Record-Setting Spacewalk at Space Station Falls Short on Repairs

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Two astronauts took an extra-long spacewalk outside the International Space Station Saturday to repair a crucial cooling system, but ran out of time before they could replace a broken ammonia pump – their primary goal. The delays stretched the work out into the longest space station spacewalk in history, NASA said.

NASA astronauts Douglas Wheelock and Tracy Caldwell Dyson had hoped to at least remove the broken ammonia coolant pump and install a new one during their spacewalk. But a stuck ammonia hose, and later an ammonia leak, pushed them far behind schedule, preventing them from completing their work. [Graphic: Space Station's Cooling System  Problem Explained]

The excursion lasted 8 hours and 3 minutes – making it the longest spacewalk at the space station without a NASA shuttle present and the sixth longest spacewalk in history. The record for the longest spacewalker ever is 8 hours, 56 minutes was set by shuttle astronauts in 2001.

Saturday's spacewalk was the first of two needed to complete the complicated repair. The next spacewalk is set for Wednesday.

The problems for Wheelock and Caldwell Dyson cropped about four hours into the spacewalk, when one of four ammonia hoses they needed to disconnect stuck fast.

"Wow, that is not budging," Wheelock said as they spacewalkers fell behind schedule. The delays ultimately prevented the astronauts from disconnecting the stricken pump and installing its replacement. A second spacewalk is planned for Wednesday to continue the work.

The astronauts began their spacewalk outside the International Space Station at 7:19 a.m. EDT (1119 GMT) as the outpost flew 220 miles (354 km) above Earth.

Stubborn ammonia hoses

The astronauts managed to disconnect the other hoses, some of which released frozen bits of ammonia coolant, which the astronauts said resembled tiny snowflakes. Any ammonia that attached to their spacesuits was expected to bake off in the sunlight.

Aside from the ammonia leaks, those hoses were disconnected swiftly.

"Don't you wish they were all that easy?" Mission Control radioed the spacewalkers.

Finally, Wheelock returned to the stuck hose. He banged on it with a lever tool and managed to free it, only to see a large amount of ammonia leak out.

"You can see, it's got a pretty good snowstorm there," Wheelock said. "Boy that's a lot of pressure in there."

The astronauts waited while Mission Control decided what to do and described the ammonia flakes leaking out.

"That one looked like a giraffe," Wheelock said.

Ultimately, the astronauts left the hose connected to the faulty pump as time ran out.

At one point, the carbon dioxide sensor inside Wheelock's spacesuit failed. As a precaution, Mission Control told him to make sure to report any symptoms related to elevated carbon dioxide levels – though none were reported.

Pump failure in space

The 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.

Space station managers have said the pump malfunction is a major failure for the 12-year-old orbiting lab, enough so that engineers had already drawn up repair plans on the off chance it occurred.

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. 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.

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 tall (almost 1 meter). Caldwell Dyson has said the pumps are about the size of a laundry dryer.

Wheelock and Caldwell Dyson were initially slated to perform a different spacewalk on Thursday. That spacewalk was aimed at preparing the space station for future construction work. Those tasks have been postponed.

Saturday's spacewalk marked the first for Caldwell Dyson and the fourth for Wheelock.