Mitchell kept his plan a secret from NASA, knowing that the agency would be completely unreceptive to the idea. He said nothing about it to his crewmates. The test subjects had also agreed to keep quiet.
Trouble began on the sixth day of the flight, November 17. The capsule developed an atmospheric leak, the pressure first dropping from 760 to 380 mm of Mercury. With the drop in cabin pressure all the animal test subjects died. It would have killed any Cosmonaut not wearing a spacesuit.
The first attempt for a Zond 4 follow-up launched on April 22. It failed when the Launch Escape System sent an erroneous abort command at T+260 seconds and shut down the Proton booster’s second stage. The escape rocket fired and pulled the descent module to safety.
When the Gemini IX-A Agena fell into the Atlantic Ocean, Gemini XII was threatened with a major hardware shortage of an Agena and an Atlas to launch it. Replacing the Agena was no real problem. Lockheed’s first production model, 5001, used for development testing at the Cape, had already been sent back to the Sunnyvale plant for refurbishment. Now it was simply a matter of tailoring it to the Gemini XII mission…
The rotation rate checked out at 55 degrees per minute, and the crew could now test for a minute amount of artificial gravity. When they put a camera against the instrument panel and then let it go, it moved in a straight line to the rear of the cockpit and parallel to the direction of the tether. The crew, themselves, did not sense any physiological effect of gravity.