♪ ♪ NARRATOR: Earth.

Over eight billion people live here.

But not all of them are home.

Since the turn of the century, a few have been living here... ♪ ♪ ...in the most hazardous scientific outpost ever built: the International Space Station.

♪ ♪ Altogether, over 280 people have ridden a controlled explosion 250 miles into space, risking their lives to maintain a continuous human presence off planet.

One day, we may be getting ready for a spacewalk and the next day, we'll be doing some world-class science.

♪ ♪ NARRATOR: Its unique environment provides a research laboratory like no other.

But conducting science in space brings its own challenges.

RAJA CHARI: Very quickly, you can have an out-of-control situation from a very innocuous start.

♪ ♪ NARRATOR: As astronauts push the boundaries of science and exploration, they face daily threats.

(over radio): CO2 sensor bad.

MAN: Copy, Luca.

LUCA PARMITANO: You are constantly at risk of dying for a multitude of reasons.

♪ ♪ WENDY LAWRENCE: You've got orbital debris that can hit your module and put a hole in it.

♪ ♪ CHRIS CASSIDY: In the military, we say that problems happen at the fold of a map and at night.

And that's exactly how this was shaping up.

ZEBULON SCOVILLE: How do we respond to that unknown?

Understanding situations may have the ultimate consequences.

(alarm beeping) MAN (over radio): Station, Houston on 2.

This is an actual.

NARRATOR: This is the story of innovation and scientific exploration aboard the I.S.S.

"Operation Space Station: Science and Survival."

Right now, on "NOVA."

♪ ♪ NARRATOR: June 2024.

The world is gripped by a drama playing out far above Earth.

NASA astronauts Butch Wilmore and Suni Williams had been scheduled to spend just eight days aboard the International Space Station while testing a new commercial spacecraft, the Boeing Starliner.

WILLIAMS (over radio): Into the spacecraft!

NARRATOR: Instead, when the vehicle encounters technical issues, their mission stretches into months.

Despite the difficulties, the pair pivots to become routine crew members, drawing from their years of training and a deep love of the job to embrace their extended mission.

Because this job is like no other.

We're still pioneers in space, so there's still a lot that we have to learn.

(over radio): Okay, my yellow hook, closed and locked.

NARRATOR: Space is incredibly risky.

The safest thing to do is to not go anywhere, but that's not going to really teach you much about your species or the planet you live on.

You have to take risk to get the reward.

NARRATOR: Butch and Suni's extended stay is just the latest in a long line of unexpected challenges faced in orbit.

MAN (over radio): And lift off.

The International Space Station is underway.

NARRATOR: Since the space station's first module was launched in 1998, there have been countless moments of risk and uncertainty.

ROBERT CURBEAM (over radio): There's ammonia all over the place.

NARRATOR: Requiring calm and quick thinking to resolve.

TOM JONES (over radio): You just pulled the bail?

CURBEAM: Yeah.

And that stopped the leak.

NARRATOR: All to create a unique research laboratory, a platform for international science and discovery through cooperation, uniting humanity in the pursuit of knowledge beyond our planet.

♪ ♪ From monitoring hurricanes on Earth to cultivating plants in space.

From detecting X-rays released by neutron stars to developing cancer therapeutics.

RENITA FINCKE: We have taken technologies, people, systems from all over the world, and we've put it together into this amazing laboratory.

♪ ♪ The research that we've been able to do has directly benefited life on planet Earth, and we're only just getting started.

♪ ♪ NARRATOR: Every minute of an astronaut's day on the station is carefully planned, and life on board might even come to feel routine.

But it's always just a matter of time before the universe finds a way to remind the crew of just how vulnerable they are.

♪ ♪ In 2013, plans are underway to further extend the International Space Station's scientific footprint.

A new module called Nauka, Russian for "science," is to be added to the station the following year.

Created to conduct materials science and biotechnology research, Nauka includes a base for the European Robotic Arm and will also feature a specially designed airlock to carry out experiments in the vacuum of space.

♪ ♪ But to unlock this groundbreaking science, astronauts must brave the most unforgiving extremes.

CASSIDY: In my personal operational experience in the military, in the SEAL teams, the bad guys were the enemy that's shooting at you.

In a space mission, the "bad guy" is the environment.

♪ ♪ The environment is trying to kill you.

PARMITANO: Space is not really made for life.

It's very cold and very hot at the same time.

It doesn't have an atmosphere.

It really doesn't want to keep you alive.

NARRATOR: A spacewalk, also known as an extra-vehicular activity, or E.V.A., is especially dangerous.

