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

Cosmonauts Sergey Prokopyev and Dmitri Petelin were dressed in spacesuits, with the airlock depressurized, when flight controllers told them to standby while the leak in a Soyuz spacecraft was investigated. The spacewalk was subsequently called off shortly before 10pm ET (03:00 UTC Thursday).

The leak appears to have originated in an external cooling loop located at the aft end of the Soyuz MS-22 spacecraft. Public affairs officer Rob Navias, who was commentating on the spacewalk for NASA Television, characterized the spacecraft as leaking “fairly substantially.” Video of the leak showed particles streaming continuously from the Soyuz, a rather remarkable sight. This was likely ammonia, which is used as a spacecraft coolant, although Russian officials have not confirmed this.