With the first post-Columbia shuttle flight a month and a half away, engineers are still assessing what sorts of damage might result from impacts of foam debris falling from the shuttle's redesigned external fuel tank. Engineers are confident the new tanks will be the safest ever launched and that expected debris will not harm the shuttle's sensitive wing leading edges, which experience the fiercest heat during re-entry.

But officials said today test data indicates expected debris, in a worst-case scenario, could cause potentially catastrophic damage if the debris in question fell off at the worst possible moment and hit a sensitive area of heat-shield tiles at the proper angle and impact velocity. The precise nature of that risk is not yet known and engineers say they cannot provide details until after a three-day debris verification review, which starts Thursday.

"In terms of the foam, we have our best estimates of what the foam debris loss is going to be," said John Muratore, manager of shuttle systems engineering and integration at the Johnson Space Center in Houston. "And with our best estimates of the foam debris loss and our best estimates of the (wing leading edge) impact tolerance, we don't believe that expected foam can cause any damage" to the reinforced carbon carbon - RCC - wing leading edge panels.

A hole in the leading edge of Columbia's left wing, caused by the launch day impact of a 1.67-pound chunk of foam debris, led to the orbiter's destruction during re-entry Feb. 1, 2003.

Since then, the tank has been redesigned to eliminate the foam in question and to improve the application of foam in other areas to minimize debris shedding.

The shuttle Discovery's external fuel tank is not expected to release any foam heavier than 0.023 pounds - 0.37 ounces - from the upper portions of the tank during launch. Debris from the upper part of the tank poses the greatest threat to the ship's nose cap and reinforced carbon carbon leading edge panels. The 0.023-pound limit is for foam covering the liquid oxygen tank at the very top of the structure.

Foam from the intertank area, where bipod struts attach the nose of the shuttle and where the foam originated that doomed Columbia, can be slightly more massive: 0.03 pounds, or 0.48 ounces. Pieces of foam from the bottom of the tank pose less of a threat and thus can be larger still: up to 0.75 pounds.

Here is a partial table of allowable debris as approved by shuttle program managers:

REGION.................................................FOAM (pounds/cubic inches)

Oxygen tank...........................................0.023/15.9
O2 tank to Intertank flange....................0.026/20.7
Intertank...............................................0.03/20.7
Hydrogen tank and intertank flange........0.03/20.7 to 0.075/51.8
LH2 tank................................................0.075/51.8

The most sensitive areas to damage are the edges of the landing gear doors and smaller doors that cover external tank propellant line connections after the external tank is discarded in space.

"The sizes that we're expecting to be liberated from the tank in a worst-case condition can cause significant damage," Muratore said. "It all depends on the location and the depth (of the damage) that actually occurs and the underlying structure."

But such worst-case scenarios have never happened before - "that is significantly outside of our flight experience," Muratore said - so the question becomes one of how conservative NASA's assumptions might be and "we're struggling with that problem right now. It's a very difficult engineering problem."

Those issues will be discussed in depth during the upcoming debris verification review. Those results, in turn, will be presented to senior NASA managers later this month and ultimately to an independent panel monitoring NASA's implementation of post-Columbia safety upgrades.

That panel, led by former Apollo astronaut Thomas Stafford and former shuttle commander Dick Covey, has delayed a final public hearing and completion of its report in part because of the time NASA has needed to complete the impact test analysis.

While the numbers aren't yet available, Muratore cautioned that NASA has not eliminated all risk of foam impact damage.

"So there is some risk we're accepting that if the conditions are different than our best estimates, that we could take serious damage," he said. "And that's the risk we're going to have to take."

Asked if NASA could gain any additional safety margin by taking more time to improve the tank still more, Muratore said "we feel that the main way we can get smarter about this problem is go fly."

"We have come to the point, as it comes in every experimental vehicle, where the amount of analysis and modeling and ground tests has sort of saturated the problem and now the only way to really know what's going to happen in the real world is to go actually perform a flight test."

Launch currently is targeted for May 15, but getting Discovery to the pad has taken longer than expected and used up what spare time was built into the schedule to handle unexpected problems. NASA hopes to move the shuttle to the pad Wednesday and many observers expect the launch date ultimately will slip five days to a week when all is said and done. But shuttle program manager Bill Parsons has said he does not plan to consider possible changes until after a fueling test around April 14. As of this writing, Discovery can still, in theory, make May 15.