NASA will decide Friday evening whether the spacewalk is needed Saturday. One of the spacewalk veterans slated for the job is due to return to Earth on Monday, one of the reasons NASA wants to do it 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 certain box, but isn't certain. There's a spare box right near it and spacewalking astronauts can swap it out if that's the source, Navias said.
"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."
If needed, U.S. astronauts Chris Cassidy and Tom Marshburn would make about a six-hour spacewalk. They have trained for this type of repair. It is what NASA calls one of the "Big 12" types of emergency repair work that all spacewalking astronauts prepare 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. Humphries said if a spacewalk is needed, NASA would like the experienced duo of Cassidy-Marshburn to do it. NASA Administrator Charles Bolden told reporters in Virginia on Friday that the return of the three astronauts will go ahead as planned Monday.
Another reason to do it quickly: There may be some residual ammonia left which will help the spacewalkers find the leak, which is generating visible white flakes. If they wait longer, it will be harder to find the leak if there is no more ammonia left to come out in 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.