Chris Cassidy during training for the spacewalk in NASA's Neutral Buoyancy Laboratory. Credit: NASA

Floating in the International Space Station's Quest airlock module, astronauts Christopher Cassidy and Luca Parmitano switched their spacesuits to battery power at 8:02 a.m EDT (GMT-4), officially kicking off a planned six-and-a-half hour spacewalk.

The primary goals of the excursion are to replace a Ku-band communications transceiver; to install cabling needed by a Russian laboratory module scheduled for launch late this year; to retrieve a pair of space exposure materials science experiments; to mount a pair of radiator servicing attachment fittings; and to carry out routine maintenance.

For identification, Cassidy, call sign EV-1, is wearing a suit with red stripes using helmet camera No. 20. Parmitano, EV-2, is wearing an unmarked suit. The first Italian to walk in space, Parmitano is using helmet camera No. 17.

This is the 170th spacewalk devoted to station assembly and maintenance since construction began in 1998, the fourth so far this year, the fifth for Cassidy and the first for Parmitano, representing the European Space Agency.

Both men plan to carry out another spacewalk next Tuesday to complete a long list of maintenance and assembly tasks.

"Why spacewalks now? The program has collected a number of tasks over the last couple of years," said flight director David Korth. "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" is 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)."

Cassidy, a former Navy SEAL, was first out of the Quest hatch, followed by Parmitano. 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 Cassidy works on the transceiver swap out, Parmitano will move to the right side of the station's solar power truss to retrieve a pair of space exposure experiment pallets that will be returned to Earth later this year aboard a commercial SpaceX Dragon cargo ship.

Parmitano also plans to photograph 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 will team 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 if a swap out is ever required.

Parmitano, anchored to the end of the space station's robot arm, will carry one RGB to the right side of the truss where he and Cassidy will bolt it in place. Astronaut Karen Nyberg, operating the arm from inside the station's Destiny laboratory module, then will move Parmitano back toward the port side of the truss.

Along the way, he plans to remove a failed camera assembly from the robot arm's mobile base so it can be returned to Earth for refurbishment.

While that work is underway, Cassidy will install 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, 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.

After installing the MLM wiring, Cassidy will take the failed camera assembly from Parmitano and carry it back to the Quest airlock. Parmitano, meanwhile, will carry the second RGB to the port side of the truss.

After helping Parmitano install the left-side RGB, Cassidy will begin 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 works to install the bypass jumpers -- the work will be completed during next week's spacewalk -- Nyberg will maneuver Parmitano back to the center of the power truss where he will get off the robot arm and stow the foot restraint that anchored him in place.

Cassidy and Parmitano will meet back up for the final major task of the day, installing a protective cover over the station's forward port where space shuttles once docked. After that, the duo will head back to the Quest airlock and call it a day.