Fifty years ago, before most people living today were born, the beep-beep-beep of Sputnik was heard round the world. It was the sound of wonder and foreboding. Nothing would ever be quite the same again — in geopolitics, in science and technology, in everyday life and the capacity of the human species.

The Soviet Union had launched the first artificial satellite, a new moon, on Oct. 4, 1957. Climbing out of the terrestrial gravity well, rising above the atmosphere and into orbit, Sputnik crossed the threshold into a new dimension of human experience. People could now see their kind as spacefarers. Their enhanced mobility might someday prove as liberating as the first upright steps of hominid ancestors long ago.

The immediate reaction, though, reflected the dark concerns of a world in the grip of the cold war, a time of fear and division in which the two superpowers, the Soviet Union and the United States, stared each other down with the menace of mass destruction. Sputnik altered the nature and scope of the cold war.

It was an unprepossessing agent of alarm. A simple sphere weighing just 184 pounds and not quite two feet wide, it had a highly polished surface of aluminum, the better to reflect sunlight and be visible from Earth. Two radio transmitters with whiskery antennas issued steady signals on frequencies that scientists and ham operators could pick up, and so confirm the achievement.

The Russians clearly intended Sputnik as a ringing statement of their technological prowess and its military implications. But even they, it seems, had not foreseen the frenzied response their success provoked.

When the Soviet dictator Nikita S. Khrushchev received word of the launching, he was of course pleased, and he and his son, Sergei, turned on the radio to listen to the beeping Sputnik. They went to bed, the son remembers, without realizing “the immensity of what was happening during those hours.”

The Soviet press published a standard two-column report of the event, with a minimum of gloating. But newspapers in the West, particularly the United States, filled pages with news and analysis.

Sputnik’s signal reverberated through chambers of the powerful and down ordinary streets. People listened and, from rooftops and backyards, saw in the night a moving point of light, like an errant star. The interrogatory of invention used to be “What hath God wrought?” Now it was “What are the Russians capable of next?”

“No event since Pearl Harbor set off such repercussions in public life,” Walter A. McDougall, a historian at the University of Pennsylvania, has written. A younger generation may draw comparison with the terrorist attacks of Sept. 11.

Sputnik plunged Americans into a crisis of self-confidence. Had the country grown lax with prosperity? Was the education system inadequate, especially in training scientists and engineers? Were the institutions of liberal democracy any match in competition with an authoritarian communist society?

In “The Heavens and the Earth: A Political History of the Space Age” (1985), Dr. McDougall wrote that before Sputnik the cold war had been “a military and political struggle in which the United States need only lend aid and comfort to its allies in the front lines.” Now, he continued, the cold war “became total, a competition for the loyalty and trust of all peoples fought out in all arenas of social achievement, in which science textbooks and racial harmony were as much tools of foreign policy as missiles and spies.”

At the time of Sputnik, John F. Kennedy was the junior senator from Massachusetts with no particular interest in space. Yuri A. Gagarin was an unheralded Russian military pilot. John H. Glenn Jr. was a Marine Corps pilot who had recently set a record for the fastest transcontinental jet flight to New York from Los Angeles. Neil A. Armstrong was testing high-performance aircraft in the California desert. Their lives were soon to be changed, as were those of hundreds of thousands of engineers, technicians, other workers and ordinary people everywhere.

Thomas J. O’Malley, an aviation engineer in New Jersey, would move in a few months to a forlorn spit of land at Cape Canaveral, Fla., to be a test conductor in the accelerated development of the Atlas missile, which would eventually lift American astronauts into orbit. “We had one goal,” he recalled recently. “To get something up there as quickly as possible.”

Christopher C. Kraft Jr. soon found himself working with a task force planning an American response to the challenge. He would become the first flight director of astronaut missions, but at the start, he has written, the morale of American engineers was low. “I wasn’t the only engineer who was stunned at how much I didn’t know and how much I had to learn,” he said.

