The breakdown 10 days earlier left one of two identical cooling loops too cold and forced the astronauts to turn off all nonessential equipment inside the orbiting lab, bringing scientific research to a near-halt and leaving the station in a vulnerable state.
Mission Control wanted to keep the spacewalkers out even longer Saturday to get even further ahead, but a cold and uncomfortable Mastracchio requested to go back. The spacewalk ended after 5½ hours, an hour short on time but satisfyingly long on content.
Earlier, Mastracchio managed to unhook all the ammonia fluid and electrical lines on the pump with relative ease, occasionally releasing a flurry of frozen ammonia flakes that brushed against his suit. A small O-ring floated away, but he managed to retrieve it.
"I got it, I got it, I got it. Barely," Mastracchio said as he stretched out his hand.
"Don't let that go, that's a stocking stuffer," Mission Control replied.
"Don't tell my wife," Mastracchio said, chuckling, as he put it in a small pouch for trash.
Mastracchio, a seven-time spacewalker, and Hopkins, making his first, wore extra safety gear as they worked outside. NASA wanted to prevent a recurrence of the helmet flooding that nearly drowned an astronaut last summer, so Saturday's spacewalkers had snorkels in their suits and water-absorbent pads in their helmets.
To everyone's relief, the spacewalkers remained dry. But midway through the excursion, Mastracchio's toes were so cold that he had to crank up the heat in his boots. Mission Control worried aloud whether it was wise to extend the spacewalk to get ahead, given Mastracchio's discomfort.
Not quite two hours later, Mastracchio had enough as he clutched the old pump. When Mission Control suggested even more get-ahead chores, he replied, "I'd like to stow this old module and kind of clean up and call it a day." He said a couple of things were bothering him, not just temperature, and declined to elaborate when asked by Mission Control what was wrong.
Flight controllers obliged him. Once the old pump was secured to a temporary location, the spacewalkers started gathering up their tools to go in.
Adding to the excitement 260 miles up, a smoke alarm went off in the space station as the astronauts toiled outside. It was quickly found to be a false alarm.
The pump replacement is a huge undertaking attempted only once before, back in 2010 on this very unit. The two astronauts who tackled the job three years ago were in Mission Control, offering guidance. Mastracchio promised to bring back a wire tie installed on the pump by the previous spacewalkers. "Oh, awesome, thanks Rick," replied the astronaut in Mission Control who put it on.
The 780-pound pump is about the size of a double-door refrigerator and extremely cumbersome to handle, with plumbing full of toxic ammonia. Any traces of ammonia on the spacesuits were dissipated before the astronauts went back inside, to avoid further contamination.
NASA's plan initially called for the pump to be disconnected Saturday, pulled out Monday and a fresh spare put in, and then all the hookups of the new pump completed Wednesday in what would be the first Christmas spacewalk ever. But if the work is finished Monday, a third spacewalk will be unnecessary and most everybody on the team will have the holiday off.