HOUSTON - Two astronauts are set to step
outside the International Space Station (ISS) Monday and make a critical repair
to aid future
construction of the orbital laboratory.
Spacewalkers
Piers
Sellers and Michael
Fossum, who spent more than seven hours working
outside the ISS Saturday, will once again don their U.S.-built spacesuits
to restore the station's mobile crane to full operations. The spacewalk is set
to begin at 8:13 a.m. EDT (1213 GMT).
The primary
task for Sellers and Fossum, both mission specialists for NASA's STS-121
shuttle mission aboard Discovery, is to replace a reel-like power and data
cable system that was severed late last year due to a still-unexplained glitch.
Known as a Trailing
Umbilical System (TUS) Reel Assembly, the system transfers electricity,
data and video to the space station's railcar-like Mobile Transporter. The
transporter itself is a critical movable platform for shifting the ISS robotic
arm or massive station components along the orbital laboratory's main truss.
"It's
mobile now, but it's only dependable on one string," Sellers told reporters Sunday
during a space-to-ground video link. "And when we're done tomorrow, it will
have two strings and therefore be more reliable."
There are
two reel assemblies that supply transfer power and video between the ISS and
Mobile Transporter. The ribbon-like cable to one, the TUS to be replaced today,
was severed by an emergency cutting
system originally designed to fire its spring-loaded blade in the event the
line snagged while the Mobile Transporter was in motion. The system inexplicably
cut the trailing cable in December 2005.
An attempt
to safeguard the surviving cable failed during a February 2006
spacewalk by the space station's Expedition
12 crew, though the two astronauts were able to remove the TUS line
entirely from the cutting system. The fix stranded the Mobile Transporter in
place until Saturday, when Sellers and Fossum installed a
blade blocker that will prevent cutter tool from biting into its cable
target even if it fires on its own, NASA said.
"Right now,
it's dead on one side and just limping along on two wheels," Sellers said of
the Mobile Transporter after Saturday's fix. "We need that thing to be working to
move big pieces around during the station's assembly."
NASA's next
shuttle mission - STS-115 aboard the Atlantis orbiter - is set to deliver a
massive truss and new set of solar arrays to the ISS in just over one month,
with launch currently set for Aug. 28.
"It's kind
of important to get that done before the next lot of guys show up," Sellers
said of the TUS repair.
Tony
Ceccacci, lead shuttle flight director for Discovery's STS-121 mission, said
today's spacewalk will be challenging because the space station has no stowage
areas for the large, piano-sized TUS assemblies and Sellers will have to hold
both at the same time at one point in the repair.
During
their Monday spacewalk, Sellers and Fossum will also install a spare pump
module, which is used to move the liquid ammonia used in the space station's cooling
system.
Fossum
added that the TUS on board the ISS has also never been physically tested for
use in the shuttle payload bay fittings that will secure the hardware in place
during Discovery's return flight home.
"We're
going to be holding our breath a little bit," Fossum said Sunday. "But we feel
good about it, we've practiced it a lot."
Today's
spacewalk begins at 8:13 a.m. EDT (1213 GMT) and will be broadcast live on NASA
TV. You are invited to follow along with the STS-121 spacewalkers'
activities using SPACE.com's
feed of NASA TV by click here.
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