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December 24, 2013 11:48 AM ET

Two NASA astronauts are on a rare Christmas Eve spacewalk outside the International Space Station, working to replace a pump that caused a cooling malfunction on the orbiter.

This is the second spacewalk in four days for the two astronauts. On Saturday, NASA flight Engineers Rick Mastracchio and Mike Hopkins removed a malfunctioning pump from the outside of the space station.

NASA tweeted this photo of astronaut Mike Hopkins riding on the end of the International Space Station's robotic arm during a spacewalk to replace a malfunctioning coping pump. (Photo: NASA via Twitter)

With that part of the job complete, Mastracchio and Hopkins are replacing the degraded ammonia pump module on the station's starboard truss with a new one.

They left the station to begin their efforts at 6:53 a.m. ET today. Ground engineers expect the spacewalk to continue until about 12:30 p.m.

Three hours into the spacewalk, the two astronauts began to install the 780-pound replacement pump.

Hopkins, attached to the end of the station's robotic arm, grabbed hold of the new, refrigerator-sized pump and, with Japanese astronaut Koichi Wakata guiding the robotic arm from inside the station, maneuvered the pump into place.

The astronauts may go out on another spacewalk on Christmas Day if there is still work to be done to get the station's cooling system running fully again.

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NASA astronauts on critical Christmas Eve spacewalk