It came as a shock to American intelligence officials, to the Kennedy administration — and to the original seven Project Mercury astronauts. Fifty years ago Tuesday, the Soviet Union, which had trouble producing quality refrigerators or television sets for its own people, sent a man into space and returned him safely to Earth.

His name was Yuri Gagarin, a forgotten figure in most of the world today, and he did more than brush his shoulder against outer space, which is essentially what America’s first space veteran, Alan B. Shepard, did a month later in a 15-minute flight that took him 115 miles high. As he whistled strains of Shostakovich, Gagarin piloted his Vostok spacecraft beyond the bounds of the atmosphere, achieved Earth orbit and spent 108 minutes in space.

Shocked by Soviets

“We didn’t think they were as far along as they were, and we thought that whoever rode that first Redstone would be the first person in space,” John H. Glenn Jr., who with M. Scott Carpenter is one of the only two original American astronauts still alive, said in an interview this spring. “They had better boosters but we thought we were ahead. When they announced Gagarin had gone around the Earth, it was a shock.”

The shock has worn off. Since then, more than 500 people have flown in space, men have stepped on the moon, and serious people speak of a mission to Mars within the lives of those now walking the Earth. But Gagarin’s feat, the culmination of the human species’ dream for generations, remains an important marker — and this week’s anniversary means that humans have been in space for a half century.

Space travel is hardly routine even now; two space-shuttle tragedies have underlined the danger of missions that seldom attract attention today. But it is difficult to remember the effect that the dawn of the Space Age had on the world in 1961, in part because it is hard in a world without bitter ideological superpower struggle to imagine the Cold War tensions that produced the space race.

In that atmosphere, the astronauts were both spacemen and symbols — emblems of American virtues and values, test pilots whose outlook married adventure, daring and technology. They were heroes of a sort that does not exist today. “They were revered and extolled,” Tom Wolfe wrote, “songs and poems were written about them, every reasonable comfort and honor was given them, and women and children and even grown men were moved to tears in their presence.”

These astronauts were supposed to be just like us even as they were different from us. Two Air Force doctors in a presentation to the American Psychiatric Association’s annual meeting in 1959 said that specialists had dismissed early worries that “extreme interest in high performance aircraft might be related to feelings of inadequacy in sexual or other areas.” Instead, they found that these volunteers for space flight had an unusually high tolerance of stress and “uncomplaining acceptance” of discomfort.

These men were given classroom, aircraft, weightlessness and survival training — plus skin-diving instruction. They were outfitted in uniforms with an aluminized nylon covering that gave them a silver color and protection from the extreme heat of re-entry. Dressed for flight, the astronauts had 15 different zippers.

As the American astronauts trained, a parallel effort was under way in the Soviet Union. Indeed, the Soviets indicated as early as 1951 that they were prepared to go into space, and as early as 1953 their specialists declared that Soviet science “had reached such a stage that the launching of a stratoplane to the moon” was a reasonable achievement.

Political overtones

The space race was constructed on the architecture of the Cold War; its assets, after all, were missiles, its pilots were military aviators and its computers had military origins or uses.

The touchstones of the space race were intertwined with the big events of the superpower struggle. As the East Germans finalized their secret plans for the Berlin Wall later in 1961, for example, the radio station Deutschlandsender distracted the East German public with a feature on cosmonaut Gherman Titov’s 17 orbits of the Earth.

The Soviet lead in space didn’t hold. With a series of dramatic rendezvous in space and technological advances, America’s Gemini and Apollo missions leaped ahead of the Soviets and, despite the death of three astronauts on a Cape Kennedy launch pad in 1967, reached lunar orbit and, soon afterward, the lunar surface. The space race produced upswells in pride and patriotism and introduced to a post-war generation the terms pitch and yaw. They would remember them for the rest of their lives.

They would remember, too, the pure excitement of the endeavor and would yearn to recapture it even in the years of irony when modern technological achievement would rob spaceflight of its thrill and its novelty.

Sergei Krikalev, a Russian cosmonaut, spent nearly two and a quarter years in space, and another Russian, Anatoly Solovyev, spent nearly three and a half days in spacewalks over the course of 16 tries. Those achievements, inconceivable when the Space Age began, have produced a generation blase about the blast of a rocket into space.

Mankind needs a dream

Though great challenges — poverty, disease, environmental degradation — press on humankind, the fading of the space dream nonetheless worries national leaders, educators, philosophers, theologians — and astronauts.

“Unless people keep striving for things that excite them — that have a bit of a utopian vision to them — they will stagnate,” said Jay Apt, a Carnegie Mellon University physicist who has been on four space shuttle missions. “An endless source of that utopian energy can be found around our heads as we take the annual journey around the sun. There is no better way of stretching your imagination than … reaching out to the night sky.”

One person who understands that is Franklin Chang-Diaz, one of only two men to have made seven trips into space. He was an 11-year-old in Costa Rica when the Space Age began with the Gagarin mission. Later he would have pictures of the Mercury astronauts on the walls of his room.

“I had been dreaming about someday becoming an astronaut,” he said in a phone interview from Costa Rica, where he operates a company making a plasma rocket that might one day cut a mission to Mars from six months to a month. “But of course in those days there were no astronauts, only heroes from science fiction. It was a time I wish we could recover.”