KENNEDY SPACE CENTER, FL (CBS) - NASA hopes to resume shuttle flights Wednesday, weather permitting, with a three-spacewalk mission to repair the international space station's stabilization system, to deliver critical supplies and equipment and to prove the design defects that led to the Columbia disaster have been corrected.

Implementing an overlapping suite of approaches, NASA managers believe the chances of Columbia-like damage to Discovery during launch are minimal. But if damage does, in fact, occur, they are equally confident they will be able to detect it, determine if it is entry critical and, if it is, carry out at least rudimentary spacewalk repairs.

Those repair procedures are not fully tested and formally certified for use by Discovery's crew. But NASA managers say certified repair procedures are not required for flight because of the elimination of major debris, improved damage detection, a better understanding of the consequences of impacts and the crew's worst-case ability to use the space station as a "safe haven" until another shuttle, already prepped for flight, could be launched on a rescue mission.

"I believe in our flight rationale, which says we are fixing the vehicle," said LeRoy Cain, the ascent-entry flight director for Columbia's final mission and now, for return to flight. "We are eliminating critical debris from being liberated from the tank and the boosters, the launch pad, every source that we can think of, we think we're eliminating critical debris. That's number one for me.

"I really feel like the chances of us having something come off of the stack and create a problem for us in our flight are exceedingly low."

Low, but not zero. Even after two-and-a-half years of research, testing and analyses to minimize the amount of foam insulation or ice that can shake off the shuttle's external fuel tank during launch, NASA engineers say catastrophic impacts are still possible in worst-case scenarios.

Not to mention the ever-present possibility of a main engine or booster failure in a machine that weighs 4.5 million pounds at launch and accelerates from zero to more than 100 mph - straight up - in less than 10 seconds.

"People seem to have forgotten that when we fly the shuttle with the technology that we human beings own today - and there is none better - it's still not good enough," said new NASA Administrator Michael Griffin, a refreshingly candid rocket scientist with a doctorate and five master's degrees.

"This is a very risky venture," he said. "The people who are doing this are risking their lives in support of objectives the United States has in the pursuit of space exploration. Many people risk their lives on behalf of the United States and our flight crews, our astronauts, are in that group also. Everybody should understand that."

Shuttle commander Eileen Collins agreed.

"There are millions of things that have to go right on launch day and entry day, there are all kinds of sensors and transducers and wires and electrons and flow paths going all through the orbiter, any of which could break at any time," she said in an interview with CBS News. "People ask me, what worries you the most? It's really not what we know about but what we don't know about that worries me."

Even so, she said, "it's time for us to go fly."

"It's been over two years now and a huge effort has been put into getting the space shuttle back flying again," she said. "I think we're ready to do it. My only concern is that after my mission flies that we continue to make things better, we don't just drop it and say return to flight has happened and now we can go back to business as usual."

A "business as usual" attitude among senior NASA managers played a role in the decision to launch Columbia Jan. 16, 2003, despite a major foam debris strike two missions earlier.

During Columbia's launching, a piece of foam broke away from the same area of the ship's external tank and blasted a hole in the leading edge of the shuttle's left wing. During re-entry sixteen days later, on Feb. 1, 2003, super hot plasma entered the breach and melted the wing from the inside out, triggering the shuttle's destruction.

The Columbia Accident Investigation Board, or CAIB, made 29 recommendations to improve safety and management, including 15 that were to be implemented before the resumption of shuttle flights.

An independent panel charged with assessing that implementation concluded the agency had failed to fully address the three most critical recommendations to eliminate all debris sources; to make the shuttle's thermal protection system more resistant to impacts; and to develop repair techniques to fix any such damage that might occur.

The Return to Flight Task Group's conclusions rekindled a long-running debate among managers, engineers and even reporters about the intent of the CAIB given the deliberately vague wording used in some of the recommendations.

But the Task Group, led by former Apollo astronaut Thomas Stafford and former shuttle commander Richard Covey, adopted a literal interpretation of the recommendations. And by that standard, they concluded, NASA came up short on debris elimination, shuttle hardening and development of reliable repair techniques.

NASA managers did not argue with the panel's conclusions. But they pointed out that engineering reality had dramatically changed since the CAIB's initial report was released and that it had turned out to be impossible, given the shuttle's design and the time and funding available, to eliminate all debris.

While some tiles and seals around the landing gear doors were "hardened" to resist impacts, NASA managers called off efforts to toughen the shuttle's wing leading edges, concluding it did not make sense to mount such a major engineering effort given a recent presidential directive to retire the shuttle fleet by 2010. Only 18 to 20 shuttle flights are expected before the shuttle era ends.

As for repair techniques, NASA managers say they ended up in a "Catch-22:" To fully develop the required repair procedures, astronauts needed to test them in the space environment to make sure they could be trusted to bring a crippled ship home.

Instead, Discovery's crew will test two rudimentary leading edge repair techniques, one meant for cracks and one for small holes, and one procedure intended for use on tiles with minor impact damage. The results of the tests aboard Discovery, coupled with additional tests aboard the next shuttle flight in September, may provide the data needed to formally certify one or more repair techniques.

Then again, they may not.

