CAPE CANAVERAL, Fla. Expect the most electrifying mission ever staged in space.

Five astronauts strapped into shuttle Endeavour are scheduled to blast off Thursday for the International Space Station, now occupied by its first full-time tenants.

Once the shuttle is there, the station crew will watch anxiously as spacewalkers wearing "helmet-cams" exit Endeavour and scale the outpost, mounting a dramatic bid to erect a $600 million electrical power tower atop the 13-story complex.

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With the future of the $60 billion station construction project hanging in the balance, the Endeavour astronauts then will attempt to unfurl a giant pair of solar panels with a wingspan greater than that of a 747 jumbo jet.

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The entire package will become the largest and heaviest element delivered to the station by a shuttle when Endeavour docks at the outpost around 3 p.m. EST (20:00 GMT) Saturday.

A major make-or-break moment for the mission will come the next day.

Clad in cumbersome spacesuits, Tanner and crew mate Carlos Noriega will climb up to the stations so-called Z-1 truss, a 9-ton piece of metal framework mounted atop the outpost by a visiting construction crew in October.

In the front row, from left to right, are Pilot Michael Bloomfield, Mission Specialist Marc Garneau and Commander Brent Jett. In the rear are Mission Specialists Carlos Noriega (left) and Joseph Tanner.

Wielding Endeavours 50-foot (15-meter) robot arm like a construction crane, Canadian astronaut Marc Garneau then will try to maneuver the solar array package to a point within 3 feet (0.91 meters) of the top of the Z-1 truss.

The towering station will block Garneaus view out shuttle windows, so the "Yuri Gagarin of Canada"
who became that nations first space flier in 1984 -- will rely on a computerized, closed-circuit television system to do the job.

Tanner and Noriega then will help Garneau guide the solar-array package into a mechanical claw designed to secure it to the top of the station. Four king-sized bolts also must be driven by the spacewalkers to make certain it is firmly in place.

"If we cant do that, then weve either got to bring it home or throw it away, and we dont want to do either one of those," Tanner said.

The next order of business for the spacewalkers: routing electrical power and computer cables between the P-6 and Z-1 trusses. A series of bolts, latches, winches and cinches then must be released so that the solar arrays, their mast and the three radiators can be deployed.

All of the high-wire construction work will be captured by color cameras in the shuttles cargo bay and then beamed back to Earth for broadcast on NASA TV.

And for the first time, spacewalking astronauts will be outfitted with "helmet-cams" that will give viewers an up-close look at the burgeoning station.

"Itll be fun to watch," Tanner said. "We have three little cameras little mini-cam, lipstick-type cameras mounted on our helmets, so everything that were doing, youll be able to see."

The unfurling of the massive arrays, however, wont come until after Tanner and Noriega retreat back into the shuttles airlock some 90 feet (27.3 meters) below.

The reason: NASA engineers say there is a remote chance that the high-voltage arrays could create an electrical shock hazard for the spacewalkers.

Only cargo bay camera views, consequently, will be broadcast as the growing station spreads its solar wings, which will generate 64 kilowatts of direct current (DC) power or enough electricity to run 30 average American homes -- minus air conditioning.

Endeavour commander Brent Jett will do the honors, sending computer commands to unfold the arrays one at a time. Each is expected to take 13 minutes to fully extend.

Thats not to say, however, that Tanner and Noriega wont be prepared to respond if motors designed to roll out the arrays for some reason fail.

In an emergency, the spacewalkers would be sent back outside with pistol-grip power tools that could be used to do the job. Less sophisticated ratchet wrenches also could be used to manually crank the arrays to their fully extended position.

Still, theres one scenario in which the astronauts simply would be out of luck. And that is if a partly deployed array for some reason got stuck and could not be reeled all the way out, or all the way back in.

"If theres a jam or something in the mechanism, thats a different story," Tanner said. "We cant do much about that."

In that case which Tanner terms "fairly unlikely" project officials would have a tough decision on their hands: Either learn to live with the partially deployed array or toss the entire multimillion-dollar package overboard.

"I dont want to speak for the program, but there would be a lot of talking about what to do about that should it happen," Tanner said.

Assuming the arrays unfurl without problems, Tanner and Noriega will head out on two more spacewalks during the planned 11-day mission.

In both cases, the stations new solar wings will be shunted to prevent them from generating electricity that could shock the astronauts something that cannot be done during the deployment of the arrays.

During the second excursion, Tanner and Noriega will finish wiring the arrays to the station, work that will enable electrical power to be routed to outpost systems.

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A key communications antenna will be repositioned atop the P-6 truss, and other work will be done to ready the outpost for the planned delivery early next year of the stations first science lab.

The final foray will involve setting up a device designed to characterize the potential for dangerous electrostatic discharges that could present a shock hazard to spacewalkers working outside the station.

The station already is equipped with electrical grounding rods that theoretically should neutralize any hazard. The so-called Floating Potential Probe, however, is expected to give NASA engineers data needed to prove whether the grounding rods in fact work as advertised.

An entire week, meanwhile, will pass before the shuttle and station crews crack open hatches between the joined spacecraft and actually meet face-to-face.

Thats because the atmospheric pressure aboard the shuttle must be kept significantly lower than that on the station to expedite preparations for all the work outside the outpost.

The lower pressure enables shuttle astronauts to spend less time -- 45 minutes rather than four hours breathing pure oxygen before a spacewalk, an act crucial to avoiding dangerous bouts of decompression sickness, which scuba divers call "the bends."

Unclear at this point is whether the joined crews will stage some sort of hatch-opening ceremony to mark the arrival of the first visitors to what now is an inhabited station.

One thing, however, is certain:

The station construction project which is a joint effort of 16 nations and 100,000 workers on four continents -- will come to a standstill if the huge solar panels cannot be successfully mounted and activated by Endeavours crew.

The electrical power supply on the station now is so sparse that its resident crew U.S. astronaut Bill Shepherd and two Russian cosmonauts, Yuri Gidzenko and Sergei Krikalev -- is restricted to working within just two of the outposts three pressurized modules.

Existing electrical supplies primarily are being used to run oxygen generation, carbon dioxide removal, water production and other systems essential to keeping the station crew alive in the deadly vacuum of space.

More electricity, meanwhile, is a must before the bus-sized U.S. Destiny lab which will be the scientific heart of the station can be delivered during a shuttle mission now scheduled for launch in mid January.

"Without electricity, the station is dead," Tanner said. "And if for some reason we dont generate the electricity the lab needs, the lab stays on the ground until we fix the problem with power generation."

And in that case, an intricately orchestrated series of 41 U.S. shuttle and Russian rocket flights still needed to complete station construction would come to a screeching halt.

Dont think the Endeavour crew isnt fully aware of that fact.

"Even if I wasnt totally biased about the importance of this mission," said Tanner, "I would have to agree that it is critical in the assembly sequence."

Added Jett: "Well know when we leave the station whether or not weve accomplished what we set out to do."

Endeavour and its crew is scheduled to return to Earth Dec. 11.