Crew members returned safely to the U.S. segment of the International Space Station Wednesday afternoon after evacuating earlier in the day.
Concern about a possible ammonia coolant leak early Wednesday prompted the three astronauts to evacuate their portion of the complex, joining three cosmonauts in Russian modules while flight controllers studied telemetry to figure out if alarms were triggered by an actual leak, a sensor problem or some other issue, officials said Wednesday.
A few hours later, engineers studying telemetry found no evidence of any leaking ammonia, raising suspicion that a circuit board, or card, in a specific computer known as a multiplexer-demultiplexer, or MDM, might have suffered a failure that took four critical sensors off line. That, in turn, triggered a sequence of events that may have combined to indicate a leak in the station's ammonia coolant system.
"At this point, the team does not believe we leaked ammonia," Mike Suffredini, the space station program manager, said at 11 a.m. EST (GMT-5). "What we are dealing with is this failure of probably a card inside one of our multiplexer-demultiplexers, it's just a computer that sends telemetry down and brings commands back up. This card has a number of measurements on it and those were the measurements we lost."
The alarms were triggered just after 4 a.m. EST (GMT-5) when an apparent pressure increase was detected in a water coolant loop in the forward Harmony module. Water is circulated inside the station to carry away the heat generated by the lab's electronics. The water then flows through components called heat exchangers, transferring the heat to ammonia coolant that flows through huge external radiators to keep the station within temperature limits.
The apparent pressure spike in coolant loop B was a possible indicator of an ammonia leak, and playing it safe, flight controllers told Expedition 42 commander Barry "Butch" Wilmore, Terry Virts and European Space Agency astronaut Samantha Cristoforetti to don masks and move into the Russian segment of the space station.
The telemetry was confusing at first and the initial evacuation was briefly called off. But when flight controllers saw indications of a slight pressure increase in the crew's air supply -- a more convincing sign of an actual leak into the station -- they told the crew to head back to the Russian segment.
Joining cosmonauts Alexander Samokutyaev, Elena Serova and Anton Shkaplerov, Wilmore, Cristoforetti and Virts closed a hatch between NASA's Unity module and the Russian Zarya module, isolating all six crew members in the Russian segment of the lab complex, which uses a different cooling system.
Flight controllers then powered down critical systems tied into coolant loop B and shut down an external pump to reduce the pressure in the system. After studying the telemetry, however, engineers saw signs that something in that powerdown process might have triggered the slight change in air pressure that was detected.
"It looked like a no-kidding pressure increase, now we're thinking this may just be normal reactions to the events that started to unfold," Suffredini said. "So the team's working through that."
Water loop B was back in operation within a few hours of the initial alarm and engineers have downlinked data from the computer in question. They hoped to recover use of the internal circuit card in question after additional troubleshooting.
In an earlier exchange with the crew, astronaut James Kelly at the Johnson Space Center in Houston told Wilmore and company to stand by pending additional analysis.
"We're still trying to figure out exactly what happened," Kelly said. "We're not entirely convinced this is an ammonia leak. ... There's a possibility of a combination of sensor problems, MDM (computer) partial failures and thermal effects all thrown together in the exact wrong way to make this thing look like it was your classic ammonia leak.
"Bottom line is we've got all the experts coming now. Everybody's poring over the data. We've got all the smart folks taking a look at it, and we're trying to figure out exactly what's going on."
"Thank you, we really appreciate that summary," Wilmore replied. "We'll just stand by ready to do anything from our end that you have for us."
Kelly told the crew to stand by for additional updates and in the meantime, "enjoy your impromptu day off."
"We'll keep you guys informed as to what's going on, and we'll also let you know as the conventional wisdom comes around on the story," Kelly said. "But like I said, the good news right now is we're not utterly convinced that we had a very bad problem that we had indications of. Clearly, we did the right thing with the indications we had, but we're still trying to figure out what the actual event is."
By mid-afternoon, the astronauts were cleared to return to the U.S. segment of the station, and the hatch was re-opened at 3:05 p.m. EST. Crew members went in with masks on to sample the air, and when no ammonia was detected, they were able to remove their masks.
A statement from NASA Wednesday afternoon said flight controllers were continuing to analyze data in an effort to determine what triggered the initial alarm. It said the crew is expected to resume their normal research activities on Thursday.
The International Space Station is equipped with two independent coolant loops that use water and ammonia circulating through a complex arrangement of heat exchangers, pumps, valves and radiators to get rid of the heat generated by the lab's electrical systems.
While either loop can handle the heat produced by critical life support, communications, stabilization and key computer systems, both are needed to cool those components, the station's major science experiments and other non-essential equipment.
Inside the station's pressurized modules, electrical components are mounted on "cold plates" that use water flowing through internal lines to keep equipment cool. The warmed water in the "moderate temperature loop," or MTL, is pumped to heat exchangers that transfer the thermal load to the ammonia coolant that circulates through the station's external thermal control system, or ETCS.
Powerful pumps in each coolant loop push the ammonia through an intricate system of valves and lines to large radiators mounted on the back side of the lab's main solar power truss where the heat is radiated to space. The cooled ammonia then is returned to the heat exchangers for another cooling cycle.