WASHINGTON — Two astronauts will make a precedent-setting spacewalk Saturday to try to fix an ammonia leak in the power system of the International Space Station.
Spacewalks are rarely done on such short notice, but the space agency says the six-member crew is not in danger.
The ammonia leak was discovered Thursday and forced the shutdown of one of eight solar panels that power the station, but the outpost can operate fine with only seven, spokesman Kelly Humphries said.
One of the spacewalk veterans slated for the job is due to return to Earth on Monday, one of the reasons NASA wants to tackle the problem this weekend, he said.
Station Commander Chris Hadfield of Canada told NASA flight controllers Friday that the six-member crew is completely ready for the spacewalk.
"I think it's really smart the way we're all proceeding here," Hadfield radioed down to Earth. "It's the right thing to do."
Hadfield tweeted that the crew was working "like clockwork" and said the two astronauts were already getting their spacesuits ready, adding "Cool!"
The leak is in one of the radiator lines that chill the power systems. NASA spokesman Rob Navias said the line was expected to run out of ammonia coolant Friday. Power has been rerouted and is operating normally, he said.
NASA suspects the leak might be on the far left truss of the station from a pump box, which will be swapped out with a nearby spare during the spacewalk.
"What's causing the leak is unknown because there's a lot of plumbing underneath the box itself," he said. "We've had lots of experience in installing and replacing coolant loop hardware."
U.S. astronauts Chris Cassidy and Tom Marshburn will do the six-hour spacewalk. The repair is what NASA calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts train for in advance, Navias said.
In 2009, Cassidy and Marshburn flew to the space station on the shuttle Endeavour and walked in space together to swap out a battery in the same location, so "they know this work site inside and out," Navias said.
Marshburn, Hadfield and Russia's Roman Romanenko are set to return to Earth on Monday. NASA Administrator Charles Bolden told reporters in Virginia on Friday that their return will go ahead as planned, leaving three astronauts remaining.
Another reason to do the repair quickly: There may be some ammonia left which will help the spacewalkers find the leak, which is generating visible white flakes, Humphries said.
Last fall, station instruments revealed a radiator leak that was so small that it wasn't visible. It was in the same general area, but NASA isn't sure if it is the same leak or not, he said.
In November, two other spacewalking astronauts tried to reroute coolant lines to bypass the tiny leak but it wasn't successful, he said.
Seth Borenstein can be followed at http://twitter.com/borenbears. AP writer Brock Vergakis contributed to this report from Hampton, Va.