The typical astronaut: young to middle-aged male in top-notch physical and mental condition.

The possibly ideal space-shuttle passenger: 55- to 65-year-old man leading a fairly sedentary life.

According to recent NASA tests, the perfect specimen manning the space shuttle may not be the ideal candidate for passenger.

In fact, if you're a 55- to 65-year-old man who doesn't exercise much, you could fare better.

"It came as a surprise to us that many do it withstand space travel better as they get older," says Dr. Harold Sandler, chief of the biomedical research division at NASA's Ames Research Center.

"People in good condition are more at risk."

The tests at the Ames' Human Research Facility in Moffett Field, Calif., were geared to determining through simulated space-shuttle flights how the average person reacts to weightlessness and the return to earth's gravity.

"One day," predicts Sandler, "the space shuttle will go back and forth like a truck, with persons mining for resources on the moon and asteroids.

"We wanted to find out if persons other than the classic astronaut can fly."

NASA selected a separate group of eight men and eight women in each age category: 35-45, 45-55, 55-65. The volunteers were admitted for nine days of controlled observation, seven days of bed rest, five days of recovery and post-bed rest tests of cardiovascular systems.

The seven days of bed rest, says Sandler, produced the same physiological effects as seven days of space travel. The volunteers were strapped into a centrifuge -- which accelerates the body to produce a greater gravitational pull -- simulating the effects of a space-shuttle landing. They were subjected to 1 1/2, 2, and 3 times the pull of normal gravity. (The space traveler returning to earth on the shuttle withstands 1 1/2Gs to 2Gs for 20 to 30 minutes.)

"We always stopped the test at the first signs of faintness -- when the blood pressure fell or the heart beat changed," says Sandler. "That enabled us to derive a tolerance time for each person."

Researchers speculate that older, sedentary men -- basically 45 to 65 -- withstood the stresses better than younger, more athletic men because the older man's less flexible blood vessels prevent the blood from pooling quickly in the lower parts of the body and away from the heart.

In contrast, young men's larger hearts, greater blood volume and increased number of blood vessels in the legs all add up to a greater proclivity for pulling the blood farther away from the heart.

Also, since older, less athletic men tend generally to have higher blood pressure, their blood pressure does not fall as dramatically when subjected to increased gravitational pull.

The older men also reacted more slowly to a change in the environment.

"Our theory," says Sandler, "is that the aging process seems to dampen rather than exaggerate the capabilities" in response to stress. But while older persons are slower to respond to space stress, they don't recover as quickly, he says, when returned to earth's gravity.

Women, on the average, did not last as long as men on the centrifuge (although some women withstood the pull of gravity longer).

Women usually have a smaller heart and a smaller volume of blood, making them more susceptible to fainting spells.

"But that doesn't mean," says Sandler, "women can't go into space."

Sedentary, post-menopausal women tended to have a longer tolerance on the centrifuge. "There is a possibility," says Sandler, "that the process of menstruation may decrease tolerance to the effects of weightlessness."

The researchers found they could help prevent space stress by outfitting volunteers with anti-gravity suits (G-suits) sometimes worn by pilots. The air-filled trousers "squeeze" on the legs and stomach and keep the blood from pooling into the lower part of the body.

"We want to fool the body in space so that it doesn't adapt to weightlessness [making reentry more difficult]," says Sandler.

Some other ideas -- still being tested -- that some researchers believe may ease reentry:

* The consumption of a lot of fluids or certain medications.

* Specially designed suits to create the effect of gravity while in space.

* Exercises to minimize the effects of weightlessness.

Researchers are also investigating the effect of hormonal responses, such as the presence of noradrenaline which increases blood pressure.

"The important thing to remember," says Sandler, "is that it's hard to give an absolute judgment" about how a certain type of person will react to space travel.

"It all depends on the length of space travel and the individual person. But all of our work shows it's amazing how well the body tolerates space flight."