Apollo 10 was the fourth manned mission in the United States Apollo space program. It was an F type mission—its purpose was to be a dress rehearsal for the Apollo 11 mission, testing all of the procedures and components of a Moon landing without actually landing on the Moon itself. The mission included the second crew to orbit the Moon and an all-up test of the Lunar Module (LM) in lunar orbit. The LM came to within 8.4 nautical miles (15.6 km)[2] of the lunar surface during practice maneuvers.
Apollo 10 was the first of only two Apollo missions with an entirely flight-experienced crew. Thomas P. Stafford had flown on Gemini 6 and Gemini 9; John W. Young had flown on Gemini 3 and Gemini 10, and Eugene A. Cernan had flown with Stafford on Gemini 9. In addition, Apollo 10 marked the only Saturn V flight from Launch Complex 39B, as preparations for Apollo 11 at LC-39A had begun in March almost immediately after Apollo 9's launch.