Astronauts usually go through the airlock in pairs so that once outside, they can assist each other if needed.

But still, they are isolated and vulnerable to hazards like radiation, micrometeoroid impacts, and equipment malfunctions.

On July 16, 2013, American astronaut Chris Cassidy and flight engineer Luca Parmitano don their spacesuits.

Spacewalks are not something that you just willy-nilly do-- "Hey, it's Tuesday.

You want to go out and knock out a spacewalk?"

No, it doesn't work like that.

Takes about four hours to go through the oxygen pre-breathe process and all of the checks leading up to opening the hatch.

(over radio): All right, I have a waist tether, gate closed and locked.

(voiceover): All spacewalkers are tethered to the space station with a retractable metal cable that pays out to 85 feet.

And just like if you walk a pair of dogs on dog leashes, those leashes get all tangled up.

PARMITANO (over radio): On my way.

CASSIDY (voiceover): Safety tethers are no different.

So, oftentimes, we intentionally take separate routes, so the safety tethers don't tangle up on themselves.

NARRATOR: Luca and Chris come together on top of the Tranquility module to carry out their task: connecting data and ethernet cables for when the new module arrives.

38 minutes into the E.V.A., a warning alarm is triggered in Luca Parmitano's spacesuit.

The carbon dioxide levels are too high.

PARMITANO (over radio): CO2 sensor bad.

MAN: Copy, Luca.

It measures 48.6.

CASSIDY: Copy that, Luca.

(exhales) NARRATOR: Ordinarily, exhaled carbon dioxide would be controlled and removed by the Portable Life Support System in Luca's backpack.

The alarm could signal a dangerous buildup of the gas inside his spacesuit, which could lead to severe headaches, impaired judgment, and even death.

PARMITANO (voiceover): We have a checklist telling us to check different parts of the suit to understand whether it is an actual alarm or whether the sensor itself is at fault.

CASSIDY: It said, "Did the reading go from normal to ludicrous instantly?"

If that happened, then it's probably the sensor's bad.

MAN (over radio) And Luca, nice work on the cuff checklist, but no further action on your part.

NARRATOR: Doubtful of the carbon dioxide reading and satisfied that Luca has no symptoms, Mission Control concludes that the sensor is faulty, and the E.V.A.

can continue as planned.

(static hisses) But six minutes later, Luca calls Mission Control again: there's another issue.

PARMITANO (over radio): F.Y.I., I feel a lot of water on the back of my head.

MAN: Are you sweating?

Are you working hard?

PARMITANO: Um, I am sweating, but feels like a lot of water.

MAN: Yeah, Luca, we copy.

MAN: Copy.

PARMITANO: Just F.Y.I.

(voiceover): There should never be any water inside your space suit, especially not in the, in the helmet area, and so I communicated that to the ground.

(chuckling): And I said, "I don't think it's a big deal."

I just wanted to keep going with the E.V.A.

MAN (over radio): Hey, Luca, while you're working there, can you give us, uh, maybe some more words on the water?

Um, maybe identify the source, do you think, and then, is it getting any worse or is it the same?

PARMITANO: Um, good questions.

It's still the same, and I cannot tell you the source.

NARRATOR: On Earth, water falls under the force of gravity.

In space, it's much less predictable.

CASSIDY: It's surface tension that drives the behavior of water.

That surface tension is a pretty strong force, and the water just wants to stick to whatever it's stuck to.

NARRATOR: In Luca's case, the unexplained ball of water is sticking to the back of his head and threatening to creep around toward his face.

PARMITANO: My biggest concern was the water getting inside my ears.

(static hisses) Because if it did, it's really hard to get it out.

You cannot tap your head-- there's nothing you can do.

(static hisses) CASSIDY: I finished what I was doing and got over close to him, and I remember, I could see, it looked like half of a grapefruit just kind of jiggling on his head.

(Parmitano panting softly) CASSIDY: And then little bitty pieces of water droplets moving around inside.

(on radio): I see beads of sweat.

PARMITANO: No, it's not sweat.

CASSIDY: Oh, it's not sweat.

MAN: Hey, Luca, can you clarify, is it increasing or not increasing?

PARMITANO: It's hard to tell, but it feels like a lot of water.

CASSIDY: Oh, I see, yes.

I see it now, wiggling.

PARMITANO: And, um, I, I can't, don't understand where it's coming from-- it can't be the water... CASSIDY: It has to be the bag.

NARRATOR: At the European Space Agency, astronaut instructor Hervé Stevenin watches a live feed of the mission.

(Stevenin speaking French) (translated): They thought it might be the small water bag that the astronauts carry.

