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Several missions of the Apollo program were canceled during the 1970s, due to budget limitations or schedule constraints. The most notable of these were three Moon landing missions, Apollos 18, 19 and 20, which had received some level of planning, but there were a variety of other, later planned flights. Some of these were incorporated into the Apollo Applications Program , of which the only result was the Skylab space station.
In August 1971, President Nixon had even planned to cancel all remaining lunar landings (Apollo 16 and 17 ). His OMB Deputy Director Caspar Weinberger was opposed to this, persuading Nixon to keep the remaining Moon missions, but recommended that if such cancellation would happen that it be "on the ground that Apollo 15 was so successful in gathering needed data that we can now shift, sooner than previously expected, to the Space Shuttle , Grand Tour, NERVA, etc."