Space Station Astronauts Take 2nd Spacewalk to Make Repairs

This story was updated at 11:51 a.m. ET.

Two spacewalking astronauts floated outside the International Space Station Wednesday to take another shot at removing a faulty ? but crucial ? coolant pump, this time taking special steps to free a stuck hose and avoid the type of toxic ammonia leak that thwarted their initial attempt.

NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson ventured outside the space station on what they expect to be a six-hour job to remove the disabled pump, which is about the size of a kitchen oven.

About two hours into the spacewalk, they finally detached the stubborn ammonia hose that hampered their first repair attempts in a Saturday spacewalk. Wheelock had to use brute strength to loosen the hose, but ultimately detached it to the delight of the astronauts and Mission Control. With the hose out of the way, the spacewalkers finally pushed forward with the pump's removal.

"This will allow us to get our station back," Wheelock said. "Back in action."

The spacewalk began at 8:27 a.m. EDT (1227 GMT). It is the first of three aimed at replacing a faulty ammonia pump and restoring the space station's cooling system to full strength.

"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 the jammed hose ? and later an unacceptable ammonia leak ? stalled their efforts. [Graphic: Space Station's Cooling System  Problem Explained]

Wheelock and Caldwell Dyson did not have to close two valves in hoses upstream of the pump or 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 ammonia (which freezes into flakes when it leaks) 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.

Wheelock said he only saw a few flakes of ammonia while detaching the pump's final fluid hose Wednesday, far less than he saw during the Saturday spacewalk.

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.

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. 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.

Spare parts are a key concern for the International Space Station, which is now slated to keep flying through at least 2020. After next year, no NASA space shuttles will be flying to the space station to deliver large spare parts or supplies.

NASA plans to fly two more shuttle missions (in November and February) before retiring its orbiter fleet for good next year. NASA would need approval to fly a potential third shuttle mission, which would fly next summer if approved. Last week, the Senate passed a NASA authorization bill that would approve that extra shuttle flight, though the House vote on its version of the bill is still pending.

• Graphic: Inside and Out: The International Space Station
• FAQ: International Space Station Cooling System Pump Failure
• Record-Setting Spacewalk at Space Station Falls Short on Repairs

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

This report has been corrected to reflect that the spacewalkers did not have to vent ammonia or close extra valves for today's repair work.