Tuesday's potentially deadly spacesuit soaking demonstrates just how much astronauts depend on the more than 300 pounds (145 kilograms) of clothing and equipment they have to wear when they go outside the International Space Station.
It also demonstrates how risky spaceflight can be, even during a seemingly routine fix-up job.
"This shows there are always problems that can occur," retired NASA astronaut David Wolf, a veteran of seven spacewalks, told NBC News on Tuesday.
Water in the helmet
The scheduled six-hour spacewalk, aimed at preparing the station for the arrival of a Russian science module later this year, had to be cut short after just an hour and a half when water accumulated inside Italian astronaut Luca Parmitano's helmet. The zero-gravity water built up at the back of Parmitano's head, then crept around and drenched his ears, eyes and nose. It fouled up his communication system as well.
Parmitano and his fellow spacewalker, NASA's Chris Cassidy, made a safe return to the station's Quest air lock. Other crew members helped Parmitano take off his helmet and mopped up the globs of water.
The fluid could have come from Parmitano's bag of drinking water, but it's more likely that the source was the spacesuit's cooling system, which relies on circulating about a gallon's worth of water to keep the internal suit temperature stable. The system, which is supposed to vent recycled air into the back of the helmet, could have suffered a leak.
"There are many points in the cooling system where a leak can take place," said Wolf, who once served as head of the EVA branch of NASA's Astronaut Office. "Thousands of tubes are covering your body."
Moisture from sweat and condensation can build up inside the suit under normal conditions. Sometimes it can pool up inside the spacesuit, which would affect the suit's internal sensors. But lead spacewalk officer Karina Eversley said Parmitano's situation was far more serious. "We have not seen a problem like this before," she told reporters during a news briefing at NASA's Johnson Space Center in Houston.
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In the days and weeks ahead, experts on the ground will work with the station's crew to figure out exactly what happened.
Blobs of jelly
Wolf couldn't comment specifically on what went wrong for Parmitano, but he thinks he knows how the Italian felt. He found himself in a similar, though less serious, situation during one of his own spacewalks. "It feels like blobs of jelly are sticking to you," Wolf said.
In the station's microgravity environment, excess water doesn't necessarily come off in free-floating drops: Because of surface tension, the water sticks to the body — as Canadian astronaut Chris Hadfield demonstrated in his videos about crying in space and wringing out a zero-gravity washcloth. In a worst-case scenario, an excess of water could pool up around a spacewalker's nose and mouth, posing a potential drowning risk in the midst of outer space.
"Choking or drowning is definitely a possibility," Eversley acknowledged.
Fortunately, it never got to that point for Parmitano, although a good amount of water built up in the helmet and the rest of the suit. "Anywhere between a liter and a liter and a half of water (1 to 1.5 quarts) had accumulated in the suit itself," NASA flight director David Korth told reporters.
That much water can interfere with the suit's sensors as well as the helmet's communication headset, which would explain why Parmitano had trouble communicating. In that scenario, Wolf said it's standard procedure to use hand signals if necessary to coordinate the tricky return to the airlock.
No time for fear
It sounds like a scary situation, but Wolf said astronauts are trained to deal with such emergencies methodically.
"The training and the situation leaves one little mental space to be scared, as one is working with the situation," he said. "You look back on those situations and shudder, because there's really very little margin for malfunctions and errors in spacewalks."
To stay on the safe side of the margin, spacewalkers have to rely on their own training — and on the expertise of their crewmates and mission controllers. "Clearly they did an excellent job," Wolf said.
Kenny Todd, chairman of NASA's Mission Management Team, seconded that sentiment during Tuesday's briefing. "There wasn't any doubt as to how to get this done successfully," he said. "All the training paid off."
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Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the NBC News Science Facebook page, following @b0yle on Twitter and adding +Alan Boyle to your Google+ circles. To keep up with NBCNews.com's stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.