When Alan B. Shepard Jr. became the first American in space, he rose only 115 miles off the Earth's surface atop a converted military missile. But the success of his daring launch on May 5, 1961 raised the hopes of a nervous and dispirited nation immeasurably higher.

In the uneasy spring of that year, the United States -- the world's paramount postwar technological power -- was still reeling from the shock of the Soviet Union's four-year string of space triumphs. It had begun with the launch of the first satellite, Sputnik I, in October 1957, and continued to April 12, 1961, when the USSR made cosmonaut Yuri Gagarin the first human in space and the first to orbit the Earth.

Compounding the crisis in American self-confidence, it was only five days after Gagarin's return that the world learned of the abortive U.S.-controlled incursion into Cuba known as the Bay of Pigs invasion.

"There was gloom and doom all over the country," said NASA Administrator Daniel S. Goldin, who was an engineering student at the time, until Shepard "went up and lifted our collective spirits." Shepard, who died of leukemia Tuesday at the age of 74, was "a rare American hero -- tenacious, committed and courageous," Goldin said. "It's the kind of stuff that gives one goose bumps and inspires a generation."

Space shuttle launches are now so routine, and the notion of space exploration so familiar, that it is nearly impossible to recall the uncertainties that faced humanity's off-world pioneers, especially Shepard and the other six astronauts chosen in 1959 for Mercury, America's first manned spaceflight program.

Shepard's mission lasted just 15 minutes and 28 seconds, as he roared 302 miles "downrange" from Cape Canaveral and splashed down just east of the Bahamas. But it was very risky in a number of ways.

For one, "we didn't even know what was going to happen to the human anatomy" under the stress of launch, Goldin said. "Would the eyeballs stay in their sockets? Nobody knew. What would happen to normal bodily functions?"

Of course, Gagarin's flight, which completed one orbit of the Earth, "more or less took those objections away," said John M. Logsdon, director of the Space Policy Institute at George Washington University, "but not totally." Shepard's mission was still "a biomedical experiment of the first order," Logsdon said.

Although Gagarin had apparently emerged healthy after his 108-minute circuit, no one in America could be certain. The Soviet Union only announced the event after its cosmonaut had already returned to Earth, and few details were made public. So some American physicians remained worried about the possible effects.

And for good reason: Shepard's vehicle was not intended to carry people. It was a Redstone rocket, a direct descendant of Germany's World War II-era V-2, designed by Wernher von Braun's engineering team as a battlefield missile with 200-mile range. As modified for Shepard's mission, it was capable of speeds above 5,000 miles per hour, producing 11 G's -- that is, 11 times the normal force of gravity. Not only would a 175-pound astronaut feel as if he weighed a ton, but it would be extremely difficult for arteries to maintain blood flow to the brain. After enduring that, the astronaut would then abruptly become entirely weightless before plunging to Earth in a parachute-rigged space module barely larger than a phone booth.

While physiologists pondered the medical consequences, other experts worried about potentially devastating effects on national self-esteem if the launch should fail.

"These were military missiles that didn't have the reliability we've become used to in later years," said Frederick I. Ordway III, who at the time of the launch worked at the Army Ballistic Missile Agency as a member of von Braun's rocketry team.

Several different kinds of missiles had experienced spectacular failures in the months prior to Shepard's mission, though "that was a risk everybody accepted," Ordway said. But unlike the Soviet efforts, "we did it in front of the whole world and announced it in advance."

An American space catastrophe on live TV would have damaged the nation's reputation, Logsdon said, and "it was debated right up to the last minute whether it would be shown {on television} or not." President John F. Kennedy, who had been in office only four months, "ultimately made the decision that we were going to do this thing in the full view of the world," Logsdon said.

Mercury officials had narrowed the list of possible astronauts to Shepard, Virgil I. "Gus" Grissom and John Glenn. "It wasn't until a short while before the launch that it was announced that it would be Shepard," Ordway said.

At 9:34 a.m. on Friday, May 5, after more than 24 hours of delays to fix technical problems and wait for good visibility, Shepard found himself strapped to the nose of Spacecraft No. 7, a 70-foot-long, 6-foot-wide Redstone whose alcohol- and liquid-oxygen-fueled engines were about to explode into 75,000 pounds of thrust. "The probability of success was nothing like what it is today," Goldin said. "Yet he just crawled into that little tin can and went into space."

It was over in minutes, and did not equal the Soviet achievement. (Gagarin was later quoted ungenerously as saying that the USSR had already done the same thing with a dog -- a reference to the second Soviet satellite launch in 1957, which carried a dog named Laika.) But it determined the course of U.S. space research for decades and "had a remarkably reinforcing impact on the decision to go to the moon," Logsdon said.

"Over the weekend, the recommendation that Kennedy adopt a lunar landing goal was put together by NASA, the Defense Department and the Bureau of the Budget," Logsdon said, and it arrived at the White House on Monday -- just as Kennedy was awarding Shepard the Distinguished Service Medal.

In 1963 -- while training for the first Gemini manned mission -- "Big Al" Shepard contracted a rare inner-ear ailment that left him grounded, albeit as chief of NASA's astronaut office. The first American to leave the Earth, it appeared, would never set foot on the moon. But then in 1969, a few months before the first Apollo lunar landing, he underwent experimental surgery. It worked. Shepard was returned to active flight status, and soon named mission commander of Apollo 14, which landed on the moon in 1971. Among other accomplishments on that mission, Shepard became the first person to hit a golf ball on the lunar surface.

To the nation's original astronaut corps, Sen. John Glenn (D-Ohio) said yesterday, Shepard stood out as "a highly intelligent . . . dedicated leader whose motivation towards accomplishing our mission was a true inspiration to all of us." CAPTION: THE FIRST STEP: MERCURY-REDSTONE 3 (This graphic was not available)