s early as 1997, a senior NASA engineer warned that hardened foam popping off the external fuel tank on the Columbia shuttle had caused significant damage to the ceramic tiles protecting the vehicle from re-entry temperatures.
The warning was sure to receive new scrutiny after NASA said yesterday that its investigation into the cause of the destruction of the space shuttle on Saturday was focusing on damage to tiles that may have been caused by foam or ice or a combination of the two. NASA officials also acknowledged that they might have underestimated the potential seriousness of damage sustained by the tiles when the shuttle lifted off.
Gregory N. Katnik, a NASA engineer at Cape Canaveral, said in a report dated Dec. 23, 1997, that the Columbia had sustained damage to more than 300 tiles on a recent flight. The inspection after a Columbia mission in 1997 showed that the tiles had sustained damage that was "not normal," Mr. Katnik said.
In a number of other shuttle flights, tile damage from falling foam also caused smaller amounts of damage, but NASA decided that over all, the problem did not threaten the survival of its spacecraft.
Now the agency is re-examining that assumption as it struggles to explain the mystery of how the Columbia broke up as it soared back into the atmosphere.
Ron D. Dittemore, the shuttle program manager, told a news conference yesterday that damage to the tiles was the leading focus of the investigation, but he cautioned that what appeared to be the most likely solution might prove illusory as the complex inquiry moved ahead.
"There's some other event; there's some other missing link that we don't have yet that is contributing to this temperature increase," Mr. Dittemore said. "It's a mystery to us."
But new evidence that surfaced yesterday suggested that damage to the tiles may have been more severe and covered a wider area than first estimated.
A videotape made by a team of NASA scientists at the Jan. 16 liftoff appeared to show a bushel-basket-sized chunk of debris breaking away from the external fuel tank and striking the fragile protective tiles on the underside of the left wing. A NASA analysis suggested the impact could have damaged a swath of tile as large as 7 inches wide and 32 inches long, according to an agency memorandum made public yesterday.
The possibility that damaged tile could have caused the ship to break apart when it re-entered the atmosphere was supported by several outside experts in aerodynamics. They said in interviews that even slightly damaged tiles — perhaps only roughened or cracked — could generate turbulence near the tiles during the tremendous speeds of re-entry, creating potentially dangerous heating of Columbia's aluminum skin.
"There is a possibility that just damage to the tile to the point that they are rougher could create increased heating," said Dr. Michael Holden, director of the Calspan University of Buffalo Research Center in New York, which does aerodynamic testing of the shuttles.
"If it went turbulent, you'd be in more jeopardy," possibly affecting the survival of the entire craft, Dr. Holden said.
While NASA has had trouble working out the details of how the foam impact could have led to the breakup, the accumulating evidence could force the agency to alter its initial determination that the damage seen on the Columbia was not significant, said Dr. Edward Crawley, chairman of the department of aeronautics and astronautics at the Massachusetts Institute of Technology.
"It's possible that more insulation fell off than they thought," Dr. Crawley said. "It's possible that it hit in a way that caused more damage than their model suggested."
But it is also possible, Dr. Crawley said, that "there's still sort of a missing event here" — the link that will make a new pattern, and a new theory for the catastrophe, emerge.
The newly released NASA video shows a whitish object soaring backward, striking the Columbia's left wing and bursting into a cloud of dust. The oblong chunk appears to be the size of the shuttle's astronaut hatch, which measures about 40 inches by 40 inches.
Mr. Dittemore said NASA's analysis determined that the piece probably weighed just under three pounds, though he said that new and more intensive work was being done that could revise that number. With a range of assumptions about the exact angle at which the piece ricocheted off the spacecraft, he said, NASA determined that anywhere from a single tile to a swath 32 inches long could have been damaged.
He said that NASA's computer model generally predicted more damage than would actually take place. The debris was first noticed the day after the shuttle liftoff when engineers were reviewing film. But Mr. Dittemore said no one on NASA's study team initially thought the damage posed a serious concern.
"At the time I was not aware of anybody that had those feelings, at least to the point where they would want to come forward and identify that there's still something that they think remains undone," Mr. Dittemore said.
Reports prepared during the flight of the Columbia were released yesterday by NASA. They showed that the falling debris occurred 81 seconds into the flight and was first identified the day after the liftoff.
"At approximately 81 seconds Mission Elapsed Time, a large light-colored piece of debris was seen to originate from an area near the ET/Orbiter forward attach bipod," the report said. "The debris appeared to move outboard and then fall aft along the left side of the Orbiter fuselage, striking the leading edge of the left wing."
The report said that more detail would come from analysis of high-speed tracking films.
A report two days later said analysis of the films could not identify individual tiles, but it concluded, "no indications of larger scale damage were noted."
On Jan. 28, a report said that a study of films and analysis of temperatures onboard the spacecraft indicated a potential for significant damage to the tiles. But it concluded that there was "no safety of flight issue."
But the idea that somehow the tile was extremely vulnerable to damage received support from the earlier report on similar damage.
In 1997, Mr. Katnik, the senior NASA engineer, worked in a division that analyzed data from inspections of the shuttles. He is now a technical manager in the Space Shuttle Program Launch Integration Office at the Kennedy Space Center.
He said on the 1997 mission the shuttle sustained a significant amount of damage to its heat tiles. In a normal mission, a shuttle will sustain damage to up to 40 tiles because of ice dropping from the external tank and hitting the tiles, Mr. Katnik reported. But on that mission, he said, "the pattern of hits did not follow aerodynamic expectations, and the number, size and severity of the hits were abnormal."
Inspectors counted 308 hits. Of those, 132 were "greater than one inch," Mr. Katnik said. Some of the hits measured up to 15 inches long with depths of up to one-and-a-half inches. The tiles were only two inches deep, so the largest hits penetrated three-quarters of the way into the tiles, he noted.
The damaged tiles were mostly around the shuttle's nose. After the mission, more than 100 tiles were taken off because "they were irreparable," Mr. Katnik said.
The report went on to speculate as to why the foam dropped off. As it turned out, to be environmentally friendly, NASA had eliminated the use of Freon in foam production, Mr. Katnik reported. The Marshall Space Flight Center in Huntsville, Ala., later concluded that the absence of Freon led to the detachment of the foam.
While the formulation was later improved, the episode revealed potentially dangerous new ways in which tiles could be damaged.
"The tiles still had plenty of material left," Mr. Katnik said in an interview yesterday. "There was a margin of safety."
Nonetheless, he said, the shuttle "was coming back with an irritating amount of damage that we had to repair."
