The manager of NASA's shuttle program said today he has all but dismissed the possibility that a piece of flying foam could have damaged the left wing of the doomed Columbia during launch and caused the shuttle's fiery disintegration on Saturday.

Repeated tests and analyses have failed to produce evidence that the lightweight foam could have caused such severe damage, said shuttle program manager Ron Dittemore, who is heading the investigation into the tragedy.

The foam is waterproof, and conditions during the Jan. 16 launch precluded the buildup of much ice, Dittemore said today at the Johnson Space Center. That meant that the piece of flying debris that was seen hitting the wing on videotape was probably the size of an empty backpack and not much heavier.

"It does not make sense to us that a piece of debris could be the root cause for the loss of Columbia and its crew," Dittemore said. "There has got to be another reason."

From the beginning of the investigation, NASA officials had said they doubted that the impact of the foam hitting the tiles during launch could have caused enough damage to endanger the shuttle, although for a time they said that event had to be considered a leading suspect. But today, Dittemore said further analysis had convinced them anew that the first assessment was correct, and left them hoping for some new piece of evidence to explain Columbia's fate.

Speaking at a news briefing, Dittemore showed low-resolution video images taken several seconds apart of the moment that the chunk of foam struck the wing.

"It does show that, as you look at it both before and after impact, there really is no gross large area of damage," he said. The images do not show the vulnerable underside of the wing, however, and Dittemore conceded that they were not sharp enough to capture fine damage -- such as a deep gouge to a single tile or a shallower indentation in a larger number of tiles. That left NASA specialists to deduce the amount of damage by calculations, modeling and guesswork.

Dittemore held a chunk of foam in his fingers as he spoke -- it was yellow on one side and white on another. "When it hits the wing, this foam disintegrates, it disintegrates into dust," he said. He also said that the foam could not have been coated in ice.

Dittemore said shuttle flights have averaged about 100 strikes of debris to the delicate tiles, which protect the spacecraft during the infernal heat of reentry. Of those, he said, an average of 25 to 35 impacts have usually been deeper than an inch.

Officials at the space agency have rejected critics' suggestions that a careless attitude toward safety had led to the disaster. Had there been any real concern, the flight would have been scrapped or aborted, officials said today.

"A shuttle launch is 20,000 people looking for a reason not to go," Mike Kostelnik, a senior shuttle administrator, said earlier in the day.

At the same time, Dittemore acknowledged that the overwhelming preponderance of the evidence continues to suggest that some kind of damage to Columbia's left wing triggered its demise. Eight minutes before ground controllers lost contact with the spacecraft, data from temperature sensors in the left wing abruptly ceased, and others recorded a rise of about 40 degrees Fahrenheit in the left wheel well on the wing's underside. Subsequent computer analysis indicated that the shuttle's computerized flight control system was battling mightily against something creating increasing drag on the left wing.

"The flight control system was trying to overcome a disturbance, but it is losing the battle," said Dittemore, indicating with his hands the struggle taking place more than 200,000 feet in the sky as the Columbia hurtled toward Earth at 12,500 miles per hour. "More and more flight control muscle is being added to keep the vehicle pointed straight ahead; eventually that muscle is going to run out and you will lose control," he said.

NASA officials said their investigation would go beyond physics and materials science to solve the riddle of Columbia's destruction. Kostelnik said NASA and an outside investigative panel were going to look at how the agency does business and would "reconsider everything in the system."

Asked what could have caused the drag on the left side if not damage from the foam striking the wing during launch, NASA officials said again they need to find "a missing link" to explain what happened. They are hoping that an all-out hunt for debris from the spacecraft may turn up a crucial piece that will reveal the event that triggered the crash.

Any wreckage that landed farther west more likely came off as the problem was starting, and would provide the best clues into the accident. Every tile on the Columbia's skin is marked with a special code, so that if one of them is found relatively intact, officials would know precisely where it came from.

Dittemore said he was evaluating the small but distinct possibility that Columbia was hit by a piece of space debris, such as from long-destroyed rockets or satellites or even a meteorite. The shuttle administrator called this scenario remote, but that is also true of other possible scenarios.

In Florida, beaches were being combed to determine whether a piece of debris that wasn't caught on film hit the shuttle during launch and may have washed on shore. In the forests of Arizona, New Mexico and California, officials were searching on foot and horseback for shuttle pieces. And in Texas, the collected debris was mounting rapidly. The wreckage is being sent to the Barksdale Air Force Base, and will be moved to the Kennedy Space Center in Florida to try to reconstruct the shattered spacecraft.

NASA expects the recovery of debris to accelerate "exponentially" in the next few days.

Although officials said they had not found any red-tag fragments -- items that could point to the earliest moments of the disaster -- teams were being dispatched to investigate reports of debris findings in California.

Engineers are also poring over the 32 seconds of fragmentary data that computers on the ground recorded after communications with the shuttle went dead. This data is corrupted, meaning that accurate information is mixed with garbled signals. Officials were trying to separate the useful information to determine what happened in the final seconds before the disintegration and to evaluate how aware Columbia's crew was of the coming disaster.

Remains of the seven astronauts were moved today to Dover, Del. A memorial service is planned Thursday at Washington National Cathedral.

Officials at the space agency also acknowledged today that significant portions of the scientific research that filled the 16-day mission, especially projects involving plants, animals and biology in general, had been destroyed with the spacecraft.

"The loss is quite substantial and in some cases, complete," said Howard D. Ross, NASA's acting associate deputy administrator for biological and physical research. Ross said the astronauts had transmitted the results of other experiments in the physical sciences, including several dealing with combustion, back to Earth before Columbia began its catastrophic descent.

For the first time, officials also raised the possibility that the shuttle's ability to support the international space station may be seriously threatened, and that there was a small possibility that the space station would be left unstaffed, especially after June, when supplies are due to run out.

"If worst comes to worst, we would de-man the station," Kostelnik said. "It's not the best option, but we could do that."

Lin Marcantel of the Texas Parks and Wildlife Department uses an all-terrain vehicle to pull the space shuttle Columbia's nose cone out of the woods near Hemphill, Tex. Ron Dittemore, NASA's space shuttle program manager, holds a piece of the insulation foam that is used to surround the shuttle's external tank. "When it hits the wing, this foam disintegrates, it disintegrates into dust," he said. Members of the New Mexico National Guard recover shuttle wreckage near Nacogdoches, Tex. NASA is going to try to reconstruct the spacecraft.