Set for launch today aboard shuttle Atlantis is the next major piece of International Space Station hardware, a $790 million package of electronics and plumbing that will be key to the outpost's staying power and future growth.

STS-110

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Now roughly about the size and weight of Russia's Mir space station, which was deorbited last year, the ISS has passed "go" in a conditional sense.

That is, a three-person crew can now safely keep on keeping on. Pieces yet to fly are meant to enhance the orbiting facility, upgrading its scientific utility and, perhaps, one day support an international team of six or seven people.

Fallback plan

Scheduled for liftoff at 5:13 p.m. EST (2213 GMT), Atlantis' crew is ready to unload and latch to the outpost the S-Zero truss, a major girder that serves as a structural centerpiece of the station. This hardware is packed with thousands of components, including computers, plumbing lines and electrical cables.

When the beam is locked down in place atop the U.S. Destiny science laboratory and its lines connected to the station during four planned spacewalks, the truss will become the first installment of a larger framework necessary for attaching other ISS hardware, particularly Japanese and European laboratories.

But what happens if the S-Zero truss component is a no-show?

For example, a shuttle accident on launch is always a major worry. Then there's the chance the section is somehow damaged while astronauts arm-wrestle it into place. Or maybe the piece might not work once attached.

"I hope we won't have to cross that bridge," said Mike Raftery, ISS vehicle manager for Boeing, the prime contractor in piecing together the mega-space project. "But we do have a fallback plan," he said.

"We need a safe launch. We always do," Raftery told SPACE.com.

Raftery said that there is a backup to the truss, a structural test article that could be flown later to the ISS.

"It would probably require some rework," he added, in attaching all the wiring, piping and other gear to the replacement girder.

Two more truss segments -- known as P1 and S1 -- being readied for launch this year also have backup counterparts, Raftery said. The S-Zero truss that's now tucked into Atlantis' cargo bay is the more complex of the other two sections, he said.

Without a hitch

Berthing the large truss segment to the Destiny module is a top priority goal this shuttle trip.

"That's probably the thing I'll be watching closest," Raftery said. Although the Canadian-built ISS robot arm needed to move the S-Zero truss has had problems, a solid plan is now in place to assure the arm will do the job at hand.

"I'm very confident that it's going to go off without a hitch," Raftery said.

One key confidence-boosting tool is use of Digital Pre-Assembly technology, evolved out of the airplane-manufacturing arena. By digitally mapping ISS elements, computer-generated match-ups of hardware can be done on the ground. That lessens the number of surprises in orbit, Raftery said.

Outfitting the ISS with the new truss element sets the stage for further enhancements to the orbiting complex.

"It's going to be an exciting year," Raftery said. Few appreciate the magnitude of what's taking place in Earth orbit, he added, contrasting the building of ISS to the Apollo moon landing project.

"Perhaps it doesnt have the first time sense of drama that Apollo had. You can't match that. But in terms of engineering difficulty, what we are doing is right up there in the same league," Raftery said.