There is a small bag which is attached to the stomach with Velcro and contains around one to one-and-a-half liters of water.

This has a little straw, to enable the astronaut to stay hydrated.

CASSIDY (over radio): And you've sucked it dry?

PARMITANO: Okay, the bag is dry now.

NARRATOR: But the situation doesn't improve.

CASSIDY: The bag is empty now.

And, uh, you know, there's something less than a liter in the back of his head.

800 milliliters, maybe.

MAN: All right, Chris.

NARRATOR: More water collects in Luca's helmet.

PARMITANO: Yeah, I'm thinking that it might not be the water bag.

The only other option, Chris, that I'm thinking is the LCVG.

NARRATOR: The LCVG, or Liquid Cooling and Ventilation Garment, circulates cold water in the spacesuit via a system of tubes to prevent astronauts from overheating.

As Chris and Luca correctly suspect, Luca's LCVG has malfunctioned and is leaking water into his helmet.

The very suit designed to protect him from the dangers of space may now be putting him in mortal danger.

CASSIDY: That's the moment, I think, where it solidified in everybody's mind, "Okay, we're done for today."

NARRATOR: 67 minutes into the E.V.A., Mission Control terminates the spacewalk.

MAN (over radio): All right, Chris and Luca, just for you guys, we think we're are gonna terminate E.V.A.

case for EV2.

So, Luca, we'll have you head back to the airlock.

Chris, we'll get a plan for you to clean things up here and then join him in a minute.

NARRATOR: Luca needs to get back inside the space station and remove his helmet.

He begins retracing his route back to the airlock, while Chris clears away their tools.

PARMITANO: I remember leaving Chris, and as I made the corner and disappear from sight, I remember thinking that I really wish that Chris could come with me, because that's how we train.

We train to be together-- that's why we go outside as a team, so that we have each other's back.

CASSIDY: Before I saw Luca disappearing behind the Z1 truss... And I wanted to say out loud, "Luca, stop, hold on, I'm coming with you."

It just felt like that's what we should do.

NARRATOR: But as tempting as it is to hurry, Chris follows orders and forces himself to move deliberately.

In the military, we say that problems happen at the fold of a map, at the junction of grid squares, and at night.

And that's exactly how this was shaping up.

As soon as he started to move, the sun set.

PARMITANO: And everything turns gold, bright gold, for one split second.

Incredibly beautiful, and then it's lights out.

(static hisses) (static hisses, water drips on radio) NARRATOR: The I.S.S.

takes around 90 minutes to orbit the Earth.

For half this time, the space station is in total darkness.

(static hisses, water drips on radio) CASSIDY: Now, once Luca started to move, well, what happened to his motion, it gets transferred into the water, too, so that stationary jiggly ball of Jell-O starts flushing around his head.

♪ ♪ PARMITANO: I felt it coming closer and closer to my forehead and I thought, "Okay, this might cover "both of my ears at the same time.

"It may be more than a simple hindrance.

It may be an actual, an actual problem."

And I just completed a maneuver where I put myself upside down, so all my terms of reference had completely switched 180 degrees.

That's when the water really completely covered my nose... ...and I found myself isolated, unable to see anything, unable to breathe through my nose.

I didn't really know how much time I had before the water reached my mouth.

NARRATOR: If too much water covers both Luca's nose and mouth, he'll be unable to breathe or drink his way out of trouble.

(radio static hissing) And to make matters worse, water enters Luca's comms, causing it to malfunction.

PARMITANO (over radio): Hey, Chris?

CASSIDY: I hear you, Luca, go ahead.

(radio static hissing) Luca, I hear you.

PARMITANO: Chris?

Yeah... (radio static hissing) (translated): There are two mics here, and when Luca talked, all we could hear were gurgling sounds.

This was terrifying to hear for us here in the control room.

At this stage, we realized that something serious had happened, very bad and very dangerous.

(water bubbling) My memories are of just trying to feel around, making sure that I would grab the correct handles in a pressurized glove, which gives you no feedback, no tactile feedback.

And I called Chris, telling him, "Hey, Chris, I, I think I'm a bit lost," but nothing came out, and I, I didn't have any return in my earphones, so I knew that he couldn't hear me.

(water bubbling) And at the time, my thoughts were, "Okay, I don't think Chris is going "to come and get me right away.

How much time do I have?"

That was my thought in my head.

"How much time do I have before this helmet fills up with water?"

There was a risk that if the water increased dramatically, I would drown in space.

NARRATOR: For the next five minutes, Luca will be completely silent and alone.

