Challenger's catastrophe yesterday came at the end of a long chain of delays with causes as various as a sandstorm in Senegal, a recalcitrant titanium bolt and a tired ground crew.

It was the first shuttle launch from Cape Canaveral's pad 39B; it ended 74 seconds later as NASA's biggest disaster and the first in 19 years involving the loss of a crew.

Teacher Christa McAuliffe was on board because of a campaign promise. On Aug. 27, 1984, President Reagan, speaking to 20 winners of the Education Department's Secondary Schools Recognition Program, said, "It's long been a goal of our space shuttle . . . to someday carry citizen passengers into space. Until now, we hadn't decided who the first citizen passenger would be. But today, I'm directing NASA to begin a search in all of our elementary and secondary schools and to choose as the first citizen passenger in the history of our space program one of America's finest -- a teacher."

More than 11,000 teachers applied and McAuliffe was chosen on July 24, 1985, from among 10 finalists. She "had some concerns," she told an interviewer, that the other crew members "would see me as excess baggage."

But spacecraft commander Francis R. (Dick) Scobee, married to a teacher himself, told McAuliffe, "No matter what we do in this mission it's going to be known as the 'teacher in space' mission . . . and that's okay."

Scobee and the other members of his crew -- Navy Cmdr. Michael J. Smith, Judith A. Resnick, Ellison S. Onizuka, Ronald E. McNair and Gregory B. Jarvis -- were originally scheduled in 1984 for another mission, but were substituted sometime last year for another crew of astronauts, NASA said. Such crew changes are not unusual months before a mission, NASA said.

The mission was originally scheduled for last Wednesday, when the series of delays set in, but there were earlier problems with Challenger dating to the end of its last mission. In November, when the craft last returned from space to Edwards Air Force Base in California, there was damage to the landing gear and a booster rocket segment.

The landing gear damage occurred when Challenger got stuck in the mud that sometimes collects on Edwards' dry lake runways. The shuttle was being towed to a Boeing 747 for transport to Cape Canaveral, and NASA said it took some force to free the ship.

Then, at Cape Canaveral, as one of the eight cylindrical segments that make up the two solid-rocket boosters was being lifted by a crane, workers heard a "sharp cracking sound." Inspections disclosed a broken pin on the handling ring by which the segment was moved. An investigative board was selected to review the mishap and determine whether Challenger needed further repair before flight. The results of that investigation could not be learned yesterday.

Last week began the long series of delays at the launch pad that seems recently to have become a standard part of space launches. The Columbia, used on the previous shuttle mission, had set a record with seven launch delays and was brought back a day early because of other problems.

Columbia's difficulties became a part of Challenger's history. James Ball, a NASA spokesman at the Kennedy Space Center, said the rescheduling from last Wednesday to Thursday was because of time needed to complete ground crew training for this mission.

"This is related to the mission before," he said. When one mission bumps into another, "it's very complicated, but you do lose time."

Before yesterday's explosion put the shuttle program on hold, NASA had an ambitious plan to launch 15 missions this year using all four shuttles and the launch pads at both Cape Canaveral and the new West Coast spaceport at Vandenberg Air Force Base in California. Four classified missions were to be flown from Vandenberg.

At 10 a.m. Thursday the countdown began, but a sandstorm at an emergency landing site in Senegal was expected to continue, so NASA built a 23-hour, 16-minute delay into the schedule, forcing liftoff to Saturday evening.

By late Saturday, the weather in Florida had deteriorated to the point that launch was not possible. To avoid further problems in Senegal, another emergency landing site in Casablanca, Morocco, was chosen. Launch was rescheduled for Sunday morning.

The Florida weather scrubbed the Sunday launch and Vice President Bush's plans to watch the blastoff. Jesse W. Moore, NASA associate administrator for space flight, told reporters, "We're not going to launch this thing and take any kind of risk because we have that schedule pressure . . . . We'll sit on the ground until we all believe it's safe to launch."

Monday morning came. Electronic switches failed to confirm that the shuttle's hatch was closed properly. An engineer entered the cabin to check. With that issue resolved happily, workers were confronted with another problem. They could not remove the titanium bolt holding a hatch handle that is not needed in flight. The bolt apparently was stripped.

A battery-operated drill proved unequal to the task. More specialists were summoned to make sure no explosive gases were wafting around that would endanger the user of a stronger, plug-in drill. That done, a hacksaw was used to cut away part of the cylinder in which the bolt rested, and the bolt was freed by the new drill.

While workers toiled, the weather window of opportunity came and went. By the time the handle was removed, wind gusts were up to 30 knots, more than the shuttle can safely handle.

Liftoff was rescheduled to yesterday at 9:38 a.m., but the combination of electronics and unusually cold weather was to intrude again.

The launch team called an unscheduled hour-long hold so workers could scrape large icicles off the launch pad. There was concern that the vibration of the launch would throw chunks of ice against the shuttle and damage its heat-shielding tiles.

Then an electronic fire-detection system on the launch pad, heeding a faulty computer card, began spewing inaccurate readings. It is needed to ensure that the fire-suppression system is functioning on the launch pad. It was repaired.

Those problems cost two hours, and the launch was set finally for 11:38 a.m.

What happened then has been seen and heard dozen of times on television. It is Hugh Harris' voice saying, "Ten, nine, eight, seven, six, we have main engine start. Four, three, two, one and liftoff. Liftoff of the 25th space shuttle mission. And it has cleared the tower."

Christa McAuliffe was to teach two televised classes from space, including a tour of the shuttle. She hoped students watching her might learn something that would help prepare them for space careers, and she spoke about that to an interviewer.

"That's a new world out there, a new frontier, and there are a lot of people who will be living and working in space," she said.