In this handout provided by NASA the International Space Station is seen from NASA space shuttle Endeavour after the station and shuttle began their post-undocking relative separation May 29, 2011.

In this handout provided by NASA the International Space Station is seen from NASA space shuttle Endeavour after the station and shuttle began their post-undocking relative separation May 29, 2011. / Getty Images

A planned spacewalk by two Russian cosmonauts has been called off after ground teams noticed a significant leak from one of the spacecraft docked at the orbiting International Space Station.

The leak appeared to originate from the Russian Soyuz spacecraft, spewing an unknown substance into space for several hours.

During their spacewalk, cosmonauts Sergey Prokopyev and Dmitri Petelin were scheduled to move a radiator from an older Russian module to a newer science module that arrived at the station last summer. The spacewalk was called off at the last minute — while the duo were suited up and in the space station's airlock — after ground teams discovered the leak.

The leaking Soyuz capsule transported the two Russian cosmonauts scheduled for the spacewalk, along with U.S. astronaut Frank Rubio, from the Baikonur Cosmodrome in Kazakhstan on Sept. 21.

Ground teams at NASA in Houston and at Roscosmos in Moscow are evaluating the potential impacts on the integrity of the Soyuz spacecraft, which is also responsible for bringing them home. The trio is scheduled to return in the capsule in late March.

A NASA spokesperson said at no point were any of the seven members of the ISS in danger.

"The best plan of action tonight was to focus all of our attention to sorting what's going on exactly with the Soyuz spacecraft and then we'll regroup tomorrow," said NASA Chief Flight Director Emily Nelson.

Russian mission controllers asked the cosmonauts on board the space station to photograph at the best resolution possible what appeared to be a coolant leak, and instructed the crew to not open certain window shutters on the station. Cosmonaut Anna Kikina used a robotic arm attached to the station to get a closer look at the spacecraft.

Four others are currently on the station: NASA's Nicole Mann and Josh Cassada, Japan's Koichi Wakata, and Kikina. Those four arrived on SpaceX's Dragon capsule on Oct. 5. That capsule is only capable of transporting up to four crew members back home.

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Tags: Soyuz  International Space Station