♪ ♪ (water drips) CASSIDY: He had the amazing presence about himself, when everything kind of went dark and silent, to just stop.

(water bubbling) NARRATOR: At that point, Luca remembers he is tethered to the space station's airlock.

STEVENIN (translated): His cable extends all the way along the route, creating a safety line.

If the astronaut were to float off into space, this cable would slowly pull them back towards the airlock.

(speaking French) CASSIDY: That tether has a little three- or five-pound pull to it, and he waited to feel the direction of that pull.

And then he just put his hand over and grabbed the next piece of metal that he could find, and inched his way back that way.

Unbelievable awareness by Luca.

♪ ♪ NARRATOR: The gentle tug of the waist tether leads Luca back to the airlock.

But he's not safe yet.

Before he can take his helmet off, the airlock must be sealed and repressurized.

With no time to waste, the I.S.S.

crew runs through their checklist.

Five minutes later, Chris joins Luca, squeezing in feet first.

CASSIDY: The only thing that mattered at that point was the hatch being closed.

Because if we couldn't close the hatch, we couldn't get good air pressure.

If we couldn't get air pressure, we couldn't get Luca's helmet off, and we can't get Luca's helmet off, pretty soon, he's not going to have air in his helmet to breathe.

NARRATOR: With Luca's comms malfunctioning and water now obscuring his vision, Chris must ready the airlock before crewmate Karen Nyberg can repressurize it.

(man speaking faintly on radio) As they speed through the normally 15-minute-long airlock procedure, Ground Control attempts to check in with Luca.

MAN (over radio): Hey, Luca, from Houston.

How you doing?

Give us a status.

(water bubbling) NARRATOR: But there is still no response.

CASSIDY: Luca, do you hear that?

(water bubbling) NARRATOR: The pair are positioned head-to-foot, making it difficult for Chris to see into Luca's helmet.

(static hissing) CASSIDY (over radio): Shane, I don't think he can, uh... His voice is going out or you, he can't hear you, but... I'm trying to see him.

Right about then, I grabbed his hand.

(over radio): Squeeze my hand if you're fine, Luca.

(static hisses) Luca, squeeze my hand.

Everything okay?

You okay?

And we had never pre-briefed this, never talked about it, but I squeezed his hand.

(static hisses) (over radio): Roger that.

Shane, visually confirmed, he's doing okay.

♪ ♪ Sorry to my wife, but, there'd been never a more comforting squeeze back of a hand.

♪ ♪ That sensation of feeling his hand squeeze back in a nice, controlled way was, like, "Okay, we're going to be all right."

(over radio): He looks fine.

He looks miserable, but, uh, okay.

MAN: Copy all, Chris, uh, we'll just status you from now on to check on him.

(Cassidy's mic crackling) CASSIDY: 14 PSI, Karen.

NYBERG: Hatch coming open.

(static hissing) MAN: Airlock, Houston.

Uh, if you could have some towels ready, that would be great.

NYBERG: We have them here.

PARMITANO (voiceover): As my crewmates from the inside were opening the hatch, I looked up, and I could see Karen, Karen Nyberg's face, and Fyodor.

The whole crew had come into the airlock to help me, and their faces, they were so worried.

CASSIDY (over radio): Hatch is open, Shane.

Luca's going through the airlock.

MAN: Copy, Chris, thanks.

The sense of friendship, camaraderie, it's a form of love that, that we should explore more, I think.

♪ ♪ I could feel it, I could sense it, I could see it on their faces, and I was moved.

MAN (over radio): Okay, Chris, just hang tight for a bit.

Once we get Luca taken care of, we'll work on you.

CASSIDY: No problem, I'm in no rush.

I got a ringside seat.

PARMITANO (voiceover): I was obviously relieved.

I knew that the ordeal was just about to end when they took the helmet off.

♪ ♪ I was all smiles, and I started cracking jokes.

They were certainly happy to see me.

I was certainly happy to see them.

♪ ♪ NARRATOR: A thorough investigation reveals that a clogged filter led to the leak in Luca's suit.

To ensure that astronauts can keep breathing if such a thing happens again, NASA has an absorbent pad and even a makeshift snorkel installed into each helmet.

♪ ♪ The way water behaves in space is just one aspect of microgravity, which is one-millionth the gravitational pull experienced on Earth.

And it's what helps keep the space station in orbit.

♪ ♪ It's a challenge to live and work in microgravity, but it also allows for groundbreaking studies to be carried out on the I.S.S.

♪ ♪ Biomedical experiments in space reveal that in microgravity, protein crystals form more uniformly, with fewer defects.

