Space station astronauts tackled the last of their urgent cooling-system repair work Monday, making their third spacewalk in just over a week. Douglas Wheelock and Tracy Caldwell Dyson were so anxious to wrap up the job that they floated out more than a half-hour early, as the International Space Station circled 355 km above Earth. The orbiting lab has been operating on only half its normal cooling capability ever since a crucial ammonia coolant pump failed July 31. It took two spacewalks, but Wheelock and Caldwell Dyson finally removed the broken pump last week. Early Monday, the spacewalking team was trying to install a spare pump. If it succeeds, NASA expects to have the disabled cooling loop back in action by Thursday. NASA said a fourth spacewalk eventually will be needed to move the failed pump into a better storage location, but managers are uncertain whether this crew or another will carry out the work. The bathtub-size pumps – weighing 355 kg apiece – are needed to drive ammonia through cooling loops and keep equipment from overheating. Four spare pumps are on board; the one being installed Monday was the oldest of the bunch. It flew up in 2006.