When the Sputnik news reached Huntsville, Ala., Wernher von Braun was beside himself with restless frustration. Mr. von Braun, a German-born rocket scientist working for the United States Army, said this country could have beaten the Russians into orbit if not for Pentagon orders to resist any thought of adding a small satellite to the Jupiter-C missile he had been testing.

To make matters worse, the first American attempts to launch a tiny Vanguard satellite were embarrassing failures. It was the end of January 1958 before Americans succeeded with Explorer 1, boosted into orbit by a multistage version of Mr. von Braun’s Jupiter-C. But the much larger Sputnik 2 had already carried the dog Laika into orbit, a harbinger of human spaceflight. The original Sputnik — in Russian, “satellite” or “fellow traveler” — was no onetime fluke.

•

The post-Sputnik dynamic even reached out and recruited me. I was then a soldier in the cold war. Along with nearly every able-bodied young American man (even Elvis had to put in his two years), I was fulfilling my obligation to interrupt life and career for military service. I had completed college and was a reporter on military leave of absence from The Wall Street Journal, at the Army base in Fort Dix, N.J.

The morning after the Soviet triumph, I was on a one-day pass in Trenton. I bought the papers and spread them out on a coffee shop table. Banner headlines trumpeted the news. The recondite language of rocketry and orbits tied up my head, but I read on. I gave a passing thought to the coincidence of Sputnik’s going up on my birthday; at least I should never forget the date the space age began.

My story should at this point resound with destiny’s thunderclap or a sudden gust swinging open the door, scattering the papers and leaving me strangely moved. But I had no premonition that Sputnik had set in motion events that would shape my career. It was not until 1959, soon after I returned to The Journal from service in West Germany, that I felt the Sputnik effect.

Newspapers and other media, influenced by Sputnik, were scrambling to expand coverage of science, medicine and technology. I agreed to the managing editor’s suggestion that I try my hand writing about medicine. One thing led to another, from medicine to science and space exploration, to Time magazine and eventually to the staff of The New York Times to cover the most ambitious American response to Sputnik: the Apollo program.

•

Sputnik should not have come as such a surprise. Both the Soviet Union and the United States had embarked on the development of ballistic missiles for carrying nuclear warheads to great distances. They had also announced plans to launch artificial satellites in the International Geophysical Year, a cooperative 18-month scientific undertaking to study Earth and its atmosphere, beginning in 1957. Khrushchev had reiterated Soviet intentions only two months before.

But a shock it was, a wake-up call. One of the intriguing might-have-beens of history is: What if Americans had deployed the first satellite?

Alex Roland, a historian of technology at Duke University and a former NASA historian, said that a first launching by Americans would have merely confirmed their reputation for technological superiority. The costly rivalry for dominance in space, he said, would have probably been waged with much less driving urgency.

John M. Logsdon, director of the Institute of Space Policy at George Washington University, agreed. “If not for Sputnik,” he said, “there would probably not have been Apollo.”

But after Sputnik, there was no stopping the momentum of the space race. Critics attacked the administration of President Dwight D. Eisenhower, who at first had dismissed Sputnik as an event of only “scientific interest.” Soon the Defense Department stepped up missile development. The Democratic Congress established the National Aeronautics and Space Administration.

The perception of a threatening Soviet advantage in missiles persisted. Necessity had dictated the Russian concentration on missiles. Ever since World War II, American bombers had been more capable than those of the Russians, who also had no air bases in striking distance of their adversary’s heartland, in contrast to the American bases that ringed the Soviet Union.

An exaggerated estimate of the “missile gap” became a rallying cry of the 1960 presidential campaign and may have been crucial in Kennedy’s narrow victory. Not long after he took office, the Russians scored another stunning triumph. In April 1961, Gagarin became the first human to fly in Earth orbit.

After weeks of closed-door consultations, Kennedy went before Congress, on May 25, and declared, “Now it is time to take longer strides — time for a great new American enterprise — time for this nation to take a clearly leading role in space achievement which, in many ways, may hold the key to our future on Earth.”

He committed the country to “the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth.”

•

How brief the space race was, the 12 years from the wake-up call to the first walk on the Moon, but thrilling, mind-boggling, even magnificent at times.

