Gemini 8

There was some suspicion on the ground that the Agena attitude system was acting up and might not have the correct program stored in it (this suspicion was subsequently found to be incorrect). Just before they went off contact with the ground, the crew of Gemini 8 were informed that if anything strange were to happen, they were to turn off the Agena.

After the Agena began execution of its stored command program, which instructed the Agena to turn the combined spacecraft 90° to the right, Scott noticed that they were in a roll. Armstrong used the Gemini's Orbit Attitude and Maneuvering System (OAMS) to stop the roll, but the moment he stopped using the thrusters, it started again. They immediately turned off the Agena and this seemed to stop the problem for a few minutes. Then suddenly it started again.

Scott noticed that the Gemini attitude fuel had dropped to 30% indicating that it was a problem on their own spacecraft. They would have to undock. After transferring control of the Agena back to the ground they undocked and with a long burst of translation thrusters moved away from the Agena.

It was at that point that the Gemini spacecraft began to roll even faster, and approached one revolution per second. The astronauts were now in danger of impaired vision and loss of consciousness due to the violent motion. At this point Armstrong shut down the OAMS and used the Re-entry Control System reaction control system (RCS) to stop the spin. After steadying the spacecraft, they tested each OAMS thruster in turn and found that Number 8 had stuck on. Mission rules dictated that the flight be terminated once the RCS had been fired for any reason, so Gemini VIII prepared for an emergency landing.