SEATTLE — Two spacewalking astronauts troubleshot an ammonia leak in the international space station’s cooling system Nov. 1, accomplishing the chief objective of their marathon excursion outside the orbiting lab.

NASA astronaut Sunita Williams and Japan’s Akihiko Hoshide ventured outside the space station at 8:29 a.m. EDT. A little more than five hours later, they had reconfigured some coolant lines and deployed a spare radiator, isolating the leak.

“Suni and Aki, heartfelt congratulations to you,” NASA astronaut Mike Fincke, who helped walk the astronauts through their tasks from mission control at the agency’s Johnson Space Center in Houston, said at 1:41 p.m. EDT. “We accomplished just about everything we set out to do today.”

“A big, huge congratulations to you guys on the ground for putting this together,” Williams responded.

The spacewalk was slated to last about six-and-a-half hours, and it proceeded pretty much on schedule. By 2:58 p.m. EDT both astronauts had re-entered the space station through its airlock, and the spacewalk was officially over nine minutes later.

Each of the space station’s eight huge solar panels has its own associated power system, and circulating ammonia helps cool this gear down. The leak, which NASA and its space station partners first noticed in 2007, has the potential to affect one of these power channels if it is not fixed.

At the start of the spacewalk, Williams — commander of the station’s current Expedition 33 — and Hoshide made their way over to the port side of the station’s backbone-like truss. They reconfigured some lines in the affected coolant system and installed a spare radiator.

NASA officials hope this stops the leak, which they suspect may have been coming from the old radiator.

“We’re real suspicious of the radiator,” spacewalk flight director Mike Lammers of Johnson Space Center told reporters Oct. 26 during a pre-spacewalk press conference. “It kind of stretches out there and is susceptible to micrometeorite impacts.”

Because the leak is so slow, it will likely be several weeks before it is known if the fix worked, officials have said.

If ammonia continues to escape, the station’s operators will eventually have to try something else. For example, astronauts may need to swap out some of the coolant system’s pump gear on another spacewalk. But there likely will not be an urgent need for a new fix, since the spare radiator should provide enough ammonia to keep the coolant system operating until next October or so, officials said.

Williams and Hoshide, both seasoned spacewalkers, ticked off their tasks efficiently. In late August and early September, they replaced a vital power unit on the station over the course of two spacewalks, defeating an unexpectedly stubborn stuck bolt in the process.

The Nov. 1 spacewalk was the seventh for Williams, the third for Hoshide and the 138th to depart from the station overall.