flown to the Moon's orbit without an LM, was considered a "C-prime" mission, but its success gave NASA the confidence to skip the "E" mission, which would have tested the full Apollo spacecraft in medium or high Earth orbit. Apollo 10, the dress rehearsal for the lunar landing, was to be the "F" mission.

NASA considered skipping the "F" mission as well and attempting the first lunar landing on Apollo 10. Some within the agency advocated for this, feeling it was senseless to bring astronauts so close to the lunar surface, only to turn