A Russian spacecraft began leaking uncontrollably on Wednesday night

A Russian space mission was canceled at the last minute after an unexpected major leak occurred on Wednesday night in the spacecraft docked to the International Space Station.

Cosmonauts Sergei Prokofiev and Dmitry Petlin, dressed in spacesuits, were told by air traffic controllers to wait while air traffic controllers examined the leak on the Soyuz spacecraft. The spacecraft stopped shortly before 10pm ET (03:00 UTC Thursday).

The leak appears to have originated in an external coolant circuit located in the rear end of the Soyuz MS-22 spacecraft. Commenting on the spacewalk for NASA television, public affairs officer Rob Navias characterized the spacecraft as leaking “quite significantly.” Video of the leak showed particles streaming continuously from the Soyuz, a remarkable sight. It could be ammonia, which is used as the spacecraft’s coolant, although Russian officials have not confirmed this.

None of the crew members aboard the space station at any time were in danger, including Prokofiev and Betlin, and their fellow astronaut Anna Kikhina; NASA astronauts Frank Rubio, Nicole Mann and Josh Cassada; and Japanese astronaut Koichi Wakata. The leak was outside the station, not inside the orbiting lab.

However, the leak raises questions about the reliability of the Soyuz spacecraft, the return trip to Earth for Prokofiev, Bedlin and NASA’s Frank Rubio. They were launched back to the space station in September in this Soyuz vehicle, and are due to return to Earth next spring. The leak continued three hours into Monday night and showed no sign of abating.

The Soyuz is a rigid spacecraft, so it is plausible that there will be no impact on its ability to exit the space station and return to Earth. However, Russian engineers and those from NASA, assuming Rubio will be on board, will fly an alternate Soyuz to the station if they determine there is a problem. Soyuz vehicles are capable of autonomous launch and docking. However, this meant that the three crew members had no emergency escape vehicle until a replacement shuttle arrived.

The other four astronauts on the station flew aboard a SpaceX Crew Dragon spacecraft in October. Seven people cannot travel in that vehicle back to Earth.

Given the long duration of the leak, NASA is likely concerned about the impact of all the ammonia on the surfaces of the space station and other docked vehicles. Much of the ammonia will boil off the surface of the hardware over time, but that will certainly complicate operations as the U.S. space agency launches its own spacewalk on Dec. 19 to install new solar arrays.