A Russian spacecraft began leaking uncontrollably on Wednesday night

A Russian spacewalk was canceled at the last minute on Wednesday night when a spacecraft attached to the International Space Station developed an unexpectedly large leak.

Cosmonauts Sergey Prokopyev and Dmitry Petlin were wearing spacesuits, with airlock pressurized, when flight controllers told them to prepare while investigating the leak in the Soyuz spacecraft. The spacewalk was subsequently canceled shortly before 10 p.m. ET (03:00 UTC Thursday).

The leak appears to have originated in an external cooling loop located at the rear end of the Soyuz MS-22 spacecraft. Public affairs officer Rob Navias, commenting on the spacewalk for NASA TV, described the spacecraft as leaking “fairly dramatically.” The leak video showed particles constantly pouring in from the Soyuz, which is a rather cool sight. This is likely ammonia, which is used as a spacecraft coolant, though Russian officials have not confirmed this.

At no time were any of the crew on the space station endangered, including Prokopyev and Petlin, their fellow cosmonaut Anna Kikina; NASA astronauts Frank Rubio, Nicole Mann, and Josh Cassada; and Japanese astronaut Koichi Wakata. The leak was outside the station, not inside the orbiting laboratory.

Still, the leak raises questions about the viability of the Soyuz spacecraft, the return trip to Earth of NASA’s Prokopyev, Betlin and Frank Rubio. They blasted off to the space station again in September aboard the Soyuz spacecraft, and are scheduled to return to Earth next spring. Three hours later Monday night, the leak was still going, showing no sign of abating.

Soyuz is a very rigid spacecraft, so it’s plausible that there might be no impact on its ability to disengage from the space station and return to Earth. However, if the Russian engineers—and those from NASA, considering Rubio will be on board—determine that there is a problem, a replacement Soyuz plane will need to be flown to the station. Soyuz vehicles have the ability to launch and dock independently. However, this would mean that until the replacement starship arrived, the three crew members would not have an emergency escape craft.

The other four astronauts flew aboard the station on the SpaceX Crew Dragon spacecraft in October. That vehicle cannot accommodate seven people to return to Earth.

Given the length of the spill, NASA also likely had concerns about the impact of all that ammonia on the surfaces of the space station and those of other docked vehicles. It’s possible that a lot of ammonia will boil off the surface of the device over time, but it will certainly complicate operations as the US space agency is working on its own spacewalk on December 19th to install new solar arrays.