The S0 truss fills most of Atlantis' payload bay, seen here at launch pad 39B. Photo: NASA

But Ben Sellari, NASA's S0 launch package manager, makes it clear S0 is "more than just a truss element that some people may generally think of as just a girder or a piece of structure."

With more than 475,000 parts, S0 measures 44.2 feet long, about 14 feet wide and weighs 26,716 pounds. In terms of sheer mechanical and electrical complexity, only the Destiny laboratory module can eclipse it.

"All of the power distribution for space station, once we're in the assembly complete phase when we have our outboard arrays, will come through S-zero's switching units and be able to be distributed to the other modules, the other truss elements," he said.

"It'll also provide us, for the first time, with a cross-strap capability so that if we were to lose a (solar array power) channel in one area, we could then augment that from another array channel through the switching unit."

Major components mounted inside the S0 truss include:

• Power distribution systems to handle the flow of electricity from the solar arrays into the space station
• DC-to-DC converter units to step down solar array voltage to levels needed by the station's internal systems
• Secondary power control and switching systems
• Two computers to control systems inside the S0 truss segment
• Two computers to control systems in future outboard truss segments
• Global Positioning System antennas and two laser ring gyros to permit on-board U.S. determination of the station's orbit and orientation for solar array and antenna aiming
• A three-switch video distribution system that will route television from all externally mounted cameras, including those on the Canadarm2, into the station or down to Earth
• Strain guages, accelerometers and other instrumentation to monitor the truss's structural rigidity
• Instrumentation to monitor the space environment, including radiation sensors
• The mobile transporter and its associated rails and subsystems

Until the solar arrays are mounted on the end of the outboard truss segments, the Z1 truss atop the Unity module will feed electricity to S0 through connectors that will be hooked up by the Atlantis spacwwalkers. Once the outboard arrays are in place, S0 will send power to Z1 and the rest of the station.

The S0 truss provides the structural attach points for the port and starboard - P1 and S1 - trusses that will be attached later this year, along with pass-through ammonia cooling interfaces. Utility trays on S0 will provide power, cooling and data to Node 2, a module virtually identical to Unity that will be launched later. That power then will be routed to international modules.

The completed solar array truss will have 10 Canadarm2 work sites, two of which are located on S0.

Illustration of S0 truss being lifted out of Atlantis' payload bay and maneuvered over the ship's wing on the trek up to the space station. Photo: NASA TV/Spaceflight Now

The day after Atlantis docks with the space station, Ochoa, operating the Canadarm2 from inside the U.S. lab module, will pull S0 out of the shuttle's cargo bay and mount it on top of Destiny, using a powerful claw already in place to temporarily lock it in place.

Moments later, Smith and Walheim will exit Quest and begin work to attach power lines and to permanently latch the in place, deploying two bipods on the forward face of the truss and bolting them to fittings on Destiny's hull. The next day, Ross and Morin will deploy and attach two massive tripods on the aft-facing side of S0 to complete the truss' mechanical attachment.

"Our busiest day by far is flight day four, the day after we rendezvous and the first EVA," Bloomfield said. "S0 has a timeline on it, a lifetime, once we take it out of the payload bay it's got a 16-hour clock and once that 16 hours expires, then the possibility exists that we could do damage to S0 because it got too cold. So that's the clock we're running against where we have to get power on it.

"If we start getting behind, we have a second set of cables up there called LTA cables, which is launch-to-activate, which is basically a way to bypass the normal way of hooking up S0 and give us power to the critical boxes just for a short amount of time," he said.

The mobile transporter is mounted on the forward face of S0, attached to a pair of rails that run the full length of the truss segment. The transporter measures 102 by 107 inches, weighs 1,923 pounds and is capable of a top speed, unloaded, of one inch per second. When it crosses joints between truss segments, the speed will drop to a glacial 0.06 inches per second.

A "linear drive unit" inside the transporter "is basically like the motor, transmission and brakes in your car," Sellari said. "This is what makes the mobile transporter go. It has two drive devices on it that engage the rail and through friction, allow the mobile transporter to go up and down the rails."

The transporter also is equipped with a quartet of load transfer units, or LTUs, claw-like devices that lock the platform in place at various truss work sites so the Canadarm2 can safely maneuver large components.

A device called a roller suspension unit keeps the mobile transporter on its rails in the weightlessness of space, using five spring-loaded wheels to apply the force and tension necessary for the linear drive unit to properly engage.

The transporter, and eventually Canadarm2, will route data and video back to the station through a flat cable called the trailing umbilical system, or TUS. The TUS will unroll behind the transporter as it inches along and roll back up when it goes the other way.

At each of the 10 Canadarm2 work sites along the completed truss, two umbilicals will be available to provide the power needed to actually operate the Canadarm2.

During the third spacewalk of Atlantis' mission, Smith and Walheim plan to remove launch locks and other equipment to free the transporter for movement. The next day, the transporter's linear drive system will be powered up to drive it from its launch position to work site 4 on the starboard end of S0 and then to work site 5 on the port side. Assuming all that goes smoothly, the transporter will be moved back to work site 4 and locked in place for use during the next assembly mission.

"We'll actually translate it from its launch position to one of the work sites and then we'll translate it to the other work site," Castle said. "We'll leave it in the position UF-2 wants it, because UF-2 brings up the mobile berthing system, which attaches to the mobile transporter. We want to put it in the right spot for them."

Overall, S0 is "the centerpiece" of the station's main solar array truss, Castle said. "It's what the rest of the truss elements build out on."

"The truss elements are very important because that gets all the power modules up and that's what allows us to add all the partner elements," he said. " We're rapidly approaching the point where we're going to be power limited, we need to go ahead and build the truss out and get the other power elements on line."

CONTINUE TO NEXT PAGE