Astronauts in the US section of the International Space Station (ISS) were preparing for a possible spacewalk to inspect and fix an ammonia leak in a cooling loop, NASA said Friday, hours after the leak was detected.

“This ammonia leak is designated as one of the big 12 situations which require immediate spacewalks, if possible,” NASA said in a briefing from Houston, that was streamed live.

If the go-ahead is given for the spacewalk, astronauts Chris Cassidy and Tom Marshburn – who NASA described as “veteran spacewalkers with three spacewalks each” – will inspect the far left-side of the ISS’s truss structure, where the leak was detected, and, if they are able to, repair the source of the leak.

A final decision on whether to go forward with the spacewalk will be taken late Friday, NASA said.

“The suspected culprit is the station’s pump and flow control system,” a device used to drive coolant to the space station’s power channels, NASA said, although mission control in Houston was “not absolutely certain” that they had pinpointed the source of the leak.

“The station continues to operate normally otherwise as the ammonia continues to slowly deplete out of the power channel. The crew is in no danger,” NASA said.

The leak was reported to mission control in Houston on Thursday, when the ISS crew said they could see “a very steady stream” of small white flakes floating away from an area of the truss structure.

The astronauts used handheld cameras and, with help from mission control, were able to narrow down the location of the leak to the truss. They also determined that the rate at which ammonia was seeping out of the cooling loop was increasing.

“We’re expecting the loop to shut down in the next 24 hours,” mission control told the astronauts during the morning briefing on Friday.

Chilled liquid ammonia is used to cool the power channels on the ISS’s eight solar array panels, which supply electricity to the ISS. Each solar array has its own cooling loop.

The ammonia loop where the leak was detected is the same one that spacewalkers tried to fix a leak on in November last year, NASA said.

In Moscow, an official from the Russian space agency said the leak had occurred in the US portion of the vessel.