NASA is on the hunt for the source of a leak that prompted a hasty and early end to a spacewalk outside the International Space Station on Tuesday.

NASA astronaut Chris Cassidy and Italy’s Luca Parmitano were less than an hour into a planned 6-hour, 15-minute spacewalk when Parmitano reported that the back of his head felt wet.

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Thinking his drink bag had sprung a leak, Parmitano emptied it, but water continued to build up inside his helmet.

“It was clear that he was having trouble,” flight director David Korth later told reporters on a conference call.

“We don’t expect any water — above and beyond just normal sweating — to be inside the helmet and when he started reporting that it was accumulating around his ears, that’s the point at which we said we are not comfortable.”

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In microgravity, water collects into blobs that float around. Too much water in the helmet could cause an astronaut to choke or even drown.

“He certainly had that risk today and that’s why we took it so seriously,” said lead spacewalk officer Karina Eversley.

NASA has run into problems with spacewalkers’ suits before, but never anything like what happened on Tuesday.

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Initially, engineers focused on Parmitano’s drink bag as the source of the leak, but that doesn’t seem to be the problem, Korth said.

Another option is the spacesuit’s liquid-cooled undergarment. For now, NASA doesn’t know if the problem is limited to Prmitano’s suit or part of a wider issue.

The spacewalk was a continuation of maintenance chores started during a previous outing on July 9. Parmitano, who wore the same spacesuit last week, is the first Italian astronaut to walk in space.

NASA said he apparently has suffered no ill-effects from the incident.

Image: European Space Agency astronaut Luca Parmitano, Expedition 36 flight engineer, participates in a session of extravehicular activity (EVA) on July 16 as work continues on the International Space Station. A little more than one hour into the spacewalk, Parmitano reported water floating behind his head inside his helmet. Credit: NASA/ESA