CAPE CANAVERAL, Fla. (AP) - Despite all the talk at NASA about using John Glenn as a guinea pig for geriatric research aboard Discovery, Glenn's advanced age will be of relatively little scientific benefit.

NASA always planned to study the problems of aging in space during the shuttle mission that is set to begin Thursday. But putting Glenn aboard was an afterthought.

''This was never designed as a flight to study a 77-year-old astronaut,'' said Dr. Richard Hodes, director of the National Institute of Aging, a partner with NASA in the medical studies. ''The scientific studies for this flight were approved long before there was any crew selection.''

Researchers say the space agency is not really looking for ways to fly the elderly in space but is trying to learn how young and healthy astronauts could medically endure a long mission to other planets.

Flying in space causes astronauts to rapidly and temporarily develop many of the same disorders that strike the elderly on Earth. The institute formed a partnership with NASA in hopes that the space research will discover ways to treat aging disorders on the ground.

NASA needs to find ways to prevent the serious deterioration that weightlessness causes to the human body. If answers are not found, experts believe, humans will never be able to fly on missions that would require two years or more in weightlessness, as in Mars.

Glenn had persuasive clout as a U.S. senator from Ohio and as one of America's space heroes. And the reasons for his presence aboard Discovery perhaps have more to do with symbolism than science.

''I felt America owed John Glenn a second flight,'' said NASA administrator Daniel Goldin.

During his nine days in space, Glenn will be a medical guinea pig. The fact that he is 77 contributes little to the basic experiments, experts say.

Dr. Kenneth Baldwin, a NASA researcher and a professor of physiology at the University of California at Irvine, said to draw broad conclusions about the effect of spaceflight on older people would require many experiments over many flights. NASA has no plans to fly any other old astronaut.

Hodes said that Glenn's research ''can't be representative of all people in that age range'' because the astronaut is unusually fit for his age. One researcher said the senator has the body of a 60-year-old.

Space fliers and the elderly on Earth suffer the same general health effects. There is a serious loss of muscle mass and strength. Bones leak minerals and become more brittle. Sleep is short and restless. The immune system weakens. And about 70 percent of astronauts, once they return to Earth, experience spells of dizziness, a common problem among the elderly.

One important question that Glenn may help scientists answer is whether age magnifies the damage caused by weightlessness, an effect that could put an age limit on space fliers.

''With John Glenn, one can ask whether his muscles deteriorate faster since he is two times the normal age of other astronauts,'' Baldwin said. ''Nobody knows the answer and that is one reason to fly John Glenn.''

Some experts believe that after the flight ends, the stresses of readjustment to Earth's gravity will be far more severe for Glenn than for younger astronauts.

''I have a subhypothesis that

the stress will be more telling on him and it may take him longer to recover,'' said Arny Ferrando, a scientist at the University of Texas Medical Branch, Galveston, and one of the prime researchers on the Glenn mission.