HOUSTON -- Two
spacewalking astronauts will step
outside the International
Space Station (ISS) later today to complete efforts to rewire
the orbital laboratory and, time permitting, give a stubborn
solar array a little push.
Veteran
spacewalker Robert
Curbeam and first-time spaceflyer Sunita
Williams plan to spend about six hours working outside the ISS to shift two
of the outpost's power channels from their temporary configuration into their
final form [video
preview].
If they
finish their tasks early, the spacewalkers are due to ascend the space station's
mast-like
Port 6 (P6) truss and tap on the bottom of one of its solar array boxes in
efforts to loosen stuck guide wires that have prevented the 115-foot (35-meter)
wing from retracting properly.
"The solar
array viewing, pushing on that box, is a get ahead task," Kirk Shireman, NASA's
ISS deputy program manger, said late Friday. "The primary objective tomorrow is
to rewire the space station."
Today's
spacewalk, set to begin at 2:37 p.m. EST (1937 GMT), is the second
in three days dedicated to overhauling the space station's power system and
the last
of three planned for NASA's STS-116 shuttle mission aboard
Discovery.
Curbeam and
fellow STS-116
mission specialist Christer Fuglesang, of the European Space Agency (ESA),
successfully rewired the first half of the station's electrical grid during a five-hour
spacewalk on Thursday. Discovery pilot William
Oefelein will choreograph the spacewalk from the shuttle's flight deck.
"Hopefully,
it will go just the same and we'll be just as good," Williams, a newly arrived
member of the space station's Expedition
14 crew who will make her spacewalk debut, told reporters Thursday. "I'm
just looking forward to going outside and checking out the planet. It should be
pretty nice."
Curbeam and
Williams are expected to spend the first 90 minutes of today's spacewalk
wrangling stiff power cables, connecting 21 of them while unplugging 18, to feed
power though a pair of Main Bus Switching Units used in Channels 1 and 4 of the
space station's power grid. They are also charged with attaching a grapple bar
along with some debris panels, stored in a pyramid-like fashion dubbed the "Christmas
tree," destined to shield a Russian station module against micrometeorites to
the space station's exterior.
During the
critical power system overhaul, lights, smoke detectors, ventilation fans and
other ISS systems will be shut off as half of the orbital laboratory is powered
down while Curbeam and Williams reconfigure the station's power system to its
permanent configuration. A vital cooling system pump must perform properly
afterward to ensure the work is a success.
"We're
looking forward to a successful [spacewalk] tomorrow," Shireman said Friday.
Solar
array work possible
Only if
they complete all of their assigned tasks swiftly will Curbeam and Williams be
given the go ahead to inspect, then tap, the portside wing of the space station's
P6 solar array. Either of the astronauts could do the tapping, which includes
pushing up or forward on the afflicted array's storage box.
"It's a
pat," Shireman said of the push, adding that the spacewalkers are authorized to
apply a gentle five pounds of force for the job.
Shuttle and
ISS astronauts partially
retracted the six-year-old P6 array on Wednesday to allow a separate pair
of solar wings on the station's Port 3/Port
4 (P3/P4) truss beneath it to begin rotating to track the Sun. But one of the array's two solar
panels, known as blankets, refused to fold properly [image].
ISS flight controllers believe one of three guide wires is hung up in a series
of stainless steel grommets [image].
Efforts
to loosen the jam remotely Friday proved fruitless. ISS flight controllers
first wiggled the array itself and then asked Expedition 14 flight engineer and
ESA astronaut Thomas
Reiter to perform a vigorous workout on an exercise machine that has
caused noticeable vibrations of the solar wing in the past. Neither attempt
succeeded.
"I'm very
sorry to hear that, I was training for it for half a year," said Reiter, who
has lived aboard the ISS since July.
"We'll give
you a silver medal for that," replied NASA astronaut Terry Virts,
serving as spacecraft communicator.
Mission managers have discussed the
possibility of adding a fourth
spacewalk to the STS-116
mission, which would occur no earlier than Monday if at all. The
possibility of an additional spacewalk has always been included in Discovery's complicated
spaceflight in case major elements in the rewiring effort had to be repaired or
replaced entirely.
Sending astronauts
to work near ISS solar arrays carries added risks, particularly shock hazards,
for spacewalkers, but Discovery
astronauts said Friday they were willing to help in whatever way required.
"I'm not
concerned at all about the shock hazard, we've got a great team on the ground
working a plan that hopefully will work well for us," Curbeam said. "If nothing
else, I know it will be safe for us."
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