Astronauts replace pump on emergency spacewalk

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Two US astronauts have replaced a pump on a spacewalk aimed at fixing a leak of ammonia from the International Space Station's cooling system.

Chris Cassidy and Tom Marshburn completed the work an hour ahead of schedule, reporting no further escape.

The crew had spotted particles of ammonia drifting away from the laboratory on Thursday.

Nasa said the crew were not at risk but managers wanted to solve the problem before Mr Marshburn left the station.

He is due to return to Earth early next week along with the space station's Canadian commander Chris Hadfield and Russian cosmonaut Roman Romanenko, after six months in space.

Liquid ammonia is used to extract the heat that builds up in electronic systems, dumping that excess energy to space through an array of radiators.

Radiators Ammonia flows through radiators (white panels) to dump heat into space
'No leaks!'

"No leaks! We're bringing Tom & Chris back inside," Mr Hadfield wrote on his Twitter account, some four and a half hours into the spacewalk.

The leak was coming from the station's port side, at the far end of the backbone, or truss, structure that holds one of the laboratory's huge sets of solar arrays.

Mr Hadfield reported seeing "a very steady stream of flakes" on Thursday.

It is not the first time that the station's cooling systems have caused problems.

A very small leak was identified in 2007 in the same location, and a spacewalk was organised in 2012 to reconfigure coolant lines and isolate the problem.

While the crew may have been safe, damage to the power system from the leak could affect the station's scientific work.

The station currently has a crew of six.

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