Apollo 21

Apollo 21 was the fourth of five unofficial Apollo missions following Apollo 17, itself a successor to Apollo 11, which landed on the Moon on 20 July 1969. These unofficial missions ended with Apollo 22 in June 1980. (PROSE: Apollo 23)

Behind the scenes

 * In reality, Apollo 17 was the highest-number Apollo mission launched, with 18, 19, and 20 planned but cancelled. A hypothetical "Apollo 21" lunar landing mission has been used in other works of fiction.