Dec. 21, 2013— -- Two American astronauts conducted an urgent repair outside the International Space Station Saturday during a spacewalk that lasted five hours and 28 minutes.
The astronauts ran about an hour and a half ahead of what NASA expected to be their timeline.
Rich Mastracchio and Mike Hopkins, Expedition 38 Flight Engineers, began working at 7:01 am EST Saturday morning to replace a degraded ammonia pump module associated with one of the station's two cooling loops that keeps internal and external equipment cool, NASA said.
Mastracchio, the lead spacewalker, conducted six previous spacewalks, and holds the record for the 14th longest total number of spacewalking hours. This was Hopkins' first spacewalk.
NASA's website offered the public a live video feed showing the astronauts and Mission Control.
During the repairs, the astronauts communicated with Mission Control Houston about the procedure. The spacewalk ended after five hours, 28 minutes and 11 seconds, shorter than the 6.5-hour expected spacewalk, because Mastracchio complained about chilly temperatures in his space suit.
After the cooling line broke down on Dec. 11 at the International Space Station, flight controllers tried but failed to fix the bad valve through remote commanding, the Associated Press reported.
The 780-pound pump is about the size of a double-door refrigerator and difficult to handle, with plumbing full of toxic ammonia, AP reported.
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NASA said Saturday's spacewalk is the 175th in support of space station assembly and maintenance.
The work of the two astronauts on Saturday is part of a series of spacewalks to replace the ammonia pump module. While the astronauts prepared the pump for removal on Saturday, it is expected to be replaced during a spacewalk on Monday.
NASA says a third spacewalk would occur on Christmas Day if necessary to finalize the installation of the replacement pump module. It would be the first Christmas spacewalk for NASA.
The two astronauts received guidance on the spacewalk procedures from NASA astronauts Doug Wheelock and Tracy Caldwell-Dyson, who replaced the ammonia pump at the same location during three spacewalks in August 2010, NASA said.
ABC News' Gina Sunseri contributed to this report.