That mission was Apollo 7, the first crewed Apollo flight to successfully reach space. It was also the first astronaut launch since the Apollo 1 disaster in January 1967 killed all three passengers during what was supposed to be a routine test.
"The whole credibility of the Apollo program was riding to some extent on that being a smooth flight — as well, of course, as the whole launch schedule after that, which was very aggressive," Michael Neufeld, a space historian at the Smithsonian National Air and Space Museum, told Space.com. [Building Apollo: Photos from Moonshot History]
After the Apollo 1 fire and a Congressional investigation into what had gone wrong, NASA needed a win. So, the agency did what it needed to do to get one, stepping back from the frantic pace of development.
"It forced a top-to-bottom reimagination of the whole project," Neufeld said. NASA changed the command module's wiring, the exit hatches and more, all to make sure that future spacecraft wouldn't result in the same sort of disaster. "It's hard to imagine the lunar landing could have been made, unfortunately, without the deaths of those astronauts."
When the new launch date finally came, astronauts Wally Schirra, Donn Eisele and Walter Cunningham piled into the new command module. This time, the launch went off without any major hitches and the trio orbited Earth for almost 11 days, proving that the Apollo program was safe for humans.
But while it may have been safe, it definitely wasn't pleasant. All three men caught colds in the close confines of the command module, and they didn't handle the sniffles graciously. (Colds in space are particularly unpleasant, because without gravity, fluids don't move through the body the way they do on Earth.)
The astronauts spent large parts of the mission acting cranky with controllers on the ground and protesting their assignments. When it came time to land again, they refused to wear their helmets, which would have stopped them from blowing their noses. NASA ground personnel didn't take kindly to the insubordination, and none of the three astronauts made it onto a future mission.
"Certainly, it gave the message to the astronauts: Stay in line or risk not being able to fly again," Neufeld said of the debacle.
One moment of tension came from another factor that set Apollo 7 apart: The mission was the first time humans broadcast live television from space. At one point during the flight, Schirra canceled a scheduled broadcast because the equipment wasn't ready. Nevertheless, live video footage has now become a mainstay for NASA missions.
Apollo 7's success took place against a backdrop of national tension, including high-profile assassinations and widespread protests against the war in Vietnam, Neufeld said. But although this mission was an important achievement technologically and ensured the success of the Apollo program, it didn't sell the U.S. public on the idea of spending so much money on the project.
"One of the favorite illusions that Americans like to have is that everyone was united by the moon landing," Neufeld said. "The reality is that there was a lot of dissent."