The US space agency has ordered a series of emergency spacewalks to carry out repairs on a broken cooling line at the International Space Station.
Nasa said the work to replace a faulty pump module would require two astronauts and three spacewalks - the first of which is planned for Saturday.
Half of the cooling system automatically shut down last week after detecting abnormal temperatures.
Nasa said the situation was potentially serious but not life-threatening.
The pump is associated with one of the station's two external cooling loops, Nasa said in a statement.
The two external cooling loops 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.
Each spacewalk is scheduled to last six-and-a-half hours and will be broadcast on Nasa TV.
Nasa said last week that it suspected a malfunction had affected a flow control valve inside the pump itself.
The teams moved certain electrical systems over to the second loop, but the space agency stressed that the crew and the station were in no danger.
Some non-critical systems were powered down inside the Harmony node, the Kibo laboratory and the Columbus laboratory whilst engineers worked to figure out what had caused the problem and how to fix it.