Heavy rain early Thursday buffeted Florida's Kennedy Space Center where Discovery had already been rolled out to the launchpad after early glitches that postponed the launch three times earlier in the week were resolved.

"There is a no go for 24 hours because of the weather," a spokesman at Kennedy Space Center said.

Launch is now set for 3:03 pm (1903 GMT) Friday, with weather forecasts showing a 60-percent chance of favorable conditions, the US space agency said.

Mission experts had agreed on Thursday's launch attempt after a close study of the latest electrical glitch found that a circuit-breaker was the origin of a cockpit problem, not the main engine controller which would have been more serious.

After three postponements to Discovery's final flight before it is retired, NASA experts were eager to see it fly Thursday.

"From a vehicle prospective we are ready to go," said mission management team leader Mike Moses, although with just a 20 percent chance of favorable conditions he acknowledged the mission might be grounded by Mother Nature.

A green light from mission managers would begin shuttle fueling operations, which normally take three hours.

The launch window closes Sunday. The next window for the mission would come on December 1 and last only a few days.

Discovery's 11-day mission, originally scheduled for November 1, was delayed twice for an engine pressurization problem and again on Tuesday for an electrical malfunction of a circuit-breaker in the shuttle's cockpit which failed to turn on as expected.

While the latest glitch with the back-up controller for shuttle engine three was not considered a major problem -- the breaker worked after several tries -- NASA engineers wanted to understand its causes should it malfunction again during takeoff, Moses said.

NASA concluded the problem was residue buildup on the breaker switch.

Discovery's all-American six-member crew, including female mission specialist Nicole Stott, will deliver a pressurized logistics module called Leonardo, which will be permanently attached to the space station to provide 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 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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