Armstrong and Scott transferred control of their docked spacecraft to the Agena at which point Houston capcom Jim Lovell gave the crew a reminder about control: "If you run into trouble and the Attitude Control System in the Agena goes wild, just send in Command 400 to turn it off and take control with the Spacecraft." It was a routine transmission.
With the successful docking done, the plan was for Armstrong to go through four docking tests with the Agena and park the Agena in a 255 mile circular orbit to performing a re-rendezvous exercise. Scott, meanwhile, would prepare for his two hour and 40 minute EVA that was scheduled to start 20 hours and 25 minutes into the mission.
Throughout the mission, the crew was to conduct systems evaluations, evaluate the auxiliary tape memory unit, and finally demonstrate a controlled reentry.
But they never made it past that first rendezvous on the checklist. Lovell's mention of an out of control attitude system turned out to be almost prophetic.
While docked with the Agena, Scott noticed on the Flight Director Attitude Indicator (sometimes called the 8-ball) in front of Armstrong that the Gemini-Agena had rolled 30 degrees off the horizon. Armstrong confirmed what Scott saw, they were on the night side of the planet so had no reference apart from their instruments, and manually fixed the attitude. But as soon as he released the hand controller the spacecraft started rolling again.
Both men instinctively blamed the Agena so Armstrong gave Scott the go ahead to disable it and return control to the Gemini spacecraft. The roll stopped, but only for a minute. Then it got worse as they stared moving around three axes - pitch, yaw, and roll. As Armstrong fought against the tumbling, the astronauts realized they were running low on fuel; the Gemini control system was down to 13 percent. Their only option was to disengage from the Agena.
Armstrong wrestled enough over the spacecraft to minimize the risk of it crashing into the Agena when the two separated. Scott set the recording devices on the Agena so ground control stations could track it and gather telemetry readings to find out what had gone wrong. He also grabbed a video camera to record the separation.
The men in Houston got confirmation that the spacecraft had separated just as Armstrong pulled the Gemini back and Scott hit the undock switch. Unfortunately, the separation didn't help the astronauts. Scott called down as calmly as he could, "We have serious problems here. We're– we're tumbling end over end up here. We're disengaged from the Agena."
They tried to explain to mission control what was happening. "We're rolling up and we can't turn anything off," Armstrong said. "Continuously increasing in a left roll." Armstrong fought without success to dampen out the spacecraft's tumbling. Turning things over to Scott didn't help; he was equally unsuccessful.
By then, Gemini 8 was making one full revolution per second. The centrifugal force building up inside the spacecraft was making loose items - flight plants, checklists, and procedure charts - stick to the walls. Their heads and arms were pinned against the backs of their seats making reaching the hand controllers an effort. The sunlight coming in through the windows was flashing as fast as a strobe light.
As command pilot, the hard decision fell to Armstrong. He knew the only chance left to get home was to use the reentry system to counter the tumbling. He also knew that would kill the mission. Mission rules stated that once the reentry system was activated the crew had to return as quickly as possible. But, it was the only way to save their lives. Still fighting the roll with his hand controller, Armstrong reached to the control panel above his head and switched on the reentry control system more by feel than by sight.
Within half a minute, he had the spacecraft back under control.
During their last orbit, the crew got everything settled in the spacecraft and changed their reentry plan. Scott had to feed the new reentry data into the onboard computer line by line, one line in one line out, because of its limited storage space. Ten hours and 44 minutes after launch, Gemini 8 splashed down. The crew was safe, though nauseous. Armstrong threw up first, then Scott. Unfortunately they only had one air sickness bag between them.
HOWSTUFFWORKS: How the Gemini Spacecraft Worked
The problem turned out to be with the Gemini spacecraft and not the Agena. Thruster number eight of 16 was stuck open, firing constantly. In space, without air resistance, that one thruster was enough to endanger the lives of the crew. Instead, Scott's keen eye and Armstrong's quick thinking and skilled flying saved their lives, allowing both men to walk on the moon as commanders of Apollo missions.
Image: The Agena target vehicle as seen from Gemini 8 shortly before docking. Credit: NASA