Astronauts take a walk to repair space station's AC

Astronauts aboard the International Space Station are on a space walk Wednesday to make repairs on the space station's cooling system

In this image taken from video and made available by NASA, astronaut Douglas Wheelock, works on the International Space Station attempting urgent repairs to restore a crucial cooling system on Wednesday. The International Space Station has been operating with only half its usual cooling capability ever since an ammonia pump failed one-and-a-half weeks ago.

A pair of astronauts floated outside the International Space Station Wednesday to once again try to remove a faulty – but crucial – coolant pump during the second in a series of complicated spacewalks to replace the device.

NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson ventured outside the space station at 8:27 a.m. EDT (1227 GMT) on what they expect to be a six-hour job to remove the disabled pump, which is about the size of a kitchen oven.

"Sunlight – sunlight coming out," Wheelock said as he looked out of the station's open hatch just before stepping out to begin the spacewalk.

"Wow, that's nice and bright," Dyson responded.

"Beautiful," Wheelock said.

The astronauts tried to remove the ammonia coolant pump during a Saturday spacewalk, but a jammed hose – and later an unacceptable ammonia leak – stalled their efforts. [Graphic: Space Station's Cooling System Problem Explained]

During this spacewalk, Wheelock and Caldwell Dyson will close two valves in hoses upstream of the pump in an attempt to stop the leak. They'll also vent any remaining ammonia coolant overboard before removing the last of four ammonia hoses from the faulty pump.

The International Space Station uses liquid ammonia to cool its onboard systems by transporting waste heat to a network of radiators mounted to its main truss.

Spacewalking astronauts take care to avoid exposure to the ammonia while working outside and have lengthy decontamination protocols to clean their spacesuits when they do see leaks in order to avoid bringing the toxic chemical inside.

The faulty ammonia pump failed July 31, knocking out half of the space station's cooling system and forcing astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating. A tripped circuit breaker, likely caused by a power spike, caused the malfunction, station managers have said.

There are two main cooling system loops – Loop A and Loop B. The failed pump is in Loop A, while the other cooling loop remains operational.

Wheelock and Caldwell Tyson don't plan on completing their space station repairs during Wednesday's spacewalk. That comes on Sunday, when the astronauts plan to perform a third spacewalk to retrieve a spare pump from a storage platform and install it in the old pump's place, station officials have said.

The failed ammonia pump is located on the station's right side truss and will be replaced with one of four spare pumps stored at the orbiting lab.

Each pump weighs 780 pounds (353 kg) and is 5 1/2 feet long (1.6 meters) by 4 feet wide (1.2 meters). They are about 3 feet (almost 1 meter) tall.

NASA is broadcasting the International Space Station spacewalk repairs live from space on NASA TV. Click here for space station mission updates and's NASA TV feed.

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