CAPE CANAVERAL, Fla. — Space station astronauts this morning tackled the last of their urgent cooling-system repair work, making their third spacewalk in just more than a week.

Douglas Wheelock and Tracy Caldwell Dyson were so anxious to wrap up the job they floated out more than a half-hour early, as the International Space Station circled 220 miles 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 today, 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.

The bathtub-size pumps — weighing 780 pounds apiece — are needed to drive ammonia through cooling loops and keep equipment from overheating. Four spare pumps are on board; the one being installed today was the oldest of the bunch. It flew up in 2006.

By The Associated Press