Spacewalk Saturday to Fix Broken ISS Cooling System

Astronauts aboard the International Space Station, or ISS, are readying for a spacewalk Saturday to replace a failed cooling pump. A second spacewalk to complete the repairs is planned for next week.

Members of the International Space Station's mission management team unanimously agreed Friday that astronauts are ready for Saturday's spacewalk. They will replace a cooling system pump in the station's truss that failed July 31.

The faulty pump is supposed to feed ammonia into cooling loops to maintain the proper temperature for the station's electrical and flight systems. When the pump failed, it shut down half of the space station's cooling system. Astronauts had to switch off some equipment to reduce the amount of heat on board the ISS.

NASA says astronauts will focus Saturday on removing the faulty 350-kilogram module that is about two meters long, one meter tall and more than a meter wide. They will also move one of four spare modules into place. During a second spacewalk, planned for Wednesday, they will connect lines carrying ammonia fluid.

Flight Director Courtenay McMillan told reporters earlier this week that astronauts will face multiple challenges during the repair mission. McMillan is on the ground, where she will lead the team supporting the spacewalk, which, In space terminology, is called "extravehicular activity" or an EVA.

"The most challenging in there is the ammonia quick-disconnect connections that the crew is going to have to release," said Courtenay McMillan. "Since the external loop uses ammonia as the cooling fluid, all of the lines are pressurized with ammonia and have to be released by the crew during the EVA."

Concentrated forms of ammonia are hazardous and corrosive.

"The crew is very well trained on how to do decontamination if they get ammonia on them during the procedures, but it presents a timeline challenge to make sure we have enough room in the timeline to account for that," said McMillan.

Astronauts Doug Wheelock and Tracy Caldwell Dyson will perform Saturday's spacewalk, while astronaut Shannon Walker will be inside the ISS controlling the station's robotic arm.

NASA has twice delayed the spacewalks after reviewing proposed timelines and tasks. Astronauts at the Neutral Buoyancy Lab at the Johnson Space Center helped develop the timeline this week. The astronauts practiced the pump removal and installation task on a mock module, while wearing space suits, underwater, in a swimming pool - mimicking circumstances similar to those in orbit.