Gemini 8
Armstrong first entered space with pilot David Scott during the Gemini 8 flight. The spacecraft launched on March 16, 1966. It was important in that it was the first successful docking of two vehicles, the Gemini (which the pilots were on) and Atlas-Agena, in space.
However, the launch quickly turned into an emergency situation after the docking due to the Gemini thruster being stuck. After the linkup of the two vehicles in orbit, Scott noticed that they were going in the wrong direction on the Gemini 8. His command pilot used the orbital altitude and maneuver system (OAMS) thrusters to stop the tumbling, but the roll continued.
Armstrong saw that the propellant level dipped below 30 percent and believed that the thruster could be the problem. He turned off the OAMS and used a re-entry control system to regain command of the Gemini 8. John Hodge, the chief flight director, decided to bring the crew home shortly after, as Armstrong had used about 75 percent of the re-entry propellant. Splashdown occurred 10 hours and 41 minutes after the liftoff, with Gemini 8 landing in Japan.