JENNIFER BUCHLI: They tend to be higher quality.

They tend to be purer and larger.

And so, this has really been a great application for the pharmaceutical industry.

NARRATOR: A team of Japanese researchers has taken advantage of the I.S.S.

's Kibo laboratory to investigate proteins associated with Duchenne's Muscular Dystrophy... ...a severe genetic disorder causing progressive muscle weakness.

BUCHLI: That series of experiments were really successful in finding some of the crystals associated with those proteins that they weren't able to do on the ground, and develop drugs to target those.

NARRATOR: Other experiments focus on the astronauts themselves and the impact the microgravity environment has on their bodies.

A vital area of research if humanity is to venture farther out into space.

And we give periodic samples of blood and urine and saliva, and, in some cases, number two to test all of the different outcomes on the body.

PARMITANO: We are both the conductors and the test subjects.

We don't like the term "guinea pigs" because, uh, because we're not, but we are subjects of the same, of the same kind of experiments.

♪ ♪ In the end, we are interesting in, what can we glean from performing science on the human body in space that we cannot on the ground?

♪ ♪ NARRATOR: Every part of the space station is crucial to conduct this science and maintain the lives of its inhabitants.

All the things we might take for granted here on Earth-- air, water, food-- have to be designed, calculated, and engineered to work properly in microgravity.

Even using the toilet.

CASSIDY: Close the lid.

CASSIDY (voiceover): You gotta be able to use all of your facilities, whether it's the bathroom or the air conditioning or the water processor.

All of those systems are there for a reason, because you need them.

Let's take a peek at the process.

NARRATOR: With so little gravity, space toilets use air flow to pull urine and feces away from the body and into the proper receptacles.

And this valve is what activates the whole system.

(voiceover): Going number one, you feel pretty confident with that.

You're effectively just turning on a fan, opening a valve, and, and peeing.

In good NASA fashion, we have a checklist.

This is called procedure 2.301, and in case things go wrong... Going number two is a little bit more... Yeah, you, you really reread every little line.

There's a plastic liner in there, where your deposit goes.

If you make the mess, you got to clean up the mess.

So you don't want to be that one.

You take this red tab, you pull it off the rim, and we do have a, a stick, and you push it down in there.

And then it's time to put a new bag on.

Ready to go.

(toilet flushing) NARRATOR: But the toilets are vital to the International Space Station in more ways than one.

CASSIDY (voiceover): Water is life.

Water is life on Earth.

Water is life on the International Space Station.

Every drop of water, whether it's your urination, whether it's your sweaty T-shirt from exercise, the moisture in our breath as we exhale, all of that moisture gets captured by the Environmental Control System and put back into the water processor.

NARRATOR: This fundamental part of the space station's Environmental Control and Life Support System allows the I.S.S.

to recycle 98% of the water that a crew brings along at the start of a mission.

CASSIDY: Every now and then, as you're filling your coffee bag, you think, "Wait a minute, this was yesterday's coffee, and it's also today's coffee," but the, the system works really well, and you get alarms long before it, it is anything but pure water.

NARRATOR: While the systems on board are designed to keep the crew alive, it is the responsibility of the crew to maintain the space station itself, a fundamentally symbiotic relationship.

Neither could survive for long periods in space without the other.

The I.S.S.

is the largest human-made structure in orbit ever built, with a surface area of well over 100,000 square feet exposed to the hostile environment of space.

One of the dangers in space that's outside the vehicle are what we call micrometeoroids.

So those can come from a number of sources.

LAWRENCE: Bits of cosmic dust traveling at a tremendously high speed can hit the shell of your module and put a hole in it.

♪ ♪ NARRATOR: Impact tests conducted on the ground reveal the destructive force of high-velocity particles, some no larger than a grain of sand.

Incoming particles can reach speeds of up to 162,000 miles per hour, up to 50 times faster than a speeding bullet.

The I.S.S.

has survived several direct hits to its solar panels and to the Canadarm2.

MICHAEL LÓPEZ-ALEGRÍA: The smaller pieces cause problems because they're going very fast.

So you multiply a very small number, which is the mass, times a very big number, which is the velocity, that's where you get the problem.

NARRATOR: The result is that even the tiniest particles can pack a huge punch.

To defend the space station, more than 100 shields protect the critical components.

A common kind is a stuffed Whipple shield... ...a multi-layered system of metal and synthetic materials, which can break up smaller meteoroids, dispersing their energy.

KWATSI ALIBARUHO: Russian modules tend to have thicker ribs and thinner skin.

So from a probabilistic perspective, the Russian modules are much more susceptible to small micrometeoroid penetrations.

