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

NASA managers today formally approved a bold plan for two spacewalking astronauts, working nine stories above the shuttle Endeavour, to tighten up a loose solar array blanket, one of four on the international space station's new $600 million solar power tower.

The improvised repair work will be carried out during an already planned spacewalk Thursday by Joseph Tanner and Carlos Noriega to install an instrument to measure the station's electrical environment.

The idea is to pull two loose cables taut and then to rewind them onto their takeup spools at the base of the slack solar blanket so a spring-driven tensioning system can keep the blanket taut.

The cables jumped off their reels Sunday when the P6 solar power system's starboard wing was deployed.

The shuttle crew was briefed on the repair procedure late Tuesday and again today. Videotape showing astronaut David Wolf carrying out the procedure on the ground was uplinked earlier today.

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

"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!"

"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 $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, 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.

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

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 loose blanket's solar cells are working normally. But NASA managers want to rewind the tension cables to keep the blanket taut and to enable a future crew to retract the wings so the P6 array can be relocated.

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 today. "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.

"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."

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

Asked how optimistic he was about pulling off the tricky repair job, Tanner said "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 tomorrow. 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."

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 Thursday and oriented edge on to the station's direction of travel to minimize any electrical buildup.