Gemini 9A

Gemini 9A (officially Gemini IX-A) was a 1966 crewed spaceflight in NASA's Gemini program. It was the seventh crewed Gemini flight, the 15th crewed American flight and the 23rd spaceflight of all time (includes X-15 flights over 100 kilometers (62 mi; 54 nmi)). The original crew for Gemini 9, command pilot Elliot See and pilot Charles Bassett, were killed in a crash on February 28, 1966, while flying a T-38 jet trainer to the McDonnell Aircraft plant in St. Louis, Missouri to inspect their spacecraft. Their deaths promoted the backup crew, Thomas P. Stafford and Eugene Cernan, to the prime crew. The mission was renamed Gemini 9A after the original May 17 launch was scrubbed when the mission's Agena Target Vehicle was destroyed after a launch failure. The mission was flown June 3–6, 1966, after launch of the backup Augmented Target Docking Adaptor (ATDA). Stafford and Cernan rendezvoused with the ATDA, but were unable to dock with it because the nose fairing had failed to eject from the docking target due to a launch preparation error. Cernan performed a two-hour extravehicular activity, during which it was planned for him to demonstrate free flight in a self-contained rocket pack, the USAF Astronaut Maneuvering Unit. He was unable to accomplish this due to stress, fatigue, and overheating.

On February 28, 1966, See and Bassett were flying from Texas to inspect the Gemini 9 spacecraft at the McDonnell Aircraft plant in St. Louis, Missouri. The conditions at Lambert Field were poor and, as a consequence, in attempting a visual approach and landing, See hit one of the assembly buildings of the factory and caused the aircraft to crash, killing himself and Bassett instantly. As a consequence, the backup crew was promoted to prime crew, the first time this had occurred since the flight of Mercury-Atlas 7 in 1962.

The promotion of Stafford and Cernan from backup to prime crew meant that a new backup crew was required. Jim Lovell and Buzz Aldrin were originally the backup crew for Gemini 10. This is significant as the standard crew rotation meant that a spot on the backup crew of Gemini 10 would have placed Buzz Aldrin on the prime crew of the non-existent mission after Gemini 12 (the crew rotation usually meant that after serving on a backup crew, an astronaut could expect to skip two missions and then be on a prime crew). Being moved up to the backup crew of Gemini 9 meant that Aldrin flew as part of the prime crew on Gemini 12, which played a major part in his selection for the Apollo 8 backup and Apollo 11 prime crews, ultimately making him the second human on the Moon.

The first mission objective was to dock with an Agena Target Vehicle, as had first been achieved on the Gemini 8 mission. Accomplishment was not possible because of a launch preparation error on the target vehicle.

A second objective was a planned extravehicular activity (EVA), or "space walk", by the right-hand seat Pilot. The plan was for him to move to the rear of the spacecraft and strap himself into the Air Force's Astronaut Maneuvering Unit (AMU), a 'rocket pack' which would allow the pilot controlled flight, independent of the capsule's life support system. Use of the AMU was not achieved due to Cernan experiencing high cardiac stress, overheating, and fatigue during EVA.

A third objective was to carry out seven scientific experiments, including a medical experiment which measured the astronauts' reactions to stress by measuring the intake and output of fluids before, during and after the flight.

Gemini 9's Agena Target Vehicle (ATV) was launched on May 17, 1966, on an Atlas launch vehicle. The Atlas malfunctioned in flight and the ATV failed to reach orbit. This forced the cancellation ("scrubbing") of the Gemini 9 launch scheduled for later that morning.

The Augmented Target Docking Adapter (ATDA) had been designed for use as a contingency for the ATV, which had failed during the original Gemini 6 launch. Built by Gemini spacecraft manufacturer McDonnell Aircraft Corporation, the ATDA replaced the Agena rocket with the reentry control section of a Gemini. It was built using already tested equipment, and successfully launched on June 1, 1966, into a 298-kilometer (185 mi; 161 nmi) orbit using the Atlas SLV-3 rocket.