American Chris Cassidy and Italian Luca Parmitano were forced to call off this morning's planned spacewalk outside the International Space Station when Parmitano suddenly reported that there was water inside of his suit helmet.

"My head is really wet and I have a feeling it's increasing," he radioed about an hour into the spacewalk.

The EVA, designated EVA-23, was one of the ones that Ars watched astronauts Cassidy and Parmitano train for late last year. That was during our visit to NASA's Neutral Buoyancy Laboratory, the giant swimming pool where NASA simulates spacewalks in microgravity. According to NASASpaceFlight's recounting of events, Parmitano was in the process of running data cabling to connect the as-yet-unlaunched Russian Nauka module when the water began to make itself apparent. The quantity of liquid in Parmitano's helmet rapidly increased, with Parmitano noting that it had begun to enter his eyes, nose, and mouth.

The decision was quickly made by controllers to terminate the EVA, and Cassidy and Parmitano immediately began to make their way back to the station's Quest airlock. The liquid caused Parmitano's suit radio to malfunction and he could no longer speak to or hear anything from either Cassidy or from Mission Control, but Cassidy monitored Parmitano while the airlock repressurized.

"He can't hear," Cassidy said of Parmitano during the repressurization. "He looks miserable, but OK." Parmitano's helmet was removed as soon as possible after the airlock repressurization was completed, with NASASpaceFlight reporting some quantity of water "floating away" with the helmet.

At present, Parmitano's in-suit drinking water bag is suspected of being the source of the leak, though the specifics aren't yet known. Since ISS EVAs can last many hours, the spacesuits contain a quantity of drinking water to keep their occupants hydrated.

This isn't the only source of water inside a suit, though. The PLSS backpacks the astronauts wear also contain a significant amount of water, which is circulated across the astronaut's body via a mesh of plastic tubes. The water helps to regulate the astronaut's body temperature.

The microgravity experienced by astronauts in orbit makes even small drops of liquid potential hazards, and the small enclosed environment of a spacesuit is a particularly dangerous place to have free liquids floating around. Even the relatively tiny amount of liquid in a drinking bag would be more than enough to potentially drown a person if it were to be inhaled—in no small part because repressurizing the airlock and removing the astronaut's helmet to provide assistance takes more than ten minutes.

NASA will be investigating the cause of the leak over the next several days; another EVA will also have to be scheduled to finish the tasks that weren't completed during EVA-23. Fortunately, there are spare spacesuits on board the ISS, so Parmitano won't have to wear the same leaky suit again.