Everyone knows something about the flight of Apollo 11, the first landing of human beings on the moon. However, that historic flight might not have been possible had it not been for Apollo 10, a mission that took place 50 years ago from May 18, 1969 to May 26, 1969. Apollo 10 replicated every step of a moon landing except for the final approach to touch down on the lunar surface, obtaining much-needed data that would be of use during later Apollo missions.

Apollo 10 was the final step in a meticulous, step-by-step program that flew increasingly challenging missions until NASA believed it was ready to shoot for a lunar landing. The careful yet quick approach that saw Apollo spaceflights flown every few months between October 1968 with the orbital Apollo 7 mission until the Apollo 11 flight in July 1969 proved to be highly successful.

The lunar module, called Snoopy after the imaginative and daring dog from the Peanuts comic strip, came no closer than 47,000 feet from the lunar surface. The command module was dubbed Charlie Brown after Snoopy’s hapless owner. The creator of Peanuts, Charles Shultz, drew a number of motivational posters for NASA using his characters.

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The crew of Apollo 10 were all spaceflight veterans. The commander, Thomas Stafford, had flown on Gemini 6 and Gemini 9. John Young, the command module pilot, had flown on Gemini 3 and Gemini 10. Eugene Cernan had flown on Gemini 9. Young and Cernan would go on to walk on the moon, Young would command the first space shuttle mission, and Stafford would fly on the Apollo-Soyuz Test Project.

The Apollo 10 was a near-perfect mission, with the exception of a few minor glitches. At one point, when the lunar module was still separated from the command module in lunar orbit, it began to roll unexpectedly because the crew had inputted the same commands twice into the flight computer as the ascent module separated from the descent module.

At this point, Stafford and Cernan, who were flying the spacecraft, were heard uttering some choice words not suitable for a world-wide TV and radio broadcast. Fortunately, they were able to recover control of the lunar module before the problem because uncontrollable, which would have caused the spacecraft to crash into the moon.

The mission planners felt that they had to load the lunar module fuel tanks with only enough propellant to take it from its closest approach to the moon back to the command module. The reason they did this was that if the lunar module had a full set of tanks, Stafford and Cernan, being the “right stuff” kind of astronauts they were, might have attempted an unauthorized lunar landing. Had they done so, they would not have had enough fuel to return to lunar orbit and dock with the command module.

Apollo 10 also encountered a mystery that has piqued the interest of UFO enthusiasts. While orbiting the far side of the moon, the astronauts started to hear an odd whistling sound coming out of their communications equipment that was said to be like a stereotypical “alien space music” from 50s science fiction movies. Since the moon was blocking any radio traffic from the Earth, the supposed transmission must have come from space.

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While the mission transcripts show the astronauts commenting about the noise, they downplayed it during their debriefing after returning to Earth. They possibly feared that NASA would start to question their mental health and not allow them to fly in space again. The mystery was not publicly revealed until well into the 21st century.

The noise was likely not caused by an alien spacecraft but by the action of charged particles interfering with radio communication between the command module and the lunar module, then separated. However, that mundane explanation has not impressed those people who think that the truth is out there, to borrow a phrase from “The X-Files.”

In any case, armed with the data obtained from the Apollo 10 mission, Apollo 11 blasted off less than two months later and, on July 20, 1969, landed on the lunar surface.

Mark R. Whittington is the author of space exploration studies “Why is It So Hard to Go Back to the Moon? as well as “The Moon, Mars and Beyond.”