Yesterday, astronauts Chris Cassidy and Tom Marshburn have carried out a spacewalk – in technical terms an extravehicular activity (EVA) – to try to fix a leak of ammonia, used as a refrigerant for the solar panels power lines of the International Space Station.

It wasn’t the first time that there was a leak on the same circuit but until last week it was at a minimum level but last year the problem got worse. A first spacewalk performed on November 1, 2012 by astronauts Sunita Williams and Akihiko Hoshide wasn’t sufficient and a new repair had already been scheduled. In recent days, however, the loss suddenly increased, forcing NASA to set up a new repair as soon as possible.

In space it’s complicated to precisely locate the faulty element but in the end the suspect was the Pump Flow Control Subassembly (PFCS) of the channel 2B of the Photovoltaic Thermal Control System (PVTCS). It was therefore decided to replace it with one of the spare parts available during a spacewalk performed by flight engineers Chris Cassidy and Tom Marshburn.

Any operation performed in space, meaning in a spacesuit that’s not really comfortable to wear, is time-consuming. Yesterday’s spacewalk lasted about five and a half hours, also because after replacing the component suspected to cause the problem the two astronauts had to perform some other operations.

First of all, Chris Cassidy and Tom Marshburn waited for the pump to be put into operation to verify that the fault was really repaired. Once they made sure that there were no further leaks, they had to carefully clean up their spacesuits to eliminate possible traces of ammonia which might have got attached to them.

The checks on the pump will continue over the next few days to make sure that the fault has been really fixed. Astronaut Tom Marshburn should return to Earth tomorrow but his trip could be postponed for he to be still available for possible more fix jobs. On May 28, three new astronauts should leave for the International Space Station but at this point we need to see if the the schedules will be modified to keep the situation under control.

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