NASA's Rob Navias, speaking on a late Wednesday evening NASA TV broadcast, called it a "fairly significant" leak. Live images during the broadcast showed liquid spewing out from the Soyuz. Navias said the leak was first observed around 7:45 p.m. ET Wednesday.
The Soyuz spacecraft is docked to the Russian segment of the space station, CNN reported. The external radiator cooling loop of the Soyuz is the suspected leak source, according to an update from NASA on Thursday afternoon.
The crew is safe, and all systems of the space station and the ship are operating normally, Roscosmos said in a statement in Russian released Thursday morning on Twitter. (CNN translated the statement.) Navias also said the astronauts inside the space station were "never in any danger."
"The crew reported that the warning device of the ship's diagnostic system went off, indicating a pressure drop in the cooling system," according to Roscosmos. "A visual inspection confirmed the leak, after which it was decided to interrupt the planned extravehicular activities by the crew members of the ISS Russian Segment Sergey Prokopiev and Dmitry Petelin."
Russian cosmonaut Anna Kikina, using the camera on Russia's Nauka module on the space station, "photographed and filmed the outer surface of the ship," according to Roscosmos. "The data was transmitted to Earth, and the specialists have already begun to study the images."
Russian managers looked over the data and consulted "with both NASA managers and engineers" and outside experts, Navias said.
More information about the leak will be gathered, NASA said.
"Plans for an additional inspection of the Soyuz exterior using the station's Canadarm2 robotic arm are underway," according to a NASA update.
The Soyuz MS-22 ferried NASA astronaut Frank Rubio and the two Russian cosmonauts to the space station on September 21 and is scheduled to bring them back to Earth in late March.
A NASA spacewalk to install a solar array outside the space station on Monday remains on the schedule for astronauts Rubio and Josh Cassada. Meanwhile, a Russian spacewalk scheduled for December 21 has been indefinitely postponed.
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