NASA took a decision to delay the next shuttle mission launch till November because engineers had been unable to figure out why insulating foam fell off the craft's external fuel tank.
A September launch was cancelled amid confusion over why pieces of insulation break off the fuel tank at lift-off.
A large piece of foam fatally damaged the Columbia shuttle in 2003, causing it to burn up on re-entry, while a smaller piece snapped off Discovery, BBC reminds.
Solving the problem may prove expensive ahead of the planned retirement of the entire shuttle fleet by 2010.
Officials had been hoping to clear up the foam problem in time for a September launch of the shuttle Atlantis.
"We didn't find any root cause" of the foam incident, William Gerstenmeier, program manager of the International Space Station, was quoted as saying by Los Angeles Times. "It was probably a combination of events. We just need to keep looking."
Nasa grounded the shuttle fleet for two-and-a-half years after the Columbia disaster, which killed all seven astronauts on board.
NASA spent more than $1bn (Ј552m) on investigating the problem.
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, BBC reports.
A November launch could be possible, but the 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.
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