Two NASA spacewalkers are troubleshooting an ammonia leak in the coolant system for one of the International Space Station's massive solar arrays, just two days after the problem was detected.

Saturday's operation ranks as one of the fastest turnarounds ever for a space station repair — a feat that impressed the orbital outpost's Canadian commander, Chris Hadfield. "The whole team is ticking like clockwork. ... I am so proud to be commander of this crew," he wrote in a Twitter update. "Such great, capable, fun people."

Hadfield is serving as the inside man for the spacewalk, and will help make sure that NASA spacewalkers Tom Marshburn and Chris Cassidy stay on track during an outing that's expected to last six and a half hours.

The spacewalk got underway at 8:44 a.m. ET, about a half-hour later than originally planned, but the two veteran spacefliers made quick progress — so quick that Mission Control had to remind them to stop and do safety checks on their spacesuits. "You guys got to the worksite a little faster than we were keeping up with," said Mike Fincke, an astronaut who was guiding the duo from NASA's Johnson Space Center in Houston.

Marshburn and Cassidy went out to check a 260-pound (118-kilogram) pump box that's thought to be the source of the leak, and possibly replace it with a spare. The spacewalkers didn't immediately see anything amiss, other than some brownish markings on the box. "I see nothing off-nominal," Cassidy said.

The procedure is considered one of the "Big 12" spacewalk tasks for long-term station maintenance, and the astronauts were trained to do the swap before they launched. Nevertheless, the two-day turnaround is "precedent-setting" for space station operations, said Norm Knight, NASA's chief flight director.

Station crew members alerted Mission Control to the leak on Thursday when they saw "snowflakes" of frozen ammonia floating away from an area around the pump box. That area had been losing coolant at the rate of about 5 pounds (2.27 kilograms) per year, said Mike Suffredini, NASA's space station program manager. On Thursday, the rate jumped to 5 pounds a day.

Suffredini said the leak isn't putting the crew in any danger, and the station could manage without that particular coolant systems if it had to. The system services only one of the station's eight 112-foot-long (34-meter-long) solar arrays. For the time being, power from that array is being routed through the station's other electrical channels, Suffredini said.

But if the problem isn't resolved, it could complicate long-term planning for operations aboard the station, particularly if another power channel goes out. The spacewalk provides an opportunity to find out if a balky box is actually the source of the leak. If it isn't, it would "take us quite a bit of time ... to figure out where this leak could be," Suffredini said.

Marshburn and Cassidy are experienced spacewalkers — in fact, they worked together during a 2009 outing to install fresh batteries at the same location, on the station's P6 truss.

Marshburn and Hadfield are scheduled to fly back to Earth with Russian cosmonaut Roman Romanenko on Monday, closing out a five-month tour of duty in orbit. Suffredini said that plan would go forward even if the leak couldn't be fixed during Saturday's spacewalk.

Their departure would leave Cassidy on the station, along with Russia's Pavel Vinogradov and Alexander Misurkin. Three fresh spacefliers — NASA's Karen Nyberg, Russia's Fyodor Yurchikhin and Italy's Luca Parmitano — are scheduled to lift off from the Baikonur Cosmodrome in Kazakhstan on May 28 and join them.