Late on a Friday afternoon, just 11 days after President Reagan appointed his commission to investigate the Challenger explosion, the panel encountered its first major crisis, and the course of the investigation suddenly swerved.

The commission had been searching for what it assumed was the unpredictable mechanical malfunction that led to the explosion, the one-in-a-million equipment failure that slipped through NASA's legendary fail-safe designs and multiple backup systems.

Instead they heard from witnesses at a closed hearing on Feb. 14 at Cape Canaveral that as late as the night before the fatal flight, high NASA officials had been warned that the solid rocket booster's O-ring seals might fail in the cold weather that gripped the Cape on launch day, Jan. 28. The National Aeronautics and Space Administration launched Challenger anyway.

"We knew right then that was a real bombshell," said a source close to the presidential commission. "The commission was shocked. A lot of people were visibly shaken. We found out NASA's process didn't work the way we all thought it would have worked."

The revelation was a major turning point for the commission. The 13 members whom many outsiders initially saw as too closely tied to NASA -- heavy with astronauts, officials of NASA contractors and NASA consultants -- suddenly found themselves united in what one person present that day recalled as "anguishing" doubt about the space agency.

The incident galvanized the commissioners just as their fledgling investigation was moving into a more highly organized phase with hiring of a staff, renting office space and establishment of a sophisticated computer database system to keep track of the flood of testimony and documents.

Most presidential commissions include distinguished members who merely meet periodically to oversee work of a hired staff. This one, under the unusually vigorous leadership of former secretary of state William P. Rogers, saw the commissioners as the chief investigators, most working full time, and often overtime.

The turning point came that Friday afternoon when the commissioners first heard officials of Morton Thiokol Inc., the booster manufacturer, say their engineers had recommended against launch. They then heard top NASA officials declare that the warning had never reached them.

At that point the shocked commissioners sent witnesses out of the hearing room and caucused for 45 minutes.

They realized that the concurrent NASA investigation was in the hands of those who made the decision to launch, including those who disregarded the warnings from Thiokol engineers. The commisioners decided to advise the president and to ask NASA's acting administrator William R. Graham to remove from the investigation all those who had a hand in the launch decision.

But the commission members, acutely aware of the deep public interest in the investigation, felt that was not enough.

"We felt we had to make a statement about what we had found," one commission source said. "We were clearly going to have to broaden the investigation from equipment failures to something a lot more troubling."

If the commission did not challenge NASA's decision-making process, it risked being seen as whitewashing the problem. If it came down too hard, it risked damaging NASA's reputation so much that the agency would be crippled for many years.

The decision was to announce that NASA officials should not be in the position of investigating themselves and to release the carefully worded statement that NASA's "decision-making process may have been flawed."

"If we had not stepped up to that," a commission source said, "we would have been challenged [as pawns of NASA] from then on out."

Rogers, believing that the commission's work would be scrutinized for many years, was keenly aware of the questions that continued to plague the Warren Commission's investigation, decades ago, of President John F. Kennedy's assassination.

Rogers wanted to make it publicly clear that his commission was truly independent. Instead of waiting to report findings at the end of the commission's official 120-day lifetime, Rogers scheduled three days of public hearings in Washington so that the same officials who testified privately would have to describe the decision-making process in front of reporters. At the end of those hearings, Rogers announced his conclusion that NASA's decision-making process "is flawed."

The openness of the Rogers commission has become a hallmark of what is already regarded as one of the most unusual presidential commissions ever. Many commission members and observers credit this to Rogers, a former district attorney who is also remembered as an effective attorney general in the Eisenhower administration.

"He's the driver. He's very much a doer," said Alton G. Keel Jr., the commission's executive director. Keel, who has a PhD in aerospace engineering and who had been assistant secretary of the Air Force for research and development, was borrowed from his current assignment in the Office of Management and Budget.

On Feb. 6, just three days after the commission was appointed and only one day after the members had met for the first time, Rogers called the first public hearing. Testimony revealed there had been concern about cold temperatures the night before the launch, but there was no indication yet that Thiokol engineers had explicitly warned against launching.

Four days before Rogers hired his first staff member, Keel, the commission was off and running on the expertise of the commissioners themselves.

All are familiar with NASA and most with spacecraft technicalities. Several have strong backgrounds in physics and engineering that have enabled them several times to challenge the scientific accuracy of testimony by NASA's experts.

Commission member Richard P. Feynman, a Nobel laureate in physics, conducted the panel's first scientific experiment -- dunking a piece of rubber O-ring into ice water to see how its resiliency was hampered by cold -- during a break in one public hearing. NASA officials had just claimed the O-rings would work even if cooled to 30 below zero. Feynman's admittedly crude experiment, which he reported immediately, suggested otherwise.

Two members have flown in space. Neil A. Armstrong, the commission's vice chairman, was the first person on the moon and Sally K. Ride, who orbited the Earth aboard Challenger in 1983, was the first American woman in space. Ride, who has a doctorate in physics, has questioned NASA officials in a style that combines an easy familiarity with the agency's acronym-laden jargon with an incredulous challenging of apparent NASA lapses in following its safety rules.

