It was a Thursday afternoon when the T38 trainers, nifty little single-engine jets, whisked in from Houston carrying six astronauts and a teacher bound for space. They wore the customary uniforms of NASA: blue jumpsuits, white sneakers and broad smiles.

The launch of the space shuttle Challenger, already twice delayed, was set for Sunday morning, but for many veterans of the space program, the action was across the continent in California, where a persistent little satellite named Voyager 2 was beaming back the first closeup shots of the planet Uranus. The notion of seven men and women climbing into a bomblike contraption and riding fire into space was no longer big news.

Still, this flight had something special that drew an unusually large crowd of reporters and onlookers: the first "ordinary" citizen to be strapped into the orbiter and sent aloft, a 37-year-old Concord, N.H., teacher named Christa McAuliffe. She was the main attraction to the hordes who came here for the shuttle program's 25th flight. Her presence on the flight, said Tom Wolfe, the author of "The Right Stuff," would take a lot of the mystique out of space exploration. Ordinary people could do it, too.

As with every space flight, the crew's arrival marked an emotional moment for the ground crews that prepare the vehicle for launch and monitor its voyage. "Their lives are totally in our hands," one veteran engineer of the National Aeronautics and Space Administration put it later. "They know it, and so do we."

For NASA and the massive support team at the Cape launch site, the Challenger flight was the second mission in an extraordinarily crowded year that would test whether the shuttle program could live up to its promise of becoming a reliable and regular transportation system to space. The first flight of 1986, which finally took off on Jan. 12, was bedeviled by delays, seven before a successful launch and three more before NASA engineers would let the Columbia return to Earth.

The pattern of delays was about to be repeated with Challenger, this time with a catastrophic conclusion that remains unexplained. But none of that was clear on Jan. 23, when the crew of seven descended from the skies to the palmetto swamps of the Florida coast.

The astronauts were put into strict quarantine from the news media and almost everyone else except their NASA handlers, as is customary before a launch. It is a kind of isolation within isolation, since the island space center, set in a flourishing wildlife refuge, is largely cut off from the surrounding communities by lagoons and security gates.

With the astronauts off-limits, the prowling, restive press corps was left to pursue McAuliffe's family, friends and neighbors, staying at various motels in nearby resort communities, as they toured the space center or went about their business. After months of cheerfully hamming it up for the media, even Scott McAuliffe, 9, was finally beginning to turn away from the cameras.

McAuliffe's mother, Grace Corrigan of Framingham, Mass., told the quote-hungry pack after a Friday visit with her daughter, "She's just bubbling all over." Thousands Gathered to See Liftoff

McAuliffe's husband, Steven, son, Scott, and daughter, Caroline, 6, other relatives, Scott's third-grade class and hundreds of others from New Hampshire were all here by week's end, being tracked on journalistic radar. About 66 bus loads of teachers and students and thousands of other spectators gathered for the launch, along with twice the number of journalists that normally covers "routine" shuttle flights. Many of these reporters had applied to be the first journalist in space, in 1987.

By the time the crew arrived, the Challenger, the workhorse of the four-ship shuttle fleet, had been pronounced in perfect condition, having just undergone a multimillion-dollar refurbishing after 10 flights. It was mated with explosive bolts to the 154-foot, rust-colored external fuel tank, which soon would be filled with 385,000 gallons of highly combustible liquid hydrogen at minus 400 degrees Fahrenheit and 140,000 gallons of liquid oxygen.

Bolted to this tank were the pencil-shaped booster rockets, each consisting of eight sections of solid rocket fuel surrounded by metal casings fastened together with a system of pins and rings.

Together, this "stack" of Challenger, main tank and twin boosters weighed 4.6 million pounds and represented a stupendous engineering achievement -- and an extremely dangerous one. Once fully fueled, the entire area for three miles around the shuttle would be evacuated, except for the crew perched in their pressurized cabin in the gleaming orbiter.

So dangerous is the shuttle that NASA's normal security arrangements, which require "more clearance badges the closer you get to it," are totally reversed. "On launch day, there's not even a guard," said Samuel T. Beddingfield, former NASA deputy director of space shuttle projects management. "You don't have to wear a badge or anything. It's one of the things the astronauts always joke about."

