An electrical malfunction forced the US space agency to announce Tuesday the third delay so far, this time due to a problem with a circuit-breaker in the shuttle's cockpit.

The new launch time is set for 1929 GMT on Thursday (3:29 pm local time), but the weather forecast was gloomier than before, with just a 30 percent chance of favorable conditions for launch, forecasters said.

The latest electrical malfunction centers around the back-up controller for engine three, which failed to turn on as expected. Engineers have narrowed down the issue "to either a cockpit circuit breaker or switch that provides power," NASA said.

"We are getting very close to have a very good understanding of the problem but we need to polish it, we need to organize it, to make sure that we are not wrong about it," said mission management team leader Mike Moses.

"We are taking an extra day to do that," he said, adding however that circuit breaker problems have appeared before on the shuttle fleet without causing major concerns.

He said the management team would meet on Wednesday at 1800 GMT to decide whether or not to green-light Thursday's launch.

"We don't fly with unknown risks and right now this risk is a little bit unknown to us... we want to truly understand the risk we do fly with," said Moses.

Should the Discovery need a repair, Moses said, it would likely take several days, making it difficult to set a new launch date before the mission window closes Sunday or Monday.

He said the next launch window for the mission would come on December 1 and last only a few days.

Earlier delays were announced so that Kennedy Space Center technicians could repair quick-disconnect fittings in the system used to pressurize one of Discovery's orbital maneuvering rocket engines.

Discovery's all-American six-member crew on this voyage, including female mission specialist Nicole Stott, will deliver a pressurized logistics module called Leonardo, which will be permanently attached to the space station to give it more storage space.

The shuttle will also bring Robonaut 2, the first human-like robot in space, and a permanent addition to the orbiting space station, as well as spare parts.

Two space walks, for maintenance work and component installation, are scheduled.

The flight to the orbiting ISS is the fourth and final shuttle flight of the year, and the last scheduled for Discovery, the oldest in the three-shuttle fleet that is being retired in 2011.

"Discovery is not going out easy, she is giving us a little bit of trouble but that is fine, she will fly perfectly when she does," said launch director Mike Leinbach.

The three US shuttles -- the other two are Atlantis and Endeavour -- are due to be sent off to become museum pieces after a final shuttle mission to the space station in late February.

That means Russian Soyuz spacecraft, a modernized version of which recently dropped off three fresh crew members to the ISS, doubling the crew to six, will for several years be the only vehicle for transporting humans into space.

However, NASA's recently approved 2011 budget has left the door open to an additional shuttle flight in June.

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