The U.S. space agency, NASA, stands by its original view that a piece of foam that struck the wing of the ill-fated shuttle Columbia during launch did not cause the spacecraft to disintegrate Saturday as it was landing. NASA technicians are struggling to identify another cause of the disaster.
Shuttle officials say lightweight insulating foam that flew off Columbia's external fuel tank broke into dust and was not heavy enough to have damaged its left wing.
That was the initial conclusion of NASA engineers during the shuttle's mission, but they reanalyzed the data following the orbiter's loss.
Shuttle program manager Ron Dittemore says the 50-centimeter long piece weighed just over one kilogram and was waterproof, so ice could not have made it heavier and more destructive. In fact, he says, inspection of the orbiter and external tank before launch showed no ice buildup, as sometime occurs. And films of the shuttle launch show no apparent wing damage, although the resolution is low.
Mr. Dittemore pointed out that engineers are looking for another cause of the shuttle's demise.
"Right now it just does not make sense to us that a piece of debris would be the root cause for the loss of Columbia and its crew," he said. "There has got to be another reason."
The foam debris incident has been the focus of attention since the accident because it occurred on the same side of the orbiter that heated to unusually high temperatures minutes before Columbia broke into thousands of pieces. The question has been whether the foam caused damage to or loss of some wing insulating tiles that would have led to overheating.
Although NASA believes the foam is an unlikely cause of the trouble, Mr. Dittemore said engineers are conducting tests to see what size and weight of the material would be necessary to damage tiles.
Three studies in the 1990s noted the vulnerability of the tiles in certain shuttle locations. Mr. Dittemore noted that tile damage is common under the stress of launch and re-entry into the atmosphere, but has always been minor in the 22-year history of the shuttle fleet.
"For all these 113 flights, we have never identified damage that would be a safety of flight concern," said Mr. Dittemore. "Tile is an area where we are constantly measuring effectiveness of the tile and the number of impacts that we receive to determine whether we are doing anything different or something has changed in our configuration."
NASA officials say the best evidence of what went wrong with Columbia is the debris strewn along thousands of kilometers below its flight path, possibly as far west as California.
Recovery teams have gone to California to check reports of debris that NASA believes could be another wing. Experts are keenly interested in the earliest pieces, assuming that these might lead to the source of the shuttle's failure.
The first truckload of Columbia debris has arrived at Barksdale Air Force Base in Louisiana, one of two collection points for inspection. But so far, NASA says it does not contain key items that technicians are looking for to explain the disaster.
There are still many truckloads to go. Nevertheless, the NASA official who oversees the shuttle and international space station programs, Michael Kostelnik, believes the collection process will be speedy.
"The operation has been, I think, a great success and now is at full steam, so I expect that this will continue unabated for the next weeks and we'll get these sites cleaned up very quickly," he said.
A top NASA priority is to remove debris that might be a public hazard. The earliest collection was around public schools in central Texas, the region where most shuttle pieces fell. The schools opened for the first time Wednesday since the disaster.