The space station's newly spread solar arrays catch the sun. Photo: NASA TV/Spaceflight Now

After four days of around-the-clock analysis, testing and planning on Earth, two astronauts working nine stories above the shuttle Endeavour today will attempt to tighten a loose solar array blanket on the international space station's $600 million power system.

Astronauts Joseph Tanner and Carlos Noriega are scheduled to float out of Endeavour's airlock around noon to begin a planned six-hour spacewalk, their third since Sunday. The shuttle's five-man crew was awakened at 7:06 a.m. to begin preparations.

Asked how optimistic he was about pulling off the tricky repair job, Tanner said Wednesday "I'm a real optimistic sort of guy and yeah, I think we're going to be able to do it."

But, he quickly added, "I don't anticipate we'll get it on our first try. On the ground, even, they had several attempts before they were successful doing what we're proposing to do. I'll be overjoyed if we get it on the first try, but I don't anticipate that."

Either way, he said, "I think it's going to be exciting."

Along with tightening up the loose solar blanket, the spacewalkers also plan install a television system that will help the next shuttle crew attach the U.S. laboratory module, Destiny, in January and install an instrument that will measure the lab's electrical environment.

Endeavour astronauts (left to right) Mark Garneau, Carlos Noriega, Joe Tanner and Mike Bloomfield review plans for the solar array repair. Photo: NASA TV/Spaceflight Now

The excursion was originally scheduled to last less than four hours, but it now is expected to run a full six hours, ending around 6 p.m.

The $600 million P6 solar power tower was installed during the crew's first spacewalk Sunday. The array is made up of two huge wings and each wing is made up of two solar cell blankets. The blankets were folded up for launch.

When the starboard wing was deployed Sunday, it's central mast extended at a constant speed, pulling its two solar blankets out of their storage boxes. As they unfurled, however, panels making up the blankets tended to bunch up and suddenly pull free, causing the blankets to ripple back and forth in an unexpectedly dynamic fashion.

As a result, two adjustable cables intended to put up to 75 pounds of tension on one of the two blankets popped off their pulleys and take-up spools, leaving the blanket slacker than engineers would like.

The port-side solar wing was opened Monday using a cautious start-and-stop extension that avoided any problems.

The loose blanket's solar cells are working normally and the four blankets making up the P6 system's two wings are generating more than 50 kilowatts of power.

"The P6 power system is functioning normally," said lead flight director Jeff Hanley. "We are presently transferring three kilowatts of power to the (Russian) service module. ... The present configuration of the system is very healthy and we're very pleased with how that has progressed."

But NASA managers want to rewind the tension cables to keep the blanket taut to provide the support needed when shuttles and Russian spacecraft dock with the station, jarring the complex and its appendages.

Slack tensioning cables may be an indication that all is not well with the starboard solar array. Photo: NASA TV/Spaceflight Now

"These two tensioning cables were designed to take the load from the solar array so when the solar array is plumed or an acceleration happens and the blanket moves outward, the load path is across these two cables and that tensioning bar at the bottom of the array," said shuttle integration manager Bill Gerstenmaier.

"What's happened now with that amount of slack in there, that's no longer the load path," he said. "It turns out the ribbon cables on the sides of the array that take the power from the solar array down into the space station, those are now the shortest elements between the array and the structure at the bottom of the blanket box.

"So in this current configuration, if we take an acceleration or a load, what will happen is those power cables, or extension cords, will take the load rather than the cables. And that's a situation we really don't want to be in for the long term."

The repair plan calls for Tanner and Noriega to anchor their feet in portable work platforms near the top of the P6's central truss, some 90 feet above Endeavour's cargo bay. The starboard P6 array wing will be rotated so the two tension cable pulleys and take-up spools are within reach.

The mast will be retracted a few feet and then the work will begin.

"They're going to rotate the blanket box so it's directly above two portable foot restraint positions," Noriega told a reporter Wednesday. "So basically, we'll be standing up so we can reach as high as we can and the blanket box will be directly above our heads.

The area where the astronauts will be working. Their foot restraints are visible on the right hand side of the picture. Photo: NASA TV/Spaceflight Now

"At that point, we'll pull the slack out of the cable through a pulley guide that's there. I'll hold the cable down while Joe winds up a tension reel and then we'll slowly feed the cable onto the reel and hopefully, that'll do it."

If one of the cables is jammed and cannot be tightened in the normal fashion, Tanner and Noriega will wrap the excess around a wrench and secure it with tie wraps, temporarily providing the tension needed to keep the blanket taut until a future crew could attempt more extensive repairs.

"What we want to do is at least put these wrenches in place to take that slack out such that that cable is the mechanism that's taking the load rather than the extension cords on the side," Gerstenmaier said.

"It's not really a concern for the attitude we're in now with the shuttle docked, it's not really a problem for shuttle undocking. The potential loading case would be when we come back to dock, either with a Progress or a shuttle on a subsequent mission.

"And it's not a guarantee that it's a problem, but it's a load path we really didn't design the array to take," Gerstenmaier said. "So to be safe, we'd like to get the thing back in more of a nominal configuration where it takes the load through the tensioning cables as it was designed."

The shuttle crew was briefed on the repair procedure late Tuesday and again Wednesday. Videotape showing astronaut David Wolf carrying out the procedure on the ground was uplinked to help them visualize the procedure.

"You say in the note that several attempts might be required," Tanner observed at one point. "What does a failed attempt look like? It's not on the right drum of the reel, is that correct?"

"That's correct," replied veteran spacewalker Jerry Ross, overseeing the repair work planning at the Johnson Space Center. "The video we sent you was probably Dave Wolf's 15th attempt or so. He had a fairly steep learning curve and so what you guys saw... I don't want to upset you guys. Dave's a slow learner!"

The floating potential probe to be installed on Thursday's spacewalk. Photo: NASA TV/Spaceflight Now

"So if we do it in less than 15 attempts, we're going to be OK, huh?" Tanner quipped.

"We're counting on you," Ross replied.

The original goal of the mission's third spacewalk was to mount a device called the floating potential probe on the station's hull. The device was designed to measure how well two other components, called plasma contactors, are working to keep potentially dangerous electrical charges from building up.

As the station plows through charged particles in the extreme upper atmosphere, its new solar arrays will build up an electrical charge. The plasma contactors emit a steady stream of xenon atoms to close a giant space circuit, eliminating that potential.

The floating potential probe will be located between the two solar arrays at the top of the P6 truss. Photo: NASA TV/Spaceflight Now.

Without the contactors, a spacewalking astronaut could get receive a potentially dangerous -- perhaps deadly -- shock from a sudden discharge.

The plasma contactors are on and performing normally as far as anyone can tell. But there are no instruments to measure the electrical environment around the station. The floating potential probe will do just that, presumably proving once and for all that lightning bolts are not on the list of threats spacewalkers need fear.

To be on the safe side, however, the P6 solar arrays will be "shunted" during the spacewalk today and oriented edge on to the station's direction of travel to minimize any electrical buildup.