While everyone can be thankful that the large chunk of insulating foam that shook loose from the Space Shuttle Discovery’s external tank during liftoff July 26 did not strike the orbiter, the incident proves that NASA officials still do not understand their vehicle. That revelation raises serious new questions about how many shuttle flights NASA will be able to conduct between now and 2010, the date the agency intends to retire all three remaining orbiters.

Discovery’s mission was correctly labeled an experimental flight, and while many aspects of the mission so far have gone well, the flight unveiled flaws so serious that the fleet had to be grounded again — for who knows how long this time. The impact of that grounding on the international space station program is large and potentially very costly.

Despite two-and-a-half years of time and effort, NASA clearly has not mastered the foam-shedding problem identified as the root cause of Columbia’s failure to safely return from orbit in February 2003. In that instance, a piece of foam that broke off from the tank punched a hole in Columbia’s left wing, allowing superhot gases to penetrate its internal structure during re-entry, causing the orbiter to disintegrate.

The seriousness of the latest foam-shedding incident was summed up perfectly by Wayne Hale, NASA’s deputy shuttle program manager, who said during a July 27 press conference that had the foam debris broken off when the shuttle was lower down in the atmosphere, “we think this would have been really bad.”

To its credit, NASA wasted no time in grounding its shuttle fleet and informing the public about what had happened. Nevertheless, the agency’s credibility on the foam issue has to be called into question. NASA officials had said the modified external tank on Discovery was the safest one to ever fly. Clearly, it was not safe enough.

One has to wonder now if the external tank will ever be safe enough, regardless of what NASA officials believe. After all, fixing the foam-shedding problem was a central focus of the post-Columbia return-to-flight effort, and yet the agency still fell short.

NASA Administrator Mike Griffin expressed confidence that the orbiter fleet will be up and flying again in short order. But if the latest grounding starts to look as if it will last a year or longer, as some experts have suggested, then NASA, the White House and Congress need to seriously consider retiring the fleet immediately. The resulting savings could be plowed into the development of a new and safer Crew Exploration Vehicle, human-rated launcher, heavy-lift launcher and perhaps even modifications to space station hardware that would allow it to launch on rockets other than the space shuttle.

NASA officials have estimated it will take 28 more shuttle flights to fulfill the agency’s international obligations to the space station program. Now that manifest — highly suspect before Discovery’s launch — is out of the question if NASA wants to hold to the 2010 fleet retirement date. Mr. Griffin has been looking for ways to trim the manifest to a more manageable 15 to 20 flights, but even that scaled-back number has to be considered a stretch if NASA sticks to the current timetable .

Grounding the shuttle fleet for good would have major negative ramifications for NASA and its space station partners whose hardware still awaits launch on the orbiter. It must nevertheless be considered. The issue is first and foremost one of safety: Even if NASA can eliminate the foam-shedding problem with a high degree of confidence, it is fair to assume there are other potentially catastrophic hazards lurking beneath the surface.

Then there is the opportunity-cost consideration. A year long interruption in space shuttle operations would effectively burn more than $4 billion, which is NASA’s rough annual price tag for maintaining the standing army of skilled technicians necessary to run the program. And that does not count the costs directly associated with investigating and finding a resolution to the debris problem, which could entail a redesign of the external tank.

Might that money be better invested in hardware that supports NASA’s future rather than its past?

That is a question NASA and its overseers in the White House and on Capitol Hill will have to think long and hard about in the weeks and months ahead.