Commander James McDivitt, along with David Scott and Rusty Schweickart, tested docking, undocking, and rendezvous maneuvers between the command module and the lunar module 'Spider,' proving the hardware needed for a Moon landing worked as designed. The mission's success cleared the way for Apollo 10's dress rehearsal and, four months later, Apollo 11.
Apollo 9 never left Earth orbit, but its mission was arguably as technically demanding as any lunar flight. It was the first crewed test of the complete Apollo spacecraft stack—command module, service module, and lunar module—together in space, and the first time astronauts flew the oddly-shaped lunar module independently.
Schweickart and Scott undocked the lunar module, nicknamed Spider, and flew it more than 100 miles from the command module Gumdrop, testing the rendezvous radar and engines that would later have to work flawlessly above the Moon with no possibility of rescue if they failed. Schweickart also battled space sickness so severe that a planned spacewalk was scaled back, a problem NASA studied intensely afterward.
When the crew splashed down in the Atlantic Ocean, NASA had proven nearly every piece of hardware and procedure needed for a landing except the actual descent to the lunar surface. Gene Kranz and mission planners moved Apollo 10 up as a final lunar-orbit dress rehearsal, setting the stage for Apollo 11's landing that July.
Key people: James McDivitt, David Scott, Rusty Schweickart