NASA launched Apollo 5 from Cape Kennedy atop a Saturn IB rocket, carrying an unmanned lunar module into Earth orbit to test its descent and ascent engines for the first time. The mission, though plagued by a computer programming glitch that cut short the first planned burn, ultimately validated the spacecraft's engines and structure, clearing a crucial hurdle toward the eventual Moon landing.
The lunar module was an ungainly, spider-legged contraption unlike any spacecraft built before—designed purely for flight in vacuum, with no need to be aerodynamic since it would never fly through an atmosphere. Engineers needed real flight data on its descent engine, which had to throttle precisely to control a lunar landing, and its ascent engine, which had exactly one job: fire perfectly to lift astronauts off the Moon with absolutely no possibility of repair or backup.
Shortly after reaching orbit, an onboard computer prematurely shut down the descent engine test, forcing mission controllers on the ground to improvise a new test sequence in real time. Working through the night, NASA engineers devised alternate 'fire and abort' sequences that ended up wringing more useful data from the mission than originally planned.
The module's engines performed well despite the scare, and NASA declared the mission a success. It was an unglamorous, unmanned flight that received little public attention at the time, but it was indispensable groundwork—eighteen months later, that same lunar module design carried Neil Armstrong and Buzz Aldrin to the Moon's surface.