NASA animation shows the P6 truss being hoisted into position for mounting to the international space station while the spacewalkers are getting into position. Photo: NASA TV/Spaceflight Now

The Endeavour astronauts are gearing up for a "daunting" spacewalk today to install a $600 million set of solar arrays on the international space station, a 17.5-ton power system with the largest solar wings ever built for use in space.

Joseph Tanner and Carlos Noriega plan to venture into Endeavour's cargo bay at midday for a planned six-and-a-half-hour excursion. The official start time is 1:31 p.m., but the spacewalk could begin up to 45 minutes early if the crew is ready.

Today's outing is the first of three spacewalks to install the P6 arrays, to wire them into the station's electrical system and to install instrumentation to measure the lab's electrical environment.

The four solar cell blankets making up the two wings of the P6 array will generate nearly 20 kilowatts of excess power, allowing NASA to launch the U.S. laboratory module, Destiny, in late January.

But installation of the massive array poses the most challenging assembly spacewalk yet attempted.

"It is a daunting EVA, a challenging EVA," lead flight director William Reeves said of today's spacewalk. "But we have workarounds developed, we've been able to spend extra time on all the what iffing and all the contingencies and we have work arounds for just about anything you can imagine that could go wrong."

The Boeing-built P6 arrays consist of two huge wings that will extend 115 feet in opposite directions from a central truss. When fully extended, the wings will measure 240 feet from tip to tip, longer than the wingspan of a 747 jumbo jet.

The new P6 truss towers above the international space station. Photo: NASA

Each wing features two solar array blankets supported by an open framework mast that, in effect, assembles itself during deployment. The central truss houses 12 batteries, cooling equipment and electronic gear.

The acordian-like blankets were folded up for launch. If all goes well, they will be unfurled by the two motor driven masts late in today's spacewalk.

Endeavour is docked to a downward-facing port on the U.S. Unity module. The P6 truss will be mounted atop another truss -- Z1 -- that is already attached to Unity's upward facing port.

As Tanner and Noriega are leaving Endeavour's airlock, Canadian astronaut Marc Garneau will use the shuttle's robot arm to move the P6 array from its overnight park position to a point about three feet above the top of the Z1 truss.

The spacewalkers will position themselves on opposite sides of the Z1 truss so they can see the mating interface and provide verbal cues to Garneau to help him precisely align the array truss.

He will not be able to directly see that interface from inside the shuttle's crew cabin as he slowly lowers the P6 truss to within a few inches of the Z1 capture plate.

But Tanner and Noriega will provide a bird's eye view of the operation using a new wireless video system and small spacesuit TV cameras equipped with wide-angle lenses.

"We call them the Joecam and the Carloscam," Tanner joked. "You'll be able to see exactly what we're seeing. ... This is going to be real thrilling, it's a tremendous EVA tool."

Once the P6 truss is properly positioned, Tanner and Noriega will engage a large mechanical claw built into the top of the Z1 truss.

NASA animation shows arrays beginning to deploy. Only one side will extend at a time, however. Photo: NASA TV/Spaceflight Now

"It is a manually operated claw by the EVA astronauts," Reeves said. "They will crank the claw closed to capture (a) capture bar on P6 and pull it down and have a solid mate between Z1 and P6. At that point, the EVA crew will go to each corner of P6 and install a bolt that will hold P6 to Z1 permanently."

There are three bolts at each corner, a prime and two backups. Only one bolt at each corner is required to firmly lock the array truss to Z1.

Once that work is done, Noriega will anchor his feet to a work platform on the end of the robot arm, now operated by pilot Michael Bloomfield, and begin connecting various electrical cables between P6 and the Z1 truss.

"Once P6 is mated, we're going to connect the umbilicals that allow us to send shuttle power to P6," Reeves said. "And at that time, we're in a pretty safe position. As long as the shuttle's there, we can take care of any contingency after that point."

Tanner and Noriega then will proceed up the P6 truss to unlatch the launch restraints on the solar array blanket boxes and to extend the two beta gimbal joints that form the base of each solar array wing. The blanket boxes then will be rotated into position for array blanket deployment.

"Once they've completed all of that activity, they will go back to the airlock, go inside the airlock and go back on the umbilicals to the shuttle," Reeves said.

"We will keep them there to extend their EVA time if we need them. And it's at that point that the ground and the crew on board the shuttle will begin the commanding for the deployment of the solar array wings."

The solar arrays are fully extended to generate much-needed power for the space station as seen in NASA animation. Photo: NASA TV/Spaceflight Now

Jett will send the actual array deployment commands from a computer on Endeavour's flight deck.

"If anything should go wrong, we have the (spacewalkers) ready to go out and assist," Reeves said. "But we're also in a safe configuration for the night if we need to work a problem and be ready to go out the next day."

NASA managers originally said only full wing extension was allowed. If one of the wings jammed during deployment and if the spacewalkers were unable to free it, the wing would be jettisoned.

But it now appears a partially deployed wing might be useable, depending on the nature of the failure and how far it had extended before the jam.

In any case, Reeves said, "once we're satisfied we don't have a major contingency that requires immediate attention, the crew will go ahead and close out the airlock, come inside and post-flight the suits."

But Tanner and Noriega will no doubt take time to enjoy the view of the arrays towering above the shuttle's open cargo bay.

"I think the view is going to be absolutely fantastic," Tanner said. "These things are huge, they're quite colorful and I think the view from the cameras in the crew module is going to be fantastic. Once the arrays are out, the station becomes the third brightest star in the heavens."

Venus is the brightest "star" in the night sky, reaching a magnitude of -4.3. It is outshone only by the sun and moon. Jupiter, currently at magnitude -2.69, is second while Sirius, at magnitude -1.47, is the brightest true star in the sky.

One assumes, based on Tanner's remarks, that the station will outshine Sirius, ranking third behind Venus and Jupiter. But other NASA officials have thrown the moon into the mix and it's not clear where the station actually ranks in apparent magnitude.

But it is certainly correct to say the outpost will be one of the brightest objects in the night sky once the highly reflective P6 arrays are deployed.

"The most thrilling time for both of us is going to be on the very top of P6," Tanner said. "We're over 85 feet from the sill (sidewall) of the orbiter. We spend a fair amount of time at the top of P6, working hard, of course, but enjoying the view. I think that's going to be fantastic."