Gemini 9A

Gemini 9A carried the "A" in its name because, like Gemini 6A before it, the mission never flew as originally planned. The intended Agena target vehicle failed to reach orbit in May 1966, and NASA scrambled to launch a backup docking target called the Augmented Target Docking Adapter, or ATDA, in its place.

When Tom Stafford and Gene Cernan launched on June 3, 1966, and closed in on the ATDA, they found a problem no one had anticipated: the protective shroud covering the docking adapter had failed to separate properly, leaving it clamped open in a way that made the target look, in Stafford's memorable description, like "an angry alligator." Docking was impossible, and after examining the target from multiple angles, the crew abandoned that part of the mission entirely.

The flight still accomplished valuable work. Stafford and Cernan practiced three different rendezvous techniques with the ATDA, refining the orbital mechanics and piloting procedures that later missions would depend on. But the mission's most demanding chapter came from Cernan's planned spacewalk, intended to test a large Air Force-developed rocket backpack called the Astronaut Maneuvering Unit.

Without adequate handholds or footholds to brace against, Cernan found even simple movements outside the spacecraft required far more physical effort than anyone had anticipated. He grew increasingly overheated as his exertion outpaced his suit's cooling system, his visor fogged over until he could barely see, and his heart rate climbed to dangerous levels. After a grueling struggle, the AMU test was cancelled and Cernan returned to the spacecraft, having survived one of the most physically punishing spacewalks in NASA history, a warning sign that reshaped how NASA approached EVA training for the rest of the decade.

Gemini 9A's failed docking turned out to be one of the program's most valuable lessons in disguise. The mission proved that spacecraft rendezvous could be achieved through multiple different techniques, giving mission planners flexibility for future flights, even when the docking itself could not be completed.

Cernan's exhausting spacewalk, more than any single event in the Gemini program, exposed how poorly NASA understood the physical demands of working in a vacuum without gravity to help stabilize movement. That hard-won knowledge directly shaped the redesigned EVA procedures, complete with proper restraints and rest periods, that made Gemini 12's spacewalks a resounding success just five months later.

Together, Gemini 9A's rendezvous data and its EVA warning signs made the mission an essential, if imperfect, stepping stone toward the docking and spacewalk techniques Apollo's lunar missions would rely on completely.