NASA's decision to send a 77-year-old John Glenn back into orbit has reignited the space race, only this time the contenders are seniors.

Some of the nation's hardier, older souls have offered to replace Glenn aboard shuttle Discovery this fall as the aging population's envoy to space. At the very least, they'd like to follow in his high-flying footsteps."This John Glenn thing has brought just a slew of requests saying, `How come you picked him? Why not me? I run triathlons. Or I do this. Or I do that,' " said David Leestma, director of flight crew operations at Johnson Space Center.

Why not me, demanded an 81-year-old triathlete.

Or me, asked a 77-year-old retired military pilot.

Why not, indeed?

The answer, NASA says, is simple. There's only one John Glenn, a Mercury astronaut and former Marine with virtually his entire medical history on file, and he's in top shape.

"He's been through the whole selection process. We're dealing with a known commodity," Leestma explained.

The fact that Glenn is a senator with political connections didn't hurt, NASA insiders admit.

It was Glenn's idea, after all, to fly an older person in space.

The Democratic senator from Ohio and first American to orbit Earth was intrigued by the many similarities between aging and the body's changes in weightlessness - weakened bones, depressed immune system, disturbed sleep. He was so confident both the elderly on Earth and astronauts in space could benefit from joint orbital studies that he urged NASA Administrator Daniel Goldin in 1996 to launch a senior as a medical guinea pig.

"If I can pass a physical, why not me?" Glenn asked.

Glenn aced all the necessary medical tests last year and then some. By the time NASA announced his appointment in January to a nine-day October research flight by Discovery, he had taken more exams than any other astronaut candidate.

"He didn't pull any strings. He played by the rules and checked out healthy," said Dr. Jerry Linenger, a former Navy physician and astronaut. "He used to be the square guy back in the '60s. He'd go out and exercise, and didn't drink, and it shows."

Even hard-nosed Russian space officials are impressed with Glenn's vigor.

"We have cosmonauts who have reached their 70s, but none is in good health," the Russian Space Agency's chief, Yuri Koptev, said after meeting Glenn. "We envy you for having older people in such condition."

Not everyone is thrilled to have Glenn back in orbital action.

Some of NASA's 121 astronauts, especially those who have yet to fly in space, resent the fact that Glenn is taking up what could have been their shuttle seat.

The medical testing planned for Glenn in orbit also has been called into question: What good is one scientific subject? And how can such a remarkably fit man represent the average 77-year-old?

"I wish we could send up 150 77-year-olds all at one time, right now, and get a database," said Glenn, who will turn 77 in July. "Well, we can't do that obviously. But we're going to start with one. Maybe there will be more later on."

"I don't look at this as a one-shot deal," he added.

Leestma says he has no idea how or where he would recruit 70-something astronauts if instructed to do so. Moreover, he wouldn't necessarily want to.

"In general, spaceflight is a younger person's game," said Leestma, a former astronaut. "It's not the Ritz. It's not driving your Cadillac around. It's not always easy, you know, and there's a lot of stress . . . "