This story was updated at 11:27 a.m.
EST.
Two astronauts will float outside
the International Space Station today to wrap up repairs on a clogged solar
array-turning gear during the fourth spacewalk of their mission, which NASA has
extended by one extra day due to glitches with the outpost's new water
recycler.
Endeavour astronauts Steve Bowen and
Shane Kimbrough are due to step outside the station at 1:45 p.m. EST (1845 GMT)
on the fourth spacewalk of their shuttle mission to clean the massive
starboard-side gear and lube up its port-side counterpart for good measure.
While Bowen and Kimbrough work
outside, flight controllers at Mission Control in Houston will continue efforts
to solve start-up glitches with the station's new recycling system, which is
designed to turn sweat and urine
back into drinking water.
Mission Control called up to the
docked Endeavour and space station to officially extend their joint 15-day
mission by one day to allow more time with the balky machine. The mission will
now run 16 days, with landing set for Nov. 30.
"We really enjoy having these guys
up here and if we can only extend by one day that will have to do," said space
station commander Michael Fincke.
"Hopefully, it will be a good extra
day for you guys," astronaut Terry Virts radioed back from Mission Control in
Houston.
The water recycler's urine processor
has been shutting down earlier than planned. The glitch is apparently due to a
sensor's interference with a spinning centrifuge used to distill water from
urine, but solving it is vital if the station is to shift to larger,
six-person crews next May, mission managers said.
Astronauts removed a set of
vibration dampeners from the machine on Sunday in an
attempt to fix it. The measure extended the urine processor's performance,
but did not appear to completely solve its early shutdown problem.
"It's definitely something we could
not have identified while we were on the ground," space station flight director
Courtenay McMillan said Sunday.
Final station tune-up
Monday's spacewalk is the last
in a series to perform an unprecedented clean-and-grease-job on the 10-foot
(3-meter) wide gear, which rotates the station's starboard solar wings like a
paddlewheel so they always face the sun. The gear has been damaged for more
than a year, when its lubrication gave out and grinding parts spread metal
shavings throughout its delicate turning surface.
"The analogy I like to draw is,
we're jacking up the International Space Station and we're taking the wheels
off," said Endeavour commander Chris Ferguson on Friday. "And when you do that,
you're bound to find a little bit of dust and get a little bit greasy, and
we're learning as we go."
On their last three spacewalks,
Endeavour astronauts have used mitten-like wipe cloths and grease-spewing guns
to clean the gear of metal grit and add a new layer of lubricant. The gear, or
Solar Alpha Rotary Joint (SARJ), has 10 new bearings thanks to the orbital
work, with an 11th to be installed today. One other bearing was replaced in a
separate mission last summer.
Flight controllers had to replan the mission's
spacewalks to rely on only one full set of gear-cleaning tools after a $100,000
bag escaped from lead spacewalker Heide Stefanyshyn-Piper during the first
excursion last week.
"I think people are going to find
that by the time EVA 4 is complete we're going to have, if not a great SARJ,
we're going to have a better SARJ," said Ferguson, using NASA's technical term for spacewalk.
The space station's port side SARJ
gear has been working fine
since its installation in 2006, but Mission Control wants to have its mechanism
lubricated as a preventative measure.
"Having the port side Solar Alpha
Rotary Joint and the starboard one working as they're planned will really help
us maximize our power," Fincke said Sunday in a series of televised interviews.
Maintenance chores on tap
In addition to servicing the
station's solar array gears, Bowen and Kimbrough plan to tackle a host of other
maintenance tasks.
Much of that work is aimed at
preparing the space station's Japanese Kibo laboratory for the arrival of a
porch-like external platform and Japanese cargo ship next year. They will also
snap photos of a damaged radiator outside the station.
Monday's spacewalk will be the third
for Bowen, but second for Kimbrough, who admitted last week that he was so
focused on his first excursion on Thursday that he didn't take enough to time
to look down and marvel at the view of Earth.
"I know it's a beautiful view out
there," Kimbrough said.
NASA is providing live coverage of
Endeavour's STS-126 mission on NASA TV. Click here for SPACE.com's
mission coverage and NASA TV feed.
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