Emergency spacewalk aims to fix ISS ammonia leak

Astronauts on the International Space Station (ISS) are due to carry out a spacewalk to fix a leak of ammonia from its cooling system.

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

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.

Nasa says the crew on the ISS are not in any danger.

It is rare for spacewalks to be performed at such short notice.

Astronauts Chris Cassidy and Tom Marshburn are due to begin carrying out the operation at 12:15 GMT. It is expected to last around six and a half hours.

The leak is 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. The astronauts will "inspect and possibly replace" a pump controller box in that area of the space station, according to Nasa.

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

"They were coming out cleanly and repeatedly enough that it looked like it was a point source they were coming from," he added.

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.

The station currently has a crew of six. Commander Hadfield, a Canadian, is due to leave the platform with American astronaut Tom Marshburn and Russian cosmonaut Roman Romanenko on Monday.

Cmdr Hadfield had asked mission controllers if the leak might prevent the undocking of his return capsule. They responded that there was no technical reason why it should, but that engineers would update the crew once they understood the issues more fully.