Apollo 9 (March 3–13, 1969) was the third human spaceflight in NASA's Apollo program. Flown in low Earth orbit, it was the second crewed Apollo mission that the United States launched via a Saturn V rocket, and was the first flight of the full Apollo spacecraft: the command and service module (CSM) with the Lunar Module (LM). The mission was flown to qualify the LM for lunar orbit operations in preparation for the first Moon landing by demonstrating its descent and ascent propulsion systems, showing that its crew could fly it independently, then rendezvous and dock with the CSM again, as would be required for the first crewed lunar landing. Other objectives of the flight included firing the LM descent engine to propel the spacecraft stack as a backup mode (as would be required on the Apollo 13 mission), and use of the portable life support system backpack outside the LM cabin.

During the ten-day mission, they tested systems and procedures critical to landing on the Moon, including the LM engines, backpack life support systems, navigation systems and docking maneuvers.

After launching on March 3, 1969, the crew performed the first crewed flight of a lunar module, the first docking and extraction of the same, one two-person spacewalk (EVA), and the second docking of two crewed spacecraft—two months after the Soviets performed a spacewalk crew transfer between Soyuz 4 and Soyuz 5. The mission concluded on March 13 and was a complete success. It proved the LM worthy of crewed spaceflight, setting the stage for the dress rehearsal for the lunar landing, Apollo 10, before the ultimate goal, landing on the Moon.