Sen. Jake Garn is jogging in the failing Texas light, on a bleak piece of undeveloped real estate near the Lyndon B. Johnson Space Center. He wears matching blue Adidas shorts and T-shirt, and a black plastic watch that bleeps now and then. Usually it is dark when Garn gets around to jogging -- about six miles a day, 35 miles a week. He waves to neighbors standing in front of their condos, and jogs up to the door of his home-away-from-home.

In recent months -- through several delays and even the cancellation of the space shuttle flight on which he was booked -- he was batching it here, plopping down to watch TV and practice maneuvers he will have to replicate in space. On Tuesday, the senator and the six other crew members left for Florida. The countdown began at 1 a.m. yesterday for the launch of the shuttle Discovery tomorrow morning, from Kennedy Space Center.

"Things are going to be different in zero-G," he says. "Things float away if you're not careful. You float away."

Astronauts have gone to Congress -- Sen. John Glenn of Ohio, former senator Harrison Schmitt of New Mexico and the late Rep. Jack Swigert of Colorado -- but Garn will be the first senator to go into space, a temporary career change that critics have called the ultimate junket. Garn will also be the first senator to have his bowel sounds recorded in flight, picked up by microphones strapped to his midsection and fed into a specially adapted Sony Walkman.

"Sometimes the gut just shuts down in space," said Garn some weeks ago, during training here. "They NASA scientists want to know what happens in the digestive tract. This will help them understand."

He kept a foot tucked under a chair during the practice sessions. He wore his spacesuit, equipped with many pockets for storing the Walkman, extra batteries, film, electrodes and wires to be fastened to his bald head for the electroencephalogram, and cotton swabs for collecting saliva that will go into pink plastic vials for analysis.

He also wore the microphone belt, and another for something called an "electrogastrogram," never before done in space, the other half of the digestive duet.

"They won't monitor it during flight," he said. "The circuits will be too busy for bowel sounds."

He practiced putting on a plastic stocking to measure the girth of one leg, to detect fluid shifts. There's another tape recorder to be carried, for the electrocardiogram, and an inflatable cuff for taking his own blood pressure. Also a device for measuring eye-hand coordination, a miniature computer programmed with games for determining dexterity ("my kids would love them") and a paper thermometer.

"I told them, 'I don't want to go just to watch. You can do anything to me you want.' The medical department picked up on that. A lot of people have said they cooked up these things for me to do. Well, these were all ongoing experiments before they ever heard of Jake Garn."

His wife, Kathleen, sat on the edge of the couch, a slight, pleasant woman with blond hair and a ready smile. It was her first visit to Houston, and she watched her husband approvingly as he went through his warm-downs.

She said, "Jake's lost 10 pounds. I said, 'You ought to be in an advertisement.' One of my neighbors called after Jake's picture appeared in a magazine and said, 'I didn't know how good a 52-year-old man could look.' I told Jake, 'Don't you ever pose in your shorts again.' "

She is 36, Garn's second wife. (His first wife died in an automobile accident in 1976.) The Garns are Mormons, and have seven children, five by Garn's previous marriage. She describes her husband as a "homebody who loves the kids, and loves to bake." The muffins Garn made in his Houston kitchenette were from a recipe used by his mother. At home in suburban Virginia he bakes bread twice a week, grinding his own flour from 50-pound bags of Utah whole wheat. "There's no room here for making bread," he said, holding up a frozen loaf that Kathleen had brought him.

"He's not easily swayed," Kathleen Garn said earlier. "He has strong character, and is very predictable because of it. After he jogs at home he records how far he ran, how long it took, the weather conditions, how he feels, where he jogged, his weight and heartbeat.

"Sometimes he gets emotional. But he's never tacky on the Senate floor."

Garn is a conservative, and a hard-liner on defense. During a budget debate in 1982, protesting the deficit, he said, "I am angry at this body. I am angry at Congress -- I do not care which party, Republican or Democrat -- because there are weak-kneed, gutless politicians on both sides" unwilling to cut entitlement programs.

As chairman of the Senate Banking Committee, he opposes government intervention in domestic and consumer affairs. "He's so close to the banking industry," says Ralph Nader, "that you can say he's the first banker to go into outer space, rather than the first senator."

Garn and Nader clashed during a 1979 committee hearing on the troubled Chrysler Corp., when Nader said that if required safety equipment had been better on American cars, "some senator's wife might be alive today." "What kind of human being are you?" Garn demanded, in a now-famous display of indignation.

Nader said Garn was "grandstanding."

Kathleen Garn disagrees. "It hurt the whole family. One of his sons was delivering newspapers when he read about what had been said. He came home very depressed. As a matter of fact, his wife was wearing a seat belt."

There is a boyishness about Garn more fitting in an Explorer Scout than a member of the world's greatest deliberative body. He unabashedly championed his own candidacy for space flight during committee hearings. And when the Challenger flight was canceled, and before Garn was reassigned to the Discovery, he said, "We're not going to waste the training level I have achieved . . .This is not a personal cancellation. I will fly. It's not a question of if, but when."

