YOU'RE HAVING DINNER with physicist and space dreamer Gerard K. O'Neill in Washington. Listening to his ideas about the future, you begin to get the feeling you're somewhere else. You keep asking about these new worlds in space, and what it might be like for a child who's born there. Suddenly, the child seems to come alive:

THE SCENE IS THE SPACESHIP Gerard K. O'Neill as it throttled its ion-drive engines down from its 3 million-miles-per-hour cruising speed. Sally wiggled in her chair. It was Sept. 1, 2085, her 10th birthday, so at least she got to pick what scenes she wanted to see on the video screen. She liked images from the Greek Isle Colonies founded back in 2020, near Mars. It was much more interesting than all that really old stuff about Earth that her mother was always trying to get her to look at. But now, she remembered, Mars was the other way, behind her, back toward her own "Pennsylvania" in the Fox Cluster group of colonies near Pluto. And now that Earth was getting closer, it looked better, especially with that big sun that kept rising and setting every 45 minutes. At home, the sun was like a far-off orange, its light trapped in huge collecting mirrors.

Sally's three-month journey was nearly over; in a few minutes they would dock in Low Earth Orbit -- everybody called it LEO -- and make the short trip down to Earth on a shuttle. That, she knew, was about as far as the early astronauts regularly went a hundred years ago. Sally had even been named for one of them -- Sally Ride -- but felt a bit silly about that. She wondered if they'd have old-fashioned names like Sally in Waterford, grandfather's hometwn in the west of earthly Pennsylvania . . .

GERARD K. O'NEILL, not the spaceship of 2085 but the 58-year-old space visionary of 1985, moves like a rocket between planets. He has retired from his professorship of physics at Princeton, but lectures on space around the world and serves as president of Geostar Corporation, a satellite firm that he founded. He also functions as a major goad of the National Aeronautics and Space Administration. After appointments in Asia and before some in Europe, he has scheduled a few days in Washington to meet with colleagues on President Reagan's National Commission on Space, whose 14 commissioners are assigned to make recommendations on the nation's future course in space.

Lingering over escagots and Napa Valley red wine, he discourses eloquently about his favorite topic -- solving mankind's poverty and overcrowding by moving into space -- and easily imagines the impressions of a space-born child. "Sally" is a composite of that conversation and of places and experiences O'Neil vividly describes in two of his books, the award-winning The High Frontier -- Human Colonies in Space, published in 1976, and 2081: A Hopeful View of the Human Future, published in 1981

O'Neill began thinking about the human habitation of space in the heady days after the historic 1969 landing of men on the moon. Apollo fired imaginations because it was magnificent and "macho," to quote Jesco von Puttkamer, a German-born scientist who had come to work on Apollo as a prot,eg,e of Wernher von Braun. But by standards of the time, it was also extraordinarly expensive. Former President Dwight Eisenhower had spoken for many Americans when he declared in 1963 that the idea of spending $40 billion to go to the moon was "just nuts."

What use was this moon walk? O'Neill tried the question out on his best physics students, asking them to examine the possibility of humans living in space. To his astonishment, for nearly every question they raised, some ready answer seemed to be at hand within known technology.

It wasn't long before O'Neill began to project a vision of the future. By 2001, a space colony could be established. Thirty-five years after the initial colony, as many as 7.3 billion people could be living in space, mining raw materials from the moon and the asteroids, growing their own food and capturing their energy needs from the sun. Their life style would be sumptuous compared to the 1985 norm for Earth's estimated 4.8 billion inhabitants.

O'Neill's vision of the "High Frontier" consists of a profusion of enclosed colonies that orbit in free space. By rotating on their axes, they will have Earth-normal gravity in the inhabited areas and low-gravity areas on the colonies' high points for recreation. Residents will live in spacious garden apartments or houses, enjoy parks, birds, flowers, water sports, and all the urban amenities of shops, restaurants and theaters, while separate factories and greenhouse agricultural areas will be maintained outside the cylindrical habitat. "The space colony," says O'Neill, "is a very low-tech artifact. There's basically one moving part, and once you get it rotating right, it just stays." As for selecting the weather and length of the day, "It's all a matter of temperature, just a question of when you open and close the window shades."

The early colonies O'Neill envisions would be relatively small cylinders (about a mile in circumference), big enough to support only about 10,000 people. But within a few years, space residents would be ready to move on to the more ambitious models. Even with no technological advances beyond those of 1985, the second-generation colonies could attain the staggering land area of 8,000 square miles, more than half the size of Swizterland, large enough to maintain several million inhabitants.

