Under glowering skies and a thick fog that matched the mood at the Kennedy Space Center here, several thousand employes listened this morning as top officials urged them to build a better, stronger space program in the wake of the Challenger tragedy.

"Clearly, our most important job over the next few weeks, and maybe even a few months, is to understand what happened," center director Richard G. Smith told the workers. "I hope none of you gets discouraged because of that."

The space shuttle program, he acknowledged, has come to a temporary standstill. The mission that had been scheduled to leave in early March to study Halley's comet will be scrubbed, he said, and two planetary probes will probably be postponed until next year.

Even so, Smith said, "I am convinced that we will be flying again, probably sooner than we think right now. And I know that the demands of flying many missions as rapidly as we can will be there again."

The audience of about 2,800 listened from the stands where an exuberant crowd had gathered last week to watch the Challenger launch. It was chilly that day. Today it was warmer, but the morning fog obscured the upper reaches of the massive Vehicle Assembly Building nearby.

Smith took comfort in his recollections of an earlier tragedy, in 1967, when "a fire on the pad" killed three Apollo crew members. Until Challenger, it was the only NASA accident in which astronauts died.

"We, the team, came out better, stronger and more dedicated than before," Smith said. "And . . . what we did in that adverse time led to the success in the Apollo programs and the Skylab programs beyond that."

By all accounts, many in the 15,000-member work force here, 2,000 of whom are NASA employes, remain gloomy and depressed in the aftermath of Challenger's destruction. NASA has set up a 24-hour telephone "care line," staffed by volunteers who listen to fellow workers' frustrations about the tragedy and offer sympathy.

Dozens reportedly have called. Others have vented their feelings in writing. Smith said one letter -- from an employe so distraught over the disaster that "he wanted to run away" -- made him cry.

Smith admitted that he was still baffled by the accident. "You know, as I reflect back over the events leading up to this loss -- the meetings that we had, the weather problems that we had, the discussions that we had -- there was nothing in any of that that said we did not do the right thing at that time in attempting a launch . . . . That flight, looking at real-time data, was going right down the middle of where it should go."

The Coast Guard announced today that it was ending its search for floating debris from the shuttle. The Navy, meanwhile, dispatched 22 divers aboard the USS Preserver, a salvage rescue ship, to an undisclosed location.

Cmdr. Arthur E. Norton, a Navy spokesman, said diving operations would not begin until the morning. He said the team includes members of an ordnance disposal unit but would not say whether they will be expected to defuse anything underwater.

Space agency officials think that sonar soundings may have located both of Challenger's solid rocket boosters (SRBs). Recovery of them, particularly the right-hand booster, would be invaluable to the investigation of the explosion.

The divers, who wear helmets and pressurized suits, normally operate in water 190 to 230 feet deep, and were believed headed for the area where the left-hand booster is thought to be. The waters there are several hundred feet shallower than the area where the right SRB is thought to lie.

The search here for an answer to what went wrong is gripped in a secrecy that is growing daily and has proven infectious.

Those answering the "care line" refused to discuss how many calls have been coming in. And after Smith's remarks this morning, some employes brusquely told reporters, "No interviews."

At the press center, officials have refused even to disclose the prelaunch temperatures, which may have played a role in Challenger's explosion. Announcements concerning offshore search and recovery operations have dropped off from a trickle to almost complete silence. Queries about what has been found have been met with "no comment."

Out of respect for the Challenger crewmembers and their families, NASA officials have declined to comment on reports that remains of the astronauts and/or their personal effects have been recovered. But tonight, in response to news inquiries, NASA issued a statement saying that "no identifiable human remains" have been recovered.

Today's speeches here were designed to be inspirational. Rep. Bill Nelson (D-Fla.), the chairman of the House space science subcommittee who flew on the shuttle Columbia last month, assured workers that the federal budget will still be growing for them. "When we know the cause of this tragedy, we'll fix it," he said.

Smith concluded his remarks with a biblical reference to the rainbow after the flood. He likened it to the appearance of a group of dolphins last Saturday as a helicopter wound up memorial services for the seven Challenger astronauts by dropping a wreath into the waves.

Dolphins are a good-luck symbol for mariners, and space center workers like to think there were seven of them frolicking in the waves that day.

Smith said that television film showed "at least five dolphins that surfaced by the wreath . . . . I kind of believe that's a sign of the good and better that's going to happen."