Mayor Peter C. Knudson had an 11-year-old girl in his orthodontist's chair when he heard something on the radio, above the whine of the suction tube, that bothered him.

He switched off the equipment and listened, feeling sick, as the announcer reported the explosion of the space shuttle Challenger.

About 25 miles from Knudson's town is the Morton Thiokol plant where thousands of his constituents made Challenger's solid fuel rocket boosters, now the focus of NASA's investigation into the accident that killed its crew of seven.

The plant is the source of Brigham's prosperity and its largest employer. Knudson and his young patient, whose father works at Thiokol, looked at each other in shock and for a while said nothing.

Thiokol employes willing to talk about the accident describe a burst of shock at the loss of lives, followed hours later by thoughts about its impact on Brigham City. A week later, many of the town's 17,000 citizens appear to feel better about their future.

Many company executives have followed news reports carefully and now suspect a mistake in the rocket parts' assembly in Florida, not in their manufacture here. "The part that seems involved is more a matter of assembly than engineering," said Thiokol computer specialist Mark Walker, 32.

After dropping from $37.50 to $31.25 a share last week, the stock of the Chicago-based Morton Thiokol Inc. rose $1 today. Surprisingly, past and present executives, and families and friends of company employes, said they were relieved that the investigation narrowed so quickly to the boosters.

When the initial pictures gave few clues, Walker said, "I thought it was going to take months" to find the cause. Now, he said, he expects an answer "in five to six weeks."

Even if some blame falls on the brightly painted plant -- perhaps a mispouring of the quickly congealed fuel into the rocket casings or a crack somehow undetected by routine gamma-ray inspections -- many Thiokol employes assume that it will be an easily preventable error. And the sooner the investigation is complete, they believe, the less likely their operations will be interrupted.

The facility is "the world's largest solid fuel rocket production plant," said former Thiokol president Harold C. Ritchie. If the unthinkable happened and the government canceled the contract, he said, another company would take years to arrange to match its production.

"We've never before had a solid fuel rocket fail in flight," said Ritchie, 73. He said he still wonders how a fuel leak could have caused the accident when the rockets managed to fly straight and true for so many seconds. But he said he does not think the incident will hurt the company seriously.

Gilbert Moore, director of external relations at the plant, would not be drawn into speculation. He said the company will help NASA with its investigation.

Thiokol's arrival in Brigham City, an old Mormon settlement, in the mid-1950s rescued the desert town from a slump. The plant made solid fuel rockets for the Minuteman missile in the safety of a 20,000-acre patch of wasteland, and Brigham became a thriving town of church steeples, baseball fields and McDonald's arches at the foot of the Wasatch Mountains.

Despite earlier tragedies -- a flash fire in August that killed a construction worker, a lightning fire in June that caused $3 million damage, a 1984 fire that injured 14 people -- labor-management relations remain strong in a plant that, like much of Utah, has no unions.

People have roots here. Children work at the plant where their parents worked. "Family is very important here," Knudson said. "That was what shocked people here most of all, the loss to all those families."