Senior NASA managers today cleared the shuttle Discovery for launch Oct. 23 on a critical space station assembly mission, concluding concern about the integrity of a protective coating on three of 44 wing leading edge panels did not warrant a lengthy delay. While there were no official dissenting opinions, NASA's chief engineer opted to write down his concerns about the decision to proceed with flight and a NASA engineering panel stuck to an earlier recommendation to replace the panels in question.

In a worst-case failure, one in which some unknown mechanism caused the protective coating to somehow come off after the crew's normal heat-shield inspections in orbit and before peak heating during re-entry, the shuttle could suffer a catastrophic leading edge burn through. Replacing the panels in question would eliminate the threat but the work would delay launch for two months or more.

NASA is attempting to complete the international space station and retire the shuttle by the end of fiscal 2010. At a news conference late Tuesday, Hale did not address how the prospect of a long delay might have played into the launch decision. But he made it clear he believes it is safe to proceed with Discovery's flight while testing continues, saying there is no engineering data to support the worst-case scenario.

"We certainly explored it in a great deal of depth," Hale said. "Everybody got to ask questions, everybody got to give their understanding of it down to the working-troop level. And at the end of the day, the flight readiness review board decided we were in an acceptable risk posture to go fly. Which is not to say we completely and perfectly understand the problem that's been laid out. We're going to continue to work very hard on it as the data comes in. We will continually re-evaluate our position from flight to flight and if the risk grows to an unacceptable level, we will take action, whether that's to change some hardware or to delay some flights while we do testing or what have you.

"I really think this was a credit to the lessons that we learned since Challenger and Columbia to be able to listen to all the opinions, to think very clearly about what they mean, apply some critical thought processes and, I trust, come to a good decision that provides us with an acceptable reason to go fly. We have a very important mission ahead of us and the crew is going to have a very intense time on orbit. We need to focus on what they are getting ready to do ... because it's absolutely critical to the next stage of building the international space station which is, after all, the reason for which we're flying the space shuttle."

Discovery's crew - commander Pam Melroy, pilot George Zamka, Scott Parazynski, flight engineer Stephanie Wilson, Doug Wheelock, Italian astronaut Paolo Nespoli and space station crew member Dan Tani - is scheduled to fly to the Kennedy Space Center Friday for the start of the shuttle's countdown Saturday afternoon. Launch is targeted for 11:38 a.m. Tuesday.

Bill Gerstenmaier, NASA's chief of space flight operations, said the crew, represented by the astronaut office at the Johnson Space Center in Houston, agreed with the decision to press ahead with launch. So did NASA's new chief engineer, Michael Ryschkewitsch, although he apparently had reservations. Gerstenmaier said Ryschkewitsch wanted to write down his concerns as part of a process that allows managers to go beyond a simple yes-no vote.

The primary goal of Discovery's mission is to deliver a new multi-hatch module called Harmony that will serve as the connecting point for European and Japanese research modules scheduled for launch in December and early next year. The astronauts also plan to move a stowed set of solar arrays to its permanent mounting point on the far left end of the station's main power truss and stage a recently added spacewalk to test heat shield repair techniques.

Discovery's flight is the first to use a new management approval process, splitting up the traditional flight readiness review into separate program- and headquarters-level meetings. The idea behind the change was to make it easier for mid-level managers and engineers to express their views and opinions, part of NASA's on-going drive to improve communications between engineers and managers.

The program-level review was held Oct. 10 and during that meeting, shuttle project and wing leading edge subsystem engineers recommended launching Discovery on time despite concern raised by the NASA Engineering and Safety Center - NESC - that the coating on three reinforced carbon carbon (RCC) wing leading edge panels might be susceptible to failure.

The issue involves a protective silicon-carbide coating on the shuttle's RCC nose cap and wing leading edge panels. The nose cap and 44 RCC leading edge panels - 22 on each wing - protect the shuttle from the most extreme heating during re-entry when temperatures exceed 3,000 degrees Fahrenheit. A breach in Columbia's left wing leading edge, caused by the impact of foam debris from the ship's external tank, led to the shuttle's destruction in 2003.

Since then, NASA and contractor engineers have paid close attention to the RCC panels and nose cap, devising sophisticated non-destructive tests to assess the health of the critical carbon composite material before each flight. One of those new techniques is called thermography, which measures how heat dissipates in the carbon composite material. The technique can show areas where the protective coating on the panels might be degrading.

"Before Columbia there were two instances where we landed and some of this coating, visibly little amounts ... was off the vehicle when it landed," Hale said. "Nothing bad had happened, the vehicle survived. There was a theory as to why this happened, we developed a screening technique that we thought would detect the problem before it became critical, before it became a safety-of-flight issue."

