Nasa has nine shuttle flights left before it completes the space station
Nasa has delayed the first space shuttle launch of 2009 for the fourth time because of safety concerns.
US space agency managers and engineers met for 13 hours on Friday to discuss a potentially dangerous problem with fuel valves in its engine compartment.
The Discovery shuttle's crew is due to fit the International Space Station with its fourth and final set of solar arrays.
No new launch date has been set for the orbiter's two-week mission.
During the last shuttle mission in November, a tiny part of one valve cracked.
There was no danger to the crew or the shuttle, officials said.
But Bill Gerstenmaier, Nasa's associate administrator for space operations, said: "We need to understand a little bit more the consequences if a piece comes off."
He added: "When we're in this mode, we need to be careful about jumping to conclusions and picking solutions."
Nasa has nine shuttle flights left before it completes construction of the station.
The fleet is scheduled to be retired in 2010.