The gauges told him he was weightless. But he felt nothing . . . No doubt he should say something to the ground about the sensation of weightlessness. It was the great unknown in space flight. But he didn't feel anything at all! He noticed a washer floating in front of his eye . . . That was the only evidence his five senses had to show that he was weightless . . . It was as if he were standing still, parked in the sky.

-- Tom Wolfe, in "The Right Stuff"

A quarter century into the space age, it's easy to forget the dire predictions of what would happen to Alan Shepard during his 15-minute flight into space inside a cramped Mercury capsule in 1961.

Before Shepard's launch, some of President Kennedy's top scientific advisers worried that Shepard's pulse would race to 180 beats a minute and that he might go into cardiac arrest as he hurtled weightless through space.

That didn't happen, of course. Shepard's heart rate during the brief flight peaked at 136. And the NASA medical team was as relieved as the astronaut himself.

"That was an entirely different time," recalls Dr. Charles A. Berry, former medical chief for the space agency. "It was sort of like being alive at the time of the Wright brothers."

It was, after all, the new frontier. No one knew for sure how a person would react to the strain and confinement of space travel -- and particularly to weightlessness. Although weightlessness can be achieved for brief times in free-falling airplanes, and some of its effects can be simulated in whirling centrifuges, on tilting tables or under water, zero gravity cannot be achieved on the ground.

Before Shepard, the Soviets already had completed an orbital flight. But NASA's only emissaries to outer space had been a couple of spider monkeys and a chimpanzee named Ham.

"No one was convinced that man was going to be able to perform in space," Berry recalls. "We had great arguments before Shepard's flight. I had to keep reminding them other doctors that we were talking about normal, healthy people. They were thinking about patients, people who were ill."

In fact, Berry says, the most enduring and important spinoff from the nearly three-decade-old space program is not Teflon or food-in-a-tube or flame-resistant foam but a whole new emphasis on preventive health.

As NASA's chief medical officer from 1959 to 1971, Berry was responsible for monitoring and protecting the astronauts' health during their training and space flights. Instead of treating sick patients, he had to anticipate -- and prevent -- illness that could strike otherwise healthy men in the extraordinary environment of outer space.

NASA's medical team, dubbed the "white smocks," put the astronauts through test after test to collect files of "well data" for study and comparison.

"The whole concept was really a foreign one: doing tests on healthy people," Berry says. "Nobody was doing it. Doctors were thinking about ill people and raising all those questions because of their background with illness instead of health.

"There was even concern about us being able to monitor astronauts at a distance without being able to touch them."

Sophisticated stress tests -- now a mainstay of many cardiac fitness programs and health evaluations -- were unavailable when the first seven Mercury astronauts began their training. A stress test amounted to walking repeatedly up and down a few stairs before getting an electrocardiogram.

NASA doctors couldn't even measure an astronaut's blood pressure, until they figured out a way to insert a remote-control blood-pressure cuff inside a pressurized space suit. Scott Carpenter, the second American astronaut to circle the earth, was the first to have his blood pressure taken in space.

What Berry calls "remote medicine" -- the effort to "stretch our stethoscopes 240,000 miles into space" for moon flights -- speeded development of monitoring and telemetry devices that are now standard equipment in intensive care units, coronary care units and paramedic ambulances.

Skeptical doctors weren't NASA's only doubters. The super-fit, right-stuff astronauts themselves weren't exactly keen on the white smocks' need for all those medical tests.

"You know test pilots," says Berry. "You give them a white scarf and a jock strap and they're gonna take that aircraft and go. They said, 'You're gonna do tests on us?' "

Astronaut Michael Collins, in his book "Carrying the Fire," said some of the NASA white smocks were "prophets of doom and gloom from the very beginning. It had been like pulling teeth to get them to admit that man had escaped unscathed from each successive foray into weightlessness.

"When Al Shepard seemed to be his normal, healthy, obnoxious self after 15 minutes of weightlessness aboard Freedom 7, the medics moved the decimal point over one place and said, 'Well, yes, man can endure for minutes and perhaps even for hours, but for days -- Horrors!' "

But as evidence of humans' ability to adjust to weightlessness accumulated, NASA gradually lengthened the space missions and turned its attention to other logistical challenges.

After the Gemini 5 mission in 1965, when L. Gordon Cooper and Charles (Pete) Conrad stayed in space for eight days, Berry and the astronauts were sent on a goodwill tour to Europe and Africa. During their triumphal visit to Ethiopia, Berry was surprised to have Emperor Haile Selassie pull him aside behind his throne to ask the one question that had been driving the emperor crazy: "How did they go to the bathroom?"

The emperor's question had no easy answer -- then or now. During the Apollo era, astronauts used plastic bags -- with a one-way valve for urine and a self-sealing opening lined with double sticky tape for feces.

"You sort of dropped your pants and stuck the bag on your hind end," says a NASA spokesman. "It took about an hour to go to the bathroom. A pretty brutal experience."

Efforts to design and test a unisex space-shuttle toilet -- using a series of fans to create differential air pressure to keep the waste from scattering throughout the cabin and a sort of "washing machine" to separate liquid from solid waste -- have encountered "ongoing difficulties." On the last three missions NASA has gone back to the Apollo bags.

Berry, who left NASA in 1974 to become president of the University of Texas Health Sciences Center in Houston, now heads a Houston consulting firm on preventive and aerospace medicine. The firm works with employers and insurance companies to encourage healthy life styles.

Just as he did when he was doctor to the astronauts, he sees his job as keeping people well -- determining what "normal" is for each individual and then trying to maintain or improve that condition.

"And you don't have to be in space to be in an abnormal environment," Berry says, shaking his head in disgust. He has just left a meeting of insurance executives, two of whom were chain-smoking cigarettes.

One of the difficulties in evaluating the effects of space flight on health was that every person is unique physiologically. What is "normal" varies from person to person. If one astronaut got space-sick and another didn't, was the difference due to environmental or individual factors? Compiling "well data" helped provide the answer.

Shortly before the Apollo 9 launch in 1969, Berry put the three-man crew through extensive laboratory tests. He found only one troubling sign: mission commander James A. McDivitt's white blood cell count was 8,000.

Because of his burgeoning file of "well data" on all the astronauts, Berry knew this was a bad sign. Even though 8,000 is within normal range for most people, it was twice McDivitt's usual white blood count and signaled an imminent infection. Sure enough, McDivitt came down with the classic symptoms of a cold.

Berry postponed the Apollo 9 launch for three days, "being told every day how many millions of dollars I was costing." But the alternative was the risk of sickness overcoming the entire crew in outer space.

The astronauts themselves had to make some sacrifices, quitting smoking (three of the original seven had been smokers), modifying their diets and exercising regularly to stay fit.

"We had a great motivating factor: This is what you've got to do if you want to fly," Berry says.

Back on earth more than two decades later, the challenge isn't all that different. More and more Americans are trying to maintain health and prevent illness. The motivating factor for individuals is: This is what you've got to do if you want to stay healthy. For businesses and insurance companies, the motivation is cost.

"For the first time, preventive medicine has come into its own," Berry says. "Prevention is the name of the game if you're really going to cut costs."

The major killers at the turn of the century, he points out, were viruses and bacteria: pneumonia, influenza, tuberculosis and diarrhea. Today, those diseases have been controlled in most industrialized countries by improved sanitation, antibiotics and vaccines.

"We have a totally different ballgame," he says. "Today, the big killers are heart disease, cancer, stroke and accidents. No viruses, no bacteria. The viruses and bacteria today are what we call risk factors.

"The vaccines we need today are smoking cessation, low-fat and high-fiber diet, stress management and exercise."