AUDIE CORNISH, HOST:

We want to warn claustrophobics about our next story and alert aspiring screenwriters because in space, no one expects to drown. Yesterday, more than 200 miles above earth at the International Space Station, Luca Parmitano was about 90 minutes into a spacewalk when he noticed that his head was wet and getting wetter. Water then got into the Italian astronaut's eyes.

His spacewalking buddy, Chris Cassidy, took a quick look and saw nearly a pint of water floating around in Parmitano's helmet. The pair returned to the space station and removed his helmet as quickly as possible. Parmitano is fine. Joining me now is NPR's Geoff Brumfiel. He's been following the story. And Geoff, to start, what is NASA saying happened here, about the possible cause of this water leak?

GEOFF BRUMFIEL, BYLINE: Well, not very much. They had a press conference yesterday and talked about possible sources, but believe it or not, there are actually multiple sources of water in a space suit, so they're not sure where it came from.

CORNISH: I don't understand this. Why would there be water in a space suit in the first place?

BRUMFIEL: Well, the first reason is for the astronauts themselves. Spacewalking is kind of hard work. You're out there for several hours. You're doing physical labor, so they actually have drink bags inside the suits. The other reason is that even though you might think space is really cold, and it is, there's a need to regulate the temperature because you basically have no way to radiate your heat so they actually have these built in radiators that circulate water through the suit that help keep the astronauts cool.

CORNISH: So we're joking around here, but how serious was this yesterday for Parmitano?

BRUMFIEL: It was pretty serious. You know, they stayed calm, as everyone at NASA always does, but they did get him into the airlock quickly. Not as quickly as they would have done in a full-blown emergency, but they got the helmet off right away. They got the gloves off and they had to get him clear. I mean, the problem here is that water in space does not behave like water on earth.

Surface tension causes it to sort of glom together like mercury does, you know. Little beads of mercury will tend to clump up. And so what potentially could have happened is he would have this big floating blob of water in his helmet that he couldn't clear away 'cause it's in his helmet and he could've inhaled it. He could've choked or drowned. So it was potentially, actually, a pretty dangerous situation.

CORNISH: So what happens now? Will Parmitano get a chance to get out of there, to at least get a different suit?

BRUMFIEL: Well, the situation at the moment is that they have to figure out where this water actually came from. The leading candidate at the press conference yesterday was the cooling system, but they really don't know. And they also don't know whether it was an isolated incident or whether it's a problem with all of the space suits.

So in order to get the astronauts back out there, they're going to have to troubleshoot this and they spent all day doing that, the astronauts and mission control. They do have a backup space suit, so if it turns out to be a problem with this one spacesuit, they will probably go back out there at some stage.

And Parmitano's up there till November, so he may have a shot at it.

CORNISH: That's NPR's Geoff Brumfiel. Geoff, thank you so much.

BRUMFIEL: Thank you. Transcript provided by NPR, Copyright NPR.