Astronauts Michael Lopez-Alegria and John Herrington are gearing up for their third and final spacewalk today, a planned six-and-a-half-hour excursion to complete the outfitting and activation of a new $390 million solar array truss segment. The spacewalk, the 49th devoted to space station assembly and maintenance, is scheduled to begin around 2:20 p.m. EST.

The shuttle astronauts were awakened shortly after 8:30 a.m. by a recording of "Walking on Sunshine" beamed up from mission control, a request from the family of Endeavour pilot Paul Lockhart.

"Good morning to everyone down there in mission control. It's great to hear your voice this morning," Lockhart called. "Walking on sunshine? I think that's what John and Mike are going to be doing for several hours today."

The goal of the spacewalk is twofold: To install 33 spool positioning devices, or SPDs, on various coolant line quick-disconnect fittings to prevent ammonia leakage; and to test the techniques and procedures needed to move the station's robot arm back and forth along the truss for upcoming assembly missions.

The 11-segment solar array truss eventually will stretch some 365 feet, carrying the huge solar arrays that will provide the bulk of the space station's power and the ammonia cooling system and radiators needed to dissipate the heat generated by the station's electronics.

The truss currently consists of three segments: The $600 million central S0 truss mounted atop the U.S. Destiny lab module; the $390 million S1 truss, attached to S0's right, or starboard, side; and the $390 million P1, carried up by Endeavour and attached to the left, or port, side of S0. A fourth truss element, a $600 million solar array segment known as P6, is mounted atop the Unity module. It provides interim power and cooling and will be moved to the port side of the main truss next year.

Rails run along the forward face of the main truss, allowing a $190 million mobile transporter to move back and forth along the beam. The mobile transporter, in turn, was designed to serve as a mobile platform for the station's $600 million Canadarm2 spacecrane. To complete the truss, the robot arm must be able to move to various work sites along the beam.

But first, engineers want to make sure the mobile transporter can move along the rails on its own before sending the robot arm down the line.

"What you're going to see on this EVA, while the spacewalkers are in the airlock performing their pre-breathe activities, you're going to see a ballet, or a choreographed sequence of events that will move the mobile transporter from its current location at worksite 4 (on S0), down the tracks to worksite seven (at end of P1)," said space station flight director Mark Kirasich.

The next elements in the port-side truss build-up "can only be installed with the arm on the mobile transporter and the mobile transporter at worksite 7," he said. "So it's very important we can get the MT to that location."

To make that happen, the robot arm was walked off the MT to the lab module earlier in the mission, the astronauts removed the keel pins and drag links that held P1 in the shuttle's cargo bay - and blocked the rails - and moved a new CETA rail car from P1 to S1.

Before the third spacewalk begins today, the mobile transporter will be commanded to move about 55 feet, from worksite 4 to worksite 7 near the end of P1.

"The sequence for that day is the crew starts getting into the suits, the MT moves out to the end of the P1 truss," said lead EVA officer Dana Weigel. "Then the SSRMS will do a walkoff onto the MT, and by the time we get out the door, it's in position for our first task."

"The entire (MT) procedure is bookkept for an hour and a half. Part of that is the unlatching of the mobile transporter, unplugging it from its electrical source and the drive time. It moves at about 1.2 inches per second."

Once the transporter is plugged into its power-and-data socket at work site 7, Canadarm2's free end will lock onto one of the transporter's grapple fixtures. The crane then will release a fixture on the lab module to complete the inchworm-like "walk off" to the transporter.

With the arm now ready for work in its new position, Lopez-Alegria will install two SPDs on ammonia lines at the interface between the Z1 and P6 trusses atop the Unity module. He will install two more in the "rat's nest" of coolant lines and cables between Z1 and the lab module.

Finally, Lopez-Alegria will hook up two more SPDs on a heat exchanger below the outer skin of the lab's end cone.

The SPDs were designed after engineers discovered internal leakage in the quick-disconnect fittings could have the result of pressurizing the internal cavity and preventing astronauts from disconnecting them as required during the assembly sequence. The SPDs effectively pull the QDs apart slightly, just enough to open one of two internal seals. That, in turn, allows any ammonia leakage to vent harmlessly into space.

While Lopez-Alegria works at the Z1-P6 interface, Herrington will install 18 SPDs on the rotary beam valve modules on the back of P1 where quick-disconnect fittings link ammonia lines leading from the truss to the radiators. Twelve of those fittings will be attached while Herrington is on the Canadarm2. He then will get off the arm, the arm will walk back off to the lab and Herrington will install the final 6 SPDs as a "free floater."

Lopez-Alegria, meanwhile, will install four SPDs on P1's ammonia pump module, reconfigure power cables, hook up the truss ammonia tank and prepare the ammonia system for filling during a flight next year.

The spacewalk will end after installation of a final set of SPDs on the flexible hose rotary coupler, part of the system that enables the radiators to rotate to ensure maximum cooling.

At the conclusion of today's spacewalk, the MT will be moved back to work site 4 on the S0 truss. At that point, P1 will be ready for attachment of the next port-side truss segment and 76 of 78 required SPDs will have been installed. The final two will be installed during the next station assembly mission in March.

"This will be the first time the mobile transporter has moved across an interface between two truss segments," said station program manager Bill Gerstenmaier. "The reason we're doing this on this flight is to set ourselves up for next year's activities. We need to operate off of that worksite to install some of the outboard truss elements and we want to gain some experience in using the mobile transporter, seeing how it operates, actually moving it in a time critical fashion out to this worksite.

"You'll also notice when this activity occurs that we don't move the arm on top of the mobile transporter. The arm will be walked off the mobile transporter to the lab. Then the mobile transporter will be moved out and then the arm will walk back onto the truss. So when you see that activity, that's dramatically different from what we did back on the last flight on the S1 install.

"On the S1 install, we stayed on one worksite, we used the arm all from that worksite. On this flight, you'll see a lot of mobile transporter movement, you'll see a lot of arm activity moving to different locations. That was done really because of where we have to reach on this P1 truss and it's also driven by the fact that we want to test in advance these new capabilities we're going to use on future flights.

"So just as we've been doing throughout the assembly sequence, anything we can do ahead of time to make sure it's operating, to make sure it works ... we do that," Gerstenmaier said. "So we're really getting prepared on this flight, this P1 flight, for future activities that we're going to have to do next year."