Spokeswoman Laura Brown says, "The Board is not trying to redesign the Shuttle, and Board chairman Ad, Hal Gehman says, "I seriously doubt the Board will address an issue like [whether NASA should build any new Shuttle orbiters to replace Columbia]... We will attempt to restate the risks and make sure we understand the costs and the benefits. We'll try to baseline the debate, but I doubt that the Board will make a recommendation like that."

But -- simply by virtue of its assigned role in determining the measures necessary to minimize the chances of another Shuttle accident -- the Board's recommendations may end up having very sweeping repercussions.

Gehman also says that, while 'That's really for the Administration and Congress to decided something like that... we probably will want to characterize the issue as well as we can and put that debate in context."

If the Board concludes that the Shuttle is a very dangerous vehicle and needs not just limited but radical changes to make it less dangerous, that fact alone will place massive pressure on NASA and the federal government to follow its recommendations.

And any honest appraisal of the Shuttle cannot help but reach such a conclusion.

NASA's clear desire -- as openly expressed by Administrator Sean O'Keefe -- is to make a new relatively small pro-safety modifications in it and then resume flying before the end of this year, if possible -- or at least by next spring.

Those changes are virtually certain to include considerably greater care in inspecting both the foam on the External Tank before each launch, and the condition of the Shuttle's tiles and thermally protective "RCC" panels after each flight.

They will also certainly involve radical redesign of the area around the "bipod" -- the yoke of two diagonal struts that fastens the Shuttle's front to the External Tank -- from which the chunk of foam that is the probable cause of the disaster was dislodged. (One possible scheme involves covering all the local area in a metal casing, either overlaid on top of its foam or containing electric heaters to replace the foam.)

The Board has also mentioned the possibility of installing a new design of tougher tiles on the Shuttle's belly. A recently developed, more impact-resistant kind of "TUFI" tile is already installed on many upper parts of the Shuttle -- and while these cannot be used on the Shuttle's belly and other areas exposed to maximum heating because their thermal protective properties are inferior to those of the Shuttle's softer LI-900 and LI-2200 tiles, one witness has told the Board that NASA is "very close" to perfecting a new kind of "BRI-8" tile which combines the impact resistance of the TUFI tiles with the greater thermal resistance of the LI tiles.

But the fact is that the effectiveness of such changes will inevitably be limited. There is always the chance that a large fragment of foam will come off some other part of the external tank -- NASA isn't even entirely certain that the half-kilogram fragment that struck Columbia's belly in June 1992 and plowed a gouge 18 cm long and 1.2 cm deep came off the bipod ramp -- or that some other foreign object will strike the Shuttle during launch. In December 1988, a chunk of thermal coating from one of the Solid boosters knocked half a tile off a Shuttle.

Such an object might very well damage even tougher tiles -- and no one has yet proposed a way to greatly increase the impact resistance of the "carbon-carbon" RCC panels on the leading edges of the wings, which seems to be where the fatal damage on Columbia occurred, and which must resist the most ferocious reentry heating anywhere on the Shuttle.

The only way to eliminate this danger is to engage in a radical redesign of the Shuttle -- such as dividing the External Tank's metal wall into two layers with the insulating foam between them, or designing entirely new, tougher versions of thermal protective materials such as "shingles" made out of metal rather than silica or carbon.

Such a redesign would certainly cost billions of dollars -- and require years, during which a substantial part of the Space Station's remaining design lifetime would be eaten up. And even such a system would have limited ability to survive an impact by a small but high-speed piece of orbiting "space garbage", a problem that is gradually increasing in seriousness as a direct safety hazard to any manned spacecraft - including the Station.

Nor is this the only serious safety threat still connected with the Shuttle. The plain and simple fact is that it is impossible to improve its safety more than modestly, by its basic design.

As Adm. Gehman says: "The parts of the STS which were viewed to be the most dangerous have not failed -- it's always something else which has gotten us, it seems."

He's referring to the "SSMEs", the liquid fueled reusable rocket engines on the Shuttle Orbiter itself -- which must endure tremendous mechanical and thermal stress repeatedly and still be reusable.

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