SPACE CENTER, Houston -- Spacewalking astronauts are about to enter uncharted territory at the International Space Station a potentially shocking environment that presents a real but remote chance of death by electrocution.

Heres the situation:

A visiting construction crew aboard the shuttle Endeavour will set out this week to mount a giant pair of power-producing solar panels atop the 13-story station. The problem, however, is that NASA is not entirely certain that grounding rods already attached to the outpost will work well enough to prevent the high-voltage arrays from creating a hazard much more treacherous than touching an electric fence.

[inset]

~

The high-voltage arrays, however, are so powerful that under certain circumstances, they can create an electric arc that would shoot out from metal station structure into the surrounding environment, potentially creating a shock hazard for spacewalking construction workers.

Two electrical ground rods known as Plasma Contactor Units or PCUs -- were mounted on the station in October, but recent analyses have raised concerns about their ability to neutralize dangerous electrostatic discharges from the station. 

STS-97's crew

"The source of this issue or concern comes from the fact that the solar arrays are so big, and they are rather high-voltage. They can generate, unregulated, about 200 volts," said NASA station flight director Jeff Hanley. "Its been studied extensively and unfortunately, we dont have a lot of data to draw on with high-voltage arrays of this sizeto make really detailed and confident calls on whether or not it is a concern."

Complicating matters is the fact that the new power system is absolutely critical to the $60 billion space station construction project, which is a joint effort of 16 nations and 100,000 workers on four continents.

The stations resident crew now is relying on limited power supplies from two Russian-made modules to run outpost systems, including crucial oxygen generation, carbon dioxide removal and water production devices key to sustaining life in the deadly vacuum of space.

One of the stations three pressurized wings, in fact, remains off-limits to U.S. astronaut Bill Shepherd and his two Russian colleagues Yuri Gidzenko and Sergei Krikalev because the existing power supply is not sufficient to open it up.

Whats more, the new American-made electrical system must be in place before the stations first power-hungry science lab the U.S. Destiny module is delivered during a shuttle mission now scheduled for launch in mid January.

Under the circumstances, NASA mission managers have put in place a strict set of ground rules to protect Tanner and Noriega during their three spacewalking excursions outside the international station, dubbed "Alpha" by Shepherd and his crew.

First, the veteran astronauts will be ordered to retreat to a safe haven the shuttles protective airlock before the power-producing arrays are unfurled at the end of their first spacewalking excursion.

Then, during the two subsequent walks, station flight directors will be required to shunt the arrays, thus preventing the generation of electricity while the spacewalkers are working outside the station. And in situations which that cannot be done, the entire 80-ton station will be maneuvered into a position that inhibits the ability of the arrays to generate electricity.

The Z-1 truss, with its large white antenna dish, is seen attached to the ISS as Discovery's robot arm pulls away on Oct. 14, 2000 during mission STS-92.

Finally, Tanner and Noriega will set up a specially designed monitoring system on their third and final foray outside the station a job mission managers scrambled to add to the flight less than three weeks ago.

Dubbed the Floating Potential Probe, or FPP, the device will gauge electrical potential outside the outpost to determine if the stations grounding rods are in fact neutralizing any shock hazard that might exist.

"The problem is we dont know exactly how effective the PCUs are doing their job and thats the whole point of the Floating Potential Probe," Tanner told SPACE.com in a recent interview here at JSC.

If the grounding rods are working as advertised, then NASA officials will have one less worry during the more than 150 spacewalks still required to raise the station. If not, the agency will have the data it needs to put in place a grounding system that will more adequately protect future station construction workers from any potential shock hazard.

Tanner and Noriega, meanwhile, arent taking the situation lightly. "We dont want to be the lightning rod, as it were, for the discharge between any potential energy difference between the [station] structure and space," Tanner said. "If they say we ought to take certain precautions, then by golly, we ought to take those precautions."

Both spacewalkers believe NASA mission managers are taking a prudent approach to circumstances that present only a "potential" problem, and neither feels any undue risk is being foisted upon them.

"Im very excited about having FPP on board because that will allow us to characterize the environment, and it needed to go up as soon as possible," Tanner said. "So Im glad its going to make the flight just for the sake of my buddies down the road."