Says a staffer who worked with Garn on the Hill, "I wasn't surprised that he pushed. He loves to fly, to fly anything."

Garn has even been lampooned in "Doonesbury" as "Barfin' Jake."

"I'm not mad about 'Doonesbury,' " he says.

"You don't get mad," says his wife. "But they're mad. They say he's making fun of some very important stuff."

"They" are the NASA doctors and scientists. One of them, James M. Vanderploeg, M.D., says Garn "is very easy to work with. His age isn't important, since we're comparing him to himself . . . He picks up immediately on the benefits to science. He gets on well with the crew -- it's his natural environment."

He says he has not been nauseated during training. The doctors confirm this. Garn claims that Barfin' Jake is a figment of the eastern media's imagination.

"I did get a little queasy in training," he concedes. "I tried to do a flip in zero-G and did it too hard. I went around three times."

Garn had 200 hours of training when the Challenger was grounded -- and an unusual amount of press coverage.

"Usually we exclude the press from contact with astronauts 30 days before flight," says a NASA spokesman. "But this flight has been . . . " He pauses. "It's been odd . . . "

"I can't wait to get into isolation," Garn says happily, "so I won't have to worry about picking my nose."

He is not a senator to his crew mates, with whom he will live in a mere 2,000 cubic feet, but a "payload specialist." (His payload is the medical paraphernalia he carries.) A payload specialist on a previous shuttle flight, who worked for McDonnell Douglas, paid about $80,000 for the experience. The cost of sending Garn aloft, according to NASA, is negligible. He is paying for his own accommodations and transportation during training, out of his Senate travel allowance.

"Naturally there's been some resentment among people who have waited 10 and 12 years to fly," says Steve Nesbitt of NASA's public affairs office, "but it has been minimal."

"I'm not replacing an astronaut," Garn says. "I'm not even getting an astronaut pin."

Critics consider Garn a privileged passenger rather than a "payload specialist."

He shakes his head at the mention of criticism he has received. "Jane Pauley asked me on the "Today" show , 'Isn't the only reason you're going that you're chairman of the Senate appropriations subcommittee responsible for NASA's budget?' I said, 'Yes. I've dealt with NASA for 10 years. I'm working on the space station right now, setting policy.' "

President Reagan promised that a teacher would be the first civilian in space. The announcement of Garn's flight prompted a member of the American Federation of Teachers to say, "I guess it's the first campaign promise that's been broken." The same spokeswoman said recently that she has heard no complaints about Garn's flight among the AFT membership.

"That campaign promise has not been broken," Garn insists. "That teacher will still be the first civilian in space . I have a military background. I'm a pilot. I'm not flying the orbiter, but what a difference it makes to know the environment, to have worn an oxygen mask, a harness, a Mae West, a G-suit and a helmet, to have been confined. I told shuttle commander Karol J. Bobko that my flight training was very valuable. Bo said, 'More than you know, Jake.' "

Bobko and the McDonnell Douglas payload specialist, Charles D. Walker, are the only space veterans on Friday's Discovery flight. Garn was a Navy pilot during the Korean War, and has logged thousands of hours, lofting mostly propeller-driven planes over oceans and, later, the badlands of his native state as a member of the Utah Air National Guard, where he served for 24 years before resigning. (Garn's father was commander of the Utah Air National Guard.)

"After we were married," says Kathleen Garn, "I said, 'Jake, there are three things in your life. Me, the Senate, and the Air National Guard. One of them has to go.' "

"I gave up the camaraderie of the flight crews," Garn says, "without knowing how much I would miss it. I thought, 'That's a phase of your life that's gone.' Now it's great to be back.

"I'm always candid. I don't want any wishy-washiness. I shouldn't tell you these personal things, but this flight is the fulfilment of a fantasy."

Not the ultimate junket, but the ultimate locker room experience.

"The crew is good. I know because you watch each other. They are good. We did a bunch of simulated landings . . . They didn't miss a one. They're the cream of the crop. We've gotten along very well. If there's a breakdown, the commander graciously invites me up and explains the system. They're just fantastic people.

"I'm carrying my own weight. I knew I had to prove myself."

Once in space, Garn will work out of a locker, his foot secured under a metal brace on the floor to keep him from drifting off while he changes batteries and attaches the electrodes. He will take part in what are known as "mundane duties," which include galley service and cleaning the air purifier.

"You ride up the elevator 195 feet," he says of the Countdown Demonstration Test, a dress rehearsal for blastoff they went through last Saturday. "You go out on the catwalk and look down at the orbiter mounted on the external booster. You go into that white room and put on the helmet. You get strapped in. There's the countdown. Houston's on, the whole system's up. Even though it's only an exercise, the knowledge that you're in the actual craft is ecstatic. We all broke out in a sweat."

Garn finishes his stretching exercises.

"Something could go wrong," he says. "That's part of the game."

"NASA's track record is good," says Kathleen. "But I don't want him walking around outside the spaceship."

"I'll be safer than on the George Washington Parkway." Jake Garn checks his stopwatch. "Especially if there's a little ice on it."