Illustration shows S0 truss mounted to the space station by Canadarm2. Photo: NASA TV/Spaceflight Now

Assisted by Bursch, Ochoa will "move it out over the port side of the combined stack and then move the orientation around, come up over the top of the lab and then very precisely bring S0 down into what's called the module-to-truss-segment attach system, or MT-SAS," Castle said.

"You'll also see the lab cradle assembly being used. They'll align them very precisely, then activate a capture latch. The capture latch is actually on the lab, it looks like a claw. We'll capture S0, pull the structure together and then some alignment guides will line it up very precisely.

"That will all be done in the very first part of the crew day," Castle said. "We're timing it so just as we're finished with the capture latch operation, we'll be depressing the airlock to take the first EVA crew outside."

Smith, wearing a spacesuit with red stripes around the legs, is a veteran of five previous spacewalks while Walheim, wearing an unmarked white suit, will be making his first such excursion.

Smith will deploy the telescoping struts making up the forward bipods and Walheim will "rigidize" them like the legs of a camera tripod and bolt them to fittings on Destiny's hull. Smith, meanwhile, will open a trap door near the center of S0 and float part way into the truss to loosen a set of clamps allowing a large cable tray to deploy.

The cable tray is on the aft side of S0 facing the Z1 truss. It is known as the "rat's nest."

"It's buried between several modules and trusses and this area could be called the heart of space station for spacewalkers," Barclay said. "This is where many electrical and fluid lines mate between the modules."

Before making any such connections, however, circuit interrupt devices, or CIDs, will be installed and activated like circuit breakers to ensure no electricity is flowing through any of the cables.

Smith then will disconnect a set of diagnostic cables and release a pair of bolts to actually lower the tray before connecting 10 cables between the tray and connectors on Z1. On the front side of the truss, Walheim, occasionally assisted by Smith, will deploy another tray and make another 20 electrical connections.

If time is available, Smith will install two more CIDs inside the truss to allow future assembly crews to "break electrical circuits in the middle and reduce the amount of space station power down that's required during later assembly missions," Barclay said.

The final major goal of the first spacewalk is to power up the mobile transporter's internal heaters through the trailing umbilical system. Working together, Smith and Walheim will electrically connect the TUS and install about 20 feet of cable in guides along the mobile transporter's rails.

"So at the end of EVA-1, we've got two out of the four strut groups attached, so we're structurally in good shape for quite a while, we'll have all the power hooked up so all the avionics are up and working and we'll activate S0 and all the equipment on it that evening, after the crew finishes the EVA," Castle said.

The next day, the combined station-shuttle crew will work inside the lab complex to transfer equipment to and from Atlantis, configure cameras and to top off the airlock module's oxygen and nitrogen supplies.

The second spacewalk will be conducted by Ross and Morin on flight day six. Ross has more than 44 hours of spacewalk time during his six previous missions while Morin is a rookie. For identification, Ross' spacesuit has broken red dashes around the legs while Morin's has diagonal candy cane stripes.

The primary goal of the excursion is to deploy and latch the two tripods on the aft side of S0 that will complete its structural attachment to Destiny.

"Each of these deploy out, attach to the lab, there's an expanding collar that allows their length to change," Castle said. "So they attach them to the lab and then tighten down the collar so the length can't change any more. So you wind up with a fairly rigid structural member when you're done."

Ross and Morin also will remove the two large keel pins that helped anchor S0 in the shuttle's cargo bay for launch. The keel pins and two other components called drag links must be removed to allow the mobile transporter to move back and forth along its rails.

Finally, the spacewalkers will attach a second TUS to the mobile transport to provide redundancy.

The third spacewalk, by Smith and Walheim, is scheduled for the next day. For this excursion, the shuttle's robot arm will be used to move Smith about while the two spacewalkers reconfigure the cabling for the station's Canadarm2.

The Canadian crane currently draws its power from connections to Destiny's electrical system. The goal of the third spacewalk is to connect the robot arm to power feeds on the S0 truss, allowing it to operate from the mobile transporter.

"We really don't want to power down all of Canadarm 2 all at once, we're going to bring it down one redundant string at a time," Castle said. "So we'll power down one string, do the power reconfiguration, power that string up, verify it works, then we'll power down the other string of avionics and chance the power configurations there."

The arm's redundant power lines will be connected to the truss at two points. While ground engineers test the first cable reconfiguration, Smith and Walheim will remove launch locks holding the mobile transporter in place. After making the second set of robot arm connections, the spacewalkers will split up and reposition tools while the ground runs another series of tests to make sure the crane is once again fully operational.

The final task for the spacewalkers is to install a ladder-like "spur" between the Quest airlock and S0 to make it easier for future assembly crews to get up to the truss work sites.

The next day, the crew will work with ground engineers to test the mobile transporter, commanding it to roll between the two work sites on S0. The day after that, Ross and Morin will stage the mission's fourth and final spacewalk.

"EVA-4 is a bunch of get-ahead stuff," Castle said. "If nothing on EVA-4 gets done, we've had a very successful mission. All the critical things that have to get done, get done in the first three EVAs. That's by design. If anything doesn't get done on the first three EVAs, we'll do it on EVA 4. And if something falls off of EVA 4, it's things that can be picked up down stream ... fairly easily."

Ross will ride on the end of Canadarm2 to install a pair of external lights while Morin torques three bolts to complete the structural load path between S0 and Destiny. The two spacewalkers then will finish deploying several small pieces of equipment around the truss, including a work platform and a tool stanchion.

Ross also will position two "energy absorbers" on either side of the mobile transporter.

"These are essentially large shock absorbers," Barclay said. "Future spacewalkers will have hand-propelled carts to wheel down the truss rail and these energy absorbers allow for attachment of these carts to the mobile transporter and they also would cushion any collision between the cart and the transporter."

Morin will deploy a charged particle directional spectrometer that ultimately will be used to sniff out any leaks in the ammonia coolant system and others. He also will deploy a swing arm near the rat's nest carrying connectors that later will be hooked up to the Unity module.

At the same time, Ross will be conducting a video survey "to make sure alignment marks are in good shape for later mating fo the Canadarm2 base system," Barclay said. He also will install five spacewalk handrails on S0 and inspect the GPS antennas.

"A lot of these tasks are kind of generic, driving a bolt, removing a blanket, that sort of thing," Barclay said. "But in terms of overall criticality towards the next space station mission and survival of the element, that sort of thing, this is a pretty complex mission."

The day after the fourth spacewalk, Atlantis will undock from the space station. With pilot Frick at the controls, the shuttle will fly one and a quarter loops around the station before leaving the area for good. Landing is targeted for just before 2 p.m. on April 15.

"I think there's probably a great chance we'll have a lot of fun," Ross reflected. "We've already got some things planned for the (station) crew to be able to enjoy some quiet times together. And just being there is enough fun for me.

"I mean, it's an incredible experience. God created a beautiful place for us to live on and to be able to observe from 200-and-some miles up, it's just [an] incredible experience. Living in zero gravity, being able to share that with ten other individuals is something that's very, very hard to express to other people, the impact of that."

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