The Endeavour astronauts and their space station counterparts are gearing up for the most critical day of the 112th shuttle mission: Installation of a new $390 million solar array truss segment, the third of 11 sections needed to complete the huge beam. Running well ahead of schedule, shuttle commander James Wetherbee, operating Endeavour's robot arm, began pulling P1 from its perch in the ship's cargo bay at 10:22 a.m.

The P1 truss segment will be attached to the left, or port, side of the beam by outgoing station astronaut Peggy Whitson, operating the lab's Canadarm2 spacecrane. As the segment is being locked down with motorized bolts, astronauts Michael Lopez-Alegria and John Herrington will exit the Quest airlock module for the first of three spacewalks to activate and outfit the new component.

The six-and-a-half-hour spacewalk, the 47th devoted to station assembly, is scheduled to begin around 3:25 p.m. Going into today's excursion, 34 U.S. fliers, one Canadian, one Frenchman and seven Russian cosmonauts have logged 285 hours and 25 minutes of spacewalk time building and maintaining the orbital complex.

The solar array truss anchored to the top of the Destiny lab module eventually will extend some 180 feet to either side of the station's centerline. Two of the beam's 11 segments are currently in place while a third - the $600 million P6 solar array segment - is mounted atop the Z1 truss on the Unity module to provide interim power and cooling. P6 eventually will be moved to the port side of the main solar array truss.

The fourth truss segment - P1 - is being installed today. Cost of the first four segments: $2 billion.

The completed truss will feature huge solar arrays at each end while ammonia lines, pumps and massive radiators mounted inboard of the arrays will dissipate the heat generated by the station's electronics.

Rails running the length of the truss on its forward face will allow the station's robot arm to move to various work sites atop a mobile transporter. The rails also will allow spacewalkers to move equipment using small railcars called CETA carts.

The $600 million central segment of the truss, S0, was bolted to the Destiny module in April. The first starboard-side segment, S1, was attached in October. The P1 truss segment aboard Endeavour is the first port-side section of the beam.

S1 and P2 include the lab's ammonia cooling system and radiators, three folding sets of panels on each truss segment that will extend 75 feet toward the rear of the station when fully deployed. An interim cooling system currently is in place atop the Unity module, along with a set of solar arrays that eventually will be moved to the end of the port truss.

S1 and P1 are virtually identical. But installation of the new truss segment will be very different from the techniques used to install S1 in October.

During that flight, the station's Canadarm2 space crane pulled S1 from the shuttle Atlantis' cargo bay and attached it straight-away to the right side of S0. Two spacewalkers then made a variety of electrical connections, attached ammonia jumpers between S0 and S1 and installed nearly three dozen "spool positioning devices," or SPDs, to prevent quick-disconnect fittings from locking together too tightly.

They also installed an S-band antenna system and mounted two remotely controlled television cameras, one on the lab module and one on the end of S1. The latter provided spectacular views of Endeavour's docking Sunday.

For Endeavour's mission, installation will follow a very different course, one designed to set the stage for build-out of the port-side of the truss next year.

"I'd say probably 60 to 70 percent of the tasks are identical, sort of a mirror image, although we are executing a lot of them differently," said Lopez-Alegria.

After pulling P1 from the shuttle's cargo bay, Wetherbee will hand it off to Whitson, operating the station's Canadarm2 crane, who then will move it into position for attachment to the left side of S0.

The hand-off is required because the station arm can't reach into the cargo bay from its current position without hitting one of the shuttle arm's supports.

"The P1 is unberthed from the shuttle using the shuttle (robot) arm, said station flight director Mark Kirasich. "We tilt it up to get a little bit of additional clearance from the tail and then the shuttle arm performs what we call an arc maneuver that moves the truss from inside the shuttle bay out over the port wing of the space shuttle to what we call the handoff position.

"At the handoff position, the space station arm goes and grapples a second grapple fixture on the truss, we back the shuttle arm off and the shuttle arm is maneuvered to a position where it will provide camera views of the mating interface.

"At that point, the station arm maneuvers the truss to the pre-install position where we do a fine alignment," Kirasich said. "And then we perform a maneuver we call 'press to readies.' We bring the truss close enough where the truss can be captured by the capture latch on the segment-to-segment attachment system."

While the truss is being moved into position and attached to S0 with the capture latch and a set of four motorized bolts, Lopez-Alegria and Herrington will be working through their pre-spacewalk routine, breathing pure oxygen to purge nitrogen from their bloodstreams.

As the motorized bolts drive home, pulling S0 and P1 together with 13,500 pounds of force, the spacewalkers will exit the Quest airlock module and get to work.

Lopez-Alegria, call sign EV-1, will make the first of two sets of eight electrical connections needed to power-up the P1 truss' computers and other systems. Herrington, meanwhile, will release launch locks holding a new CETA cart in place on the truss rails, the station's second. The station's first CETA cart, carried into orbit attached to S1 last month, currently is coupled to the mobile transporter atop S0.

While Herrington - EV-2 - works to release the various CETA cart launch locks, Lopez-Alegria will install six SPDs on flexible hose radiator coupler fluid lines between the truss and its folded radiator panels.

The SPDs were ordered after an engineering analysis showed ammonia leakage past one of two seals in a typical quick-disconnect fitting could have the effect of pressurizing the fitting, making it difficult for future spacewalkers to release. The SPDs effectively pull each quick-disconnect apart slightly, just enough to allow any leakage to vent harmlessly into space.

Thirty-three SPDs were installed during Atlantis' mission last month. During the current flight, 43 more will be installed, six on the first spacewalk, four on the second and 33 on the third. Two more will be installed on the next shuttle visit in March.

After the first six are installed on the first spacewalk of Endeavour's mission, Herrington and Lopez-Alegria will remove and stow structural supports called "drag links" that helped hold P1 in the shuttle's cargo bay. They must be removed, as well as two massive keel pins, to clear the rails for eventual movement of the mobile transporter and CETA carts.

Once that work is done, Lopez-Alegria will make a second set of electrical connections between S0 and P1 before working with Herrington to mount a spacesuit "helmet cam" TV antenna on the back of the Unity node.

At present, video from the helmet cams of spacewalkers wearing U.S. spacesuits can only be seen inside the station when a shuttle is docked. The new WETA - wireless EVA television antenna - system will allow station crews to receive such video during spacewalks when the shuttle is not present.

At the end of the spacewalk, the Canadarm2 robot arm, attached to the mobile transporter on the S0 truss, will "walk off," inchworm style, and attach itself to a grapple fixture on the hull of the laboratory module. This will clear the way for the mobile transporter to be moved out onto P1 during the third spacewalk.