Hours after the Jan. 28 explosion of the shuttle Challenger, Jesse W. Moore, the head of the National Aeronautics and Space Administration's shuttle program, declared in a news briefing: "All of the people involved in this program, to my knowledge, felt that Challenger was quite ready to go . . . . There was absolutely no pressure to get this particular launch up . . . . "

"It was probably one of the better weather days we've ever experienced," said Jay Greene, the Challenger flight director in Houston at a briefing the same day.

These comments were made just hours after the explosion that destroyed Challenger and its crew of seven. NASA statements then, as well as later declarations in news conferences and hearings, portrayed the prelaunch discussions as normal, repeatedly assuring questioners that the cold weather had been little cause for concern and was an unlikely factor in causing the disaster.

The disclosures of the past two weeks, however, paint a very different picture of the situation in the months preceding the launch and on the eve of the liftoff and are raising questions on Capitol Hill and elsewhere about the credibility of the space agency. Such questions have often been raised about other government agencies, but rarely NASA.

These disclosures show that, rather than everyone being in agreement about the launch, a disagreement erupted the night before between NASA officials and engineers at Morton Thiokol Inc., the company that makes the shuttle's solid rocket boosters (SRBs). At issue were the temperatures -- lower than any previous launch -- at the Kennedy Space Center and the fear of the company's engineers that the cold could compromise the safety of the flight.

The disclosures show that officials at Rockwell International, another contractor, also advised against the launch on account of the weather.

Rather than there being "no pressure to get this particular launch up," closed-door testimony to the presidential commission investigating the explosion, as well as numerous news reports, strongly suggest that NASA officials fought the doubtful engineers for hours the night of Jan. 27, insisting that they approve the flight against their better judgment. Commission members were said to be "shocked" by this testimony.

However, three commission members conducted interviews yesterday at the Thiokol plant in Utah, and one of them, David Acheson, told reporters afterward that while there was a "thorough discussion and some argument" among Thiokol engineers the night before the launch, ". . . You have to look very very hard for pressure from NASA."

Asked if that meant they had found little evidence of pressure, he said, "That is a fair statement."

When Morton Thiokol finally agreed to the launch, the document by which it did so alerted NASA once again to the abnormality of the weather conditions, noting that they were 20 degrees lower than previous launches.

Finally, NASA documents released since the Challenger explosion show that the space agency and the manufacturer had been deeply concerned for three years about the integrity of the O-ring seals designed to prevent leakage of hot exhaust gases from the solid rocket booster. The seals were subject to failure, NASA was told repeatedly, a failure that could destroy the shuttle, its crew and cargo.

None of these matters were volunteered by NASA in the days after the disaster. Instead, Moore told a news conference questioner who asked about ice at the pad, "All the NASA people involved, as well as the contract people involved . . . did feel that the conditions at the launch pad were acceptable for launch, and basically recommended that, you know, we launch."

It is probably premature to suggest that top NASA officials purposely misled the public. Statements from the presidential investigating commission have stated that at least three of the key decision-makers were, in fact, unaware of the concerns about the cold, a possibility that the commission found disturbing.

And the officials themselves have been unwilling to sit for interviews to answer these and other questions raised, saying they prefer to tell their story to the commission.

Yesterday, NASA's Marshall Space Flight Center in Alabama released this statement approved by center Director William R. Lucas: "The press has expressed intense interest in talking with key NASA managers . . . . These individuals have declined to be interviewed because they believe that in light of the commission's stated intent to examine these facts in the next few days, it would be inappropriate to preempt those hearings by discussing the important issues first in the press."

But the pattern suggested by NASA's public statements and the revelations have prompted increasing challenges to NASA's credibility on Capitol Hill.

"There's been a systematic pattern of confusion, misstatements and obfuscation by NASA," said Rep. Edward J. Markey (D-Mass.), chairman of a House Energy and Commerce subcommittee that has begun an investigation of the space agency. "Whether there is a cover-up or plain, outright incompetence is impossible to determine, but that is the question," he said.

