The reasoning, once again, seems to have been that earlier incidents had not produced fatal damage -- and this, instead of being regarded as lucky, was regarded as actual evidence that the phenomenon wasn't really as dangerous as had been thought, and that it was therefore okay to make only limited minor fixes and otherwise continue flying.

Indeed, as a Feb. 23 Washington Post article points out, the Shuttle team actually applied the same reasoning to an additional solid booster joint problem itself seen on the STS-35 flight in 1990: "Hot gas from inside the rocket had seeped several inches into a joint that holds booster sections together, approaching the innermost of three O-rings, said Richard D.

Jarvinen, who studied the phenomenon for NASA.

"Despite the intense focus on the O-ring problem after the Challenger disaster, similar problems were observed on some later Columbia flights and those of the three remaining shuttles.

" 'Some people in NASA thought this was a serious problem and should be corrected, and other thought it was tolerable because it hasn't led to anything more serious,' Jarvinen said. The latter group reasoned, 'If we worry about every little thing, we'll never fly,' Jarvinen said."

That is precisely the same reasoning that led to disaster on Challenger -- and, as I say, there's considerable evidence that it was also applied to earlier tile damage from foam impacts.

A detailed Feb. 17 New York Times article notes that the team assigned by the Shuttle's manufacturer Boeing after Columbia's final launch to assess the possible danger from the big foam impact seen during launch "relied on Crater, a NASA software program, as its 'official evaluation tool'.

The program is based on laboratory data gleaned from blasting bits of foam at tiles.

"In Columbia's case, Crater predicted 'significant tile damage', with gouge depths that actually exceeded the thickness of tiles in several critical spots. For example, the tiles protecting the wheel well are about two inches thick. Yet Crater calculated that the foam would penetrate 3.4 inches in this part of the wing.

"The Boeing team did not accept Crater's predictions at face value. The program was 'designed to be conservative, they noted, and so has sometimes 'overpredicted penetration of tile coating significantly.'

Another factor in their judgment was data from a 1992 Columbia mission that they believed had absorbed a 'potentially' similar debris impact. In that case, the debris left a gouge only a half-inch deep...

"There was a difference between the two flights, though. In 1992, the debris was thought to have struck at an angle of 3.2 degrees. This time, the predicted angles were far sharper, which the Boeing team was careful to point out would alone significantly increase the damage.

Still, for reasons not spelled out, the engineers concluded that 'even for worst case', the foam debris would leave behind at least the last quarter-inch of tile, giving the wing at least some thermal protection.

"The report did not spell out another thing: why the Boeing team never calculated how much worse the damage would have been if the debris was actually ice, or partly ice, and not foam.

"As the Boeing team noted, previous NASA research had documented the dangers of ice debris: if it struck the wing's leading edge at a sharp enough angle -- greater than 15 degrees -- it could penetrate the tough carbon coating that protects the edge from reentry heat, potentially leading to a catastrophic burn-through. In Columbia's case, NASA calculated that the debris could have hit the edge at an angle of 22 degrees...

"Having calculated the dimensions of the damage, the engineers struggled to understand its effect during reentry...[T]he Boeing engineers assessed six possible worst-case situations.

"These ranged from a single tile being knocked off near an access panel on the wing, to the loss of carbon coating on the leading edge, to the possibility of 'several' tiles lost over the wheel well.

"In four cases, they concluded that the Shuttle would return safely, though perhaps requiring significant repairs. But for the two direst possibilities -- those involving the loss of multiple tiles --the engineers reached no conclusion in their Jan. 23 report.

Indeed, they said that their overall conclusion that the Columbia would return safely was 'contingent' on the multiple-tile-loss analysis 'showing no violation' of [NASA's earlier tile-loss consequence studies].

"It is unclear what that final bit of analysis concluded. Karin Allen, a spokeswoman for Boeing, said that 'all open work was completed', and that the Boeing team had given mission managers an oral briefing on Jan. 24. She declined to say what the completed analysis showed and said Boeing had no written records of the analysis."

On Feb. 26, NASA released additional E-mails showing that on the day before the landing, several senior engineers at the Johnson Space Center and the Langley Research Center worried that there was some chance that the left wing might burn off and endanger the crew, although they considered this unlikely.

William C. Anderson, an engineer for the United Space Alliance -- the multi-corporation contractor responsible for Shuttle maintenance and safety since 1995 -- wrote, "Why are we talking about this on the day before landing and not the danger after launch?" Two days earlier, another engineer wrote: "Any more activity today on the tile damage or are people just relegated to crossing their fingers and hoping for the best?"

Jeffrey V. Kling -- the flight controller whose announced the first sign of trouble during the descent when four sensors went out on the left wing -- had written the day before that his engineering's team's recommendation, if signs of danger did occur, "is going to be to set up for a bailout" from the Shuttle "assuming the wing doesn't burn off before we can get the crew out."

The Langley engineers did contact Langley center director Del Freeman and asked whether Freeman thought William F. Readdy, NASA's associate administrator for space flight, should be contacted.

But both Freeman and the worried Langley engineers themselves agreed that, on balance, they tended to trust Boeing's analysis indicating that any such serious accident was unlikely -- and so did the worried engineers at the Johnson Space Center (as Kling has now specifically stated). And so, apparently no contact was made between the worried engineers and NASA's top echelon.

The new revelations of discussions about the possible consequences of tile damage -- even though it was still seen as unlikely -- go considerably beyond the already revealed E-mail complaint from Langley engineer Robert Daugherty about NASA's apparent reluctance to look into the possible danger in more detail during the flight: "We can't understand why getting information is being treated like the plague... [Some mission analysts] have used words like they think things are 'survivable' but 'marginal'."

According to the Feb. 22 Washington Post, "NASA engineers in Texas had consulted Daugherty during the flight because they were worried -- for reasons not fully explained -- that one or more of the Shuttle's tires might be deflated during the landing.

Daugherty gave them a telephone briefing and then a lengthy E-mail spelling out his concerns about excessive heating of the shuttle's wing, which NASA released last week.

"NASA officials said yesterday they are unsure if this was the only E-mail communication between the Langley and Johnson centers during the flight.

They also said they were not prepared to release copies of E-mails written at Johnson in response to Daugherty's warnings. Nor was NASA willing to make available any of the officials who wrote or received the emails.

"NASA has said that none of these E-mails were seen at senior levels within the agency before the reentry. Flight controllers in Houston discussed the debris issue three times during the 16-day flight and dismissed all safety-related concerns during a five-minute discussion on the 12th day, well before the Langley-Johnson dialogue had concluded, according to participants.

Daugherty, once again, indicated at the time that he tended to trust the Boeing team's conclusions and considered a serious accident unlikely -- but the Post also says: "Other E-mails and documents released yesterday revealed that after the flight, two other Langley engineers [Daniel Mazanek and Dennis Bushnell] privately expressed alarm that the NASA officials improperly dismissed any possibility that debris known to have struck the Shuttle's left wing might have included ice..."

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