratic support. "Each mission became more scientifically productive as the program went on," says Paul Spudis, senior staff scientist at the Lunar and Planetary Institute in Houston. But "the simple fact that each mission was providing a great scientific return didn't really impress very many people other than lunar scientists."

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.

Other sites considered included the Marius Hills, Hyginus Rille and Schroter's Valley, the latter noted for its variety of geologic materials. Whereas some areas were more scientifically attractive than others, "anyplace you land on the moon and collect samples intelligently would've been valuable," says Don Wilhelms, a retired U.S. Geological Survey geologist who during Apollo was part of an interagency group vetting possible landing sites. Canceling the missions was "a missed opportunity," Wilhelms says. "You had existing technology at the peak of its effectiveness. So it was a waste."

In Schmitt's view, the greater error was terminating the Apollo program, not just its last three missions. "We never should have stopped building Saturn 5s and Apollo spacecraft," Schmitt says. "Everything that's happened since, including space stations, could have been done with that technology base. In addition, you would have continued to have the ability to reach out into deep space, a capability that included being able to divert asteroids in case one looked like it might be on a collision course with the Earth. For a brief, shining three or four years, we could do that with the Saturn 5."