"Dr. Ride is a real confidence-builder. She's very smart," Keel said. "This really is an activist commission. The commissioners do go out and work and they work hard. The time and expertise they contribute is really invaluable. There's no way we could go out and buy this kind of talent."

Most of the commissioners have taken leaves from regular jobs. Some with unavoidable prior commitments have found themselves shuttling repeatedly between NASA facilities and their homes.

Some commissioners have even pressed their spouses into the work. Air Force Maj. Gen. Donald J. Kutyna, a former manager of the Pentagon's space shuttle program, once kept his wife up long into the night typing a report that had to be ready the next morning.

Arthur B.C. Walker Jr., a physics professor at Stanford University, once found himself aboard three late-night, "red-eye special" cross-country flights in four days. He went from California to Houston for two days, back to California for one day and then back to Cape Canaveral for an early-morning meeting.

Even Rogers, long accustomed to flying first class, has been taking tourist seats to keep government expenses down. Once when heading from Washington to the Cape he took a ribbing from a network TV crew, on its way to cover Rogers, flying the same plane in the first-class section.

"The commission's dedication to the difficult task assigned to it by the president is quite remarkable," Rogers said. "I believe their work represents public service of the highest order."

Rogers would not comment on what would appear to be the one exception, retired test pilot Charles (Chuck) Yeager, who has been conspicuously absent from most commission meetings.

Although the commission's expenses are being billed to NASA, Rogers turned down the agency's offer of office space in the NASA buildings. Not wanting to appear too cozy with NASA, he rented a suite of unfinished rooms in a new commercial building across the street from the space agency.

While commissioners were criss-crossing the country, shuttling between their homes and various NASA installations, or holding public hearings in a borrowed auditorium at the State Department, workmen were putting up walls, painting and carpeting the new offices.

Along with rented and borrowed furniture, one of the first pieces of equipment installed was a computerized database management system to cope with the mass of information from scores of hours of testimony and thousands of pages of documents. Every word of evidence is being entered into the computer in a way that allows commissioners or staff members to call up facts within seconds.

If there is a question about what was said by whom concerning, for example, the O-rings' ability to function at a given temperature, the computer will find and print out all the relevant information. If there is a suspicion that a witness's testimony has changed over time or that witnesses have contradicted one another, it takes only seconds to get transcripts for comparison.

The computer includes a security system to prevent anyone outside the investigation, even NASA officials, from seeing or altering the information.

To gather the information, the commission is divided into four specialized groups.

The first is looking into how the space shuttle was designed, produced and tested. It is under Joseph F. Sutter, who helped develop the Boeing 747 and is executive vice president of the Boeing Commercial Airline Co. Also in this group are Walker; Albert D. (Bud) Wheelon, a physicist who is senior vice president of Hughes Aircraft Co., a major NASA and Pentagon contractor; Eugene E. Covert, a professor of aeronautics at Massachusetts Institute of Technology and a NASA consultant on rocket engines, and Robert W. Rummel, an aerospace engineering consultant and former vice president of Trans World Airlines.

The second panel, led by David C. Acheson, a Washington lawyer who formerly was senior vice president of the Communications Satellite Corp., is focusing on the assembly of Challenger in preparation for the Jan. 28 flight and on how NASA determined the spacecraft was ready to fly. It includes Ride, Feynman, Walker and Wheelon.

The third team, examining how NASA planned the Jan. 28 Challenger mission, especially how it balanced schedule pressures and crew safety considerations, is under Ride's leadership. Working with her are Rummel, Acheson and Robert B. Hotz, retired editor of Aviation Week & Space Technology, an industry magazine.

The fourth group is charged with analyzing the accident, trying to determine what went wrong. Kutyna, who once ran an Air Force investigation of a Titan missile accident, is the leader, with Armstrong, Covert, Feynman and Yeager.

Supporting the commission are 12 professional staff members under Keel. The commission that investigated the accident at the Three Mile Island nuclear power plant, by contrast, had about 70 full-time workers and 20 part-time consultants.

The Rogers Commission staff includes professional accident investigators, various technical experts including astronaut Brewster Shaw and lawyers, some of whom have worked for the Federal Aviation Administration in investigating plane crashes. The lawyers also write the equivalent of legal briefs summarizing testimony.

Each panel has a list of questions, part of what Keel calls an "investigative matrix of boxes" that must be filled in to answer every possible question about what happened when and how and by whom.

Sometimes commissioners work and meet at the rented office but, more typically, they travel the country, visiting all the NASA and contractor facilities to interview officials, engineers and even technicians on assembly lines. Keel arranges the visits through Navy Rear Adm. Richard H. Truly, who was appointed head of the shuttle program and of NASA's review of the accident after the commission called for removal of those who participated in the launch decision.

"We call the shots," a commission source said. "We tell NASA or Thiokol or whomever what we want and when we want to come. It's worked pretty well."

So well, in fact, that commission sources say they believe they already have a fairly complete picture of what went wrong -- the main points have already been made public, they say -- and even hint that the commission may be able to complete its work before the 120-day charter runs out in early June.