HOUSTON - For the first time in
three years, the U.S.-built attitude control system aboard the International
Space Station (ISS) is complete after a successful repair job by two
spacewalking astronauts Monday.
Discovery's STS-114 astronauts
Soichi Noguchi and Stephen Robinson swapped out one of four control moment gyroscopes
(CMG) used to orientate the space station, paving the way for future
construction of the orbital laboratory. While the space station can run fine on
just two gyroscopes, all four will be needed as new modules and trusses are
added on, NASA officials said.
"Hello CMGs, we're here for ya,"
Noguchi said as he and Robinson approached their space station work site at the
Z1 truss.
The broken
gyroscope failed in 2002, leaving the space station with two primary gyroscopes
and one spare. That spare shut down
in 2004 when a circuit breaker failed. Though subsequently repaired, it failed
again in March 2005. Robinson rerouted power for the energy-starved gyroscope
during a Saturday spacewalk,
restoring it to operation.
Monday's
spacewalk began at 4:42 a.m. EDT (0842 GMT) as the Discovery-ISS complex flew
220 miles over Asia. After seven hours and 14 minutes in space, they were
safely back inside Discovery at 11:59 a.m. EDT (1559 GMT). They were about 30
minutes late at the start, but made up some time with their deft handling of
the gyroscope repair.
"Great work
today," said astronaut Michael Massimino, serving as capcom for ISS mission
control. "It's been great working with you guys."
During the gyroscope replacement,
the spacewalkers played what Noguchi has called an "orbital shell game,"
placing the broken gyroscope in a holding area, plucking out its replacement
from a berth in Discovery's payload bay and sliding the faulty unit into that
same berth.
"Outstanding, great work Soichi,"
Robinson said as Noguchi hauled off the faulty, washing machine-sized gyroscope
out of its ISS berth from the end of the station's robotic arm. "Alright my friend, let's take this CMG home."
Noguchi and Robinson did encounter a
slight glitch during the gyroscope installation. After connecting all the power
cables necessary to both heat and power the new gyroscope, flight controllers
reported it was not working properly.
But a few cable checks by Noguchi
found one connector out of place and power was restored. The news was so good
to flight controllers it prompted lead spacewalk officer Cindy Begley to jump
out of her chair in applause.
"Congratulations, mission control,"
Robinson said.
It will take up to eight hours to
spin the new gyroscope's flywheel up to its full speed of about 6,600
revolutions per minute.
The two spacewalkers were able to fit
several extra tasks into their extravehicular activity (EVA). In addition to
gathering tools to retrieve an ISS rotary motor, they also relocated a foot
restraint to be used in a future shuttle mission and grabbed a set of tools
that could be used to repair a pair of gap-fillers
if mission managers decide it is necessary.
The gap-fillers, small strips of
ceramic fiber cloth, are jutting out from between the black heat tiles along
Discovery's belly a bit farther than shuttle managers have seen in the past.
They could be snipped free or pulled out completely during the third STS-114 spacewalk,
though whether any action is necessary is still under discussion.
The results of those discussions, as
well as a final evaluation of the integrity of Discovery's wing leading edge,
are expected later today.
A tale of two gyroscopes
Control moment gyroscopes are vital
components for the International Space Station (ISS), allowing the orbital
laboratory the ability to change its orientation while conserving propellant by
not firing its Russian-built thrusters. At least two working gyroscopes are
needed to maintain the station's attitude without the thrusters.
Only two of the four gyroscopes have
running to date, though the two spacewalkers successfully restored power to a
third one - CMG-2 - during Saturday's extravehicular
activity (EVA). Flight controllers powered down one of the station's
working gyroscopes, CMG-3, while Discovery is docked at the ISS after some abnormal
vibrations led them to suspect the massive load was putting too much of a
strain on it. When docked, Discovery and the ISS together mass about 300 tons.
CMG-1 failed
on June 8, 2002, seizing up with enough force to cause a vibration that sounded
like a loud growl to astronauts inside the space station's Unity node.
"We're hearing a pretty loud,
audible noise. A kind of a growling noise in the node," astronaut Carl
Walz told
flight controllers after hearing the sound.
Engineers later suspected
that a lack of lubrication led to the gyroscope's failure, but engineers hope
to pore through the broken unit to be sure.
"We're going to bring CMG-1 back to
the ground and see what it can show us," said Paul Hill, lead flight director
for Discovery's STS-114 spaceflight, in a Sunday mission update.
"Once the shuttle is gone, we intend
to bring CMG-3 back online," Hill said.
Discovery mission specialist Andrew
Thomas choreographed today's spacewalk from inside the orbiter while shuttle
pilot James Kelly and mission specialist Wendy Lawrence operated the space
station's robotic arm for Noguchi.
Today's spacewalk was the 60th
extravehicular activity in support of the ISS, and the 27th station
spacewalk staged from a U.S. space shuttle. It was also the second spacewalk
for both Noguchi and Robinson, who have amassed a total of 14 hours and four
minutes working outside a spacecraft.
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