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ISS Panels




   
By Todd Halvorson
Cape Canaveral Bureau Chief
posted: 07:00 am ET 
28 November 2000     
Endeavour Crew to Spread Station Alpha's Solar Wings

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.
Here’s 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.

Orbiter: Endeavour (15th flight).
Launch Time: 10:06 p.m. EST Thursday (Friday, 03:06 GMT).
Launch Pad: Kennedy Space Center pad 39B.
Docking At Space Station: 2:57 p.m. EST (19:57 GMT) Saturday.
 

Spacewalk No. 1 (Start time): 1:31 p.m. EST (18:31 GMT) Dec. 3.
Spacewalk No. 2 (Start time): 1:06 p.m. EST (18:06 GMT) Dec. 5.
Spacewalk No. 3 (Start time): 12:06 p.m. EST (17:06 GMT) Dec. 7.
Station Hatch Opening: 9:36 a.m. EST (14:36 GMT) Dec. 8.
Shuttle Undocking: 2:17 p.m. EST (19:17 GMT) Dec. 9.
Landing: 6:19 p.m. EST (23:19 GMT) Dec. 11.
Mission Duration: 10 days, 20 hours, 13 minutes.
 


"There is quite a bit of disagreement among the (NASA engineering) community 
as to how dangerous it is," said Glenda Laws, a lead engineer in the agency’s 
spacewalk projects office here at Johnson Space Center. "But most of the 
community believes that it is extremely dangerous – that it would be more 
like a lightning bolt strike," she said. "So we take it very seriously."

The potentially perilous situation will begin to unfold this week as 
Endeavour and a crew of five astronauts -- including spacewalkers Joe Tanner 
and Carlos Noriega – blast off from NASA’s Kennedy Space Center in Florida.

The precisely timed launch – which is scheduled for 10:05 p.m. Eastern 
Standard Time Thursday (Friday, 03:05 GMT) – will put the shuttle on course 
for docking two days later at the station, which now is occupied by its first 
full-time resident crew.

Over the course of the following week, Tanner and Noriega will perform a trio 
of spacewalks primarily aimed at erecting and activating a massive, $600 
million electrical power tower atop the growing outpost. The tower’s most 
striking feature: A pair of blue-and gold solar panels that will have a 
wingspan greater than that of a 747 jumbo jet, stretching some 240 feet (73 
meters) from tip to tip once unfurled in space.

Big enough to cover nearly the entire length of a football field, the mammoth 
arrays – which are designed to convert sunlight into electricity - by far 
will be the largest ever deployed in space. 

Each wing weighs 2,400 pounds (1,080 kilograms) and together they will 
generate a staggering 64 kilowatts of direct current (DC) power – or enough 
electricity to run 30 average American homes without air conditioning.
 
 
 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.


"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. 
"It’s been studied extensively and unfortunately, we don’t have a lot of 
data to draw on with high-voltage arrays of this size…to 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 station’s 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 station’s three pressurized wings, in fact, remains off limits to 
U.S. astronaut Bill Shepherd and two Russian colleagues – Yuri Gidzenko and 
Sergei Krikalev – because the existing power supply is not sufficient enough 
to open it up.

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

The harsh reality, as a result, is that 41 U.S. shuttle and Russian rocket 
missions still required to complete station construction will remain on hold 
until the new U.S. solar arrays can be deployed and activated.

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 shuttle’s 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.

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 station’s grounding rods 
are in fact neutralizing any shock hazard that might exist.

"The problem is we don’t know exactly how effective the PCUs are doing their 
job – and that’s 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, aren’t taking the situation lightly. "We 
don’t 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.

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