*Updated at 11 pm CDT — dates have changed for EVAs.
Two spacewalks will be required to remove and replace a pump on a critical cooling system on the International Space Station, and NASA is hoping to have station astronauts and ground teams ready by
Thursday, August 5 Friday, August 6 for the first EVA. The pump module failed over the weekend, prompting a shift to a backup system, while other systems were shut down as a precaution. The backup system is working perfectly, and having a second system failure is highly unlikely, but NASA does not like to operate on a “single-string” system without redundancy. “Having another failure would be a serious situation for the program that we want to avoid,” said ISS program manager Mike Suffredini.
NASA had originally scheduled the first EVA for Thursday, August 5 and the second for Sunday, but decided late Monday that they needed more time for both astronauts and ground crews to prepare. Now, the first spacewalk will be Friday morning, starting at 6:55 a.m. EDT, (1155 GMT) with the second EVA on Monday, August 9.
An EVA was already scheduled for August 5 for astronauts Doug Wheelock and Tracy Caldwell Dyson to install part of a robotic crane and to prepare for a new module due to arrive in November aboard space shuttle Discovery. But that work will be postponed in order for the astronauts to do the priority fix.
“Typically, we allow two weeks to prepare for this type of EVA, but we decided to take advantage of having an EVA already scheduled, so this is a very aggressive approach,” said Flight Director Courtenay McMillan, who is leading the team supporting the spacewalk.
This type of EVA is part of what is called the “Big 14” set of contingency spacewalks that all ISS astronauts train for in the event of system failures like the one that happened on Saturday.
Suffredini said he is confident everyone can be ready. “The crew is in great spirits and are ready to do this,” he said. “These Big 14 EVA’s cover major systems repairs that an increment crew might have to do without the shuttle there. They train for both specific and generic spacewalks, and this particular one they have trained for. So they have some familiarity with the tasks they will be asked to do.”
Additionally all the operations teams have been working around the clock to prepare for the fix. “The ops team is full up for support,” Suffredini said. “If you were to go in the control room, it looks more like a shuttle flight is going on right now. We do train for these kinds of anomalies, and have been fortunate that we haven’t had to deal with anything like this before, but we have good plans in place.”
The pump — which was installed in October 2002 — failed Saturday night after a spike in electrical current tripped a circuit breaker. When the 350 kg (780-pound) pump failed, it shut down half of the station’s cooling system. Efforts to restart the pump, which feeds ammonia coolant into the cooling loops to maintain the proper temperature for the station’s electrical systems and avionics, were not successful. Suffredini said data suggests the motor is not frozen, as it did begin to pump when they did the restart, so that tells them there is likely a short in the electrical system.
The station’s crew worked with Mission Control to put the station in a stable configuration. The crew, which is in no danger, has resumed normal work activities and most of the systems are up and running. One freezer was shut down, but frozen specimens which are to be returned to Earth were transferred to another freezer.
McMillan said astronauts Cady Coleman and Sunny Williams are working today in the Neutral Buoyancy Lab — the big pool where astronauts train for EVAs — doing development runs for the emergency EVAs.
“They are doing the tasks that they forsee we’ll need to do, trying to work out details of the timeline,” she said. “And depending on how things go today we’ll see if we’ll be ready for the EVA.”
There are spare pump modules already on board the station, and Suffredini said that if more modules are needed, they can fit on board the Japanese HTV resupply vehicles or SpaceX’s cargo vehicle, and don’t need to be brought up by the space shuttle.