Apollo 10, launched on May 18, 1969, was the dress rehearsal for the first moon landing, planned for two months later. The veteran crew of Tom Stafford, Gene Cernan and John Young would do everything for a lunar mission short of initiating powered descent, the braking de-orbit of their Lunar Module for a landing on the moon.

Apollo 10 sim S69-32615-orig
Cernan, foreground, and Stafford during simulator training. (NASA)

LM-4, Snoopy, was deliberately short-fueled in the ascent stage, partially for weight savings. Cernan believed it also as a clear warning to the crew from any funny ideas of attempting a landing. Snoopy was also too heavy to attempt a landing , even if fully fueled. Most importantly, the final software changes needed for a landing weren’t in place yet: Snoopy’s mission included tests of the flight software.

So Snoopy descended to 50,000 feet (20 km) to make the final tests on the lunar landing hardware and software and to get some close-up photographs of the first planned landing sites. Above, the Command/Service Module Charlie Brown tested communications and telemetry between the spacecraft.

With the tests and reconnaissance done, mission commander Stafford and LM Pilot Cernan prepared Snoopy to separate the descent stage and fire the ascent stage. Most of you know the story told: Just before separation, Snoopy gyrated wildly, causing Cernan to exclaim “Son of a bitch!” on a live mic. After staging, Stafford quickly stabilized the spacecraft with no ill effects and eventually rendezvoused with Charlie Brown.

In Gene’s biography, The Last Man on the Moon (a great book, by the way), he described Snoopy’s behavior as pitching end over end several times, seeing the horizon pass by several times before Stafford stopped the tumble, mere seconds away from loss of control. NASA’s recount of the event suggests the crew was not in any serious danger.

Tom Stafford’s account differs. So who’s retelling the story more clearly?

The best arbiter of the event comes from the event camera mounted above a LM window inside Snoopy. It recorded the entire event.

The blunder was a crew error. Stafford and Cernan had practiced ascent stage separation many, many times. One of the tasks to do is to switch the Abort Guidance System (AGS) from automatic to attitude-hold, a stand-by mode.

AGS is a rough, emergency guidance system (as compared to fine-tuned navigation of the Primary Guidance and Navigation System, PNGS) that would try to get a quick fix on the Command/Service Module in an emergency abort, and this was an opportunity to test it.

But after Cernan had switched AGS to the proper setting for stage separation, Stafford mistakenly returned AGS to automatic.

Snoopy gyrated because it was immediately trying to lock on to the CSM as designed with AGS, but not as desired as that moment.

Spacecraft Films’ Mark Gray makes a detailed explanation of what actually happened and what was seen on the gyration and separation in this YouTube video.

Everyone loves a good “big fish” story, so we can still appreciate Cernan’s account without impugning the good astronaut’s reputation or memory.