ISS spacewalk canceled after leak from docked Soyuz spacecraft

A spacewalk from the International Space Station by two Russian cosmonauts was canceled Thursday after U.S. and Russian flight controllers noticed a stream of particles exiting a docked Soyuz spacecraft, NASA said.

Why it matters: The leak occurred as cosmonauts Sergey Prokopyev and Dmitri Petelin were suiting up to venture outside the station and move a radiator from one module to another on the Russian portion of the ISS.

• The stream was viewable on NASA's live video feed of the station Thursday morning.

What they're saying: "The crew members aboard the space station are safe, and were not in any danger during the leak," NASA said in an update.

• "This is Mission Control Houston. As spacewalk preparations have been ongoing over the past hour or so, Russian flight controllers and the flight controllers here at mission control in Houston have been noticing a stream of particles coming out of the Soyuz MS-22 vehicle that is attached to the Rassvet module on the Earth-facing side of the Russian segment of the International Space Station," NASA commentator Rob Navias said during the live feed.
• "We do not know what the source of this stream of particles at this point," he added, but later said it appeared to be coolant.
• Experts in Moscow were evaluating what the fluid was and whether the leak will affect the Soyuz spacecraft's integrity, Navias said.

The big picture: The spacecraft had shuttled Prokopyev and Petelin and astronaut Francisco Rubio to the ISS in September.

Go deeper: NASA's Orion capsule is back on Earth after journey around the Moon