While the Russians forge ahead, Americans begin catching up with the Mercury and Gemini flights in orbit. As the goal comes into sight, there are the countdowns of tingling anticipation. In the dark before dawn, we drive toward the shining light enveloping a spaceship that looks like an obelisk out of antiquity, waiting to be launched. The blast of the Saturn 5, just three miles of sand and scrub away, beats on your chest and shakes the ground you stand on. Once at full thrust, and unbound, the huge rocket at first appears to be losing its fight against gravity, then slowly rises to the occasion and is off over the ocean, fire and vapor trailing behind. Spacefarers are on their way to the Moon.

Three lunar voyages are most sharply etched in memory. The Apollo 8 astronauts, in December 1968, are the first to reach the Moon, circling it 10 times. Out their windows they see the achingly beautiful Earth, blue and green under swirls of white clouds. On Christmas Eve, the men take turns reading verses from Genesis. It is a gift from on high at a time of turmoil and despair in the year of assassinations, rioting cities and a divisive war.

Then there is Apollo 11. On July 20, 1969, Neil Armstrong steps down the landing craft’s ladder and takes “one giant leap for mankind.” Buzz Aldrin joins him for the first walk on the Moon. In contrast to exploration’s previous landfalls, the whole world is watching on television.

In the current documentary film “In the Shadow of the Moon,” Michael Collins, the Apollo 11 pilot who remained in lunar orbit during the landing, recalls that on the crew’s world tour afterward, people they met felt they had participated in the landing, too. “People, instead of saying, ‘Well, you Americans did it!’ ” he said, “everywhere, they said, ‘We did it!’ We, humankind, we, the human race, we, people, did it!”’

The warmth of shared experience was remarkable, given the origins of the space race in an atmosphere of fear and belligerence.

Apollo 11 essentially ended the space race, and public interest in spaceflight was flagging by the time of Apollo 13, in April 1970. The residual self-assurance that committed the country to Apollo in 1961 had given way to self-doubt. The war in Vietnam, another chapter in the cold war, shoved Apollo to the periphery of the national mind.

Apollo 13 is the mission that failed, but a drama of epic dimensions worthy of Homer. Three astronauts go forth on a daring quest, meet with disaster, face death and barely limp back to the safety of home. If anything, this brush with death put a more human face on spaceflight and made it seem more exciting, and dangerous.

By the end of 1972, the last of the 12 men to walk on the Moon packed up and returned home, and no one has been there since. At the conclusion of that flight, Apollo 17, I solicited historians’ assessments of the significance of these early years in space. Arthur M. Schlesinger Jr. predicted that in 500 years, the 20th century would probably be remembered mainly for humanity’s first ventures beyond its native planet. At the close of the century, he had not changed his mind.

•

In succeeding years, the Russians and Americans continued spaceflights, at a reduced pace. Most American money went into the space shuttles, the reusable vehicles confined to orbit that never lived up to their promise to make human flight more routine. The public’s most lasting images of the program are the Challenger’s deadly explosion shortly after liftoff in 1986, and the Columbia’s disintegration on re-entry 17 years later.

It was left to the relatively low-budget robotic spacecraft to sustain the impression of exploration and discovery on this new frontier. In that respect, they alone exceeded early promises. Russian and American craft explored Venus. American vehicles landed several times on Mars, and a European capsule reached the surface of Saturn’s moon Titan. Two Voyager craft made a grand tour of the four giant outer planets and are now approaching the edge of the solar system. The Hubble Space Telescope still sends images from deep in cosmic time.

Carl Sagan, the astronomer and author, often spoke of this as the golden age of planetary exploration. “In all the history of mankind,” he wrote, “there will be only one generation that will be first to explore the solar system, one generation for which, in childhood, the planets are distant and indistinct disks moving through the night sky, and for which, in old age, the planets are places, diverse new worlds in the course of exploration.”

