Twenty years ago, Robert Truax was one of the bright young men of John F. Kennedy's New Frontier, a visionary who saw the stars and to reach them.

Since then, though, while the nation's passion for space may have waned, Truax remains a true believer. But today, at 63, the still-trim, crew-cut retired Navy captain is working with a group of youthful volunteers in his cluttered back yard, building a missile that he hopes will send a man into space next fall for a 10-minute, 15 mile flight over the Pacific Ocean.

While many of his former aerospace colleagues scoff, Truax has collected about 200 recruits, mostly in their 20s and early 30s, eager to make the maiden flight on his homemade 25-foot-long X3 rocket, which lacks life-support equipment. Equally important, Truax has persuaded about 40 space quest, which he calls Project Private Enterprise.

To Truax, one of the architects of the Navy's Polaris missile system, the fearless, even brash enthusiasm of his young supporters reflects a growing sentiment, particularly in California, for the return of a visionary commitment to space like the one that marked the 1960s.

"These kids perceive the future of the human race is in space just like we used to do," Truax said. "They are really the hope because you can't do much with the old dogs in power today. It will take new dogs to get us into the new future in space."

With Truax and other Apollo-era veterans as their examples, thousands of young people are becoming increasingle active in promoting the cause of space exploration. Some are joining fast-growing organization like the L-5 Society, which takes its name from the point in space selected by Princeton physics professor Gerard O'Neill as ideal for the first space colony.

Since its founding in 1975, L-5 has grown to more than 4,000 members nationwide. Howard Gluckman, the 25-year-old chairman of the large Los Angeles chapter, believes the current low-profile national space effort will soon be dramatically jolted by his age group, a generation raised on the moon landings by American astronauts, the "Star Trek" television series and the "Star Wars" movies.

"You've got a lot of young people who grew up with the space program and now they're in responsible positions to do something about it," Gluck-man said. "We've believed in space all the time and that's the difference between us and the old people are afraid to dream, afraid to talk about the real reason for going into space."

The "Real reason," according to Bluckman, is the belief that only can provide the minerals, energy and habitats to accommodate the growing numbers of people on Earth. He sees solar space stations as a possible solution for energy shortages, lunar mining as the means to replace increasingly scarce minerals and large space colonies as the new homes for millions of the world's population.

In dramatic contrast to the early 1970s, when interest in space and high technology waned in the bitter aftermath of the Vietnam war, students now are flocking to the lecture halls to hear speeches on space colonization by such veteran enthusiasts as O'Neil and Dr. karfft Ehricke.

Ehricke, a longtime NASA consultant and associate of the late Wernher von Bruan dating back to the early experiments in rocketry conducted by Nazi Germany during World War ii, now finds himself in demand as a campus speaker on his favorite topic -- the need to establish a large colony on the moon.

"In early '70s I was booed as a member of the military-industrial complex. Now I find I have a wide-eyed audience," Ehricke said. "These young people are lookimg for their future. Space offers them that -- room unlimited.

While increasing numbers of students study aerospace engineering and attend space career seminars, some space cadets," as they sometimes call themselves, are deeply frustrated by the severe limitations placed on NASA by its budget.

Last month, a group of California-based space activists initiated an effort to raise $1 million to underwrite the cost of analyzing data being beamed back to earth from the Martian surface by the Viking probe. Without these funds, according to NASA spokesmen, large amounts of this data will remain uninvestigated at the Jet Propulsion Laboratory in Pasadena.

The Viking Project, as this volunteer effort is called, has raised $50,000 from 5,000 individuals, many of them young people.

"What we're doing with the Viking fund is saying that if you're willing to spend two hours on line and spend $5 to see a science fiction movie, why not put the same amount of money into building for the real thing? said the fund's director, Stan Kent, a 24-year-old British-born aerospace engineer.

While many space activists applaud the Viking fund's efforts, some are beginning to turn toward lobbying to pressure Congress into increasing the funds for the national space program. Groups such as L-5 have approached the aerospace industry in recent months in an effort to promote a newly formed organization called the Space Coalition.

More than 130,000 workers involved in space-related activities provide a potent space special group, the Space Coalition's organizer; Lee Ratiners, believes. By linking the aerospace industry with enthusiasts active in such things as Project Private Enterprise, Ratiner foresees the space lobby someday emerging as an important force on Captiol Hill, much as the farm or labor union lobbies are today.

Still in its early phases, the Space Coalition is likely to present its first laundry list to Congress this fall. It may include proposals for permanent space stations, tax credits for private space endeavors and studies of space endeavors and studies of space colonization.

"There are a lot of people who want to be a part of the new future, a new society," Ratiner said. "It will take a coalition of economic interests and grass-roots support to get it going. It won't be easy -- ther's still a lot of cynical b . . . . . . in Washington -- but the movement is growing and we're getting off the ground.