Apollo 9

Apollo 9, the third crewed mission of the Apollo program, featured the first flight of the Command/Service Module (CSM) with the Lunar Module (LM). Astronauts James McDivitt, David Scott, and Rusty Schweickart tested several aspects critical to landing on the Moon, including the LM engines, backpack life support systems, navigation systems, and docking maneuvers. The ten-day long mission proved the LM worthy of human spaceflight. The mission also featured the first space docking of two vehicles with an internal crew transfer between them.