After the first post-Columbia mission, however, thermography revealed an area of concern on an RCC panel from the shuttle's right wing. The panel - 8R - was removed and returned to the vendor, Lockheed Martin, for refurbishment. In the course of post-flight inspections, Hale said, engineers discovered "there was more sub-surface damage than we would have expected on that panel."

"That kicked off this whole concern and starting in about May, we have been trying to understand do we really have a flight safety concern?" Hale said. "Because we don't know that we do. There are some hypothesized, proposed failure modes that would say you potentially could have a safety-of-flight issue. So we're working through that engineering data.

"Now that's not a simple sound bite," he said. "There is disagreement over the interpretation of results from this panel, which we have now taken and cut little slices of and looked at under a microscope and compared that back with what the thermography readings were before. What does all this mean to us? It's a very complicated problem, it's a very complicated system and we absolutely need to make sure it works right and I can tell you right now, today, there is some question whether or not all these panels will work right. And the question is, do we stop and wait until we completely understand this problem, do we remove three or four or five of these panels and try to replace them with newer and potentially better panels? What do we do? That's what we've been grappling with, that's the issue."

The area of concern is near the apex of the curved RCC panels where they join together with so-called T-seals. Three panels on Discovery - RCC panels 9 right, 13 right and 12 left - were known to have small areas of degraded coating.

Until recently, the leading theory for the cause of such coating degradation was a slow process of oxidation, one that would not be expected to lead to a sudden loss of protection. The areas of concern on the three panels aboard Discovery had not shown any signs of worsening after three flights.

But the NASA Engineering and Safety Center, an independent review group set up after the 2003 Columbia disaster, concluded ongoing analysis of test data did not support the presumed root cause of the coating degradation and that as a result, engineers could not predict how the damage might evolve over time or accurately assess the danger it might pose. The NESC recommended replacing all three panels.

But Hale said there is no actual test or flight data that would suggest a sudden coating failure is a credible scenario. If any such loss did occur during launch or the crew's orbital operations, it would be detected by now-standard post-Columbia heat shield inspections. If any such damage was detected, it could be repaired, in theory at least, by spacewalking astronauts. For severe damage, the crew could take refuge aboard the space station and await rescue by another shuttle.

But if something caused a coating loss after the crew's normal inspections, the shuttle could re-enter with an undetected, potentially catastrophic heat shield defect. The problem for NASA is the recent conclusion that the previously held oxidation explanation may not be valid and as such, engineers do not understand the underlying causes of coating degradation or how that degradation might change over time.

"I would love to be in the position of saying we understand all our problems completely and we have resolved them all and there is nothing that's worrying anybody," Hale said. "The fact of the matter is this is a very complicated vehicle, it's an old vehicle and there are a lot of loose ends out there. We fly every time without having solved every one of our problems, found a root cause of every one of our issues.

"This is an absolutely critical subsystem for the safety of flight and the potential is a catastrophic loss of vehicle. So therefore, we have to pay particularly close attention to it. And we are committed to (finding) a root cause. ... The question you have to ask yourself is do we have sufficient understanding and sufficient mitigation - and in this case, mitigation is things like inspection and repair - to go proceed to fly while we're proceeding to work root cause?"

Gerstenmaier said the question comes down to "where is that line, when is the right time to ... take some remedial action or when is it not a problem? And frankly, we don't know and that's what the teams are struggling with."

"Without an underlying cause mechanism, you can go in one direction that says you ought to (replace the panels) at this value, another direction at a different value. When we thought we had oxidation as a root cause it was clear then we had margin to go fly for an extended period of time. Now, because we don't know what that failure mode is, depending on which failure mode we hypothesize, we may not have as much margin as we like. Then we have to go look at other mitigating circumstances."

Asked how NASA could proceed with flight with major unknowns about a potentially critical failure mode, Hale said "I don't know what else to say other than what we've told you."

"We have a new technique to inspect these panels," he said. "It's showing us some interesting things, we're trying to understand what that means. In the process of understanding, some folks that I highly respect, who are good engineers, have hypothesized this could lead to a very bad situation. We haven't demonstrated that, we have a test program to go out and understand all of that.

"So you ask yourself, should we quit flying? Should we do some minor repairs on these piece parts? What should we do? You look at the mitigations. If it happens during the launch phase, we can detect it on orbit and repair it. And we think if it happens late in entry it won't be a problem. If it happens early in entry, we've done an awful lot of work and calculations and it probably, to a fairly high degree, won't be a problem although it could be. That's the kind of logic we go through."

Said Gerstenmaier: "We would have to lose the coating sometime prior to peak heating to have this potential problem during entry based on our conservative entry tools. So when you factor all those things together, even though you have all these unknowns, even if you extrapolate those unknowns to the worst case, we have enough rationale that says we're OK to continue to fly while we continue to aggressively investigate this."

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