On May 22, 1969, almost four days and six hours after leaving the Earth, the crew of Apollo 10 was enjoying a delightfully uneventful mission. Rather, it was as uneventful as a mission to the moon could be.

Commander Tom Stafford and Lunar Module Pilot Gene Cernan had just returned from their close pass by the lunar surface and were readying to go through the staging maneuver that would bring them into the correct lunar orbit to rejoin Command Module Pilot John Young waiting in the Command-Service module. On schedule, the LM’s ascent engine fired.

Then all hell broke loose.

The crew saw the lunar horizon swivel past their window half a dozen times as Cernan yelled out “Son of a bitch!” Apollo 10’s lunar module, with two astronauts on board, was careening out of control a quarter of a million miles from home.

Apollo 10 marked NASA’s last step before going for the full lunar landing with Apollo 11. To that point, the space agency’s approach to landing on the moon had been incremental. Apollo 7 had tested the command-service module (CSM) in Earth orbit in October of 1968. Two months later, Apollo 8 had taken that same spacecraft for a test flight to the moon, ensuring it would be able enter and leave lunar orbit without any problems. In March of 1969, Apollo 9 was the first to take the full Apollo stack for a test drive, flying both the CSM and the lunar module (LM) on a simulated lunar landing mission in the relative safety of Earth orbit.

Apollo 10’s mission plan was in effect a full dress rehearsal of a lunar landing that would stop just short of the surface. This would give NASA a final check that the CSM and LM could fly properly in lunar orbit. The lunar lander, later nicknamed Snoopy, would descend almost to the moon's surface and then reascend and re-dock with the command module.

There was some concerns that the irregular gravitational environment around the moon from mass concentrations—so-called masscons—would throw off the spacecraft’s trajectories. It would also be a test of the communications systems, both between the two spacecraft and the spacecraft and Earth, at lunar distances. And it would give NASA a chance to take close-up images of the proposed landing site for Apollo 11.

All in all, Apollo 10’s mission was a tall order that wouldn’t bring the cachet of a lunar landing.

It took the crew three days to reach the moon and get into lunar orbit where they went through all the phases leading up to a lunar landing. First, Stafford and Cernan in the LM Snoopy separated from the command module, nicknamed Charlie Brown. Then they used the larger descent engine to lower Snoopy’s orbit, stopping less than 50,000 feet above the surface. From that altitude they saw the moon’s mountains and craters in more detail than any human had before.

Having demonstrated the LM’s descent engine could successfully control the spacecraft towards the surface, they pressed forward with staging. This was meant to mimic the moment of launch from the moon’s surface. On a landing mission, the bottom descent stage of the module would serve as a launch pad for the crewed ascent stage whose smaller engine would propel it from the surface into orbit to meet and dock with the waiting command module. On Apollo 10, Stafford and Cernan would do the same thing in orbit.

Stafford moved a switch from the Safe position to Stage, activating the small explosives that forcibly separated the ascent stage from the descent stage. But rather than a smooth flight, the spacecraft started gyrating wildly, rolling, pitching and yawing around all three axes in turn. Almost instantly, Stafford saw a yellow Gimbal Lock light illuminate on a nearby instrument panel. The computer was close to losing its orientation in space, which would mean the crew could have no idea where they were and how to get home.

“Son of a bitch!” Cernan yelled as they got a quick sight of the separated descent stage passing by a window.

“We’re in trouble,” Stafford concurred.

The spacecraft never went into gimbal lock. Stafford reacted quickly and began manually correcting the spinning and rolling to get the spacecraft back into the correct attitude for their continued ascent. Less than four minutes after the initial staging, everything had calmed down.

“I think we have got all our marbles,” Stafford called down to Houston.

It was only later that the crew learned exactly what had happened. A series of small errors on the crew's part had left the LM’s guidance system pointing in the wrong direction. An errant setting in the spacecraft left the LM’s abort-guidance system searching for the CSM at a time when it shouldn't have been, and since it wasn’t near where the crew was aiming, it sent them on their frightening spin.

Stafford and Cernan got the LM back on track, and less than two hours later the whole crew was reunited when Snoopy docked with Charlie Brown on the far side of the moon. Emerging from the radio blackout, Stafford gave Houston the good news. "Snoopy and Charlie Brown are hugging each other.”

The rest of Apollo 10's mission passed without incident, right up to splashdown in the Pacific Ocean on May 26. Despite the spin out incident, Apollo 10 was considered sufficiently successful that NASA chose to go for a landing with the next mission, Apollo 11.

As for the crew, after coming so close to the moon, both John Young and Gene Cernan would go on to walk on the surface in 1972 as the commanders of Apollo 16 and 17 respectively. Tom Stafford, meanwhile, commanded the American half of the first join US-Soviet mission, the Apollo-Soyuz Test Project in 1975.