When veteran astronaut John M. Fabian won his third shuttle assignment five months ago, he was ecstatic. His new assignment put him on the crew of the first space shuttle carrying a cargo destined for another planet, Jupiter, in May 1986.

But two months later, Fabian gave up his mission and resigned from the astronaut corps. The job, he said, was putting too much pressure on his family life.

Fabian's was the eighth astronaut resignation in the last 16 months, a sign that perhaps all is not well in the corps. At the least, the resignations suggest that the workaholic way of life that is the hallmark of astronaut service may have begun to rub away some of the glamor.

Fabian's abrupt and unexpected resignation shocked the corps, whose spirit and solidarity are said to be second to none.

"A person can only continue to be an astronaut a certain length of time and that's it," said Fabian, an Air Force colonel and seven-year veteran astronaut. "So I came home one night and told my wife, 'I put the job first for 24 years, and I'm not doing it any more. I quit.'

"There's a payoff to astronauts working 16-hour days, six or seven days a week, and that is they get to fly in space," Fabian said. "Their families don't get that payoff. All they see are the missed dinners and the trips out of town to Cape Canaveral or some contractor's factory in California."

The spurt of resignations is considered a "wave" in the astronaut corps, where one resignation a year has been an average for the last 20 years.

Besides Fabian, the National Aeronautics and Space Administration in the last 16 months has lost Joseph P. Allen; Terry Hart; William Lenoir; Jack Lousma; Thomas K. Mattingly; Donald Peterson and Richard Truly. All eight were veterans in the prime of their careers and whose combined experience covered 12 of the 21 shuttle missions flown so far.

"Are these resignations something we worry about? Yes, especially the younger ones like Allen 45 and Fabian 44 , who still have a lot of tread left on them," NASA Administrator James M. Beggs said in an interview. "We've now begun to lose the guys we've educated and trained to do the most difficult things we do, like spacewalks, and if this trend starts to increase, it's going to disturb me."

Fabian, explaining his family pressures, said his son just graduated from the Air Force Academy and his daughter started college this fall in upstate New York, leaving his wife home alone for the first time in years.

"I came home one night and my wife told me, 'I'm ready to move out of Houston this year, and I hope you are ready to move with me.' I got the message," Fabian said, "and when I came home in July after an eight-day trip to France with the rest of the crew from my last mission, I decided it was time to leave Houston."

Joe Allen's story is similar, though he stayed in Houston to work with a firm called Space Industries Inc. when he quit the astronaut service six months ago. "My wife kept saying one thing to me, over and over again: 'Joe, when are you going to get a real job?' "

Stripped of its glamor, astronaut service is demanding work that keeps its members away from home. One female astronaut, who just began training for a flight late next year, said she has seen her husband twice in the last two months. "The last time it was for 12 hours," she said, "and for six of those we were both asleep."

Astronaut training ranges from studying the physics and biology of space flight to making parachute jumps and three-day survival visits to the jungle of Panama. Classroom hours match the time PhD candidates spend at study. Physical fitness is a must, but astronauts don't get time off for exercise. They must do it on their own time.

Once an astronaut is assigned a mission, the training pace steps up. At Houston's Johnson Space Center and Florida's Kennedy Space Center, there are around-the-clock computer simulations of astronaut tasks and potential problems in space. Astronauts also visit other NASA centers and contractors' factories to become familiar with the equipment they will use on their missions.

"I remember finishing my last flight in June and starting training for my next flight the next day," Fabian said. "Our first assignment was a trip to California to be briefed on the Galileo spacecraft we were going to carry on the shuttle."

NASA officials said there is nothing they can do to slow the training pace, mostly because they believe it is the reason the United States has never had a fatal accident in space. NASA Administrator Beggs indicated that the way to keep astronauts happy and in the corps is to keep them busy and assigned to a mission.

But that didn't stop John Fabian from leaving. "I'll miss flying and I'll miss the people, but I don't want to be known to posterity as the oldest astronaut to fly in space," he said. "It's time to move on to another life."