"They are very hazardous propellants, far more hazardous than people want to accept," said Clyde Netherton, a retired NASA countdown planner whose principal job for more than a decade was to allow for every possible countdown calamity. "The key to safety is to have time to stop and take a good look when there's trouble. You don't want that thing to light up and leave."

While the launch crews continued their preparations, the Challenger crew tried to stay loose. McAuliffe and mission commander Francis R. (Dick) Scobee flew some practice runs in a T38 trainer jet Friday. Their walk to the plane was a photo opportunity.

While NASA officials watched the weather, which seemed then their chief nemesis, restive journalists peppered official spokesmen with questions about ground crew fatigue. Single Launch Team Was Kept Busy

NASA has only one launch team for the shuttle program, and it works round-the-clock shifts before a launch. The mission of the orbiter Columbia had just suffered through seven delays. Then weather delays caused it to return three days late, and to California instead of to the space center. It had returned to the space center on the back of a 747 only a few hours ahead of the Challenger astronauts, to be prepared for a March 6 launch.

But NASA spokesmen discounted suggestions of worker fatigue. If there was any anxiety evident, it was about the chance of rain and thunderstorms during Sunday's three-hour launch "window."

Officials also were concerned about bad visibility over emergency landing strips in Casablanca, Morocco, and Dakar, Senegal.

On Saturday, presumed to be launch-day-minus one, NASA held its usual series of day-before briefings on the mission's payloads and purposes. They attracted perfunctory notice, but they portrayed by subtle accretion the immense complexity, expense and hope invested in a single shuttle mission.

One of several briefings on student experiments to be taken up in the shuttle featured Purdue University science student John Velinger, 20, explaining his experiment called "Chix in Space." Sponsored by Kentucky Fried Chicken, it was designed to study the effects of spaceflight on the development of 12 fertilized chicken embryos in White Leghorn eggs. He had dreamed it up while in high school and worked on it for five years.

There were briefings on the $100 million Tracking and Data Relay Satellite (TDRS), to be deployed by the Challenger crew on the first day of the mission. Jesse W. Moore, the space agency's associate administrator for space flight, explained that it was of particular importance to NASA, because it was designed to dramatically improve communications between future shuttle crews and ground controllers. The 5,000-pound satellite, the second in a planned system of three, was billed as the largest privately owned telecommunications spacecraft. (NASA was leasing it.)

There was also a low-cost probe to study Halley's comet and a study of the behavior of fluids in orbit, to be conducted by Hughes payload specialist Gregory B. Jarvis, for the purpose of improving the design of liquid-propelled spacecraft.

The best attendance, of course, was at the "teacher-in-space" briefing conducted that afternoon by Barbara Morgan, the teacher selected as backup to McAuliffe.

Poised and articulate, the dark-haired Morgan described the lessons planned for the fourth day of the mission, to be conducted by McAuliffe and carried to millions of schoolchildren on the Public Broadcasting Service, some by direct satellite link.

The main focus of the lessons, she said, "was to show that the space program doesn't belong just to astronauts and scientists."

She expressed awe at how complicated it was, and how many people it took, to plan for even a small experiment to be taken up in the shuttle.

She smiled and said, "No," when a reporter asked if she agreed with those who criticize the teacher-in-space program as a "political ploy" by the Reagan administration.

And she answered the inevitable questions about how "Christa" was spending her day. ("Relaxing, reviewing the mission plans.") She noted that McAuliffe had already stowed her son's stuffed toy frog, Fleegle, aboard the Challenger as a mascot.

Morgan betrayed no hint of resentment at playing second banana, but she had said earlier, with one of her trademark conspiratorial winks, that there were hints from NASA that she might be allowed to ride a future shuttle mission.

Ron Epps, a senior crew trainer, described McAuliffe's 400 hours of training, including brief periods (20 to 30 seconds at a time) of weightlessness in a KC135 training plane, flying parabolas for two hours. He showed slides of the teacher smiling as she bounced off the bulkheads. Technicians' 'Apple for the Teacher'

In disputing a suggestion by a reporter that there had been friction between the scientists and the untrained teacher, Epps likened the globe of the Earth, as depicted on the official mission badge, as "an apple for the teacher" from the technicians.

At about 10 p.m. Saturday, just before the scheduled fueling of the 18-story external tank, NASA officials decided to postpone the launch because of threatening thunderstorms.