On the flip side, the U.S.

modules have very, very thick skin.

So smaller micrometeoroids are not likely to penetrate the U.S.

module.

NARRATOR: But micrometeoroids aren't the only threat.

LÓPEZ-ALEGRÍA: Increasingly, it's orbital debris that comes from something human-made.

So a screw, a paint fleck, a rocket body-- I mean, it could be anything, right?

LAWRENCE: There are so many more objects in space.

There may be bits of derelict satellites that approach your orbit.

NARRATOR: With low Earth orbit becoming more crowded by the day, the probability of catastrophic collision increases.

CHARI: The good thing about low Earth orbit is, we've seen this huge boom of private companies and commercialization, and it's everywhere.

Now, the bad thing is, there's lots of old pieces of stuff out there, whether it's spent launch vehicles, whether it's dead satellites.

NARRATOR: Protecting the I.S.S.

now poses a unique challenge.

Today, more than 15,000 pieces of space debris are tracked in low Earth orbit, where the I.S.S.

resides, including fragments of used rockets and decommissioned satellites that, with no way to regulate their temperature, repeatedly expand and contract, until they break up or sometimes even explode.

And these are just the objects we are able to track, those larger than four inches.

Add in smaller fragments, and the total number of debris particles is estimated to be in the millions.

Each fragment can go on to hit more objects, creating exponentially more fragments and more collisions.

A potential nightmare for Mission Control.

The bigger pieces we track pretty well.

And we do debris avoidance maneuvers sometimes, because we know that there is a potential collision.

NARRATOR: In 2021, Russia launched an anti-satellite missile, deliberately destroying one of its own defunct satellites.

The impact created a massive cloud of high-velocity debris hurtling through low Earth orbit.

Although the initial pass missed the station, the danger was far from over.

Subsequent orbits of the wreckage required Mission Control to carry out avoidance maneuvers to keep the I.S.S.

out of harm's way.

But rogue impactors aren't always the problem.

Every maneuver, every docking, is painstakingly planned and monitored by the folks at Mission Control.

And during these tightly coordinated movements of the space station, controllers must be ready for the unexpected.

TRANSLATOR (over radio): The range is three meters.

(man speaking Russian) TRANSLATOR: Range... (man speaking Russian) TRANSLATOR: ...is 0.1.

We're standing by for contact.

NARRATOR: July 29, 2021.

After years of delays, the uncrewed multi-purpose laboratory module, or MLM, Nauka docks with the International Space Station.

It marks the first major expansion to the Russian segment in over ten years.

Once docking to the Zvezda Service Module is complete, at Mission Control in Houston, Zebulon Scoville takes over flight director duties.

SCOVILLE: I did a quick handover and plugged in, thinking that we were just going to be doing some minor reconfigurations and stabilizations of opening hatches, equalizing the atmospheres, and so forth.

It didn't quite go that way.

About one-and-a-half minutes after plugging in my headset, a small yellow stripe lit up across the caution warning board in the front of Mission Control.

My attitude control officer notifies me that we have a loss of attitude control, or LOAC.

NARRATOR: For reasons unknown, the I.S.S.

has begun to spin out of control.

SCOVILLE: We saw on the front screens, the video cameras coming from I.S.S., almost a snowstorm of particles around the outside of the I.S.S.

And it wasn't entirely clear what the source of this was.

This could be a coolant fluid that had vented, it could be a fuel or propellant that was leaking overboard, or it could be thrusters firing.

The fact of the matter is, we didn't know.

And so this is where we had to very quickly try to understand, what is this venting overboard?

What is it doing to I.S.S.?

How will we, first, make sure this crew is going to be safe and, second, get the vehicle under control?

MAN (over radio): Station, Houston on 2.

We need you in two decimal seven zero two.

This is an actual.

NARRATOR: French astronaut Thomas Pesquet and his six crewmates are onboard at the time.

(speaking French) (translated): "This is not training."

That's the message we don't really want to hear during a six-month mission.

♪ ♪ SCOVILLE: If a space station spins too fast, this could introduce several potential hazards.

If it were to flip upside down, now our antennas, which are normally pointing towards our communication satellites, are no longer pointing up.

So we risk losing communication with the space station.

PESQUET (translated): We have solar panels that are 35 meters across and 70 meters long and very, very fragile.

The acceleration of rotating them will generate centrifugal force, and that can tear everything apart.

SCOVILLE: We have docking mechanisms with the crew vehicles that are used as lifeboats.

If we spin too fast, if we had to undock in an emergency, those hooks might jam.

