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Tomorrow morning, two NASA astronauts will leave the confines of the International Space Station. They'll be making critical repairs to the orbiting outpost. The spacewalk will be the first in a series of what are expected to be lengthy sessions outside the station. Joining us to discuss the repairs is NPR science correspondent Geoff Brumfiel. Hey there, Geoff.

GEOFF BRUMFIEL, BYLINE: Hi there.

CORNISH: So tell us a little bit about what went wrong up there.

BRUMFIEL: Well, first off, NASA wants you to know that this has absolutely nothing to do with the recent film "Gravity." Sure, one of the astronauts, Rick Mastracchio, is a seasoned veteran. The other one, Mike Hopkins, is on his very first spacewalk. But the similarities end there. The problem is with the cooling system. There's a lot of electronics aboard the space station and it all has to be kept cool. Part of that system failed earlier this month and as a result, they've had to shut down some of the systems on the station. Without those, they can't get resupply missions and they can't do a lot of the science they were hoping to do. So NASA says this is not an emergency. It's a critical fix and it needs to get done.

CORNISH: So give us some detail about the repairs. What are they doing?

BRUMFIEL: Right. So what they're going to be doing is switching out a pumping unit. This is one of three units that's stowed outside the station, the sort of frame. And it was brought up by the space shuttle. They'll be going out there, and they'll be pulling out this unit and putting another one in. Now, the tricky part about this is this thing is really heavy. It's 780 pounds. While weight might not sound like an issue in space, if you could imagine maybe putting a refrigerator on roller skates and pushing it across the floor as fast as you can, then trying to stop it, it's hard to stop. So all that mass has to be managed very carefully.

The other tricky bit is that this cooling system uses ammonia. And ammonia is toxic. So if the astronauts got it on their suits, they came back into the station, they could pollute the atmosphere.

CORNISH: And then, Jeff, I understand that there's been an issue with the spacesuits. Tell us more about that.

BRUMFIEL: Yeah. In July, an Italian astronaut, Luca Parmitano, was on a spacewalk when he started to notice water leaking into his helmet. Now, the water was coming from the system that keeps that astronauts cool. And I'll let Parmitano describe what happened next.

LUCA PARMITANO: The water kept trickling until it completely covered my eyes and my nose. It was really hard to see. I couldn't hear anything. It was really hard to communicate. I just - I went back using just memory, basically, going back to the airlock until I found it.

BRUMFIEL: Honestly, he almost drowned.

CORNISH: And I hear that the solution is snorkels. Is that true?

BRUMFIEL: Yeah, it is true. It's not the only solution. NASA thinks they've actually repaired the suit. But they're not 100 percent sure, so they're taking some additional precautions. And one of those is snorkels. They've had the astronauts fashion out of some spare tubing a sort of breathing device that they could put inside the suit. If water starts coming into the helmet, what they can do is they can grab on to the snorkel and breathe air out of the midsection of the suit. That should give them enough time to get back inside. But again, I really want to emphasize this is not "Apollo 13." This is not "Gravity." NASA expects everything to go quite smoothly.

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

BRUMFIEL: Thank you.

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