Americans Chris Cassidy and Tom Marshburn ventured out into space and travelled 45 metres to the worksite shortly after the hatch of the ISS opened at 8:45 a.m. ET.
ISS commander Canadian Chris Hadfield was assigned to oversee the mission. It was due to end at 3 p.m. ET, but three hours into the job, NASA said it was "running over an hour ahead of the timeline."
Hadfield greeted followers of his Twitter feed early Saturday morning with news of preparations for the spacewalk, saying it will be a "complex and vital" day for the crew.
Early in the mission, NASA reported the men spotted nothing unusual at the pump and flow subassembly box on the station's P6 truss, which was suspected of leaking ammonia coolant. A few hours later, they replaced the pump package as a precaution.
"All the pipes look shiny clean, no crud," Cassidy reported as he filmed the mission with a helmet camera.
"I can't give you any good data other than nominal, unfortunately. No smoking guns."
- Read how Chris Hadfield has prepped to handle incidents like this one
ISS manager Mike Suffredini on Friday said the spacewalk's main goal was to locate the source of the leak, but about an hour and a half into the spacewalk, the astronauts reported they could see no new ammonia flakes at the pump.
NASA spokesman Norm Knight called the hastily arranged walk "precedent-setting" in terms of its quick turnaround. Between detecting the leak on Thursday and stepping outside Saturday morning, it's the most abrupt ISS spacewalk ever orchestrated.
There are concerns the leak could affect the ability of the ISS to cool the solar panels that power the station. However, NASA says the station has plenty of power, and the six-man crew is not in danger.
Asked to assess the seriousness of the problem, Suffredini characterized the leak as an "annoyance," strictly due to the time and manpower involved in fixing it. He said the cause of the leak remains unclear, but a visual inspection should provide answers, including the possibility of a meteorite strike.
Small white flakes spotted
Hadfield had contacted NASA Mission Control at 11:30 a.m. ET Thursday after crew reported seeing a "very steady stream" of small white flakes floating away from a particular area of the station. A few hours later, he tweeted that it was "a serious situation, but between crew and experts on the ground it has been stabilized."
The crew's reports, along with related imagery and space station data, confirmed that ammonia, which is used to cool the power channels that provide electricity to the station, was leaking more and more quickly from the area of the space station where the flakes were spotted.
NASA said the affected cooling loop, which is linked to a particular solar array, is the same one that spacewalkers tried to troubleshoot during a spacewalk on Nov. 1, 2012. It wasn't known, however, whether the leak — which wasn't visible in November — was the same one. NASA said work was underway Friday to reroute power channels to maintain full operation of systems normally powered by the affected solar array.
The P6 truss area has had a small leak for several years, but it has proved too difficult to detect.
"You're talking a very, very, very small hole," Suffredini said.
Spacewalks usually aren't done on such short notice, but NASA wanted to try to fix the leak before all the ammonia escaped.
Hadfield coming home on schedule
Hadfield, who took command in March, is the first Canadian to oversee the crew of the space station. The ammonia leak is the first notable glitch at the space station since he took command, and comes just days before he is scheduled to return to Earth. He seemed keen to meet the challenge.
- Chris Hadfield's view of Canada
"What a fun day!" he tweeted Friday afternoon. "This type of event is what the years of training were for. A happy, busy crew, working hard, loving life in space."
Hadfield, Marshburn and Russian cosmonaut Roman Romanenko are scheduled to undock their Soyuz TMA-07M spacecraft at 7:08 p.m. ET Monday and land in Kazakhstan at 10:31 p.m.
Cassidy and Marshburn have each performed three previous spacewalks. Hadfield performed two spacewalks in 2001 to install the space station's Canadarm2 robotic arm.