Enlarge / A Soyuz spacecraft docked at the ISS. (credit: NASA )

Today, NASA held a press briefing to describe the situation on the International Space Station (ISS) in the wake of a major coolant leak from a Soyuz spacecraft that is docked at the station. At the moment, neither NASA nor Roscosmos has a clear picture of its options for using the damaged spacecraft. If it is unusable in its current state, then it will take until February to get a replacement to the ISS.

Soyuz spacecraft are one of two vehicles used to get humans to and from the ISS, and serve as a "lifeboat" in case personnel are required to evacuate the station rapidly. So, while the leak doesn't place the ISS or its crew in danger, it cuts the margin for error and can potentially interfere with future crew rotations.

As Roscosmos indicated earlier this week , the impressive-looking plume of material originated from a millimeter-sized hole in a coolant radiator. Although the coolant system has redundant pumps that could handle failures, the leak resulted in the loss of all the coolant, so there's nothing to pump at this point.

Enlarge / A view of the aft end of the Soyuz spacecraft leaking what appears to be ammonia on Wednesday night. (credit: NASA TV)

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.