NASA survived a close call at the International Space Station on Tuesday when aspacewalking astronaut could have drowned in his spacesuit helmet — an incident that pointed up the dangers encountered daily building, living and working aboard the outpost.

With the station soaring 250 miles above the planet, the six men and women living on board face a constant array of threats, any of which can prove swiftly fatal.

Space rocks or orbital debris can poke a hole in the ship’s hull.

A medical emergency — one that could be handled with a quick trip to the emergency room on Earth — can prove insurmountable in space.

Fire, toxic air and hundreds of other potential risks are carefully studied by NASA, so that crews in orbit and on the ground understand the steps they need to take when something goes wrong.

The most danger a station astronaut encounters in orbit is when he or she steps out of the airlock and into open space, something NASA and its international partners do with far more regularity in the station era than any previous one. Every danger faced inside ISS is aggravated outside.

“You know, you are embarking in your own little spaceship, and so it has to be fully operational and as safe as the one you are leaving before you commit to it,” said Tom Jones, a former astronaut and veteran of four shuttle flights and three spacewalks.

That’s precisely where Italian Luca Parmitano of the European Space Agency got into trouble Tuesday. He was performing routine maintenance outside when water started trickling into the back of his helmet.

No big problem. Engineers thought the leak came from a quart-sized drinking water bag inside his suit. There was no immediate danger.

That changed fast. Engineers expected the bag to empty and the water flow to stop. It didn’t. The flow increased, and water started moving around his ears toward the front of his helmet.

Spacewalking partner Chris Cassidy, a U.S. astronaut, took a look: “It’s a lot of water. His hair is saturated. It’s in his eyes as well as his nose and mouth.”

Mission Control was alarmed. NASA quickly aborted the spacewalk, cut it short by five hours. Parmitano and Cassidy were ordered back to the U.S. Quest airlock, where they would be out of the deadly vacuum environment in low Earth orbit, and into the relative safety of the space station.

The tide kept rising in Parmitano’s helmet during his 20-minute retreat to the airlock. And yes, NASA said he was in danger of drowning.

“Imagine you’re in a fish bowl,” said David Korth, NASA’s lead spacewalk flight director. “So, go stick your head in a fish bowl and try to walk around, and that’s not anything you would take lightly. And certainly, (spacewalking) is dangerous already.”

Parmitano could not hear or respond to questions after he reentered the airlock.

“Hey, Luca, from Houston, how’re you doing? Give us a status,” astronaut Shane Kimbrough said from Mission Control.

“Luca, did you hear that?” Cassidy asked.

He didn’t.

“Squeeze my hand if you’re fine,” Cassidy said, peering into his crewmate’s visor.

“I’m trying to see him,” Cassidy said. “He looks fine. He looks miserable, but OK.”

NASA is investigating. The initial suspect, a drinking water bag, no longer is thought to be the culprit. Jones, the veteran spacewalker, said his best guess is the leak came from Parmitano’s astronaut underwear.

Astronauts don form-fitting garments called Liquid Cooling and Ventilation Garments. They are cooled by chilled water lines running throughout. Jones suspects a rupture in a cooling line near Parmitano’s neck.

“The closest water line to where he was experiencing (trouble) is in the neck area of the LCVG,” Jones said.

The spacewalk was the second in seven days for Cassidy and Parmitano.

It was the 171st in assembly and maintenance of the station since that work began in late 1998.

Rotating crews have staffed the ISS continuously since November 2000. Spacewalking assembly and maintenance work will continue to be part of doing business there.

Jones said cutting short the 6.5-hour spacewalk after 92 minutes was “smart.”

In weightlessness, water clings to surfaces.

“And if you get it in your eyes, you can’t clear them because of the liquid just adhering to your skin around your eyes. You could be blinded, and that could be operationally serious.”

Choking is a hazard. “You could ingest or inhale droplets of water while you’re trying to breathe, and if you get it down the wrong pipe, that could really be a serious medical hazard,” Jones said.

Kenny Todd, chairman of NASA’s Mission Management Team, which is responsible for flight safety during missions, said it is unclear when another attempt might be made to complete the unfinished maintenance.

“We’ll find the right time to go do this once we understand what happened on orbit today and how we can ensure it won’t happen again,” Todd said.

Space station risks

A sampling of “top risks” to the space station and its crew, according to a 2011 NASA study:

— A micrometeorite or orbital debris strike, causing serious damage or even catastrophic loss: 1 in 100 chance.

— Crew needing medical evacuation: 1 in 23 chance in any six-month expedition.

— A crewmember dying from illness, injury, or exposure to toxins or smoke: 1 in 94 chance during a similar period.

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