Why was the Apollo program cancelled?
Short answer: Mostly money but also safety concerns.
Long answer: The original plan for the Apollo program was to have ten missions land on the Moon: Apollo 11 to 20. Earlier Apollo missions were for testing and preparation. Apollo 11 was the successful first landing, Apollo 12 also went well, Apollo 13 failed to land on the Moon, and the next four missions up to Apollo 17 were all successes. Apollo 18, 19 and 20 were cancelled. After that, NASA went in a different direction and stopped trying to get people back to the Moon.
So what happened?
The standard answer is that the last three Apollo missions were cancelled for budgetary reasons. That's NASA's official position:
According to some reports, however, the initial reason for cancelling missions was President Nixon's concern about safety. Apollo 13 nearly killed its astronauts and this worried Nixon greatly. Nixon had made a big effort to associate himself with the success of the first Moon landing. It has been documented that he felt a very close connection to the astronauts, as well as fear for how any tragedy would affect his re-election chances .
On the other hand, Apollo 13 happened in April 1970 but the first cancellation (Apollo 20) came earlier in January 1970. This contradicts the claim that Apollo 13 was the catalyst. Still, there is solid evidence that Nixon wanted to avoid unnecessary risk with the Moon landings.
NASA did agree to at least some of the Apollo cancellations in order to divert money to the new Space Shuttle program. I don't know whether NASA leadership genuinely agreed to the cancellations or whether they were simply being pragmatic.
In the end it seems that a combination of President Nixon's fear and budgetary concerns brought about Apollo's premature demise.
Read more: Why hasn't NASA ever returned to the Moon?
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2. Logsdon, John. After Apollo: Richard Nixon and the American Space Program. Palgrave Macmillan, 2015.
Author: Dave Owen.