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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