Discovery is now scheduled for launch no earlier than July 13. The flight had been targeted for late May.
A large chunk of foam insulation from the external fuel tank punched a hole in Columbia's wing that led to the shuttle and crew's demise during re-entry in February 2003. Now, the lingering concern involves the possible buildup of ice on the tank once it's filled with super-cold fuel, and the hazard such shards would pose if they came off during the launch and hit the shuttle.
NASA's new administrator, Michael Griffin, announced the delay at a midmorning televised news conference, saying it was the result of recent launch-debris reviews.
"This is consistent with our overall approach to return to flight, which is that we're going to return to flight. We are not going to rush to flight, and we want it to be right, so we're doing what we need to do to ensure that," Griffin said.
Extra repairs to Discovery's fuel tank will be needed, namely the addition of a heater, said NASA's top spaceflight official, Bill Readdy.
The foam responsible for Columbia's demise was intended to prevent ice from building up around the fittings that attach struts holding the nose of the shuttle to the tank, reports CBS News Space Consultant Bill Harwood. The so-called bipod foam has been eliminated in favor of small heaters.
Foam application techniques were changed to minimize the chances for foam shedding in general. Engineers believe the largest piece of foam that can come off the tank today is less than a half ounce. The piece that hit Columbia weighed some 1.67 pounds.
But recent testing shows ice buildups in two areas of the tank still pose a threat. One of those areas is in the so-called inter-tank region between the upper oxygen tank and the lower hydrogen tank where an oxygen feedline bellows is located. The bellows allows the line to flex slightly during launch.
The testing shows ice can build up on the bellows or on a bracket holding the line in place. Another ice problem area is near the tip of the tank around a bracket that holds a repressurization line.
NASA managers held a second debris verification review, or DVR, at the Johnson Space Center in Houston earlier this week and still were unable to conclusively demonstrate ice was not a threat, according to sources who requested anonymity.