In a very real sense, the need to test tile and RCC repair procedures aboard Discovery is as much a matter of perception as engineering.

None of the repair procedures ultimately flown aboard the shuttle will be "certified" by the time Discovery flies. Barring a worst-case scenario that left no choice, a crew would not be allowed to attempt a re-entry with a damaged shuttle until the repair techniques in question have gone through much more testing and analyses.

And that includes in-orbit testing.

"That clearly is part of the necessary certification," Hale said. "So that said, by definition almost, you can't fly the first flight with a certified repair technique. The only way to certify it is to take it to orbit, bring it back and see how it works. So we've recognized that for a long time.

"The goal, however, has been, is, will be, to get the best capability we can as soon as we can and fly with it. And if you get caught in a tight spot, you do the best you can with what you've got. The story's still in play. Tests are continuing as we speak, they're going to continue almost right up to launch day."

During at least the first few post-Columbia missions, with the space station available as a "safe haven," the crew of a shuttle with major tile or RCC damage more likely would be ordered to remain aboard the lab complex until a rescue flight could be launched.

"If we had damage that everyone concluded would be a threat to the vehicle, we would not, I'm sure, use an untried, untested repair technique to bring the vehicle home," Thomas said. "We would go into safe haven mode and await a rescue mission."

It's a tricky issue for NASA because the Columbia Accident Investigation Board concluded that before shuttle flights resumed, NASA should "develop a practicable capability to inspect and effect emergency repairs to the widest possible range of damage" to the shuttle's protective tiles and leading edge reinforced carbon carbon panels.

The key words, of course, are "practicable" and "widest possible range of damage." That gives NASA some leeway in determining what is required before flights can resume.

To meet the intent of the CAIB, the space agency is relying on safe haven and a suite CAIB-recommended post-Columbia fixes to resume flights without having certified tile and RCC repair techniques in place.

"We think launching in May on 114 looks like a very reasonable and safe thing to do," Rominger said. "Our office has never said, given we are going to station, that we have to have repair capability."

On paper, the safe haven concept calls for the station to support a combined shuttle-station crew of nine to 10 astronauts for up to six months. But for at least the first two flights, NASA plans to be prepared to launch a rescue shuttle within 45 days or so of the declaration of an emergency.

Even that's not soon enough for Sergei Krikalev, the Russian commander who will be on board the international space station when Discovery arrives. He said last week the lab cannot safely support a combined crew for much longer than that.

"First, everything possible needs to be done to prevent a situation that may require people to stay for such a long time," he said. "Second thing, if it happened, it should be a relatively short period of time ... because station would not be able to fly for many months in this configuration. For several weeks, we can deal with it, but we need to prepare kind of a backup plan for this backup scenario."

The major concern, he said, "would be supplies and also to maintain the crew in good health and good spirits. Exercise equipment we have on orbit is limited. ... I don't think we would be able to support all nine crew members with full-scale exercise."

Hale agreed, in general, that "the rescue scenario has got a number of its own risks."

"He's representative of a lot of folks who have pointed out that when you've got a lot of people at the station, their margins are kind of thin and then trying to get the next shuttle off, even though we've put our plan together, that plan doesn't allow for things to go wrong," Hale said.

Despite the unknowns still facing return to flight, Scott Hubbard, director of NASA's Ames Research center and a member of the Columbia Accident Investigation Board, said in his view, NASA appears to be meeting the intent of the CAIB recommendations.

"During all the deliberation about whether this should be written as you would write an FAA or a National Transportation Safety Board report or whether it's to be something that's more general, we came down on the side of being more specific," Hubbard said in an interview. "However, if you look at some of the preamble language and if you talk to the individuals who were there, there was always this sense that 'we know it's going to be hard, we know this is a high-risk machine and it will remain so.'

"The sense (CAIB Chairman Harold) Gehman captured that represented that was, 'do the best you can,'" Hubbard said. "What we meant by that was, it may not be that you can meet exactly every single recommendation to the 100.0 percent level. The thing to do is to take a systems look at it and say all right, if we can't get three perfect views with a perfect on orbit inspection with a perfect safe haven, with a perfect repair capability, what can you do in combination of those things that make this as safe as you can get it?"

The CAIB made 15 recommendations for changes needed before shuttles could safely resume flights. An independent panel of experts chaired by former Apollo astronaut Tom Stafford and former shuttle commander Richard Covey is monitoring NASA's implementation of those recommendations.

"Each of those 15 recommendations, each comma, each word was debated endlessly within the board as we were writing the document," Hubbard said. "We've got to be sure that we didn't tell people to go do something that was impossible and that we left it up to the agency and the Stafford-Covey review as to what the ultimate sense would be and what would be, in effect, as good as you can do to return to flight."

Following CAIB recommendations, the space agency has beefed up ground-based camera coverage to improve detection of foam strikes or other problems during launch and sensors have been mounted in the wing leading edges to detect impact forces. The sensors will not only record the force of any strikes, they also will pinpoint their locations.

Following another CAIB recommendation, Thomas will use Discovery's robot arm, and a long boom equipped with lasers and a TV camera, to inspect the shuttle's nose and wing leading edges for signs of impact damage the day after launch. The tile system will be inspected, with one-inch resolution, by the space station astronauts using telephoto lenses prior to docking and later, by more robot arm work.

Given NASA's ability to detect damage in the first place, the elimination of the external tank foam that caused Columbia's destruction and the ability of a shuttle crew to use the station as a safe haven, agency managers say certified tile and RCC repair techniques are not required to resume flights. Some engineers question the need to rush a repair technique into orbit aboard Discovery in the first place.

"You don't need it for the flight for that very reason, because we've got all of these other things," Thomas agreed. "If we didn't have it, we could still fly. I think that's an important point."

But, he added, "I think it would be nice for the program to have a capability that we could contribute to, to demonstrate and help those engineers work all those tricky problems. If one of these techniques was shown to be viable, we could put it on the flight, although time is really getting close for us. The train is leaving the station."

Asked if that implied NASA was bowing to the sort of schedule pressure blamed in part for the Columbia disaster, Thomas said "we do believe it's important to go fly. I think it's important not just for the agency, but for the country."

"We've got a space station that needs to be supported, the mission of the shuttle is to support the space station and we need to go do that and continue to work these issues so that any techniques that are developed really are themselves not driven by an arbitrary launch schedule, but can evolve in the natural engineering development cycle that they deserve," he said.

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