MIAMI (AFP) - Two US astronauts stepped out on a risky spacewalk on Friday to complete the repair of an ammonia cooling system at the International Space Station (ISS).
Highly toxic ammonia is used to cool electronics at the orbiting outpost, and the thermal system has been plagued by problems.
Friday's outing aims to complete the final repairs to a system that broke down about three years ago.
Scott Kelly and Kjell Lindgren switched their spacesuits to battery power at 6.22am (7.22pm Singapore time) and floated outside the orbiting outpost moments later, according to a live broadcast on Nasa television.
The goal for Friday's 6.5 hour spacewalk is to "restore the port truss ammonia cooling system to its original configuration," the space agency said in a statement.
In 2012, Nasa detected a major leak in the ammonia cooling system.
Astronauts replaced the ammonia pump on the station's truss in May 2013.
The spacewalkers reported seeing some flakes of ammonia when disconnecting some of the equipment, but the crew was in no danger, said Nasa commentator Rob Navias.
About three and a half hours into the spacewalk, they finished topping off ammonia in the system with about 3.4kg of liquid, Nasa said.
The operation was a "perfect fill," Navias said.
Had any of the ammonia made its way onto the astronauts' spacesuits, they would have had to stay outside the airlock longer than planned to allow it to bake off.
In the past, following ammonia leaks outside the space station, astronauts have stayed in the sun for about an extra half-hour to allow any frozen crystals to vaporise from their spacesuits so as not to bring the toxic substance inside the station.
Friday's spacewalk is the 190th in support of the International Space Station, which just marked 15 continuous years of human habitation.
Kelly and Lindgren did another spacewalk nine days ago to perform maintenance and upgrades outside the ISS.
Kelly is more than halfway through a year-long mission at the space station that aims to help Nasa study the effects of long-term spaceflight on the body and mind.
He is scheduled to return to Earth in March 2016.