During an unscheduled spacewalk on the space station’s exterior on Saturday morning, NASA astronauts Tom Marshburn and Chris Cassidy carried out the mother of all plumbing jobs: They detached a suspect ammonia pump, replaced it with a spare and watched for any further ammonia leakage.

The emergency spacewalk was carried out in response to a troubling ammonia coolant leak that was discovered on Thursday. The coolant is used to maintain the temperature of the vast solar arrays the space station uses to generate electricity for its systems.

ANALYSIS: Space Station Video Shows Ammonia Leak

But after nearly four hours of extravehicular activity, Marshburn and Cassidy reported seeing “no snow” (i.e. no ammonia flakes) as a replacement Pump and Flow Control System (PFCS) box was switched on and ammonia was pumped around the solar array at full pressure. No trace of the phantom ammonia leak was spotted by the spacewalkers’ cameras nor the ever watchful mission managers in Houston, Texas.

Animation of a segment of the NASA video showing the ammonia leak on the space station on Thursday. Credit: NASA

Space station commander Chris Hadfield watched events unfold from the orbiting outpost’s interior, providing support for the astronauts working outside. “Houston just sent the command to start flowing ammonia through the newly-installed pump. Gloved fingers crossed,” he said as the newly replaced PFCS was switched on. Several minutes later, Marshburn reported, “we don’t see anything.”

“No leaks! We’re bringing Tom & Chris back inside. In two days Tom, Roman & I return to Earth in our Soyuz. This is an amazing place & time,” tweeted an exhilarated Hadfield as Marshburn and Cassidy packed up their tools and underwent a “bake-out” procedure. The bake-out was required to allow sufficient time for any possible ammonia contamination on the spacewalkers’ suits to sublimate and dissipate into space. Ammonia is toxic, so every precaution was taken to ensure none was inadvertently carried back into the space station.

ANALYSIS: Space Station Crew Prepare for Emergency Spacewalk

Although the replacement box appears to have solved the ammonia leak, NASA will still need several days to assess whether the fix has worked. “It will take some diagnostics, still, over the course of the next several days by the thermal systems specialists to fully determine that we have solved the problem of the ammonia leak,” said NASA commentator Rob Navias during the live NASA TV spacewalk broadcast. “But so far, so good.”

This spacewalk was the fastest of its kind; there was only 48 hours from planning to execution. As soon as the ammonia flakes were seen emanating from the P6 truss on the far left of the space station on Thursday, NASA space station managers began planning for an emergency spacewalk that was approved Friday night. At 8:44 a.m. Saturday morning Marshburn and Cassidy had exited the space station to identify the location of the leak and fix the problem.

Although the spacewalkers saw no evidence of ammonia or damage, they quickly removed the suspect PFCS box and replaced it with a spare, running about an hour ahead of schedule.

More coverage of the space station’s ammonia leak:

  • Space Station Video Shows Ammonia Leak

  • Space Station Crew Prepare for Emergency Spacewalk

  • Space Station Suffers Troubling Coolant Leak

Image: Marshburn and Cassidy work on replacing the PFCS box on the space station’s exterior. Credit: NASA TV