Today’s extravehicular activity outside the International Space Station was cut short after NASA astronaut Tim Kopra reported a “small amount” of water in his helmet.

As a precaution, NASA flight director Royce Renfrew decided to end the spacewalk early after Kopra and British astronaut Tim Peake had spent 4 hours and 43 minutes working on the exterior of the orbiting outpost.

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The planned spacewalk was scheduled to last around 6 and a half hours and the spacewalking duo had already completed their primary task of replacing a failed electrical box before they were called back into the station. Kopra reported seeing a water bubble several inches wide appear inside his helmet and as the water was cold, it suggests the spacesuit’s coolant system may be to blame.

Space exploration is dangerous at the best of times, but when astronauts exit the protective environment of the space station, they are dependent on their space suit’s life support systems. Should any fault arise, spacewalking activity is quickly called off.

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In 2013, Italian astronaut Luca Parmitano was working on the space station’s exterior when large quantities of water began to engulf his head, impeding his vision. US spacewalking partner Chris Cassidy had to guide Parmitano back to the station’s airlock before the situation became dire. In microgravity, water leaks act very differently than here on Earth and a sizable water leak could easily engulf a spacewalker’s face. Although drowning in space may sound counter-intuitive, for Parmitano it became a terrifying possibility.

"Happy to see @astro_timpeake and @astro_tim safe inside. This is how I measure success: 1) crew-safe 2) main objective-completed," said Parmitano in a Twitter update in response to today's events.

Fortunately for today’s spacewalk, the leak was less dramatic, but is undoubtedly a concern. Both Kopra and Peake returned to the space station’s airlock 15 minutes after the problem was reported and an investigation is underway as to what went wrong.

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Although every spacewalk is remarkable, today’s is historic for Britain — this is the first time an "official" British astronaut has participated in a spacewalk. This was Kopra’s third.

Once the spacewalkers reentered the space station’s airlock, their colleagues were on hand to assist. Station commander and NASA astronaut Scott Kelly, plus cosmonauts Sergey Volkov and Yuri Malenchenko helped Peake and Kopra from their suits as well as using towls to help dry Kopra’s face. Kelly even used a syringe to collect the excess water from Kora’s helmet and removed absorption pads so investigators can determine the cause of the leak.

For more details behind Tim Peake’s historic mission, follow his Principia blog.