NASA astronauts on Saturday replaced a pump during an emergency spacewalk to stop an ammonia leak at the International Space Station's power system, NASA television showed.
About three hours into the spacewalk, which was expected to last six and a half hours, flight engineers Tom Marshburn and Chris Cassidy had completed attaching a "spare pump and flow control sub assembly box," said a NASA commentator from mission control.
The old box, which they had just removed, was suspected to be the source of an ammonia leak that affected the US segment of the orbiting laboratory on Thursday. Ammonia is used to cool the station's power system.
The new pump was turned on about an hour later, as ISS commander Canadian Chris Hadfield tweeted: "Houston just sent the command to start flowing ammonia through the newly-installed pump. Gloved fingers crossed."
After several minutes of observation, the astronauts reported no initial evidence of any ammonia leakage.
"Everything is looking good internally," added a NASA commentator from mission control, recapping reports from a thermal engineer.
The astronauts were more than an hour ahead of schedule and "doing extremely well," he had said earlier.
Despite not seeing any ammonia, the astronauts had said they would take all precautions against contamination in the area around the space station.
That included a "bake-off" to allow the sun to burn off ammonia traces on their space suits before returning into the space station.
Officials said the emergency spacewalk set a precedent because it was conducted at such short notice.
It was the 168th excursion in support of the orbiting laboratory and the fourth for both Marshburn and Cassidy, who have worked together before.
Both US and Russian officials stressed that spacewalks are usually taxing tasks involving ISS crews and control mission on the grounds.
Hadfield, overseeing the mission, tweeted that it is "a workout" to wear a spacesuit which the Russian Space Agency noted weighs more than 100 kilograms.
"The reason they regularly check their gloves is for damage. Even though multi-layer, even a tiny leak requires immediate haste to airlock," Hadfield said on Twitter.
"After each such sortie guys come back like they've been through a good battle, with bruised hands and grazed shoulders," the Russian space agency quoted Vladimir Solovyov, flight director for the Russian segment of the space station, as saying in a statement.
NASA has stressed that the lives of the multinational crew were not in danger, but both Russian and US space experts called the leak "serious."
NASA said ammonia was leaking from the same general area as in a previous episode in November last year.
A meteorite or a piece of orbital debris is suspected to have hit the cooling radiator and caused the problem, which International Space Station program manager Michael Suffredini described as an "annoyance because of all the work we have to do to work around the problem."
The issue took a turn for the worse on Thursday when it began leaking about five pounds of ammonia per day, compared with a previous level of five pounds per year.
Hadfield earlier described the leak as a "very steady stream of flakes or bits" of ammonia drifting into space.
The flakes were moving "evenly and repeatedly enough that it looks like they were coming from a point source," he said in a recording of the conversation posted by NASA.
Hadfield, Marshburn and Russian cosmonaut Roman Romanenko are set to return to Earth early on Tuesday after completing their half-year stint aboard the station.
Cassidy is also set to perform two scheduled spacewalks in July, NASA said.
Saturday's six-and-a-half-hour spacewalk will not interfere with their planned departure from the space station, NASA said.