(CNN) -- Two astronauts were conducting a space walk Wednesday morning to repair a broken part of the International Space Station's cooling system.
The walk is the 149th space walk since 1998. This the fifth walk for flight engineer Doug Wheelock and the second for flight engineer Tracy Caldwell Dyson. When the space walk began after 8 a.m. Wednesday, the space station was in orbit over Alberta, Canada.
Two astronauts tried Saturday to remove and replace an ammonia pump that failed July 31, setting off warning alarms and shutting down part of the cooling system.
Saturday's spacewalk lasted 8 hours, 3 minutes -- the sixth longest in human spaceflight history, NASA said. But despite its long duration, the astronauts were unable to fix the pump.
NASA says that without controls, the temperature of the orbiting station's sun-facing side would soar to 250 degrees Fahrenheit (121 degrees Celsius), while thermometers on the dark side would plunge to minus 250 degrees Fahrenheit (-157 degrees Celsius).
But a back-up system on the space station is keeping it cool, even with the failed ammonia pump, NASA said.
Six people -- three Americans and three Russians -- are on the International Space Station.
The station's crew members are conducting more than 100 ongoing experiments in biology, physical sciences, technology development, and Earth and space sciences, according to NASA.