•

One evening in 1990, I drove across Baltimore on a sentimental journey. Every so often since the fall of the Berlin Wall, the reunification of Germany, the collapse of communist regimes in Eastern Europe and the last gasps coming out of the exhausted Soviet Union itself, I had allowed myself reflections on my two years as a soldier in an unconventional war and the nearly half-century of anxieties of living in a world primed to blow itself up.

I could hardly think of myself outside the context of the cold war. Without the intense Soviet-American competition epitomized by the space race, I would not have become a science journalist who wrote about astronauts going to the Moon to “beat” the Russians. I would therefore not be in Baltimore again, this time with astronomers who were preparing to look into the heavens via a giant orbiting telescope.

I found my way to Travelers Lounge, the bar that had been across from the gate to the Army Intelligence School at Fort Holabird. We used to tarry in the back room there, over pitchers of beer fueling arguments about politics and the American novel. I took a stool and told the bartender that it had been more than three decades since I last had a beer here, back in my Holabird sojourn.

“One of them comes in every few months and looks around,” the bartender said. “We’re about the only thing left from those days.”

So I had seen. The fort was gone. In its place stretched one corporate complex after another, buildings of glass and steel and spreading car parks. The names I saw were as unfamiliar as their digitized new-technology goods and services. I imagined I was looking on a monument to the cold war, and how apt it seemed.

The conflict we had lived through did not lend itself to heroic and triumphal iconography, nothing like the Iwo Jima flag-raising statue, nothing to glorify war or proclaim victory. So these commercial enterprises rising from cold-war technology, supplanting an old fort, were working monuments to the end of the cold war, monuments that do not look back.

At least Travelers and I had made it through this passage in history. Over my shoulder, I saw families and couples dining, not a beer pitcher or soldier anywhere. I wondered what post-cold-war memories these diners would bring back there in coming years.

I took my leave of Travelers and an era. I had to be fresh in the morning for another meeting with people at the Space Telescope Science Institute. They were tending their own monument to the cold war, which had fostered the Hubble Space Telescope’s technology. I wanted to learn more of our — and my own — expanding universe.

•

Over a long dinner, after the cold war and almost 30 years since the first lunar landing, a former astronaut who walked on the Moon and one of the Apollo flight directors got to skylarking about the good old days, something people do when they think of their past receding and the world changing all around. They laughed almost to tears telling cherished stories, one trying to top the other.

Then a cloud seemed to pass over their faces. Pete Conrad, the astronaut, who would soon die in a motorcycle accident, and Gerald D. Griffin, the flight director, wondered in perplexity what had happened to their good old days. What of those grand prospects of a few decades ago? No humans have flown to Mars, as once predicted, or established a permanent base on the Moon. A long-sought orbiting space station was finally being assembled in orbit, but no one seemed sure what it was good for, except as a demonstration of cooperation by many nations, including Russia, in a major space endeavor.

Economics and shifting national priorities had thwarted the most ambitious post-Apollo plans.

Dr. Logsdon of George Washington University called Apollo “a product of a specific time in history,” and a singular crash program responding to a perceived threat to the country. It did not represent a firm commitment by society to full-scale space exploration.

As Dr. Roland of Duke pointed out, Apollo “did just what it was designed to do, which was to convince the world and ourselves that we were masters of technology, and it wasn’t designed to do anything else.” As yet, he said, “we have not identified a mission for astronauts that was commensurate with Apollo.”

Dr. Roland noted that telecommunications was the only space enterprise that pays for itself and, he added, “It has transformed the world.” All other space activities, military and civilian, depend so far on “what states believe are in their best interest to invest in” — and those interests have changed since the cold war.

Let Neil Armstrong, known as a man of few words, have the last word.

“I think we’ll always be in space,” he said in an interview for NASA’s oral history program. “But it will take us longer to do the new things than the advocates would like, and in some cases it will take external factors or forces which we can’t control and can’t anticipate that will cause things to happen or not happen.”

Mr. Armstrong then struck a note sure to resonate with many of his contemporaries. “We were really very privileged,” he said, “to live in that thin slice of history where we changed how man looks at himself and what he might become and where he might go.”