Two astronauts removed an ammonia pump on Saturday from the outside of the International Space Station as they began trying to fix a faulty cooling system.

Americans Rick Mastracchio and Michael Hopkins spent five and a half hours outside the station to detach the 350-kilogram pump that malfunctioned December 11, the US space agency said.

NASA postponed a second spacewalk to instal the new pump until Tuesday, from the original plan for Monday.

The one-day postponement was to allow time to resize a new spacesuit for Mastracchio. When he entered the repressurisation chamber after Saturday’s spacewalk, water had pushed into part of his suit, NASA said on its website.

NASA emphasised that neither astronaut was in danger during the spacewalk.

Saturday’s spacewalk, which was completed about 30 minutes earlier than expected, was Mastracchio’s seventh but the first for Hopkins.

Mastracchio has now logged 44 hours on spacewalks.

It is the first spacewalk by NASA since July when there was a problem with a space helmet worn by Italian astronaut Luca Parmitano.

The fault with the cooling system led the six-man crew to switch off all equipment deemed non-essential, halting dozens of scientific experiments.

The crew on board are said to be in no danger.

The Russian side of the space station has a separate cooling system.

The station, a project involving 15 countries, orbits at more than 400 kilometres above the Earth.

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