US astronauts begin key ISS repair spacewalk

Two International Space Station astronauts set out yesterday on the first of two spacewalks to fix a cooling pump that dramatically failed last week.

ISS astronauts will need at least two spacewalks to remove the failed ammonia pump unit and replace it with a new one after the device failed a week ago.

Doug Wheelock and Tracy Caldwell Dyson slept in the Quest airlock module overnight to accommodate their bodies to a different pressure and were awakened at about 8 a.m. to begin final preparations for their work.

They activated battery power and other life-support systems inside their spacesuits, and opened the hatch at 1.19 p.m., a procedure that officially marked the beginning of their seven-hour walk.

The beginning of the walk was slightly delayed because of what was described by US space officials as a “communication problem” inside Caldwell Dyson’s helmet. The problem has been successfully resolved, the official said.

The first spacewalk will focus on removing the ammonia pump module that failed yesterday week and putting its replacement in place, according to officials from the National Aeronautics and Space Administration.

A second spacewalk is planned for Wednesday to connect fluid ammonia lines to the replacement pump. Mission managers plan a final review of that plan early this week, incorporating updated information on the station’s configuration after the first spacewalk, Nasa officials said.

The spacewalks are challenging because the crew will be handling ammonia lines at full operating pressure, which makes the lines stiff during reconnection and mating, experts warned.

The timeline for the spacewalk will require numerous “break-out points” to ensure adequate time to complete decontamination procedures if the crew comes in contact with ammonia. Once the failed unit has been removed, the two astronauts will have to move a 355-kilo spare unit around 10 metres from the opposite side of the truss for insertion into the gap left by the defective pump module.

“This is a big, unwieldy object, so manoeuvering it around and handing it off to crew members... could take some time and a lot of focus,” Courtenay McMillan, the spacewalk flight director for the expedition, told reporters earlier last week.

But robotics experts have devised procedures that will be used by station crew member Shannon Walker to guide the station’s robotic arm, Canadarm2, from the robotics workstation in the Destiny Laboratory.