Apollo 10: The Eight Seconds When Snoopy Went Wild
How a wrong switch sent the Lunar Module spinning above the Moon just two months before Apollo 11
On May 22, 1969, two astronauts were flying a spacecraft unlike anything humans had ever flown before, in a place no one had ever attempted to fly one.
Below Tom Stafford and Gene Cernan was the Moon.
Somewhere above them, John Young waited alone in the Command Module Charlie Brown.
Stafford and Cernan were aboard the Lunar Module Snoopy, conducting the final dress rehearsal for the first lunar landing. Apollo 10 was deliberately doing almost everything Apollo 11 would do two months later—except actually touch the surface.
They had already descended to only about 47,400 feet above the Moon, lower than any humans had ever flown over another world. They photographed the future Apollo 11 landing area, tested the landing radar and proved that the Lunar Module could operate independently in lunar orbit.
Now came another critical test.
They had to throw away half their spacecraft.
And for approximately eight seconds, Snoopy went wild.
The Dress Rehearsal
Apollo 10 launched on May 18, 1969, carrying one of the most experienced astronaut crews NASA had yet assembled: Commander Tom Stafford, Command Module Pilot John Young and Lunar Module Pilot Gene Cernan.
Its purpose was straightforward but audacious.
Perform essentially the entire Apollo 11 mission without landing.
Stafford and Cernan would separate Snoopy from Charlie Brown, descend toward the Moon, inspect the landing site, test the Lunar Module’s systems and then climb back into a higher orbit to rendezvous with Young.
NASA wanted to discover the unknowns before Neil Armstrong and Buzz Aldrin had to confront them during an actual landing attempt.
The names of the two spacecraft added an almost whimsical quality to the mission.
The Command Module was Charlie Brown.
The Lunar Module was Snoopy.
But what happened next was anything but whimsical.
Throwing Away the Bottom Half
The Lunar Module was really two spacecraft stacked together.
The descent stage contained the landing gear, descent engine and equipment needed to reach the lunar surface.
Above it sat the ascent stage, containing the astronauts, controls, guidance systems and the engine that would eventually carry a lunar crew away from the Moon.
On a real landing mission, the descent stage would remain behind on the surface.
The ascent stage would lift off from it.
Apollo 10 had not landed, so Stafford and Cernan needed to simulate that event in orbit. They would jettison the descent stage, leaving themselves inside the much lighter ascent stage, then use its engine to begin the rendezvous with Charlie Brown.
At this point Snoopy had been maneuvered into an elliptical orbit that brought it down to roughly 13 miles above the lunar surface.
Everything depended upon being in the correct attitude when the stages separated.
That included a small switch controlling the Lunar Module’s backup guidance system.
ATT HOLD or AUTO
Apollo’s Lunar Module carried two guidance systems.
The sophisticated Primary Guidance, Navigation and Control System was the main system.
The simpler Abort Guidance System, or AGS, existed as a backup. Apollo 10 was deliberately testing it because if Apollo 11 encountered a serious problem during descent, Armstrong and Aldrin might need that backup system to help get them away from the Moon.
For the staging maneuver, the AGS was supposed to be operating in ATT HOLD—Attitude Hold.
The name says essentially what it did.
Hold the spacecraft in its current attitude.
But the switch had another position:
AUTO.
And AUTO meant something very different.
With the particular steering logic configured at that moment, selecting AUTO caused the AGS to command the Lunar Module to rotate so that one of its axes pointed toward the Command Module.
That was useful during rendezvous.
It was decidedly not what Stafford and Cernan wanted just as they were about to separate the two halves of Snoopy. NASA’s postflight analysis confirmed that when AUTO was selected, the control system began responding to those steering commands.
The switch needed to be in ATT HOLD.
It wasn’t.
Twenty-Eight Seconds
The first sign that something was wrong came 28 seconds before staging.
Snoopy began slowly drifting in yaw.
Stafford and Cernan suspected a problem with a rate gyro. They corrected the attitude and the spacecraft returned close to where it was supposed to be.
Then came the final seconds.
At approximately five seconds before staging, the Lunar Module suddenly began developing a rapid roll.
Small pitch and yaw rates appeared with it.
Then Stafford and Cernan staged the vehicle.
Explosive devices separated the ascent stage from the descent stage.
And Snoopy began to gyrate violently. NASA’s crew report later described exactly this sequence and recorded that the motion was stopped in approximately eight seconds.
Inside the Lunar Module, there was no ambiguity about the seriousness of what was happening.
Cernan’s reaction went out over the communications system:
“Son of a bitch!”
Mission Control soon warned them that they were “close to gimbal lock.”
Stafford responded with perhaps the best description of the entire event:
“Something went wild during that staging.”
It had.
More Than 25 Degrees Per Second
Postflight telemetry allowed NASA to reconstruct how violent the motion had become.
Snoopy reached approximately:
19 degrees per second in pitch.
And more than 25 degrees per second in both roll and yaw.
Those are extraordinary rotation rates for a spacecraft whose crew was trying to establish an exact attitude for an upcoming engine firing and rendezvous maneuver.
NASA’s investigation found that the AGS mode indication had changed from ATT HOLD to AUTO as the gyrations began.
The spacecraft was not randomly malfunctioning.
The control system was responding to commands created by the mode that had been selected.
In one sense, that made the incident even more unsettling.
The computer had not gone crazy.
It was doing what it had been told to do.
Gimbal Lock
Then another warning appeared.
GIMBAL LOCK.
