On the eve of the flight, Gagarin and his backup, German Titov, went to bed early and were awakened at 5:30 a.m. Gagarin was joking, his pulse was an exemplary 64 beats a minute and it remained the same after he took his seat in the Vostok.

Before boarding, Gagarin saw Korolyov looking haggard after a sleepless night. “Don’t you worry, Sergei Pavlovich, he told the chief designer, “everything will be just fine.”

“It was he who was comforting me!” Korolyov would marvel later.

He thought of Gagarin as a son, and Gagarin carried Korolyov’s picture in his wallet.

The security code for use in emergency was supposed to stay in a sealed envelope for the cosmonaut to open only if necessary, but Ivanovsky was too nervous to stick with protocol. As he escorted Gagarin to the capsule, he whispered the code to him: 1-2-5. Gagarin smiled and said he already knew; his instructor, equally protective, had already let him in on the secret.

Ivanovsky helped Gagarin up the ladder and into the cockpit, patted him on the helmet and wished him luck before closing the hatch, only to hear Korolyov telling him from the control room that a light that was meant to indicate the hatch was hermetically closed had failed to turn on. Ivanovsky and his two assistants had to remove all 32 screws sealing the hatch and then put them back at a frantic pace.

Inside the capsule, Gagarin was whistling a tune. Later he would joke to Ivanovsky: “You should have seen yourself while you were working on the hatch; your face had all the colors of tarnished metal.”

Gagarin’s rocket lifted off as scheduled on April 12, 1961, at 9:07 a.m. Moscow time. “Poyekhali!” (Off we go!), the cosmonaut shouted as he took off.

Korolyov and his engineers quickly got their first jolt: a signal suggesting a problem with the booster. It turned out to be just a break of a few seconds in data transmission. Gagarin’s confident reports from orbit eased the tension, and only after the flight, it emerged that an antenna malfunction had put the Vostok into a much higher and riskier orbit.

On re-entry, a glitch involving a retrorocket made the ship rotate swiftly, and the landing capsule was slow to jettison the service module. Scientists had to take a deep breath as they lost contact with the ship during its fiery earthward plunge.

Gagarin bailed out as planned, and parachuted onto a field near the Volga River about 720 kilometers southeast of Moscow. There he was spotted by a forester’s wife and her granddaughter who tried to run away from the stranger in his bright orange space suit and white helmet. They may have thought he was a U.S. spy, given that less than a year before, U.S. pilot Francis Gary Powers had been shot down over the Soviet Union in his U-2 spy plane, an incident that had badly strained U.S.-Soviet relations.

“Hey, where are you running? I’m one of us!” Gagarin shouted.

Then others arrived, realizing he was the cosmonaut they had just heard about on the radio.

Gagarin learned to his great surprise that while aloft, he was being promoted two levels higher, to major. Korolyov and others flew to the landing area and met with Gagarin at a Communist Party guesthouse. Their raucous reunion lasted late into the night. On April 14 Gagarin was flown to Moscow, where he was greeted by Soviet leader Nikita Khrushchev and driven into town on a highway lined with cheering Russians.

“People took to the streets; everybody felt excited, it felt like V-Day,” Korolyov’s daughter Natalya recalled.

Americans, waking up as the Soviet Union was well into its celebrations, were shocked. The next day members of Congress grilled NASA officials. One demanded that the U.S. be put on a war footing.

NASA explained that the Soviets had a greater lead time, having started their effort in 1954, four years before the American space agency was founded.

Twenty-three days after Gagarin’s flight, on May 5, 1961, American Alan Shepard became the second man in space. But his suborbital hop lasted just 15 minutes. It wasn’t until John Glenn’s flight on Feb. 20, 1962, that an American managed to emulate Gagarin’s globe-circling feat.

“Now let the other countries try to catch us,” Gagarin had declared after returning from space, and the U.S. quickly set out to do so. Barely three weeks after Shepard’s launch, President John F. Kennedy committed the nation to putting a man on the moon by decade’s end. The goal was achieved in July 20, 1969.

Gagarin’s legacy, meanwhile, has been dogged by conspiracy theories. Rumors still abound of botched and fatal space missions, the result of pervasive secrecy that surrounded the Soviet space program. “The degree of secrecy created shadows in which monsters could lurk,” said Oberg, the NASA veteran.

Some early flights used mannequins which might have been mistaken for real people. But at least one death was real: cosmonaut Valentin Bondarenko died in a pressure chamber fire during ground training less than a month before Gagarin’s flight.

And then there was Gagarin’s own death on March 27, 1968. It still drives conspiracy theories that the KGB wanted him dead because he supposedly opposed the Soviet regime.

Shatalov, the former cosmonaut, sat in his own jet, waiting his turn to take off after Gagarin and his crewman. He saw his friend smile and wave, and the next thing he knew, their MiG-15 had crashed into a forest. Shatalov surmises that the shock wave from another plane’s sonic boom was to blame.

Gagarin’s flight on the Vostok was entirely automated, yet simply by having the courage to face the unknown, he taught his fellow humans a vital lesson: that they had a future in space.

“Before this first flight there were reasonable suspicions that human beings weren’t made for this environment,” Oberg said. “And once Gagarin answered that question, I think every other discovery on every other manned spaceflight was just details. He answered the most challenging, the most awesome question by his performance.”