KENNEDY SPACE CENTER, Fla.--Engineers successfully repaired and retested new quick-disconnect fittings in one of the shuttle Discovery's aft rocket pod, clearing the way for the start of the ship's countdown today and launch Wednesday on a mission to deliver critical supplies, spare parts and a final U.S. module to the International Space Station.

The countdown began at 2 p.m. EDT, setting up a launch attempt at 3:52:13 p.m. Wednesday at roughly the moment Earth's eastward rotation carries pad 39A into the plane of the space station's orbit. It will be Discovery's 39th and final flight.

The shuttle Discovery atop pad 39A early Sunday.

"So far, it's been a remarkable flow for Discovery," said NASA Test Director Steve Payne. "She's been an incredible vehicle and she caps a long and distinguished career with this particular flight. She's always amazed us with everything she can do and we expect this flight should be no different. We're looking forward to launch on Wednesday afternoon."

Forecasters are predicting a 70 percent chance of good weather Wednesday with just a chance of showers in the area, scattered clouds at 3,000 feet, broken decks at 9,000 and 25,000 feet, and winds out of the north at 11 knots with gusts to 17. The odds drop to 60 percent "go" on Thursday because of an approaching frontal system. Forecasters also are keeping tabs on Hurricane Tomas, which is expected to take a turn to the northeast later in the week.

"We'll keep an eye on Tomas, but right now it doesn't look like it'll be a factor for launch," said shuttle weather officer Kathy Winters. "Our main concern will be if we have a delay. So right now on launch day we have a 30 percent chance of KSC weather prohibiting launch. If we happen to delay 24 hours, that's when a front will move into the area and we have more concerns. We'll keep watching that trend."

Depending on how the computer models play out, Winters said, "we may have to increase that 24-hour delay number."

Discovery's launch originally was planned for Monday, but the start of the countdown was delayed Friday after engineers discovered two leaking quick-disconnect fittings in an access panel where ground lines plug in to pressurize nitrogen and helium tanks in the ship's right-side OMS pod. Nitrogen is used to move various valves in the rocket pod while high-pressure helium in a much larger tank is used to push propellants to rocket engines used for orbital maneuvers.

NASA managers initially hoped to complete repairs and retest in time to start the countdown yesterday, setting up a Tuesday launching, but it quickly became apparent more time would be needed to complete the retest operation and to repressurize the high-pressure helium tanks. The latter is a 16-hour procedure that requires engineers to evacuate the launch pad.

In any case, the helium system was vented and replacement couplings were installed yesterday. Engineers then carried out extensive pressure checks and leak tests to make sure the problem had been resolved. The helium system was repressurized overnight Saturday.

"We completed early this morning our flight pressurization of our on-orbit control system tanks and the main propulsion tanks and it all went well," Payne said. "That is beyond us now."

As it turned out, the suspect nitrogen connector may have been healthy all along. During the repair work, engineers discovered the pressure in the ground supply line connected to the suspect fitting was higher than expected, possibly preventing a poppet in the quick-disconnect mechanism from closing. But the helium issue would have delayed launch regardless.

"Initially, we had two different issues with our quick disconnects, one with the nitrogen side and one with the helium side," Payne said. "The one that was the long pole, the one that took us a long time to resolve, was the one on the helium side because it required venting down our tanks and repressurizing. That just takes a long time because you have to do it slowly to allow the thermal conditioning to happen.

"For the nitrogen tank, it's a quick process. The tank is about the size of softball, it takes a couple of minutes to fill and empty. We thought it was the quick disconnect valve. There's a little poppet inside that's held open by pressure and when the pressure goes down it closes again. The (ground system) valve that was supposed to stop providing pressure to it was providing too much and it couldn't close properly. So we just went upstream and closed the gas off to that one so the pressure could bleed down and the QD could close. And it worked very well."

The primary goals of Discovery 's mission are to deliver a 21-foot-long cargo module loaded with supplies and equipment that will be used as a storage room to give station crews more room. Discovery's crew also will deliver an external cargo storage platform with a spare set of cooling system radiators. Two spacewalks are planned to facilitate the cargo module installation, to re-stow a failed ammonia pump module and to perform a variety of routine maintenance activities.

Assuming an on-time launch, Discovery will dock with the space station around 12:36 p.m. Friday. Two spacewalks are planned, on November 7 and 9, with undocking on tap November 12 at 5:02 a.m. and landing back at the Kennedy Space Center at 9:59 a.m. November 14.