Glenn entered orbit just fine, and circled the Earth three times in four hours and 56 minutes. He sounded cheerful and calm inside the capsule, making jokes to mission control and thanking the Australians who lit up their homes in greeting as he flew over their continent, while he traveled at nearly 17,500 miles per hour. “Oh, that view is tremendous!” he said in one iconic moment.

But Glenn’s return to Earth proved nerve-racking. An indicator back at mission control suggested the capsule’s heat shield was loose. Without an operational shield, the spacecraft would burn up during the fiery descent. Shepard instructed Glenn not to deploy the capsule’s retrorocket pack, which could help keep the heat shield in place, and to take manual control of the capsule. Glenn described “a real fireball outside” as the Mercury entered the atmosphere—and then splashed safely in the Caribbean Sea, near Turks and Caicos, its passenger unharmed.

Glenn was one of seven men chosen from a pool of 508 candidates for NASA’s first class of astronauts, including Shepard, who died in 1998. Glenn’s historic flight was an honor, though not the one he had wanted. The astronaut-in-training had sought to be the first American in space, but was assigned as a backup to Shepard, who claimed that title in 1961. But Glenn’s voyage came in the midst of the space race between the United States and the Soviet Union, and his success buoyed the spirits of a watchful American public.

Glenn was born July 18, 1921, in Cambridge, Ohio. He earned a private pilot’s license as a young man and joined the Marines during his college years. He flew dozens of combat missions during World War II and in the Korean War. In 1957, he flew a jet from Los Angeles to New York in three hours and 23 minutes, setting a record for transcontinental flight. The trip garnered Glenn a reputation as one of the best test pilots in the country—and an appearance on the popular game show “Name That Tune.” That year, Glenn volunteered for research at the space agency that preceded NASA, and got whirled around inside the high-speed centrifuge that would later prepare astronauts for the rigors of launch and re-entry.

Glenn would return to space once more after his historic jaunt around Earth, but not for another 36 years. He left the astronaut class two years after the flight and worked for a soft-drink company in Atlanta. President John F. Kennedy, with whom Glenn had formed a close friendship, had other hopes for the astronaut-turned-national hero, and had encouraged Glenn to run for political office. In 1970, Glenn launched his campaign for a Senate seat in his home state of Ohio, but lost. His second attempt in 1974 was successful. During a Democratic primary debate, Glenn’s challenger, businessman Howard Metzenbaum, asked him, “How can you run for Senate when you’ve never held a job?” Glenn responded by pointing to his time in the military and the space program. “It wasn’t my checkbook, it was my life that was on the line,” he said.