This story was updated at 6:31 p.m. ET.
Two spacewalking astronauts finally removed a faulty pump outside the International Space Station Wednesday after overcoming a stubborn ammonia hose during their second attempt in less than a week to repair the outpost's cooling system.
It came down to brute strength in the end for NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson to remove the balky ammonia hose, which had stuck fast and leaked toxic ammonia coolant during an earlier spacewalk.
With the hose out of the way, the spacewalkers were finally able to remove the oven-sized cooling system pump after a slight delay attaching a handle-like bar. The pump failed 11 days ago and knocked out half of the space station's cooling system. [Graphic: Space Station's Cooling System Problem Explained]
"This will allow us to get our station back," Wheelock said. "Back in action."
Wheelock had to shake a hose connector hard to pry it free from the broken pump that his helmet camera showed him swaying from side-to-side. But ultimately he disconnected the hose to the delight of the astronauts and Mission Control. He saw only a few flakes of ammonia while detaching the pump's final fluid hose Wednesday, far less than the snowstorm of frozen ammonia he reported seeing during the Saturday spacewalk.
The astronauts parked the broken pump at a storage point outside the space station after removing it. NASA plans one more spacewalk, currently scheduled for no earlier than Monday, to complete the ammonia pump repair.
"We still have a little ways to go, but today was a great day," Wheelock radioed Mission Control after the spacewalk, which lasted seven hours and 26 minutes.
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.
The faulty ammonia pump failed July 31, forcing astronauts to turn off some experiments and systems, as well as leave others without backups, in order to prevent the station from overheating. An internal electrical short tripped a circuit breaker to cause the malfunction, station managers have said.
Broken space pump removed
NASA station managers have said the ammonia pump failure is one of the most challenging repairs for the International Space Station. Engineers initially planned two spacewalks to replace the pump, but unexpected challenges on Saturday forced the addition of a third spacewalk.
Part of the difficulty lies in the sheer bulk of the pumps. 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. So the spacewalkers had to work together to move the bulky pump.
Caldwell Dyson, who helped her electrician father work while growing up, ?disconnected the pumps's five power and data cables. They gave her little trouble.
"My dad would be proud," she added.
"I'm just going to hold it as steady as I can," Wheelock told Caldwell Dyson as he grasped the pump from the tip of the station's robotic arm. Station astronaut Shannon Walker flew the arm for Wheelock from inside the station.
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. The spare pump that will replace it was arrived at the station in 2007.
But Wednesday's spacewalk appeared to go as planned, unlike the first repair attempt. Wheelock and Caldwell Dyson expected to have close two valves in hoses upstream of the pump and vent any remaining ammonia coolant overboard before removing the stuck ammonia hose, which was the last of four connected to the disabled pump. But that wasn't needed, Mission Control 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. It was delivered to the space station in 2002, but wasn't activated until 2006.
While the space station is currently stable, the U.S. segment is running with just half of its cooling system working until the new pump can be installed. If the working cooling loop fails, astronauts would have only a short time to finish the repairs, mission managers have said.
The space station's Russian segment has its own independent cooling system, but could only support the outpost's full six-person crew on its own for a few days before station managers would have to consider last-minute repairs or abandoning the station.
More repairs ahead
Wheelock and Caldwell Tyson didn't plan on completing their space station repairs during Wednesday's spacewalk. That comes on Monday, when the astronauts will 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.
Wednesday's spacewalk began at 8:27 a.m. EDT (1227 GMT) and the astronauts were consistently ahead of schedule.
"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," Caldwell Dyson responded.
"Beautiful," Wheelock said.
While Wednesday's spacewalk was long, it fell short of the record-setting spacewalk by Wheelock and Caldwell Dyson on Saturday. That spacewalk lasted eight hours and three minutes, making it the longest spacewalk ever performed from the space station.
Wednesday's spacewalk was the fifth for Wheelock and the second for Caldwell Dyson. It was the 149th dedicated to space station assembly and maintenance. Construction began on the $100 billion space station in 1998.
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 2011) before retiring its orbiter fleet for good next year. NASA would need approval from Congress and the White House to fly a potential third shuttle mission, which would launch 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 will broadcast the next International Space Station spacewalk repair on NASA TV no earlier than Monday, Aug. 16. Click here for space station mission updates and SPACE.com's NASA TV feed.