"The credibility of the agency is now in question," Markey added. "We need the same kind of inquiries that were made after Watergate and Three-Mile Island. That means by the executive and both the House and the Senate."

On Feb. 6, Arnold D. Aldrich, shuttle manager at Houston's Johnson Space Center, told President Reagan's investigative commission that "all in attendance" at a "mission management team" conference the day before launch thought below-freezing temperatures -- "warming into the morning" -- would put Challenger "well within the specification design of all of the flight elements of the vehicle."

"We had no concern expressed for the temperatures the flight vehicle would see," Aldrich said. "We had no concern for performance or safety of the flight articles at that time, nor do I even at this time."

Asked if he remembered a Thiokol warning about cold, Aldrich replied, "I do not recall such a warning at that time.

". . . The meeting where we elected to proceed was held the night before . . . and all parties felt agreeable to go," Aldrich testified.

However, Judson Lovingood, a shuttle deputy at the Marshall flight center, told the presidential commission that NASA was aware of "a concern by Thiokol on low temperatures."

At a prelaunch meeting, Lovingood said, "discussion centered around the integrity of the O-rings under lower temperature. We had the project managers from both Marshall and Thiokol in the discussions. We had the chief engineers from both places in the discussion. And Thiokol recommended to proceed in the launch."

Though it is not yet known how widely circulated in the agency was knowledge of the vehement protests of Thiokol engineers prior to the launch, neither witness mentioned them. Nor did NASA issue any statements later filling in the missing information.

The information, disclosed in news reports based on interviews with Thiokol officials and unnamed members of the presidential commission, was that during a series of telephone conferences between NASA centers and Thiokol the night before launch, an unknown number -- reportedly as many as 15 -- Thiokol engineers fought for a postponement.

Some engineers reportedly continued to protest even after a Thiokol executive overruled them and sent NASA a letter of approval.

That document itself, released by NASA late Thursday, raised a flag about the cold even while approving the launch. It said that because of the cold, the O-rings "will take longer" to work effectively.

Although NASA officials have testified that no launch limits were exceeded, it has been revealed that because of serious erosion of seals on two flights, Thiokol engineers did not want to risk flying at temperatures colder than the coldest previous flight, a launch at 53 degrees Fahrenheit.

The air temperature at Challenger's Jan. 28 launch was about 38 degrees. The boosters, especially the suspect right one, were apparently cooled further by wind blowing across the near-zero degree surface of the shuttle's supercooled external fuel tank. Agency crew members had taken readings from the right-hand booster rocket, the one that failed, 90 minutes before the launch showing temperatures as low as 7 and 9 degrees.

Aviation Week, an industry magazine, reports in its edition to be published Monday that NASA "had wind tunnel data over a year ago that showed weather conditions similar to those" on Jan. 28 could produce such cooling.

It is not known whether the agency officials who said immediately after the launch that the cold was probably not relevant to the Challenger explosion knew either of the readings or the study.

As to pressure, Thiokol engineers have been quoted as saying that the whole episode was a complete reversal for NASA. In the past, they said, contractors were expected to show why a launch should go, while NASA cautiously said no. This time, said one Thiokol official, "We were asked to prove that no launch should occur . . . . "

One Thiokol manager told the magazine Science that his management caved in because they felt "a lot of pressure from our biggest client." Analysts estimate the firm earns about 20 percent its profits from shuttle booster revenues.

Lucas, the Marshall center director, said of the seals in a recent news conference: "I wouldn't conclude there is a safety problem."

David C. Winterhalter, acting NASA shuttle propulsion boss, told the presidential commission on Feb. 11: "At no time during that period 1985, when seals were failing did any of my people come to me, give any indication that they felt like there was any, any safety-of-flight problems in their areas."

Was seal design safe and adequate, he was asked.

"It was."

But in 1983, because of in-flight erosion of the primary O-ring, NASA had raised the joint system to a Criticality 1 level, its most serious. It said the failure of even one O-ring could result in "loss of mission, vehicle, and crew."