(NEW YORK) -- Two American astronauts are conducting an urgent repair outside the International Space Station Saturday in what was expected to be about a six and a half hour spacewalk.
About three hours into the spacewalk, NASA said the astronauts are running an hour and a half ahead of their timeline. NASA's website offers the public a live video feed showing the astronauts and Mission Control.
Rich Mastracchio and Mike Hopkins, Expedition 38 Flight Engineers, began 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, has conducted six previous spacewalks. Hopkins is making his first spacewalk.
During the repairs, the astronauts are communicating with Mission Control Houston about the procedure and chilly temperatures in their suits.
The pump reportedly ways nearly 800 pounds on Earth, but significantly less in Space due to the lower gravity.
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. In the initial spacewalk, they will prepare the pump for removal, 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 this same location during three spacewalks in August 2010, NASA said.
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