PESQUET (translated): It's really very, very dangerous.

A space station is very fragile.

It doesn't even have to carry its weight.

NARRATOR: The advantage of building a station in the vacuum of space is that you don't have to fight things like wind and gravity, so the structure can be less sturdy.

The problem is, any unexpected twisting could break it into pieces.

Ground Control desperately needs to understand what is causing the I.S.S.

to turn.

SCOVILLE: So one of my first questions was to my life support officer.

(over radio): Are we maintaining cabin atmosphere?

Cabin atmosphere is stable flight.

Okay, what about cooling systems?

Thermal systems are stable flight-- we're looking good.

NARRATOR: The team ticks off a checklist of possible causes... (man speaking on radio) ...until a plausible suspect emerges.

SCOVILLE: One of the main and most likely candidates was that the Nauka module was firing thrusters.

NARRATOR: A software error on the Nauka means that it doesn't recognize it has successfully connected to the I.S.S.

(translated): And so it will try to establish its orientation, to move, to capture the sun, to recharge its solar panels.

The problem is, it starts to take the whole station with it.

NARRATOR: Nauka's thrusters start firing, trying to pull away from the space station it is now securely docked to.

MAN (over radio): Station, Houston on 2.

We are out of attitude by at least 45 degrees and increasing.

NARRATOR: Four huge gyroscopes on board the I.S.S.

keep the station in position.

As the Russian thrusters fight with the space station's gyros, there's concern that the joints of the aging ship can't take the strain.

So the decision is made to shut the gyros down... (alarm beeping) ...reducing the stress on the attachment points that connect the station's modules together.

The I.S.S.

is now in free drift.

And there's an added complication.

SCOVILLE: Unfortunately, the Nauka module could only communicate with the ground if the station was physically over the Russian ground sites.

As it was, we were just heading off of the eastern coast of Russia over the Pacific.

And so we had no insight, no telemetry, no data, and no ability to command it or turn it off.

We weren't going to get that back for about 90 minutes.

NARRATOR: In Houston, flight controllers listen to real-time translations of the communications between Moscow's Mission Control and the cosmonauts on board the station.

(people speaking Russian over radio) TRANSLATOR: Oleg, please, can, can you confirm or not whether MLM thrusters are still firing?

Do you see, do you hear anything?

(people speaking Russian) TRANSLATOR: I can confirm that the thrusters are still firing.

They have just fired, and we could feel it.

(translated): At the time, I don't know if we fully realized the seriousness of the situation.

♪ ♪ SCOVILLE: We've got one-and-a-half hours to ride this thing out before we're going to be able to completely stabilize the system.

At this point, in order to assure we had every advantage we could, I declared a spacecraft emergency.

NARRATOR: A spacecraft emergency ensures that all attention is focused on the operation at hand.

♪ ♪ All available ground antennas across the U.S.

now attempt to make contact with the I.S.S.

TRANSLATOR (over radio): Oleg, we did not copy your last.

Could you repeat your last?

NARRATOR: But still, the connection between the ground and space is patchy and unpredictable.

(people speaking on radio) And, with the whole I.S.S.

structure now turning away from its communications satellites... (people speaking on radio) ...U.S.

operations are about to lose contact with the crew.

(alarm beeping) SCOVILLE: We were tracking the space station spin, and we could predict we had minutes to communicate several steps of procedure to the crew before we were going to run out of time.

PESQUET (translated): Step number one, make sure to secure everything that is outside the station that can move, that can rotate.

(man speaking on radio) NARRATOR: The crew are instructed to lock the station's radiators, which are at risk of shearing off, while the solar panels are repositioned.

♪ ♪ Meanwhile, at Mission Control, the team formulates a plan to fight back against Nauka's thrusters.

MAN (over radio): We want you to run the procedure... ♪ ♪ SCOVILLE: We knew, as the Nauka module's thrusters were firing, that we had to bring a bigger lever arm, a more powerful force, to bear against these thrusters.

NARRATOR: For that extra muscle, Ground Control turns to the Zvezda Service Module, primary source for the I.S.S.

's propulsion systems.

At this time, a cargo ship, Progress 78, is also docked to the station, on the opposite side to the misfiring Nauka.

Crucially, both Progress 78 and the Zvezda have thrusters.

SCOVILLE: Now, typically, there will be days of analysis.

In this specific case, this was unexpected, and so there was a little bit of engineering judgment that had to be applied, recognizing that the alternative and indecision would have been worse.

NARRATOR: Mission Control makes the decision to fire up the Zvezda and Progress 78 thrusters to set up an orbital wrestling match with the Nauka that they have to win.

