<p><br /> Astronauts in orbit and on the ground have practiced for a major repair job later this week at the International Space Station.<br /> </p><p><br /> 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.<br /> </p><p><br /> 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.<br /> </p><p><br /> The fear is that the second cooling loop could shut down at any moment and leave the station in precarious shape.<br /> </p><p><br /> For now, "everything the crew needs to survive, they're in good shape, all those systems are active," Suffredini told reporters. "What we're talking about really, it would be a significant challenge if we suffer the next failure."<br /> </p><p><br /> Two of the Americans on board, Douglas Wheelock and Tracy Caldwell Dyson, will venture out on a space walk to replace the pump on Thursday. A second space walk will be needed to finish the job, most likely on Sunday.<br /> </p><p><br /> The 780-pound (350-kilogram) pump is difficult to handle, and the astronauts will need to guard against any hazardous ammonia leaks.<br /> </p><p><br /> 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.<br /> </p><p><br /> A pair of astronauts in Houston took to a giant swimming pool on Monday afternoon to rehearse the repair procedures, with another practice session due to be carried out later.<br /> </p>
