The US space shuttle fleet is to remain grounded until November at the earliest, Nasa officials have said.
Nasa had hoped Discovery would not have foam problems
A September launch was cancelled as Nasa tries to stop large pieces of foam debris being shed from the fuel tank.
A large foam chunk fatally damaged the Columbia shuttle in 2003, causing it to burn up on re-entry, and smaller pieces were shed during Discovery's launch.
Solving the problem may prove expensive ahead of the planned retirement of the entire shuttle fleet by 2010.
Nasa grounded the shuttle fleet for two-and-a-half years after the Columbia disaster, which killed all seven astronauts on board.
It also spent more than $1bn (£552m) on investigating the problem.
Despite this, engineers at Nasa still had to contend with foam problems during the Discovery mission.
The man leading Nasa's investigation admitted there were no obvious solutions to the problem, but he said modifications would have to be made.
"Frankly, even the next time we fly the tank, I would expect to see a little bit of foam loss somewhere in the tank," said Bill Gerstenmaier.
"I think it's an extremely difficult engineering problem to solve. There's no immediate answer or problem that jumps out at us."
The chunk of foam that doomed the Columbia mission in 2003 smashed into the shuttle's left wing, causing superheated atmospheric gases to destroy the shuttle on re-entry.
The largest piece that broke off Discovery's tank weighed 1lb (450g), slightly smaller than the one that hit Columbia. Crucially, it did not hit the shuttle.
Future shuttle flights have been suspended until the foam problem is resolved. That rules out a September launch, when the next mission was due to blast off from Cape Canaveral.
A November launch could be possible, but after that the next suitable date would be next year, as Nasa requires adequate light in order to film and photograph the launch for safety reasons.
Nasa hoped to send Atlantis up next as a second test mission, and to continue work on the international space station.