It took five years for O'Neill to get his controversial ideas published; once made public, they attracted devoted followers as well as detractors. One major critic was von Puttkamer, now program manager for long-range planning at NASA. "Moving into space should be evolutionary," says von Puttkamer, "but O'Neill's ideas are revolutionary."

Like two antagonists in a celestial debate, O'Neill raises a clarion voice for visionaries, pointing out the high road to space, while von Puttkamer, a basso profundo for NASA, cries out that you can't get there from here. At least not yet. Speaking of O'Neill, von Puttkamer says, "Visions should be magnets to pull you into the future, but if you put them in the present, they become a juggernaut." For his part, O'Neill characterizes much of NASA's policy for the past 15 years as "elephantine stupidity."

O'Neill emphasizes that going to space does not mean giving up the pleasures of Earth. He likens it to the American West, which initially attracted outcasts and adventurers, and where life was at first rough. Like the West, he says, families, intellectuals, artists, religious groups and virtually every sort of person from every part of the world will go to space, bringing all the diversity and richness of Earth with them.

Curiously, the further away from Earth the colonists go, the more stable and conservative their colonies will tend to become. For that reason, O'Neill explains, a child like Sally could well be born next to a hypothetical asteroid named Fox Aggregate, "a cold, silent mass of billions of tons of ice, rock and metals," in the first Quaker colony on the fringes of Pluto.

PEOPLE KEPT CROWDING SALLY at the docking station. She'd seen the Russian ballet dancers once on their tour of the Outer System, and she'd seen the Inner System baseball champions, the Japanese, in an exhibition game, but she'd never seen anything like this -- Africans in long cloths and men with turbans on their heads. It had been fun floating out of the O'Neill into the terminal, and she had liked the little luggage cart that followed her around on voice command, but she hadn't liked the crowds at all.

Now she knew what her dad had meant when he said he had needed more space. For years he had been a musician, playing the best clubs and hitching rides between colonies in the Inner System, while mother had come up from Earth on an Inter-System exchange. But mother thought the Inner System was too competitive and materialistic. Looking for more simplicity, she put her savings into a ticket to Fox Cluster, trying to get in touch with her own Quaker heritage. Sally had heard a hundred times about how her parents had met in a docking station near Jupiter.

There was, she guessed, plenty of peace and space in "Pennsylvania" with its rolling hills, small villages and spacious farm-style houses. She suspected it was even too quiet for her dad. More and more often after doing his shift in the zero-g telescope lab, he made trips to the French Dominican colony. He said the music was better. So, she'd overheard, was the wine . . .

JESCO VON PUTTKAMER spent considerable time in the 1970s going from space convention to space convention, warning people that O'Neill's ideas were impractical. He believes that a small, influential and highly educated group of people will be the only inhabitants of space for the foreseeable future. O'Neill, he says, gives people the idea that they could just line up and go establish "a happy commune and strum their guitars . . . Even Timothy Leary supported him. It wasn't savory."

The cost alone is enough to stop anyone. It would cost about $220 billion to establish a space colony, von Puttkamer says, and even if possible it would strip every other space venture of foreseeable funds. "You can't do this on a shoestring."

O'Neill, however, has a different system for figuring costs. Overcoming Earth's gravitational pull, is what makes going into space so expensive. (The present price of getting material into low earth orbit, according to NASA, is $1,250 per pound.) When the habitat necessary to live in space can be manufactured at the site from raw materials mined from asteroids nearby, the costs become manageable, O'Neill says.

Physicist, futurist and author Freeman Dyson of the Institute for Advanced Studies at Princeton estimates that the first space colony envisioned by O'Neill would cost $96 billion and could pay for itself in 24 years by beaming solar electricity collected in space back to earth.

NASA officials say that a 1981 study by the Department of Energy shows that deriving electricity from solar power in this way would be too expensive and that the microwave system for transmitting the electricity would be dangerous. Critics of the study, including Gregg Maryniak, vice president of the Space Studies Institute in Princeton, say that the potential dangers of microwave transmission, if any, are still unknown. They also contend that the study's cost estimates are based on the erroneous assumption that the construction material will be shipped from Earth rather than mined in space.