"The fact is, several CAIB recommendations, taken word by word, are not implementable with the state of our knowledge today," Griffin said. "We do not know how to repair large holes in (reinforced) carbon carbon (leading edge panels) or even small holes, maybe.

"We are being as smart about this as we know how to be but we are up against the limits of our human knowledge. If someone wants more, they're going to have to find smarter humans. So the recommendations as they were written are not strictly speaking implementable, at least not all of them are, and the (Stafford-Covey Return to Flight Task Group) noted NASA was not able to implement them. That was not a surprise."

In the end, Griffin agreed with his engineers that NASA had done everything reasonably possible to improve shuttle safety and that only incremental improvements could be expected by keeping the fleet on the ground.

"If we ground the shuttle fleet, we're not going to be able to complete station assembly, we're not going to be able to do other things that we want to do," Griffin said. "If, of course, we believe that all debris sources have been reduced to a level low enough that the shuttle won't be damaged, then the tile repair issue becomes kind of moot.

"We're in that gray area where we believe we have greatly reduced the risk due to debris, foam and ice, but not so much we're completely comfortable with it. So the STS-114 crew ... will be lifting off in the face of a known risk."

For the record, Gehman said he believed NASA did, in fact, met the board's overall intent.

"It is our judgment that they've efforts have passed the criteria that we set up for them," Gehman said in an interview with CBS News. "But that doesn't mean they're allowed to give up on the repair. In our view, they have to keep working at it."

The critical recommendations, in his view, centered on four broad areas.

"First of all, you've got to understand foam creation and the creation of the hazard in the first place and you've got to do everything you can to prevent the creation of foam in the second place," Gehman said in the CBS interview. "The second thing you've got to do is, you've got to have much better pictures on launch and ascent to know whether or not there's been a foam event, or a debris event. You've got to know that. The third thing you've got to do is, you've got to essentially re-certify the orbiter to be ready to come back into the Earth's atmosphere. That translates into some kind of an inspection in orbit.

If serious damage is detected, "you have to have some minimal, practical kind of capability to do some kind or orbital repair, the best practicable kind of a repair. Knowing full well, depending on the size of the damage or what the nature of the damage was, there are some repairs that are beyond your capability to do in space.

"And it has been our unwritten policy ... and I told Stafford-Covey and asked Stafford and Covey to back me up on this and they have - and that is, you must attack all four of these things.

"Now you can do some better than others," Gehman said. "If you really think you've done a fabulous job of preventing the creation of debris in the first place, you've got some really good ways to take pictures to make sure your orbiter hasn't been struck or anything like that and you're really sure that it's in good condition, then you can do some of the other stuff to a lesser degree.

"But you do have to make an attempt at all four areas. Now, within those four areas, there are sometimes one, two, three or four things that you've got to do. But that was what our intent was."

In a recent teleconference with reporters, Gehman said "I know of no reason why they should not proceed with the launch. That's not to same thing as saying it's safe to go, that's a different story."

Asked to explain, he added: "I don't think the American people and the Congress of the United States realize how dangerous this is."

"We didn't realize how dangerous it was when we started this investigation," he said. "It was dangerous, it remains dangerous. We the country have got to replace this vehicle as soon as possible. ... I'm sure this next flight will be safer than the previous ones, but by any measure of 'safe,' this is not safe."

Discovery is targeted for launch on the 114th shuttle mission at 3:50:52 p.m. Wednesday, July 13. Joining shuttle veteran Collins on Discovery's flight deck will be pilot James Kelly, flight engineer Robinson, seated to Robinson's right, Japanese astronaut Soichi Noguchi.

Strapped in below on the shuttle's middeck will be Mir-veteran Andrew Thomas, Wendy Lawrence and Charles Camarda. All but Camarda and Noguchi are shuttle veterans.

The shuttle's primary cargo includes a refurbished control moment gyroscope to replace one that failed earlier aboard the space station; a tool kit and spare parts module that will be mounted on the station's airlock to enable future assembly work; and a pressurized logistics module loaded with space station equipment and supplies.

In an interview for the book "Comm Check: The Final Flight of Shuttle Columbia," by this author and Michael Cabbage, Collins was asked if she had any second thoughts about commanding mission STS-114.

"Absolutely not," she said. "In fact, I am more committed to flying this mission than I ever would have been. ... I am excited, I am going to be extremely confident because look at all this work that is being done, not just done because of (Columbia), but other things that we think are risky. I am so confident, I am so excited, I want to get our country back flying in space again, so I am not one blink of an eye worried about safety."

And her crewmates?

"It's time to go fly," Robinson told CBS News in a recent interview. "There will be debris, there will be some damage, I'm convinced of that. If there isn't, that'll be great but I'll sure be surprised. I would be very surprised if it's critical damage, damage that won't allow us to fly home on. But here's the thing. We'll know it. We won't have to wonder. We'll know it.

"We'll have the technology now for the first time on this mission to take a look at it with all the cameras and sensors. This is the way we verify all the engineering that's been done. So we'll get to look at our bird before we come home. Then, on top of that, if the worst on worst on worst happens and we do have critical damage, the space station will (be available for safe haven), we won't have to risk our lives coming back through the atmosphere. This is what gives me tremendous confidence and makes me feel very lucky I'm flying now."

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