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.
"If we have that worst day, and the tire is worn and you have a flat tire in the wrong place in traffic, next to a truck going 90 mph, could you get killed? Yes, you could. Is that a reasonable set of assumptions to plan your trip on? Probably not."
Muratore said assessing the danger from foam and other launch debris is an extremely complicated engineering problem made even more uncertain by the fact that computer models show little pieces of foam could cause catastrophic damage. NASA's flight experience over the decades has proven otherwise.
What NASA has to do to get smarter, Muratore said, is to stop relying on computer models and start flying the space shuttle again.
Discovery is scheduled to blast off in mid-May on the first shuttle flight since the Columbia disaster on Feb. 1, 2003. NASA plans to move the spacecraft to the launch pad Wednesday.
NASA will fly five types of repair kits aboard Discovery for the astronauts to test in space, but the rudimentary patches will accommodate holes no bigger than 4 inches. The gash that brought down Columbia was an estimated 6 inches to 10 inches in size.
Steve Poulos Jr., a shuttle project manager, said a repair kit to fix that big of a hole should be available in two years.