Astronauts outfitted the international space station with fresh batteries in an extra-long spacewalk Friday, moving slowly and even loitering at times to avoid a repeat of the suit trouble that cut short the previous outing.
Despite their dawdling pace, Christopher Cassidy and Thomas Marshburn managed to install all four new batteries.
For the second spacewalk in a row, Cassidy ended up with rising carbon dioxide levels in his suit. But the problem did not crop up until the battery work was complete, toward the end of what turned out to be a seven-hour, 12-minute spacewalk.
Before the spacewalk, Mission Control had requested Cassidy and Marshburn to take their time heading out to the far end of the space station, where the critical battery changes awaited them.
It was difficult dealing with all the stiff bolts, and the men paced themselves accordingly as they pulled out 9-year-old batteries and plugged in new ones.
The last time Cassidy went out, on Wednesday, he was so gung-ho and moved so fast that the air-cleansing canister in his suit could not keep up. That resulted in rising carbon dioxide levels that forced an early end to the spacewalk.
The increased levels noted Friday were nothing like the other day, said Kieth Johnson, the lead spacewalk officer in Mission Control.
Johnson attributes Cassidy's hard-charging ways to his military training — he's a former Navy SEAL.
"Sending him to do a task, it's going to get done. He likes to get at it and get going and keep busy the entire time," Johnson told reporters. He will be reminded again to "keep his pace steady" when he goes back out Monday for the fifth and final spacewalk of shuttle Endeavour's visit.
Despite Cassidy's effort to stay relaxed Friday, his metabolic rate was a little high at times and Mission Control urged him to take it easy. Marshburn, in fact, took on some extra chores early on to give his partner a break.
Six and a half hours into the spacewalk, Mission Control reported that carbon dioxide levels were rising slightly in Cassidy's suit. The astronauts were advised to start wrapping everything up.
"OK, guys, come back nice and steady," Timothy Kopra called from inside the shuttle-station complex. "Nice and slow," one of the spacewalkers repeated to himself as he traversed the 150 feet (45 meters) from the work site to the hatch. They said they were both a little tired.
"And, guys, the boss says that you've earned a day off tomorrow," Kopra said. Their reply: "Wow!"
Two of the four new batteries assigned to Cassidy and Marshburn should have been installed Wednesday. Only two fresh batteries were hooked up during that shortened spacewalk.
The extra work pushed the length of Friday's spacewalk past the usual six and a half hours.
The six new batteries — 3-foot-square (1-meter-square) bundles weighing about 370 pounds (168 kilograms) each — are designed to store power collected by the solar wings on the far left end of the space station. Each one costs $3.6 million.
The old batteries will be returned to Earth aboard Endeavour at the end of next week.
Cassidy — an explosives and combat expert who went into Afghanistan two weeks after the Sept. 11, 2001, terrorist attacks — said in a series of TV interviews Thursday that he experienced no symptoms of carbon dioxide buildup and was never worried.
Back at the launch site, meanwhile, NASA conducted more testing on the fuel tank that will be used for Discovery's launch at the end of August. Shuttle managers want to be absolutely certain that the foam insulation on the central area of the tank is attached properly.
During Endeavour's liftoff on July 15, an unusually large amount of foam broke off and a few pieces struck the shuttle, causing minor damage.