Two Discovery's astronauts worked outside their spacecraft today replacing a spinning wheel, which helps to steer the space station.
Astronauts Stephen Robinson and Soichi Noguchi spent five hours exchanging the broken down 660-pound (299-kilogram) gyroscope, which failed in 2002, with a new one.
"Oh, the view is priceless," Noguchi said as he hitched a ride to Discovery's cargo bay on the outpost's robotic arm. "I can see the moon."
With Robinson's help, Noguchi secured the controller aboard the shuttle and retrieved the new gyroscope for installation.
After hours of tedious bolting and unbolting with specialized silver drivers, the pair completed the installation.
"This is just like putting in an airplane engine," said Robinson, a pilot.
"Just wiggling until you get it," Noguchi responded, causing his partner to chuckle.
"Yeah, exactly. Prepare to wiggle," Robinson said.
Both continued bolting and wiggling until the unit was tightly attached.
Their seven-hour-plus spacewalk came a day after NASA officials said they may consider sending them to repair material dangling from Discovery's belly during a third spacewalk scheduled for Wednesday.
Before going back inside, Robinson and Noguchi gathered a pry bar and forceps from an outdoor tool box to use for the potential shuttle repair. It took both of them to force open the box, on a count of three. "Yeah!" they shouted when the lid finally popped open.
But there remains debate among engineers and others over how to handle what would be an unprecedented repair - and whether it is even necessary.
During their first spacewalk Saturday, the pair restored power to another gyroscope, which had stopped spinning in March. The gyroscopes are among four that help steer the station.
"Being outside was the most incredible experience I've certainly ever felt so far, and I almost can't believe we get to do it again," Robinson said Sunday as he prepared for his second orbital outing.
Only two of the four gyroscopes that control the orientation of the orbiting science lab have worked recently. Once power was restored to the third gyroscope Saturday, one of the two that continued spinning was given a break because its 6,600 revolutions per minute had become sluggish.
Once Discovery undocks from the station Saturday, NASA hopes to have all four gyroscopes operating simultaneously for the first time in three years.
Meanwhile, NASA officials were scrambling to determine if repairs were needed by the astronauts. Some engineers worry the material that is protruding from between thermal tiles in two areas beneath the shuttle near its nose could trigger potentially treacherous overheating during re-entry.
NASA officials stressed that Discovery and its crew could be perfectly safe flying back with the exposed filler. Space shuttles have flown with exposed filler many times before, just not necessarily with such a large protrusion.
One piece is sticking out 1.1 inches (2.8 centimeters). The other protrudes at an angle from six-tenths to nine-tenths of an inch. The general wisdom and flight history indicate that the limit should be a quarter-inch, said flight director Paul Hill.
One solution would be to pull the filler completely out or fold it back in. Another could be to cut it, said Steve Poulos, manager of the orbiter project office.
Deputy shuttle program manager Wayne Hale said more technical information is needed and the risks of causing further damage by going underneath the shuttle need to be considered.
"We certainly don't want to make the situation worse than it is," he said. "My immediate knee-jerk reaction was that we can live with this. On the other hand, this is bigger than we have seen before."
In 24 years of shuttle flight, astronauts have never ventured beneath their spacecraft in orbit and have made few repairs to their ship.
But if NASA's spacewalking specialists can come up with an easy fix, Hale says correcting the problem may be worth eliminating concern about flying home with the protrusions.
"Why would you not just go take care of it?" he asked. "Why should I lose sleep over these gap fillers if we can take care of them that easy?" The tools are aboard Discovery and the crew has already been trained how to cut the fillers, Poulos said. The filler keeps the shuttle's thermal tiles from damaging one another as the spacecraft heats up during re-entry and its protective thermal tiles expand.
Hale, however, said the analysis isn't complete.
"I certainly think the jury is out at this point as to whether or not we will do anything," he said as quoted by the AP.
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