Astronauts Christopher Cassidy, a former Navy SEAL, and Luca Parmitano, Italy's first spacewalker, ventured outside the International Space Station Tuesday in the first of two excursions needed to work through a backlog of maintenance and assembly tasks.
During a six-hour seven-minute spacewalk, or EVA, the astronauts replaced a Ku-band communications transceiver, retrieved a pair of space exposure materials science experiments and installed cabling needed by a Russian laboratory module scheduled for launch late this year or early next.
The astronauts also mounted a pair of radiator servicing attachment fittings, retrieved a malfunctioning camera assembly, installed a protective cover of the station's forward docking port and began installing jumper cables that will enable engineers to more efficiently recover from power system failures.
All of the planned tasks were accomplished ahead of schedule, giving Cassidy and Parmitano time to carry out a handful of "get-ahead" tasks, including an inspection of a robot arm mounting fixture on the Russian Zarya module and installation of a data cable that will be used by the new lab module after it arrives.
Cassidy and Parmitano plan a second spacewalk next week to complete a variety of tasks that have accumulated over the past several months.
"Why spacewalks now? The program has collected a number of tasks over the last couple of years," flight director David Korth told reporters. "We like to wait to do EVAs because EVAs cost a lot of crew time and in the era of science and utilization, we try to minimize the perturbation to the overall (schedule).
"So the program has strategically placed a couple of EVAs this summer to, as we call it, burn down the list of tasks that require EVA."
Regardless of the strategy, Cassidy and Parmitano said they were eager to venture outside.
"I remember distinctly the feeling the first time I opened the hatch and looking down at the planet," Cassidy said in a NASA interview. "I remember thinking, wow, holy cow, I'm really here!
"It probably was only half a second that I kind of froze and was awestruck by the situation, but it felt like it was probably a minute or two that I was gawking. Fortunately I moved on and quickly got about my work before (then crewmate) Dave Wolf could reach behind and smack me on the head and say, come on, new guy, let's go!"
This time around, the "new guy" was Parmitano. And during a pre-launch news briefing, he said he wasn't taking anything for granted.
"Any spacewalk is challenging. ... just because the environment is so different from anything we know here on Earth," he said. "Chris and I have been training together underwater, preparing for the tasks we will be doing. It's a special challenge.
"Previously during the shuttle times, EVAs were highly choreographed, so everything was planned and choreographed and trained over and over again until every step was perfected. On the station, we don't have that luxury to train as much. So we need to be a lot more flexible."
Space Station Program Manager Mike Suffredini said it takes about 100 hours of crew time to prepare for a spacewalk, time that is lost to research.
As a result, "what you try to do is to get in as many EVAs as you can before you have to re-check the suits, flush the cooling lines, or any number of things we have to do before we do an EVA," he said. "So there's an efficiency to try to go outside a few times in a row. If you know you've got enough tasks to keep you busy, then you try to get as much of those done (as you can)."
The spacewalk began at 8:02 a.m. EDT (GMT-4) when Cassidy and Parmitano switched their spacesuits to battery power inside the Quest airlock module.
Cassidy's first job was to move up to the Z1 truss atop the central Unity module to replace a space-to-ground transmitter receiver controller, part of a Ku-band communications link, that failed last December.
The system is redundant and station communications are operating normally. But a second failure would have a major impact and mission managers want to restore full redundancy to protect against possible problems in the future.
"We're becoming very reliant on the Ku system," Suffredini said. "So making sure we have this redundant capability is important to us. We can live without it, for sure, but it would be a big impact if we lost it."
While he had a bit of trouble with a tight bolt, Cassidy successfully installed the new box while Parmitano will move to the right side of the station's solar power truss to retrieve a pair of space exposure experiment pallets. Both will be returned to Earth later this year aboard a commercial SpaceX Dragon cargo ship.
Parmitano also photographed the massive Alpha Magnetic Spectrometer particle physics detector mounted nearby to help engineers assess its condition after two years in the space environment.
Cassidy and Parmitano then teamed up to install two radiator grapple bars, or RGBs, that were delivered to the station aboard a Dragon cargo ship last March. The RGBs are needed, one on each side of the power truss, to hold radiator cooling panels in place if a swap out is ever required.
Anchored to the end of the space station's robot arm, Parmitano carried one RGB to the right side of the truss where he and Cassidy bolted it in place. Astronaut Karen Nyberg, operating the arm from inside the station's Destiny laboratory module, then maneuvered Parmitano back toward the port side of the truss.
Along the way, he removed a failed camera assembly from the robot arm's mobile base so it can be returned to Earth for refurbishment.
While that work was underway, Cassidy installed power and data cables between the Russian segment of the station and the Unity compartment that will be needed by a new Russian multi-purpose laboratory module, or MLM, that will serve as a laboratory, docking port and airlock.
The new module, known as Nauka, or "Science," will replace the current Pirs airlock compartment. It is scheduled for launch aboard an unmanned Proton booster late this year, but NASA insiders say the flight could slip to the spring timeframe because of assembly delays in Russia.
A Proton rocket carrying three navigation satellites veered out of control, broke apart and crashed seconds after launch July 2 from Baikonur Cosmodrome in Kazakhstan. Protons currently are grounded pending an investigation, but the expected Nauka delay is unrelated to the launch failure. In any case, Protons should be back in service well before the MLM is ready for flight.
Parmitano handed the failed camera assembly off to Cassidy, who took it back to the Quest airlock. Parmitano, meanwhile, carried the second RGB to the port side of the truss.
After helping Parmitano install the left-side RGB, Cassidy began routing so-called Y-bypass jumper cables on the Z1 truss that will enable flight controllers to quickly reconfigure electrical loads in the wake of failures that otherwise would require a spacewalk.
While Cassidy worked on the the bypass jumpers -- the work will be completed during next week's spacewalk -- Nyberg maneuvered Parmitano back to the center of the power truss so he could get off the robot arm and stow the foot restraint that anchored him in place.
Running well ahead of schedule, Parmitano then installed a protective cover over the station's forward port where space shuttles once docked while Cassidy began two get-ahead tasks -- installation of a data cable needed by the Russian lab module and an inspection of a robot arm mounting fixture on the Russian Zarya module.
Engineers believed a loose grounding wire might be interferring with the robot arm mounting mechanism and Cassidy was told to pull any slack out of the line. But Cassidy said he did not see anything amiss.
"It looks already done," he reported.
Connection of an ethernet cable that would route data to and from the robot arm attachment fixture was deferred to next week's spacewalk. Parmitano and Cassidy returned to Quest and began repressurizing the airlock at 2:09 p.m. to officially end the EVA.
This was the 170th spacewalk devoted to station assembly and maintenance since construction began in 1998, the fourth of nine planned for this year, the fifth for Cassidy and the first for Parmitano, the first Italian to walk in space.
All told, 112 astronauts and cosmonauts representing nine nations have now logged 1,073 hours and 50 minutes of EVA time outside the station, or 44.7 days.