Vice President Bush, who had planned to attend the Sunday launch, had a schedule conflict on Monday and canceled his Florida trip. Events would bring him here just a day later, to pay official respects on behalf of President Reagan to the grieving families.

As it turned out, the cold front was late arriving. The next morning was mostly sunny and would have made for a fine launch, officials said.

That Sunday afternoon, the tour buses that plied the space port were filled with disappointed would-be launch-watchers. One woman who described herself as a housewife from Greenbelt, Md., said she and her husband had come for the launch, but couldn't extend their vacation another day.

She sat stony-faced through much of the tour. When the bus stopped in sight of launch pad 39B, where Challenger waited, she said, still unsmiling but with tears in her eyes, "I'd give anything to be going up with them."

On Sunday afternoon, a television crew from Channel 6 (WCPX) in Orlando happened to notice McAuliffe and Jarvis bicycling between the press complex and the giant Vehicle Assembly Building. In what is believed to be the last view the public had of her up to the fatal launch day, she called out good-naturedly, "Stay six feet away from me. I'm in quarantine!" And she laughed as she pedaled off down the road.

That evening, the astronauts were reported to be watching the Super Bowl, at least until their 7 p.m. prelaunch bedtime.

Dawn on Monday. A nearly full moon shone in a clear sky. The countdown was on. Ground crews were reporting "more ice than we normally see" on the external fuel tank, but "still only one-third of what is acceptable."

The crew had suited up and climbed into Challenger. Then a mechanic's nightmare occurred. The handle on the orbiter's hatch -- to be used by ground crews and removed before liftoff -- was stuck. The ensuing comedy of errors, which included a disorganized search for the right kind of drill and the use of a hacksaw to remove part of the handle, delayed the countdown for an hour. Then they had to reset the shuttle's guidance system, which took another hour. By that time, crosswinds up to 30 mph were whipping the spacecraft's emergency landing field.

At 12:35 p.m., officials scrubbed the mission. After about five hours strapped in their seats, the crew climbed back out of the shuttle.

The launch was rescheduled for 9:38 a.m. Tuesday.

NASA reported that the crew spent the dinner hour with their spouses. Only Judith A. Resnik was not married.

The ground crews had a short "turnaround time" in which to drain the shuttle's fuel tanks and later refill them for a launch the next morning. Because the decision to scrub came so late, they had less than 24 hours.

Delays and pressure by now had become commonplace to the ground crew. At the vast Vehicle Assembly Building, life is a constant round of stacking up new booster segments, bolting together the rocket components, then attaching the shuttles. Stacking, loading, testing, launching and recovering proceeds around the clock.

"It's a flow, you see," said John Williams, director of public affairs for Lockheed Space Operations Co, which processes the shuttles from landing to relaunching. "As an orbiter moves out of an orbiter processing facility, work is focused on the next one . . . . When you're mating one, you're stacking the solid rocket booster for the next one."

Of the launch delays, Williams said, "You learn over a period of time to deal with them. It's part of a way of life . . . . You never like them but they're part of a job."

After the Monday delay, the crews had to drain some water systems and take other precautions to keep lines from freezing, as temperatures were to drop to the low 20s by sunrise.

During the "rollback," when a portion of the support structure is rolled away from the shuttle, an extendible platform grazed a section of the craft. A team of specialists, including the designer of the scratched section, went to the launch pad to study the damage and decided it could be sanded smooth and was not significant.

On Tuesday morning, the "white room" where the crew entered the spacecraft became unusually cozy as people crowded in to get out of the frigid air. A member of the ground crew recognized McAuliffe's special role by handing her an apple. By 8:35 a.m., she and the others had crawled through the hatch for the last time for what her lesson plan described as "The Ultimate Field Trip."

At 9:30 a.m., the loudspeaker voice of mission control expressed concern about "one- to two-foot-long icicles" hanging from the shuttle's support structure. A team of specialists shortly pronounced this no problem.

At 11:10 a.m., the shuttle was cleared for launch.

Because of the cold, and possibly because of the many delays, many journalists and other visitors waited until the last few minutes of the countdown to assemble in the viewing stands. It was 11:38 a.m., "T minus zero."

The thunder began.