Of all the complexities and nuances of building a live-aboard science outpost in orbit, none is more staggering than what NASA stoically refers to as the EVA wall. The obstacle is not a tangible hurdle, but it might as well be.

To complete assembly of the International Space Station, NASA is planning to stage about 165 spacewalks — or EVAs, an acronym from the technical term extravehicular activity — spanning 1,073 hours. Sounds reasonable, but consider this: As the shuttle prepares to make its 100th flight, astronauts have spent just 634 hours spacewalking outside the shuttle. Even adding outings by Gemini, Apollo and Skylab crews to NASA's cumulative spacewalk experience falls short of what will be expected from space station assembly and maintenance crews in the future.

"We have a chart that shows the buildup of the time over the next five years. It shows what we did starting back in Gemini and then Apollo walking on the moon. Those are little bumps down at the bottom and then you get up to space station assembly and there's this huge spike over the next five years. It is really a big concern and it's something that requires constant vigilance," said Greg Harbaugh, an astronaut who heads NASA's EVA office at the Johnson Space Center in Houston.

Shuttle-based crews visiting the budding space station already have completed six spacewalks lasting a total of about 21 hours, but the real grunt work begins during the next station assembly flight, scheduled for launch on Thursday. Two teams of spacewalkers will be heading into orbit to whittle away another 26 hours of exterior chores.

"We've laid the foundation. Now we're ready to begin the real construction," said Daryl Schuck, who is overseeing the four spacewalks planned during shuttle Discovery's weeklong stay at the International Space Station.

The two-man teams will work on alternate days hooking up the station's first exterior support segment, which is crammed with communications, positioning, coolant, electrical and other gear vital to the station's future growth.

The cube-shaped Z1 truss is probably not the prettiest segment of the International Space Station, said Discovery rookie pilot Pamela Melroy. "When we just saw the bare bones (of the truss) it looked like one of those old-fashioned milk crates on steroids. It's quite large, but it was this boxy-shaped thing with just structural pieces. Now there's a lot of equipment on it and (thermal) blankets, so it looks like a large white cube."

The cube is the core of the station's structural spine, which will span more than the length of a football field when completed. The station's power-producing solar arrays will be attached to the truss, along with its communications antennas, gyroscopes, electrical converters, coolant lines, power cables and other equipment.

"The Z1 is like a backbone for the rest of the station," said Robert Cabana, a shuttle astronaut who commanded NASA's first station assembly mission and now manages the program's international operations.

"It's like the utility closet where all your transformers and power controls are located."

Unlike your home utility closet, however, the station's power cannot be completely shut down when it's time to make the electrical connections.

"It would be nice to just go hit a master circuit breaker and turn everything off. But that would shut down the entire space station, so we can't do that," said astronaut William McArthur, who will be paired with Leroy Chiao for the first and third spacewalks during the upcoming Discovery mission.

"We've had to pay a lot of attention to how the sequence of tasks is choreographed so that we have the right plugs turned off when we stick the connectors in. It's quite an interesting task to get this all scheduled and choreographed," said McArthur.

Few aspects of shuttle Discovery's assembly tasks are easy. For example, the robot arm operator Koichi Wakata will have to pick up and attach large pieces of equipment without a direct line of sight out the shuttle's window. Instead, Wakata will rely on an artificial space vision system developed by the Canadian Space Agency and the directions of spacewalkers positioned outside the shuttle.

The margin for error is slim. If the Discovery crew fails to attach the Z1 truss for some reason, the station assembly sequence grinds to a halt.

"Every mission has to work because the follow-on missions depend on you getting your things done," said Discovery commander Brian Duffy. "That fact hasn't been lost on my crew nor on all of the other crews that are assigned and training right now."