KENNEDY SPACE CENTER, FL--The shuttle Discovery's launch on a space station resupply mission tomorrow was delayed at least 24 hours, to no earlier than 3:29 p.m. EDT Thursday, to give engineers more time to troubleshoot an apparent electrical glitch in circuitry associated with a backup main engine computer.

"It's another day in paradise," joked Launch Director Michael Leinbach after the countdown was extended. "This is part of the business, you fly when you're ready and you don't if you're not, and we're not ready to go."

"Discovery's not going out easy," he said, referring to the orbiter's 39th and final flight. "She's giving us a little bit of trouble. But that's fine. She'll fly perfectly when she does."

A space shuttle main engine during ground processing.

If no major repairs are needed and engineers can get comfortable launching Discovery as is, commander Steven Lindsey and his five crewmates could be cleared for launch Thursday, weather permitting. Forecasters are predicting a 70 percent chance of low clouds and rain from an approaching frontal system that could block a launch attempt. But the forecast improves to 70 percent "go" on Friday.

Regardless of the dismal forecast, Mike Moses, chairman of NASA's Mission Management Team, said the engineering community needed the extra day to review historical data, flight rules, and performance specifications to determine if the system can be launched as is or whether more extensive repairs might be needed.

If repairs are needed, engineers will be hard pressed to complete the work before the current launch window closes. As of this writing, the window closes Sunday, with a possible extension to Monday. If Discovery isn't off the ground by then, launch will slip to December 1, the opening of the year's final shuttle launch window.

"We don't fly with unknown risk and right now, this risk is still a little bit unknown to us," Moses said. "We're going to take another day to get to know it better, for lack of a better word, and make this a known risk that we do understand and that we've quantified.

"I think we could have gotten there, I think the data is there, but it's really the better part of valor to let them have the time to go put that story together in a nice, crisp package that we can walk through tomorrow."

He said engineers are developing repair scenarios, but the focus is on developing "the rationale to fly in the condition that we're in."

"If that rationale tomorrow doesn't sound good enough, we'll see what more time we need to make it a story we are comfortable with," he said. "And ultimately, if we find that we cannot get comfortable with the scenario we have, then we'll go down the path of R & R (removal and replacement)."

Discovery's final countdown began yesterday after engineers worked through the weekend to replace and retest leaking quick-disconnect fittings in the shuttle's right-side orbital maneuvering system rocket pod. That work forced NASA to delay launch from Monday to today and, eventually, to tomorrow.

Once under way, the countdown ticked smoothly through its initial hours. But during work to power up and check out Discovery's main engine electronics early today, engineers ran into problems with a circuit powering one of two channels in main engine No. 3's controller.

Each of the shuttle's three hydrogen-fueled main engines is equipped with a two-channel controller that monitors engine operation 50 times per second during ascent, relaying the data to the shuttle's flight computers.

If the controllers spot a problem, an engine can be safely shut down before a catastrophic failure can occur. Given the critical nature of the controllers, NASA flight rules require both redundant channels in each controller to be operating normally for a launch to proceed.

During checkout earlier today, the backup channel of main engine No. 3's controller did not power on when a switch was thrown. A bit later, the controller powered up on its own, the sort of signature that can indicate contamination in a circuit breaker.

After initial troubleshooting, engineers cycled the circuit breaker in a bid to clear any "transient contamination." The controller then was powered back on and it appeared to be working normally.

But the picture was complicated about 90 minutes later when telemetry indicated an unexpected, very brief voltage drop in the same circuit. Engineers now are trying to determine whether a transient issue with the circuit breaker could explain both signatures.

Moses said he was optimistic the system is healthy. The voltage drop was well within design specifications and he said had it occurred during engine startup, launch would have proceeded normally.

But he said the team needed to develop a thorough understanding of "what if we're wrong, what's going to happen if this controller does flake out, either in the count Thursday or once we start going up hill after launch"

"The other big piece of that is to make sure we're not, to put it bluntly, crafting a solution that matches what we think the problem is and that we're actually having a good physics based understanding of the phenomenon.

"Does it make sense that a circuit breaker with a little bit of a bad contact explain both of these signatures that we saw The community feels pretty confident that that is the case, but they do need time to polish that story."