Astronauts at the International Space Station have begun a spacewalk to carry out urgent repairs.
It is the first of three spacewalks needed to mend the station’s critical cooling system.
Half of the system automatically shut down last week after detecting abnormal temperatures.
Nasa said the situation was potentially serious but not life-threatening.
The six-man crew had to turn off all non-essential equipment because of the malfunction.
Their first task is to disconnect a faulty ammonia pump, which is about the size of a refrigerator.
On their second spacewalk, planned for next Monday, the astronauts will remove the pump so it can be replaced with a spare.
It is possible that a third expedition will be needed, on Christmas Day, to make final installations.
The fault relates to the external cooling loops that circulate ammonia outside the station to keep both internal and external equipment cool.
Nasa said the repairs would take priority over the launch of a supply ship from Virginia, which has now been postponed until January.