This story was updated at 8:40 a.m. EDT.

Two astronauts are floating outside the International Space Station today (Nov. 1) in an effort to isolate and repair an ammonia leak in part of the orbiting lab's cooling system.

NASA astronaut Sunita Williams and Japanese spaceflyer Akihiko Hoshide officially begun the spacewalk also known as an extra-vehicular activity, or EVA this morning at 8:29 a.m. EDT (1229 GMT). They're expected to stay outside for about 6 1/2 hours, NASA officials said. You can watch live coverage of the spacewalk here on NASA TV.

"Aki, come on in. The water is warm," Williams called to Hoshide as he floated outside the space station's airlock.

Each of the orbiting lab's eight huge solar arrays has its own associated power system, and circulating ammonia helps cool this gear down. The spacewalk's first objective is to find the source of the leak, which could affect one of these power channels if it's not addressed soon.

"We don't know exactly where the leak is," space station manager Mike Suffredini of NASA told reporters during a teleconference last Friday (Oct. 26). "It's possible the leak is in the PVR itself, the [photo-voltaic] radiator itself. It could be in the pump system, or it could be in any one of the [coolant] lines." [Gallery: Building the International Space Station]

On Thursday's EVA, Williams commander of the station's current Expedition 33 mission and Hoshide will make their way over to the port side of the station's backbone-like truss. They'll reconfigure some lines in the affected coolant system and install a spare radiator to see if that stops the leak.

"We're real suspicious of the radiator," said spacewalk flight director Mike Lammers, of NASA's Johnson Space Center in Houston. "It kind of stretches out there and is susceptible to micrometeorite impacts."

Because the leak is so slow, it will likely take several weeks before it's known if the fix works, officials said.

If ammonia continues to escape, the station's operators will eventually have to try something else. For example, they may direct astronauts to swap out some of the coolant system's pump gear on another spacewalk. But there likely won't 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.

Read the rest here:

Astronauts Begin Spacewalk to Fix Space Station Cooling System