The roar of the rocket engines fills your ears as nearly 7 million pounds of thrust presses you back in your seat.

The shuttle escapes the ground and slides past the gantry. In a few minutes, gravity, too, will be left behind. The first sense that the force which has dominated your life is gone comes from the gentle tugging of the shoulder harnesses that anchor you in your seat.

Slipping out of the harnesses, you find the swimming motion you make in mid-cabin doesn't help much. Agility without gravity takes practice. After that, "it's like being Peter Pan all the time," says Charles Walker of McDonnell Douglas Corp., who has flown on the shuttle.

In the distance, you can see your destination: The power tower, mankind's first permanent presence in space, rises with the sun over the rim of the earth.

America's new space station looks like an oversized Erector set unifying a collection of soup cans and cereal bowls.

It has been nearly a decade and $8 billion since January 1984, when President Reagan challenged the National Aeronautics and Space Administration to create a space station. NASA was ready; so were the contractors.

Since then, through the early 1990s, the shuttle has hauled two habitation (hab) and two laboratory (lab) modules -- each about as long as a MetroBus but twice as wide -- into an orbit 300 miles above the ground.

Your shuttle carries a fifth module stuffed with food and fuel.

Another problem of weightlessness intrudes on your consciousness. Fluid normally pulled into your legs by gravity has migrated into your head and chest. It's making your nose feel stuffy, and your head feel full, as if you were suddenly hanging upside-down on earth. The full feeling will fade as your body eliminates fluids by urination.

In a few weeks, you'll be a few inches taller. Without the pull of gravity, the spine elongates slightly. For some space travelers, this causes muscle strain for a few days and changes the size of their space suit.

You also feel a general uneasiness in the pit of your stomach. The part of your inner ear that senses whether you are right-side-up or upside-down doesn't function normally without gravity. It sends signals to your brain that conflict with the right-side-up view of the world that your eyes see. The result: "space adaptation sickness," which is much like motion sickness. This passes in about three or four days.

There is a gentle but decided clunk as the shuttle docks with the floating station. Through the airlock, you get your first glimpse of home for the next 90 days.

Although there is no true up or down without gravity, the mind needs a visual up and down. The modules are built so there is a floor, with all the cabinets and consoles oriented toward the floor. And the ceiling is always "up." When you move from module to module, the floors line up.

Spinning the space station would have created the effect of gravity, says Gary Johnson, McDonnell Douglas' lead engineer on the "hab" modules, but it would have created control problems. More important, weightlessness is one of the primary reasons for going into space. It allows technological procedures impossible on earth.

All crew members -- which will include men and women of various nationalities -- get a private, sound-proof compartment for sleeping, personal storage and entertainment.

Bronze mirrors surround the top of each compartment's body-shaped couch to give a feeling of spaciousness. The wall directly before your eyes contains a computer screen; the keyboard folds down onto your lap to let you control computer functions, television, movies, games and communications.

Sleeping without gravity is a matter of individual style. Some like to float with only a loose tether to keep them from drifting too far. Others crawl into an anchored sleeping bag, or lace themselves to their couch and use a net to keep their head from floating around.

Eating, too, is different without gravity. Liquids cling to themselves. Most astronauts admit to spending some time poking a shimmering glob of water around the cabin as if it were a floating bubble and then sipping it in mid-air. Generally, however, lids are used to confine liquids to a cup. Neither cooking -- most will be done in a microwave -- nor the foods themselves will be much different from on earth. Food will be designed to be sticky so it stays on the fork. Peas will have a glaze so they hang together like Chinese rice.

Crumbs will be a problem.

Personal hygiene will be tricky. The space station's permanent shower allows a quick wash, but water doesn't go down the drain. Instead, pounds of water, quivering like a silver jellyfish, will cling to your body until you wipe it away with your hand and fling it against the wall, where some will stick and some will splat into floating globs of liquid. A vacuum cleaner is needed to clean up.

Dressing is easier. "Space is a great equalizer," says Walker of McDonnell Douglas. "Everyone puts their pants on both legs at a time." You learn to hang your clothes in mid-cabin, where they retain their shape as if occupied by an invisible man. Then you simply float into them.

After a week in the space station, you watch from the ward room window as the shuttle moves away from the station and heads back toward earth.

Your ride home just left. For the next 83 days, you and five other crew members are on your own. There are no emergency escape pods if something goes wrong or someone gets sick. Pods are too expensive.

The real endurance test is about to begin.

"Living together with most people is an art," says B.J. Bluth, a sociologist from California State University at Northridge on loan to NASA.

"Living together in an environment where you cannot leave will be high art."

In this early phase of the space station's existence, much of it is still being constructed by crew members jetting around the tower in space suits.

The suit is difficult to climb into: you crawl in through a slit the back. It is stiff and ungainly even without gravity, and the pressure inside is lower than in the space station, so a walk in space can cause decompression sickness similar to the "bends" experienced by scuba divers.

The stuggle into a suit, however, is worth it. Outside the "hab," the dazzling brilliance of the sunlight reflects from everything. The space station's speed over the water and land masses some 300 miles below becomes apparent for the first time; it's as if you were looking at the passing countryside from atop a speeding locomotive. Then you notice nothing flaps in the breeze. There is no breeze, no sound, no vibrations. "Just like a magic carpet," remembers Skylab astronaut and spacewalker Jack Lousma.

There's danger, too. Although it's easy to move a piano-sized object with one hand, it's also possible to get crushed between two moving, massive objects.

Without a nearby emergency room, traumatic injuries must be treated in the station's infirmary. It will be equipped to perform standard diagnostic tests and use imaging equipment to see inside the body -- the results of which will be automatically entered into the medical computer.

The infirmary isn't equipped for major surgery, but surgery could be performed if it were the only way to save a crew member's life.

Weightlessness on long missions can harm the body. The heart works less hard and becomes deconditioned to the rigors of gravity. Muscles weaken, bones lose density, the body stops making blood cells.

Most of the problems can be controlled with diet and exercise. Treadmills and stationary bicycles have been used in the past, and a simulated mountain climbing machine may be used in the future. Because crew members will have to exercise one to two hours a day, the machines will become boring. Engineers are working on a version of weightless handball.

"A Superball in the hab module would bounce around for a long time," observes McDonald Douglas' Johnson.

Working without gravity will be both easier and harder. Manipulating huge equipment will be easy. And the ability to hang your body in mid-air makes areas that would be hard to reach on the ground easily accessible. But working with many smaller parts is a problem. Nothing stays on the counter where you laid it. "Velcro was real important," said Walker. Skylab astronauts used a lot of adhesive tape.

After three months, the shuttle returns to take you home to earth.

Readjusting to the weight of gravity takes time. "You felt very heavy the first 24 hours after a long flight," says Lousma. After two months in orbit, "we did like to sit down a bit for the first couple of days." Although all of the physical effects are reversible, the memories of the weightless life linger, sometimes to be confused with the new reality of gravity.

"The first night back, I tried to float out of bed," says Lousma. "I saw the open bathroom door . . . I pushed my feet in the direction I wanted to go, but I did not get out. I immediately realized I was in a different environment."