Shuttle systems engineering manager John Muratore said it is a risk NASA and the nation must accept for flights to resume anytime soon.
It would take years and a total redesign of the fuel tank to completely eliminate foam loss and to ensure the 2003 Columbia tragedy would never be repeated, Muratore and other officials said.
NASA expects pieces of insulating foam no bigger than one or two marshmallows to break off the fuel tank when Discovery blasts off next month. Depending on where and when the pieces hit, they could cause catastrophic damage during re-entry, Muratore said.
By contrast, the size of the foam that shattered Columbia's left wing was the size of a carryon suitcase.
Muratore told reporters he was "trying to be scrupulously honest with you about what the potential is — but that doesn't say that's what we expect to happen." He likened the situation to trying to predict the chances of being in a fatal car accident while driving to the airport.