The day after Endeavour docks with the space station, Cockrell will attach the Leonardo module to Unity. The day after that, Chang-Diaz and Perrin plan to stage a six-hour spacewalk, the first of three scheduled for Endeavour's mission.

This will be the 39th space station assembly spacewalk, the 14th staged from the station itself and the sixth from the U.S. Quest airlock module. Chang-Diaz and Perrin are the 37th and 38th individuals to participate in a station assembly spacewalk.

Going into Endeavour's mission, 30 Americans, one Canadian and five Russian cosmonauts had logged 236 hours and 27 minutes of spacewalk time in 38 ISS assembly spacewalks.

The first objective for Perrin and Chang-Diaz during the first spacewalk of Endeavour's mission is to attach a PDGF on the P6 solar array truss. The grapple fixture will be needed next year when the array is moved from the upward-facing Z1 truss atop the Unity module and mounted on one end of the main solar array truss currently under construction.

Chang-Diaz, wearing a spacesuit with red stripes, will be anchored to the end of the shuttle's 50-foot-long robot arm for most of this first spacewalk while Perrin will be the designated "free floater."

The second objective of the spacewalk is to remove the micrometeoroid debris shields form the shuttle's cargo bay and temporarily stow them on the pressurized mating adapter, or PMA, between Unity and the Russian Zarya module. As already mentioned, the debris shields will be deployed in late July by Whitson and Korzun.

Designed and built by the Russians, the six debris shields weigh about 100 pounds altogether. They are the first in a set of about two dozen panels that ultimately will be installed on the Zvezda module. When fully deployed, the complete set of panels will reduce the odds of a micrometeoroid strike that could penetrate the module's hull by about 1 percent.

While Perrin and Chang-Diaz work to transfer and stow the debris shields, Whitson and Walz, working at a control station inside the Destiny module, will lock the station's robot arm on the MBS, still mounted in the shuttle's cargo bay. The robot arm will provide the "keep-alive" electrical power to operate critical heaters inside the MBS until it can be attached to the mobile transporter the next day.

After the arm is attached and keep-alive power is flowing to the MBS, Chang-Diaz and Perrin will remove no-longer-needed thermal covers before re-entering the station's airlock. The station arm then will pull the MBS from the cargo bay and move it to a point about six feet away from the mobile transporter where it will remain overnight.

The next day, Walz and Whitson will drive the MBS down onto the mobile transporter cart and engage a mechanical claw that will temporarily lock the platform in place. The day after that, Chang-Diaz and Perrin will stage a second spacewalk to complete the attachment.

Scheduled to last six-and-a-half hours, the second EVA is inherently more complex. First, the astronauts will connect four video/data cables and two electrical power lines between the mobile transporter and the MBS. Ground engineers then will power up the MBS through the transporter.

Next, Perrin and Chang-Diaz will deploy the MBS cargo grapple fixture and lock it in place before tightening up four main bolts needed to complete the structural attachment of the MBS to the mobile transporter. Once the bolts are torqued down, ground controllers will command the claw that initially held the MBS in place to retract.

The astronauts will wrap up the spacewalk by repositioning the MBS camera to its operational location and stowing a set of emergency cables that could be used to provide power to the station's robot arm in the event of a malfunction that stalled the mobile transporter between work site power sockets.

The first two spacewalks are considered fairly routine as such things go. But the third excursion, planned for flight day nine, is a more challenging exercise: The removal and replacement of the station arm's wrist-roll joint.

For this outing, Perrin will ride on the end of the shuttle robot arm. First, the spacewalkers will put a thermal blanket around the arm's latching end effector - the LEE - the part of the arm that actually locks on to components and grapple fixtures. The arm then will be powered down and Perrin will disconnect the LEE's internal power cable.

After loosening six expandible diameter fasteners, or EDFs, holding the 450-pound LEE onto the wrist-roll joint, the spacewalkers will remove the end effector and temporarily stow it on a nearby spacewalker foot restraint.

The faulty wrist-roll will be removed in similar fashion, that is, after loosening six EDFs and disconnecting an internal power cable. The replacement joint, which measures about two feet by two feet and weighs some 220 pounds, will be installed by tightening the six EDFs with 25 foot pounds of torque each and making the single required electrical connection. That will clear the way for reattachment of the latching end effector.

At that point, ground controllers plan to power the arm back up and begin a series of tests while Perrin and Chang-Diaz move the old wrist-roll joint from its temporary stowage point in the cargo bay to the fixture used to hold the new joint in place during launch.

"We in EVA never like to think of our tasks as simple," said lead spacewalk planner Tricia Mack. "But it is a straightforward EVA if all the hardware works correctly.

"I guess what's complex about it is the timing," she said. "We will be powering the arm up in between, after we remove the end effector and the wrist roll joint, when the crew's in the bay working and safely away from the work site, we will power the arm up to protect some of the components on it that would normally be without power. So there's a lot of commanding between the ground, the station crew and obviously the EVA crew. I wouldn't say it's simple, but you're right, as long as the hardware works it'll be pretty straightforward."

And what will the crew do if one of the expandible diameter bolts fails to collapse when the spacewalkers loosen the central bolt?

"We have a long list of contingencies we've trained for," Mack said. "If we have a faulty EDF, we are flying a spare. ... If the clevis and lug interface wouldn't come apart, we have a contingency plan - and I hate saying this because I know the Canadians don't like it - but we have a pry bar on the space station that we maybe could use to give it a little help coming apart."

Five of the six EDFs in the latching end effector and the replacement wrist-roll joint must be fully engaged for normal arm operation.

"It's going to be a long EVA, because we have a lot of bolts to deal with, unbolting and rebolting them," Perrin said. "But as far as the hardware we need to change, it's pretty simple.

"It looks like major surgery because we're going to take the end of the arm out and then swap the joint and then get the LEE back in place and we have to run against the clock and do everything in an orderly and timely fashion," he said. "It's going to be a long EVA, but I think quite simple. ... I feel very confident about the amount of training we've had."

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