Astronauts in orbit and on the ground have been practicing for a major repair job due to take place later at the International Space Station.
A weekend malfunction knocked out half of the space station's cooling system, forcing the crew of six to turn off unnecessary equipment and halt scientific work to avoid any overheating.
NASA's space station programme manager, Mike Suffredini, ranked the problem as one of the most serious in the 12-year history of the orbiting lab, but stressed the outpost could keep going indefinitely given the current situation.
The fear is that the second cooling loop could shut down at any moment and leave the station in precarious shape.
Two of the Americans on board, Douglas Wheelock and Tracy Caldwell Dyson, will venture out on a space walk to replace the pump later and a second space walk will be needed to finish the job, most likely on Sunday.
The 350 kg pump is difficult to handle and the astronauts will need to guard against any hazardous ammonia leaks.
Engineering teams have been working non-stop since the right-side cooling loop shut down on Saturday night. A pump that drives ammonia coolant through those lines failed when a circuit breaker tripped.
A pair of astronauts in Houston took to a giant swimming pool on Monday afternoon to rehearse the repair procedures.