HOUSTON--Two Discovery shuttle astronauts will venture
outside the International
Space Station (ISS) for their second spacewalk this week, this time to
rewire part of the orbital laboratory's power grid.
STS-116
spacewalkers Robert
Curbeam and Christer
Fuglesang are expected to spend six hours overhauling half of the station's
power system from a temporary set up into its permanent configuration.
"[I]t is
actually quite risky," Curbeam said of the spacewalk in a NASA interview. "But the biggest is risk
is starting up the items that have never been operated on the station."
Today's
spacewalk is set to begin at 3:12 p.m. EST (2012 GMT) from the Quest airlock at the ISS.
Curbeam and
Fuglesang will work primarily on the space station's central S0 truss, where
they will route electrical connections, cables and jumpers to plug two portside
solar arrays into two of four channels that make up the outpost's primary power
grid.
Today's
spacewalk will reconfigure the station's power channels 2 and 3, which should
be complete after the first 90 minutes, NASA's lead STS-116 spacewalk officer
Tricia Mack has said. A Saturday excursion by Curbeam and newly-arrived ISS
flight engineer Sunita
Williams will reconfigure channels 1 and 4.
To do that,
spacewalkers must disconnect and reconnect dozens of stiff cables to fold the Port 3/Port
4 (P3/P4) solar arrays, which were installed at the ISS in September, into
the stations primary electrical system.
The
spacewalk will also activate two long dormant Main Bus Switching Units
(Channels 2 and 3) that direct power where it's needed for the first time four
years, and start up a vital pump module for the station's ammonia cooling system
that will prevent the power systems from overheating.
If any one
task does not go as planned, the astronauts may have to stop their work and
back out of their complicated procedures to replace faulty electronics boxes or
perform other troubleshooting work, losing precious time, NASA officials have
said.
Since 2000,
the space station's main power has been provided by solar arrays on the mast-like
Port 6 truss, which was always meant to be a temporary system until
additional solar wings were installed and plugged into the outpost's main
truss. The P3/P4 solar arrays are the first to be plugged into the station's
permanent power system, which is vital for the outpost's expansion to accommodate
larger astronaut
crews and additional international
laboratory modules, NASA officials said.
One fear,
Curbeam has said, is that he and Fuglesang will perform all their required
tasks, unplug the station's power systems from P6 and into the main truss, only
to find that nothing happens.
"We don't
expect everything to crumple on us like that," he said. "But you just don't
know."
John Curry,
NASA's lead ISS flight director for Discovery's STS-116 mission, said that half
of the space station's systems will shut down during today's spacewalk, dimming
the lights for the eight astronauts inside and leaving many non-critical
systems without redundant backups.
"A lot of
systems go single string in the power down," Curry said.
Primary
control computers and life support systems will be safeguarded using a series
of jumper cables, he added, but some ISS communications and ventilation systems,
for example, will be affected. Astronauts will have to rely on Discovery's
systems instead.
Curbeam and
Fuglesang will not be the only ones busy during today's spacewalk.
The
astronauts will work in close concert with ISS flight controllers on Earth, who
will shut down and reactivate station systems when required. Altogether, space
station flight controllers expect to issue some 4,500 commands to the ISS
between today's spacewalk and its Saturday counterpart. On an average day, flight
controllers send about 800 commands to the ISS, NASA officials said.
Once the
ISS power system overhaul is complete, Curbeam and Fuglesang still have work to
do outside the space station.
The
astronauts expected to move two tool carts, known as Crew Equipment Translation
Aids (CETA), to get a head start on future ISS construction during NASA's
STS-117 shuttle flight next year.
Curbeam and
Fuglesang will also install two thermal covers over sensors on the space
station's robotic arm and reconfigure an electrical patch panel that routes
power to the orbital laboratory's Zenith 1 (Z1) and Russian segments.
"It's going
to be a pretty exciting day," Curbeam said.
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