LIVE VIDEO Expedition 35 crew members Chris Cassidy and Tom Marshburn conduct a spacewalk on the International Space Station to inspect and possibly stop a leak of ammonia coolant.

By Alan Boyle, Science Editor, NBC News

Two NASA astronauts began a spacewalk aimed at troubleshooting an ammonia leak in the coolant system for one of the International Space Station's massive solar arrays on Saturday, just two days after the problem was detected.

The 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 are due 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 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.

NASA is trying to fix an ammonia leak on the International Space Station. NBC's Brian Williams reports.

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.

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Spacewalkers head out to fix space station's coolant leak – if they can