Another day, another launch attempt for Discovery. Photo: NASA-KSC

Engineers removed a potentially dangerous piece of debris from the shuttle Discovery's external tank early today and refueled the ship for a fifth attempt to launch a critical space station assembly mission.

With forecasters predicting a 60 percent chance of favorable weather, Discovery is set for liftoff at 7:17 p.m., the precise moment Earth's rotation carries pad 39A into the plane of the space station's orbit.

Hoping for the best, commander Brian Duffy, pilot Pamela Melroy, Japanese astronaut Koichi Wakata and spacewalkers Leroy Chiao, William McArthur, Peter "Jeff" Wisoff and Michael Lopez-Alegria plan to begin strapping in for launch around 4 p.m.

The goal of the 100th shuttle mission -the most complex station assembly flight yet attempted - is to attach a new $20 million shuttle docking port and a $273 million truss that will serve as the mounting point for a huge set of solar arrays scheduled for deliver in December.

Other equipment on board pushes the total cost of Discovery's payload to around $300 million.

Four back-to-back spacewalks by alternating two-man teams will be required to electrically connect the new components, to hook up coolant lines and to position the station's main KU-band and S-band radio antennas.

Commander Brian Duffy returns to the crew quarters follow last night's scrub. Photo: NASA-KSC

Assuming an on-time liftoff, Discovery will dock with the station around 1:43 p.m. Friday. Wakata then plans to mount the Z1 truss atop the U.S. Unity module around 11:02 a.m. Saturday. The first of the four back-to-back spacewalks is scheduled to begin at 10:32 a.m. Sunday.

If all goes well, Discovery will undock from the station around 9:37 a.m. on Oct. 20 and land back at the Kennedy Space Center at 2:10 p.m. on Oct. 22.

Just eight days later, the space station's first full-time crew - commander William Shepherd, Yuri Gidzenko and Sergei Krikalev - is scheduled for launch aboard a Russian Soyuz rocket.

NASA managers say they need at least two days between Discovery's landing and the Soyuz launch to five engineers supporting both missions to fly from Houston to Moscow.

NASA originally hoped to launch Discovery last Thursday, but the flight has been repeatedly delayed by technical problems, bad weather and, on Tuesday, because of an oversight by ground crews.

Discovery was already fueled for takeoff Tuesday when an inspection team noticed an eight-ounce metal locking pint lodged in an inaccessible area at the base of the shuttle's external tank.

Such tethered "pip pins" are used to secure tools and access platforms at the pad and it's not yet known how this one made its way onto the external tank. Officials said it should have been spotted earlier, but technicians did not find it until the T-minus three-hour mark.

Shuttle Program Manager holds up a pin similar to the one found on the Discovery. Photo: NASA TV/Spaceflight Now

Worried the pin could fall free during ascent and damage a critical system, shuttle managers reluctantly ordered a 24-hour launch delay so a rotating gantry could be put back in place to give technicians access to the area.

See our graphic of the pin's location.

Around 2 a.m. today, technicians finally reached the offending pip pin and removed it, setting the stage for a fifth launch attempt this evening.

If the flight is delayed again for any reason, shuttle managers likely would stand down until Friday to top off the ship's on-board supplies of liquid hydrogen and oxygen, which are used to power the shuttle's electrical generators.

A launch try Thursday would require a flight-day four rendezvous with the space station while launch tries Friday and Saturday would result in dockings on the third day of the mission.

That is NASA's preferred option to maximize the time available to the crew to complete its work.