HOUSTON - A spacewalking pair of astronauts has paved the way for future construction of the International Space Station (ISS) after completing a critical repair to the orbital laboratory's mobile crane Monday.
STS-121 astronauts Piers Sellers and Michael Fossum restored a backup cable system for the station's railcar-like Mobile Transporter during a nearly seven-hour spacewalk more than 200 miles (321 kilometers) above Earth. The fix restores a dependable power and data line for the Mobile Transporter, clearing the carrier for use during NASA's next ISS construction mission - STS-115 aboard Atlantis - to launch in late August.
Today's six-hour and 47-minute spacewalk, the second of NASA's STS-121 shuttle missions, began at 8:14 a.m. EDT (1214 GMT) as Sellers stepped through the outer hatch of the station's Quest airlock and into space.
"Look at that country down there," Sellers said. "Oh, that's gorgeous."
There appeared to be only one significant hitch during today's spacewalk, when one of two small latches - referred to as towers - on Sellers' emergency thruster backpack popped free of its mooring. The latches connect the Simplified Aid for EVA Rescue (SAFER) thruster system, which is designed to help astronauts return to their spacecraft should they drift free during a spacewalk, to the spacesuit proper.
"If the second tower comes free, then the SAFER will no longer be connected to the astronaut," spacecraft communicator Julie Payette told the two spacewalkers.
After securing the SAFER to Sellers' spacesuit with a tether, the spacewalk continued smoothly until another connector latch popped free. Fossum later jammed the latches into place.
They also had difficulties installing the new cable reel into its berth on the ISS. After several attempts, the reel slid firmly into place.
"What else can happen, man," Sellers said glibly.
Restoring the redundant power cable system for the station's Mobile Transporter is a critical fix for the ISS.
On Dec. 16, 2005, a guillotine-like cutting system unexpectedly fired on the Earth-facing side of the railcar, slicing its power and data cable there for no apparent reason. The Mobile Transporter is fed power and data lines from two cable assemblies - known as Trailing Umbilical System (TUS) reels - that pay out and draw in the lines as the cart slides along rails mounted to the space station's main truss.
An attempt to safeguard the surviving cable and maintain railcar mobility failed in a Feb. 4 spacewalk, when the space station's Expedition 12 crew had to remove the power line altogether - immobilizing the carrier in the process - to ensure it would not be cut inadvertently as well.
But on Saturday, Sellers and Fossum installed a blade blocker on the cable cutter threaded by the railcar's then-only functional power cable. The quick-fix removed any danger of an unexpected blade firing, and allowed the transported to move for today's spacewalk.
"We could accomplish [STS]-115 where we are now but we wouldn't have any redundancy in the cable end that goes into that system," said John Shannon, NASA's deputy shuttle program manager, during a Sunday briefing. "It is much better on a system that complex and that critical to have redundancy."
Prior to today's spacewalk, Sellers and Fossum described the complicated task as more a dance rather than orbital repair. The two astronauts had to pass broken and pristine TUS units between them, with Sellers holding onto both cable reels at one point while Fossum worked from the tip of the space station's robotic arm.
"The TUS is certainly in," Sellers said after he and Fossum installed the new reel.
Initial checks of the Mobile Transporter found no immediate problems, though flight controllers planned a series of additional tests over the next two hours, Payette, serving as spacecraft communicator, said.
Space shuttle pilot Mark Kelly watched over today's spacewalk, with STS-121 mission specialists Lisa Nowak and Stephanie Wilson guiding the space station's robotic arm for Sellers and Fossum.
Spare ISS pump installed
In addition to replacing the Mobile Transporter's TUS reel, Fossum and Sellers delivered a 1,393-pound (631-kilogram) spare pump module to the ISS.
The pump module is a backup for two identical systems already installed on the ISS, with one on the Port-1 truss while the other sits on the Starboard-1 truss. Each pump is designed to transport liquid ammonia through the space station's cooling system.
"That went very easy, by the way, it's very easy to move," Sellers said about toting the hefty pump module. "It's basically like moving a rowing boat to the dock with your hands."
The pump module installation appeared to go quite smoothly, but flight controllers said even the simplest spacewalk activity is no easy task.
"All EVAs are difficult," lead STS-121 shuttle flight director Tony Ceccacci said Sunday. "We're doing so much that everyone thinks they're like going outside and mowing your grass, but they each have their risks and they're difficult to do."
Today's EVA marked the second career spacewalk for Fossum, who now has about 14 hours and 18 minutes of orbital work under his belt. It was the fifth spacewalk for Sellers, who has now spent almost 34 hours toiling outside a spacecraft with nothing but a spacesuit for protection against the vacuum of space.
"This is where ingenuity and persistence paid off, and flexibility," Fossum said.
The spacewalk also marked the 67th staged to support the ISS and the 20th that began from the station's Quest airlock, but it will not be the last before this week is out.
Sellers and Fossum are scheduled to step outside one last time during their 13-day mission during a July 12 spacewalk to test shuttle heat shield repair methods inside Discovery's payload bay.
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