Apollo spacecraft determined their orientation using an inertial guidance platform mounted within a system of rotating gimbals.
Under certain combinations of spacecraft attitude, two of the gimbal axes could approach alignment. The system could then lose the ability to independently measure all three dimensions of spacecraft orientation.
For a Lunar Module preparing to rendezvous in lunar orbit, losing a reliable attitude reference was not a minor problem.
Mission Control could see how close they were getting.
The warning light illuminated inside Snoopy.
Fortunately, a quick check showed that the inertial platform had not performed the coarse realignment that would have indicated the platform had actually lost its stable reference.
The guidance system was still usable.
Stafford and Cernan had caught it.
Eight Seconds
NASA’s Apollo 10 Mission Report contains an almost understated sentence:
“Spacecraft motion stopped in approximately 8 seconds.”
Eight seconds.
Read on paper, it sounds almost trivial.
Inside Snoopy, it must have felt very different.
For those seconds, the lunar horizon and black sky were moving rapidly past the windows. The descent stage had been discarded. The ascent stage was rotating in multiple axes. The guidance platform was approaching a condition the crew desperately wanted to avoid.
And hundreds of thousands of miles away, there was nothing anyone on Earth could physically do to help.
Stafford and Cernan had to solve the problem themselves.
They did.
What Had Failed?
After the mission, NASA investigated several possibilities.
Could the switch itself have failed?
Could an electrical connector have malfunctioned?
Was damaged wiring sending false commands?
Engineers analyzed the hardware and concluded those explanations were highly unlikely.
The physical switch had worked before and after the incident. NASA found no plausible switch failure capable of reproducing what telemetry showed. Electrical connectors and wiring were also investigated and largely eliminated as likely causes.
NASA’s final conclusion was much simpler:
The AGS mode-control switch had been inadvertently cycled while the crew was diagnosing the erroneous yaw-rate indication.
It ended up in AUTO.
AUTO generated the steering commands.
And Snoopy began to rotate.
But there was another part of the explanation that could not be found merely by studying wiring diagrams.
It came from Tom Stafford himself.
“It Looked Like It Was in ATT HOLD”
Later in the flight, Stafford contacted Mission Control because he believed he understood what had happened.
In weightlessness he had been floating slightly higher than his normal seated position. One restraint harness was down. He put his finger on the three-position switch and looked at it.
From that angle, Stafford explained, it looked like it was in ATT HOLD.
When he changed his viewing position, he could see that it wasn’t.
Stafford told Houston that the apparent switch position depended upon where the astronaut’s eyes were relative to the panel.
Mission Control’s response was succinct:
“There was a great recovery.”
That detail turns the Apollo 10 incident into something more interesting than a simple story about somebody flipping the wrong switch.
Stafford was not an inexperienced operator.
He was an accomplished test pilot and veteran astronaut who had trained extensively for exactly these procedures.
But the simulator had a seated astronaut looking at a panel from a predictable position.
The real Lunar Module had a suited astronaut floating in zero gravity.
A switch that appeared to be in one position could actually be in another.
That is a human-factors problem—and Apollo 10 discovered it before Apollo 11 had to.
How Close Was It?
Over the years, the Apollo 10 incident has sometimes been described as Snoopy being only seconds away from crashing into the Moon.
That makes a dramatic headline.
The original NASA Mission Report does not establish that claim.
What it does establish is serious enough.
The Lunar Module developed uncontrolled-looking attitude excursions exceeding 25 degrees per second in two axes. The guidance mode was incorrectly configured. Manual attempts initially proved ineffective. The spacecraft approached gimbal lock during a critical staging maneuver, and its two astronauts needed to regain stable control before continuing the ascent and rendezvous sequence.
Had they been unable to establish the correct attitude and complete the subsequent maneuvers, their ability to rendezvous with John Young would have been threatened.
And in lunar orbit there was no rescue spacecraft waiting to come get them.
Back to Charlie Brown
The crisis ended almost as quickly as it began.
Stafford and Cernan stabilized Snoopy.
They verified that their guidance systems remained usable.
They maneuvered to the required attitude.
Then, roughly ten minutes later, they fired the ascent engine and continued the simulated lunar-ascent sequence.
The engine performed properly.
The rendezvous worked.
Eventually Snoopy approached Charlie Brown with the lunar surface behind it—the remarkable photograph that survives from the mission today.
Stafford and Cernan docked with Young and climbed back aboard the Command Module.
Apollo 10 had completed the final rehearsal.
Two months later, Neil Armstrong and Buzz Aldrin would climb into another Lunar Module and descend all the way to the surface.
Why Apollo 10 Mattered
Apollo 10 is sometimes overshadowed by the mission that followed it.
It shouldn’t be.
Apollo 11 could make history because Apollo 10 went first and found things that were still capable of going wrong.
Some were technical.
Some were procedural.
And some involved the deceptively simple relationship between a human being, a checklist and a switch.
The Apollo program was built around testing progressively closer to the objective. Each mission pushed farther, exposed another weakness and allowed engineers and astronauts to learn before the next crew encountered the same situation with less margin for error.
That was precisely what Apollo 10 did.
The spacecraft survived.
The crew recovered.
The rendezvous succeeded.
The mission was officially a success.
But hidden inside that success were eight seconds that demonstrated why the rehearsal had been necessary in the first place.
Because two months before humanity attempted to land on the Moon, a switch that looked right wasn’t.
And in a matter of seconds, Snoopy went wild.
Apollo 10 did exactly what a great test flight is supposed to do: it discovered a problem before the mission where failure could no longer be tolerated.