♪ ♪ MAN (over radio): Hey, Aki, just to update you guys, so right now we're in a little bit of a tug-of-war between thrusters firing from both the SM and the MLM.

And we're sorting through the best course of action, uh, right now.

At this point, we have no further actions for you.

NARRATOR: For nearly 45 minutes, the I.S.S.

drifts out of control.

A combined effort of the Zvezda and Progress 78 modules pushes back against Nauka.

It's putting huge strain on the space station's structure.

SCOVILLE: This was like riding a bucking bronco and we've got to make it the full eight seconds, except this time, it was in orbit around the entire Earth.

♪ ♪ NARRATOR: It will be another 45 minutes before Russian controllers can regain command of Nauka's thrusters.

♪ ♪ But then, all of a sudden... ♪ ♪ MAN (over radio): Aki, uh, we can report that the MLM thrusters are no longer firing.

We are back in attitude control.

Rates are stable.

AKIHIKO HOSHIDE: Okay, copy.

NARRATOR: Nauka's thrusters fall silent.

Its fuel tanks have run dry.

The tug-of-war is over.

♪ ♪ SCOVILLE: As we brought the space station back to normal attitude, I think we all took a, a deep sigh of relief.

NARRATOR: After rotating one-and-a-half times, around 540 degrees, mission controllers can finally bring the station to a halt and begin a controlled return to its correct attitude.

SCOVILLE: The team that I had working in the control center that day was an example of performance that to this day I will always remember.

Not because we did everything perfect, but because of how we came together.

♪ ♪ NARRATOR: Since its launch in 1998, the I.S.S.

has been humanity's laboratory in the sky.

BUCHLI: Over the past 25 years, we have served over 5,000 researchers on the International Space Station, conducted over 4,000 different experiments... ...and published right around 4,000 papers.

ALIBARUHO: The fact that we've learned so much, not just about technology, but about ourselves, as a result of the I.S.S., that's a legacy that's not to be denied or discounted in any way.

♪ ♪ NARRATOR: For many, the value of the I.S.S.

goes far beyond the physical.

It's about perspective, hope, and the shared pursuit of something greater.

PARMITANO: The space station is not just a place where we produce science, but it's a place of emotions.

It is a place of humanity.

A place where we brought with us our own dreams.

♪ ♪ PESQUET (translated): It's absolutely phenomenal.

There is a technical feat, but I think that the dream and the conception of the world, a bit of multilateralism and scientific cooperation between the countries behind it, which for me, are even more beautiful.

CHARI: A quarter-century of peaceful cooperation, multiple countries, multiple organizations, all working with the focus of trying to make humanity better, to me, that seems like a pretty powerful and lasting legacy.

♪ ♪ NARRATOR: But with aging technology and the advent of new, more efficient space platforms, the station's days are numbered.

SCOVILLE: The I.S.S.

was never meant to be a permanent structure in space.

NARRATOR: The International Space Station is scheduled to begin its final chapter in 2030.

♪ ♪ SCOVILLE: About a year and a half before the space station deorbits, we will launch a crew that will stay there for a year.

They will be in charge of getting the last bits of science and engineering we can out of this facility.

♪ ♪ About six months prior to the deorbit process, that crew will come home, leaving the space station uncrewed as its altitude starts to lower.

♪ ♪ NARRATOR: A deorbit vehicle will attach to the I.S.S.

and gently guide the station into a slow, deliberate descent, where it will enter the thick layers of Earth's atmosphere at speeds over 17,000 miles per hour.

♪ ♪ Experiencing temperatures close to 3,000 degrees Fahrenheit.

♪ ♪ SCOVILLE: We know some of it will burn up, but large pieces of it will survive through the atmospheric re-entry, and that's why we want to make sure that it's a controlled process, so we safely hit the Pacific Ocean.

♪ ♪ NARRATOR: The I.S.S.

's fiery disintegration will be the final act in its incredible journey.

♪ ♪ But the lessons we learn from its time in space, the technologies developed, and the unity it represents will echo in future space exploration for generations to come.

RENITA FINCKE: The I.S.S.

is not just a international laboratory that's orbiting in space.

It's much, much more than that.

♪ ♪ LAWRENCE: No matter where we come from, no matter the differences in political systems and the leaders that we have, we really can work together.

We have that ability.

♪ ♪ CASSIDY: It's just really fun to think about the future of space exploration and to know that I was, in a small way, part of the International Space Station's story.

♪ ♪ The International Space Station is the crowning achievement of human ingenuity so far.

(static hissing) (device beeps) ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪ ♪