WASHINGTON - The idea of sending Sen. John Glenn, the first American to orbit the Earth, back into space is no passing fancy to research scientists or space officials.
While nothing has been decided, they say, the idea is far from frivolous.
For more than a year, Mr. Glenn, D-Ohio, has been involved in discussions with NASA and the National Institute of Aging (NIA) about medical research that could be accomplished if he went up again.
Scientists have long known that many of the medical effects on astronauts are similar to those that affect the aging on Earth.
They include weakening of the bones including osteoporosis, impairment of immune systems, disturbance of sleep and other biological rhythms and decreased ability to pump blood.
''It's not a whim,'' said Dr. Richard L. Sprott of the NIA, one of those involved in the discussions with Mr. Glenn.
''There are are some legitimate questions that could be addressed.''
Mr. Glenn chose the 25th anniversary of his Mercury space mission earlier this year to announce that he was retiring from the Senate after 1998.
But he has been letting NASA know for a long time that he is interested in going back into space, if his mission could serve a worthwhile scientific function.
''None of these seats (on the shuttle) should be filled by people just going along for the ride,'' Mr. Glenn said.
''I'm interested in it. I sure am. Whether anything works out or not is anybody's guess.''
Mr. Glenn's wife, Annie, is also agreeable.
When asked about the risk, she recounted stories of close calls that Mr. Glenn had as a fighter pilot and test pilot, not to mention concerns about his heat shield holding on re-entry from his orbital mission.
''That's just part of my life, I guess,'' she said.
Mr. Glenn, who will turn 76 on July 18, remains a licensed pilot and flew a B-2 bomber in 1995.
''We don't know whether an older person in space would do better or worse than a younger person,'' Dr. Sprott said. ''They might do better.''
NASA spokeswoman Jennifer McCarter said Mr. Glenn and NASA have been talking about the senator flying aboard the shuttle, although talks remain in the discussion stage.
''He would be a payload or mission specialist,'' she said.
Some in the scientific community would like to see Mr. Glenn go back into space.
''It would be good for America,'' said John Pike, director of space policy at the Federation of American Scientists.
For Mr. Glenn himself, he added, ''It would be fun.''
Mr. Pike said astronaut heroes such as Mr. Glenn and some of the early Soviet cosmonauts used to be considered too valuable to their countries to risk sending up a second time.
Indeed, former President Kennedy blocked any thought of a second Glenn trip after his successful 1962 mission.
''The shuttle is probably a lot safer than the Mercury was,'' Mr. Pike said.
''I'm sure they would find something for him to do while he was up there.''
Scientists are intrigued by the research possibilities involved in mixing the subjects of aging and space flight.
''Science fiction literature notwithstanding, studies of the biological effects of space exploration and studies on the biology of aging seem, at first glance, to be unlikely partners,'' Vincent J. Cristofalo of the Medical College of Pennsylvania wrote in a paper on the subject.
''However, upon closer examination, we see some apparent parallels between the changes that occur in microgravity and those that occur in senescense (aging).''
He added, ''Age-related bone loss has a parallel in prolonged microgravity in which there is substantial bone calcium loss.''
But Dava Newman, professor of astronautics and aeronautics at Massachusetts Institute of Technology, said she is skeptical whether sending someone of Mr. Glenn's age is the nation's highest space priority.
''It would be very expensive. You should do it for the best scientific reason, not for the popular press reason,'' she said.
Ms. Newman, who developed two experiments for the space shuttle and one for space station Mir, said the country still has relatively scant knowledge about the effects of prolonged space flight on astronauts in more typical age ranges.
''It could have scientific merit, but I would have to see the specific proposal that NASA has in mind,'' she said.
Mr. Glenn said he is particularly interested in the effects of space on the immune system. Learning more about how it turns on and off could be valuable in fighting disease, he said.
''Sen. Glenn has worked hard at understanding the science. He does his homework,'' said Dr. Sprott of the National Institute of Aging.
