US astronauts dodge ammonia on risky spacewalk

Two US astronauts successfully dodged hazardous ammonia flakes during a risky spacewalk Friday to complete the repair of a cooling system at the International Space Station.

Highly toxic ammonia is used to cool electronics at the orbiting outpost, and the thermal system has been plagued by problems.

Scott Kelly and Kjell Lindgren switched their spacesuits to battery power at 1122 GMT 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 was to complete the final repairs to a system that broke down about three years ago, by restoring the external ammonia cooling system to its original configuration, the space agency said.

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.

Early in the outing, the spacewalkers reported seeing flakes of ammonia when disconnecting some of the equipment, but the crew was not in any danger, said NASA commentator Rob Navias.

Just after midway into the spacewalk, they finished topping off ammonia in the system with about 7.5 pounds (3.4 kilograms) each into a primary and a backup tank, NASA said.

The operation was a "perfect fill," Navias said.

Later, the pair worked together to vent off residual ammonia from a plumbing line.

After opening the valve, they moved out of the way and inspected each other's suits for any signs of ammonia flakes.

They found none.

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 vaporize 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 went on 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 space flight on the body and mind.

He is scheduled to return to Earth in March 2016.