Washington - Nasa teams on Monday raced to get ready for two challenging spacewalks to fix a pump module on the International Space Station's cooling system that dramatically failed last week.
ISS astronauts will need a minimum of two spacewalks to remove the failed unit and replace it with a new one, and teams on the ground and in space were racing to do the first of the spacewalks on Thursday, Courtenay McMillan, the spacewalk flight director for the expedition, told reporters.
Two astronauts were holding dry-runs for the spacewalks at the Neutral Buoyancy Laboratory (NBL) in Houston, where they were choreographing what needs to be done outside the ISS and working out how long it might take.
Based on how the dress rehearsal goes, Nasa will either give the green light to ISS astronauts Doug Wheelock and Tracy Caldwell Dyson to start the first spacewalk on Thursday or, if the dry run shows more time is needed, to go over some of the steps involved in removing and replacing the failed unit.
The first spacewalk will focus on getting the failed unit out of the starboard truss on the ISS, which poses a few technical challenges including releasing lines that are pressurised with ammonia which is usually pumped into the cooling system, when the module is working, said McMillan.
Once the failed unit has been removed, the two astronauts will have to move a 355kg spare unit around 10m from the opposite side of the truss for insertion into the gap left by by the defective pump module.
"This is a big, unwieldy object, so manoeuvring it around and handing it off to crew members... could take some time and a lot of focus," said McMillan.
The crew faces a very tight lead time for such a tricky spacewalk - less than a week when Nasa usually takes two weeks to prepare for a spacewalk to fix a "Big 14 failure", when a major unit stops working.
The cooling pump going down was a "Big 14 failure", said Mike Suffredini, manager of the ISS programme, but Nasa was prepared for it.
"This is an anomaly we knew someday would happen. It's an anomaly we've trained for, it's an anomaly we've planned for, it's obviously one we've spared for. So we're in a good position to solve this problem," said Suffredini.
"But it is a significant failure, in terms of systems on board ISS. So it's one that we have to get after." "
If the second of the two ISS cooling units were to fail - a highly unlikely scenario, according to Suffredini - then the astronauts on board the ISS would no longer be able to cool most of the components on board the ISS.
The crew would not be in immediate danger, however, as they could move to the Russian segment of the ISS, which has its own cooling system.
Astronauts tried after Saturday's failure to reactivate the pump module, but the circuit breaker tripped, said Suffredini.
"The data suggest that the motor is not frozen. In fact, it did start to pump some of the ammonia when we tried to start it the second time. So this tells us that there's a short somewhere in the powerfeed of the motor." - Sapa-AFP