Colonies of ordinary people could be living in space and beaming solar electricity back to earth in about 20 years if Congress makes the right decisions now, members of Congress and their staffs were told recently.

A two-day seminar organized by the Committee for the Future, a private, nonprofit organizations, drew 150 to 250 bureaucrats, congressional staffers and civic group members, as well as several members of Congress, to hear the case for the idea.

The committee asked approval of a congressional feasibility and impact study.

"We could be producing energy for the earth by 1992, a fitting observation of the 500th anniversary of Columbus' discovery of America," said Barbara Marx Hubbard, author and cofounder of the Committee for the Future.

The proposed study would examine the possibilities and problems raised primarily by Dr. Gerard K. O'Neill, a respected Princeton University physicist and author of "The High Frontier," the bible of the movement to set up space colonies.

O'Neill told the group that his proposals would require an investment of $50 billion to $60 billion over the next 10 years, but would become "almost obsecenely profitable" around the turn of the century because of earth-bound needs for power and products that could be manufactured easily in space.

Since O'Neill first voiced the notion of space industrialization in 1969, his ideas have sparked reams of academic and scientific study. There have been several conferences involving the National Aeronautics and Space Administration, which O'Neill said would soon publish "conceptual solutions" to all problems readily apparent in his theoretical proposals.

There is even an enthusiastic organization, based in Tucson, Ariz., of people who would like to populate a space colony. It is called the L-5 Society after the theoretical point between the earth and moon where stable orbit could be achieved.

The group's regular publication, L-5 News, keeps track of serious space travel literature, reports on aerospace news and legislation and instructs its readers in lobbying techniques and on recent vote coounts in Congress.

The notion of space colonies also has fervent enemies. Many critics on the left say the money could be used much more profitably solving earthly problems and that space travel is sheer muscle-flexing by a technology-happy elite who are bored with terrestrial entertainments.

Critics on the right say cost estimates are vastly understated and time projections are too optimistic. O'Neill seems to have answer for everything.

"Satellite power generation would speak to an urgent need. The market for energy could be $200 billion to $400 billion a year by the 1900s," he said.

Providing cheap energy for the world's poor nations would help them more than any $50 billion investment in food or other direct aid possibly could O'Neill said. He said space industrialization would require no technological breakthroughs, "just a lot of sophisticated engineering," and would rely on the proven possible uses of the space shuttle.

O'Neill outlined the possible sequence of events, drawn up in conjunction with NASA, that would enable power transmission to earth from a satellite station by 1992. A two-year study, beginning next year, would be the first step, and a decision to proceed would have to be made by 1980, O'Neill said.

"Between 1981 and 1984 there would be a program of engineering development and testing on the scale of the Apollo program" that put astronauts on the moon in 1969, he continued. A mission in 1985 would establish a mining facility on the moon "on a tiny scale by earth standards, one or two remotely operated bulldozers, really."

By 1997 the mining station would begin sending lunar material - iron ore, rock, oxygen compounds - out to a predetermined orbital point where a satellite would be constructed. That would be constructed. That would take until 1992, when power transmission could begin.

Diverting a small percentage of the solar power produced to the building of a colony could enable as many as 100,000 people to live in space by the turn of the century, O'Neill said. They would live in rotating cylinders that would be able to simulate farms, weather, private homes and even trees and streams.

O'Neill showed the congressional gathering a filmed demonstration of a sort of electirically powered slingshot he invented, called a mass-driver, which would transport the lunar material to the orbital point. The mass-driver could accelerate a "bucket" full of material from zero to 80 miles per hour in 1-10 of a second, almost too fast to see.

O'Neill complained that support for further research and development of space colonization and of the mass-driver had been cut because "the new people in the administration don't know what the possibilities are and haven'thad time to brief themselves on it."

Another speaker at the conference, held in Washington 10 days ago, urged the congressional group to "hurry up and get to work" on space colonization Alex Parker, owner of the No.1 Time Square building in New York City, said he planned to circulate a petition to get 10 million signatures in support for the idea.