While Dyson generally agrees with O'Neill's cost projections, he -- like von Puttkamer -- objects to the closed colony concept on esthetic grounds: he describes O'Neill's dream as "cans of metal and glass in which people live hygienic and protected lives." On one point Dyson, O'Neill and von Puttkamer are in accord: human migration into space will ultimately mean a less crowded, less polluted, wealthier Earth. The arguments are reminiscent of 18th century European thinking about the American colonies. Dyson puts it this way: "Earth may be directly supplied from space with scarce minerals and industrial products, or even with food and fuel. Earth may be treasured and preserved as a residential parkland, or as a wilderness area, while large-scale mining and manufacturing operations are banished to the moon and the asteroids."

For his part, many of O'Neill's speculations about future life on Earth center on the Waterford, Pa., of the 2080s, a Lake Erie community that celebrated O'Neill in the 1980s for putting it on the cosmic map.

THE WORST PART about Sally's visit to Waterford had been the other kids who laughed at her accent and called her a "dumb E.T." when she missed the ball playing soccer. She'd been a good player at home, but playing in Earth- gravity was so clumsy.

Her favorite thing was to sit with grandpa in the tropical garden next to his house. When the weather was stormy, the skydome that covered the town closed much like the closing of the shades on the great windows of the space colony. But it seemed funny to look into the sky and not see the villages overhead on the other side of the cylinder.

When the weather was good in Waterford, the dome opened and she could see the stars, the real stars, in real space. There was nothing between her and Fox Cluster. Nothing but a three-month ride on the O'Neill, and that return trip would begin pretty soon, right after the side trip to the Space Station Museum to see some old things from 1993 . . .

THE SPACE STATION might seem like a small step to a child of the 21st century, but it is the next giant leap for America in 1985. Von Puttkamer, quoting NASA's official line, says the station is "the next logical step" after the shuttle. Philip Culbertson, NASA's associate administrator for space station, says the shuttle is like a giant truck: it is driven to space, is unpacked to become a working laboratory, then is packed up again for the return to earth. In contrast, the lab on the space station will be a permanent installation, catering in part -- as does the shuttle lab -- to the needs of private firms.

Its inhabitants will be the first Americans to live in space (probably for no more than six months at a time) and its living quarters will likely serve as a model for other space habitats in the future. Although the interior design is not yet settled, the dimensions of the 14-foot-by-30-to- 40-foot cylinder will be roomy compared to the small three-bedroom-house size of its predecessor, Skylab.

Until recently, NASA had no long range goals for planetary or lunar settlements. "NASA just didn't want to talk about the avant-garde," says Jim French of California Institute of Technology's Jet Propulsion Lab, and a member of an informal association of scientists called the Mars Underground. Now two of the main boosters of returning to the moon are Michael Duke and Wendell Mendell of NASA's Johnson Space Center in Houston. They advocate establishing a moon base where about 20 specialists would live in a small self-sustaining ecological system: the group would raise its own small animals, grow plants for food in inflatable green houses, recycle wastes and use solar power.

Space planners are looking closely at the permanent scientific base in Antarctica as Earth's closest approximation to life on the moon. Optimists, such as Guillermo Trotti of the University of Houston, believe that a small lunar base would be so successful that the next step would be to build a fully developed lunar city. Skeptics, among them pro-Mars scientist S. M. Welsh, say that attempting to inhabit the "dead surface" of the moon borders on . . . lunacy.

"Compared to the moon, Mars is a veritable resource Garden of Eden," claims The Case for Mars, a composite report by several scientists and edited by Welsh. "It's the best available real estate off Earth."

One substance this "garden of Eden" lacks, though, is water. In order to support life, the first humans on Mars would have to find some way to melt its abundant ice caps. One method being discussed, says von Puttkamer, is to sprinkle them with 10,000 or more tons of soot brought up from Earth. Mars' first human residents would have to live in a self-contained enviroment. But melting large quantities of ice would produce humidity. Some scientists speculate that adding blue-green algae would create carbon dioxide and other gases, which could eventually produce an Earth-like atmosphere in which plants and animals could thrive in an open Martian enviroment. The process of changing Mars' atmosphere, called "terraforming," would take hundreds of years. Von Puttkamer adds: "It is also sheer hubris to think about when we still don't understand the ecology of Earth."

