The first in a series of spacewalks to replace a refrigerator-size ammonia pump assembly aboard the International Space Station in a high-stakes attempt to restore a critical coolant loop to normal operation.
Astronauts Rick Mastracchio and Mike Hopkins switched their spacesuits to battery power early Saturday to begin their mission.
Floating in the station’s Quest airlock module, the astronauts began the planned six-and-a-half-hour excursion at 7:01 a.m. EST as the space station sailed 250 miles above the Atlantic Ocean approaching Africa.
“Quite a view,” Hopkins marveled as he floated outside the airlock to begin his first spacewalk.
“Yeah, watch that first step,” joked Mastracchio, making his seventh EVA, shorthand for extravehicular activity.
For identification, Mastracchio, call sign EV-1, is wearing a suit with red stripes and using helmet camera No. 20. Hopkins, EV-2, is wearing an unmarked suit with helmet camera No. 18. This is the 175th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the ninth so far this year.
The spacewalks were ordered after a critical valve in one of the space station’s two coolant loops malfunctioned last week, resulting in lower-than-allowable temperatures.
While coolant loop A remained partially operational, flight controllers were forced to shut down a variety of systems in the station’s forward modules, including experiment hardware, to keep those systems from over heating. Coolant loop B remained fully functional.
Engineers attempted to resolve the problem using a software patch to precisely control the position of another valve in the coolant system, and thus the temperature of the ammonia in loop A. But NASA managers ultimately opted for a series of spacewalks to replace the ammonia pump module where the suspect flow control valve is located.
The pump module in question was installed during three 2010 spacewalks after the pump in the original assembly broke down, taking out coolant loop A in its entirety. This time around, the loop A pump is working normally, cooling components mounted outside the station’s habitable modules. But the faulty flow control valve is preventing the loop from cooling components mounted inside the habitable compartments.
Expedition 38 crew member Rick Mastracchio, left, checks out the spacesuit that he will wear during a spacewalk with crew member Mike Hopkins, in the Quest airlock in the International Space Station in this undated image taken from video from NASA TV.