NASA astronauts on critical Christmas Eve spacewalk

Only other Christmas Eve spacewalk was conducted in 1999 to service Hubble

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.

The space agency said the glitch poses no danger to the space station crew or the scientific experiments being done there, but they want to get it fixed as soon as possible.

The problems surfaced Dec. 11, when the pump module on one of the space station's two external cooling loops shut down. The loops are used to circulate ammonia, a coolant, outside of the orbiter to keep the internal and external equipment cool.

Engineers suspect the problem is a malfunctioning flow control valve inside the pump.

When the trouble first arose, the ground team moved some electrical systems from the problematic loop to a second one. They also powered down some non-critical systems inside the Harmony node and two laboratories to lighten the cooling load on the station.

NASA noted on Twitter this morning that astronauts have only conducted one other spacewalk on Christmas Eve. That one was in 1999 to service the Hubble Space Telescope.

This article, NASA astronauts on critical Christmas Eve spacewalk, was originally published at

Sharon Gaudin covers the Internet and Web 2.0, emerging technologies, and desktop and laptop chips for Computerworld. Follow Sharon on Twitter at @sgaudin, on Google+ or subscribe to Sharon's RSS feed . Her email address is

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