O'Neill sees speculation about the feasibility of settling Mars as almost irrelevant. "Mars is the wave of the past," he says. Isaac Asimov dismisses notions of settling Mars as a symptom of "planetary chauvinism." To O'Neill and Azimov, as to Sally, the idea of colonies in free space seems far more reasonable -- and attractive. BY 2095, the "back-to-the-planets" movement had gained strength again. Many people felt it was more "natural," but Sally thought it reactionary. After the limited success o the American settlement on the moon and the Russian base on Mars more than a half-century ago, the idea had lost support until recently.

Sally thought anyone could plainly see that free-floating colonies were better. After all, didn't people remain healthier in Earth-normal gravity, and not suffer from calcium loss, lack of muscle tone and space dizziness? And instead of overcrowding on the land areas of planets, hadn't more land been created as needed? Nowadays, they even had huge floating wildlife colonies where the endangered species of Earth could roam unhampered.

TO VON PUTTKAMER, the whole question of free-space versus planetary colonies should be brought down to Earth. Both ideas fail to take into account psychological factors. He suggests experimenters should lock up 1,000 people in a closed-system environment for three years on Earth to find out in advance what kind of a society they could expect to develop in space.

Von Puttkamer's doubts raise an even larger issue about space: Will man try to control technology so that space suits him, or will man adapt himself to the realities of space? Freeman Dyson objects to the "gray technology" of the space colony concept, and believes the long-range hope for the human race lies in a "green technology" beyond terraforming. "What I would hope is that we adapt life to the universe, rather than the universe to life." By techniques of biotechnology, which, he says, don't yet exist, he foresees man's expansion into space -- on planets, asteroids or comets -- as a natural evolutionary process. There are already dozens of examples of the human species' ability to adapt. Dyson cites as one the Mongolian nomads with tough skin and narrow eyes to withstand the cold winds of the steppes. "If some of our grandchildren are born with an even tougher skin and an even narrower eye, they may walk bare-faced in the winds of Mars."

And, he suggests, if that is possible, animals and plants can be "tauht" to adapt too. For a tree to grow on an airless asteroid, it may have to redesign its leaves.

"Our descendants will perhaps learn to grow gardens in stellar winds and in supernova remnants . . . Ultimately life will find its own ways to expand with or without our help. The greening of the galaxy will become an irreversible process."

Whether that process is ultimately green or gray, Dyson, O'Neill and von Puttkamer all agree it is time for mankind to begin an inevitable journey into the heavens.

To O'Neill the present era, with the very real threat of nuclear annihilation hanging over the human race, represents "a terribly dangerous few decades . . . (but) already by the time you build the first space colonies and get them dispersed around the solar system, the human race is probably virtually unkillable . . . Once you get them to the other star systems, then they're really unkillable."

But why would anyone want to go to another star systems.

Von Puttkamer says that mankind will make serious moves beyond earth orbit -- and the very real material rewards to be gained in Earth orbit -- only when human nature evolves to a less materialistic, more humanistic state. Dyson suggests that people will move on for the reasons they always have, from a sense of freedom, adventure and curiosity. O'Neill agrees, but also sees other likely motivations for continuing human emigration which are as old as the species itself: the threat of domination or war.

O'Neill can easily imagine man's hostility spreading to space. He offers up a resident of Fox Cluster, sitting on the outer edge of the solar system in 2105, who might say: "I'm getting really scared about what's going on . . . So-and- so dictatorship has taken over all the inner planets, it's going out with the idea that it's going to take over everybody in the solar system. Let's get out while the getting's good."

SALLY was also worried about the direction the solar system was going. Having just turned 30, she was thinking about the future -- and the past. These days she thought a lot about the "roots" trip she had made to Earth as a child.

Since that trip, she had become an avid student of history and now firmly believed that the human race had an almost endless capacity for change. Yet at some deep level, human nature remained constant. She particularly liked rereading a passage from the 20th century space thinker, Gerard O'Neill, who had said, "I can whimsically imagine people reading Shakespeares's plays on space colonies around distant stars 1,000 years from now and saying, 'It's all the same! It's all the same!'

It had troubled her as a child to be known as an E.T., yet she had come to see that being of Earth but not from it was to be a special kind of human. She and millions of others had been born off Earth, yet had always been attached by an artificial umbilicus, imitating Earth's rhythms, making day and night according to the strength of its sun. Now perhaps it was time to find other stars, other rhythms, other ways of being human. Now that new spaceships could cruise the galaxy at 60 percent the speed of light, perhaps it was time for her to go out in search of a new sun .