NASA astronauts on spacewalk to fix ammonia leak

WASHINGTON: 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.

The spacewalk was successfully completed an hour ahead of schedule at 1814 GMT, five and a half hours after flight engineers Tom Marshburn and Chris Cassidy ventured outside the space station.

During the maneuver, Marshburn and Cassidy attached a spare pump and flow control sub assembly box to replace the one 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, and after about 30 minutes of careful observation by the astronauts and mission control, ISS commander Canadian Chris Hadfield tweeted: "No leaks! We're bringing Tom & Chris back inside."

Although the pump will continue to be observed through instruments, the early indications were positive that "we have climbed a big mountain on solving the ammonia leak," a commentator at mission control said on NASA television.

But he cautioned that it will take several months of investigation and observation before it can be confirmed that the pump was "the smoking gun" that caused the leak.

Before re-entering the space station, the astronauts took precautions against ammonia contamination, despite not seeing any evidence of it in the area around the space station.

That included an hour-long "bake-off" to allow the sun to burn off any ammonia traces on their space suits.

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.

<Center><b><i>Copyright AFP (Agence France-Presse), 2013</b></i></center>