NASA managers are hopeful the replacement of a pump module during an impromptu spacewalk Saturday will eliminate an ammonia leak that forced flight controllers to shut down a solar array coolant loop aboard the International Space Station, officials said Friday.
While the station can operate in near normal fashion with the loss of one power channel, the loss of a second channel down the road would prompt significant operational changes and could force the crew to shut down non-critical systems, possibly reducing science operations.
In the arcane world of space flight operations, concern about the impact of the next possible failure is always a determining factor in the flight control team's approach to a given problem.
"If we change out the pump and the pump's not the cause of the problem, then it's going to take us quite a bit of time, I imagine, to figure out where this leak could be, identify it, isolate it, overcome it, fix it and then recharge the system," Mike Suffredini, the space station program manager, told reporters late Friday.
"So if we have to be down one power system for a long period of time, that is a significant impact to us. It's a significant impact because the next failure is so much work on the ops team and could potentially have impacts to research."
While the loss of one power channel is more of an "annoyance" than a critical failure, Suffredini said, "if we have to live with this channel down for a long period of time, very definitely it's going to have impacts to research from time to time if we lose the next power system."
Pending final approval by mission managers, astronauts Thomas Marshburn and Christopher Cassidy plan to begin a six-hour 15-minutes spacewalk round 8:15 a.m. EDT (GMT-4) Saturday, making their way to the far left end of the lab's power truss where the port six, or P6, set of solar panels is located.
The huge P6 solar arrays feed electricity into a pair of major circuits known as power channels 2B and 4B. The power is fed into banks of batteries or delivered directly to the station's main electrical grid depending on whether the lab is in sunlight or shadow.
Independent coolant loops route ammonia through cold plates that carry away the heat generated by the array electronics. Coolant lines from both P6 solar array systems are routed through radiator panels to dissipate the heat to space.
On Thursday, the station crew reported a sudden shower of frozen ammonia streaming away from the P6 integrated electronics assembly. Telemetry on the ground confirmed a leak in the coolant loop used by power channel 2B. Analysis of video taken from several angles showed the stream mostly likely originated in or near the 2B pump flow control system box.
Engineers already knew about a very small leak in the system that was releasing about 5 pounds of ammonia per year. But the leak observed Thursday, which may or may not be related to the earlier problem, amounted to about 5 pounds per day.
Based on the observed leak rate, engineers said ammonia levels in the 2B cooling loop would drop below operational limits late Friday or early Saturday, triggering an automatic shutdown.
Acting proactively, flight controllers reconfigured the system to enable coolant loop 2A to carry the most of the load for both P6 arrays. Coolant loop 2B then was shut down.
After making their way 150 feet or so from the airlock to the far end of the P6 truss, Marshburn and Cassidy plan to visually inspect the area around the pump module to see if there are any obvious signs of micrometeoroid damage or any other problems.
On the assumption the leak is located is the 250-pound PFCS box, Marshburn and Cassidy will install a replacement from one of three on-board spares. They also will carry out a detailed inspection of the bay where the pump is housed.
"Once at the work site, what they'll be doing is performing a visual inspection around the area and that will provide better insight and complement the imagery we gained (Thursday) to try to pinpoint where this leak is," said Flight Director Norm Knight. "So we're going to gather as much information as we can from the visual standpoint.
After the suspect pump module is removed, "the crew will then perform a visual inspection down in the bay where that box came out," Knight said. "They will also look up under the box to make sure that there's nothing anomalous or anything that could provide some insight as to where this leak might be originating from."
Once the new pump module is in place, flight controllers will monitor pressure levels in the line to determine if the leak has, in fact, been eliminated. If so, the 2B coolant loop could be restarted right away or, if too much coolant has leaked out, a subsequent spacewalk could be needed to recharge the system from an ammonia reservoir mounted on the truss.
"Keep in mind that depending on how much ammonia's left, once we've changed the pump out we may not be able to start the system right up," Suffredini said. "We may have to wait until a subsequent EVA to fill the system up. That will depend on how much ammonia we lost between now and the EVA and whether or not changeout of the pump actually arrests the leak or not."
Up to 100 pounds of ammonia could be needed to recharge the 2B coolant loop. Suffredini said about 500 pounds is available in the coolant reservoir.
If the leak is still present after the pump module is replaced, engineers will have to go back to the drawing board to come up with a different solution.
"We have a very creative team, so we've got a lot of opportunities to recover this system if it's not the pump itself that's the cause of the anomaly," Suffredini said.
In either case, he said, three of the station's six-man crew -- Marshburn, commander Chris Hadfield and cosmonaut Roman Romanenko -- will undock as planned Monday and return to Earth aboard a Russian Soyuz ferry craft to close out a 146-day stay in space.
Three fresh crew members -- NASA astronaut Karen Nyberg, European Space Agency astronaut Luca Parmitano and cosmonaut Fyodor Yurchikhin -- are scheduled for launch to the station later this month, boosting the crew back to six.
If an additional near-term spacewalk is needed to resolve the coolant leak, it would fall to Cassidy and Parmitano, who already are scheduled for previously-planned spacewalks on July 9 and 16.