Canceled Apollo missions

To mark the upcoming 40th anniversary of Apollo 11, I wrote a piece for Scientific American about the Apollo missions that never flew. Excerpt:

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.

Whole thing here: "Down to Earth: The Apollo Moon Missions That Never Were."