Apollo 10 (May 18–26, 1969) was the fourth human spaceflight in the United States' Apollo program and the second to orbit the Moon. NASA, the mission's operator, described it as a "dress rehearsal" for the first Moon landing (Apollo 11, two months later). It was designated an "F" mission, intended to test all spacecraft components and procedures short of actual descent and landing. After the spacecraft reached lunar orbit, astronaut John Young remained in the Command and Service Module (CSM) while astronauts Thomas Stafford and Gene Cernan flew the Apollo Lunar Module (LM) to within 14.4 kilometers (7.8 nautical miles; 9 miles) of the lunar surface, the point at which powered descent for landing would begin on a landing mission.

Wikipedia: https://en.wikipedia.org/wiki/Apollo_10. Related NASA reference: https://nssdc.gsfc.nasa.gov/planetary/chronology.html#1969. Description adapted from Wikipedia contributors (CC BY-SA 4.0; https://creativecommons.org/licenses/by-sa/4.0/).