Astronauts return to US side of space station after leak scare


Updated at 7 p.m. EST (0000 GMT)

Astronauts return to the US segment of the International Space Station wearing masks as a precaution. Image: NASA TV/Spaceflight Now.

Concern about a possible ammonia coolant leak early Wednesday prompted astronauts aboard the International Space Station to evacuate the U.S. segment of the complex and join three cosmonauts in the Russian portion of the outpost while flight controllers scrambled to untangle initially confusing telemetry. But within a few hours, engineers concluded a computer glitch most likely was to blame, not an actual leak.

After a detailed analysis and review by mission managers, a hatch between the U.S. and Russian segments was reopened at 3:05 p.m. EST (GMT-5) and Expedition 42 commander Barry “Butch” Wilmore, Terry Virts and European Space Agency astronaut Samantha Cristoforetti were cleared to re-enter the lab’s forward modules, wearing protective masks just to be on the safe side.

But no ammonia was detected in the station’s air supply and flight controllers resumed work to restore the lab to normal operation.

“The crew is in good condition, was never in any danger and no ammonia leak has been detected,” NASA said in an afternoon update. “The alarms are suspected to have been caused by a transient error message in one of the station’s computer relay systems, called a multiplexer-demultiplexer.”

Engineers eventually sent commands to turn the box off and back on again and the computer, one of more than 40 MDMs aboard the station, booted up normally with no signs of any trouble.

“Flight controllers continue to analyze data in an effort to determine what triggered the alarm that set today’s actions in motion,” the NASA statement concluded. “The crew is expected to resume a normal complement of research activities on Thursday.”

The problem cropped up around 4 a.m. when alarms went due to readings that appeared to indicate a pressure increase 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 is pumped through huge external radiators to keep the station’s temperature within normal limits.

The apparent pressure spike in coolant loop B was a possible indicator of an ammonia leak and playing it safe, flight controllers told Wilmore, Virts and 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,” space station Program Manager Mike Suffredini said earlier in the day. “So the team’s working through that.”

Water loop B was back in operation within a few hours of the initial alarm and engineers successfully downlinked data from the computer in question for detailed analysis. The crew, meanwhile, remained in the Russian segment of the station until flight controllers were able to reboot the suspect MDM and confirm normal operation.

The astronauts then were cleared to open the hatch and re-enter the U.S. segment of the station.

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.

In the closed environment of the space station, an ammonia leak is a potentially serious problem. While either coolant loop can be vented overboard, equipment tied to that loop would have to be shut down pending repairs and ammonia servicing.