TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. The six-part documentary series "Among The Stars" gives viewers a behind-the-scenes look at NASA as it prepares missions on the ground and executes them aboard the International Space Station. One of the astronauts featured on the series is Chris Cassidy. Our producer Sam Briger spoke to the now-retired astronaut about his time in NASA and on the space station. Here's Sam.

SAM BRIGER, BYLINE: The Disney+ documentary "Among The Stars" follows Chris Cassidy on the Earth as he preps for a mission to fix an experimental module on the International Space Station that just might tell us about the beginnings of the universe. But due to a rocket malfunction, he doesn't get to actually make that trip and has to watch his friends aboard the station carry on without him. Cassidy eventually makes it back to the space station as its U.S. commander. He's been to space three times, his first aboard the Space Shuttle Endeavour in 2009 as it delivered a Japanese experiment module to the station. Before joining NASA, Cassidy was a Navy SEAL and was awarded the Bronze Star. Let's hear a scene from "Among The Stars." The first voice we'll hear is Chris Cassidy.

(SOUNDBITE OF DOCUMENTARY, "AMONG THE STARS")

CHRIS CASSIDY: You know, there's a few things that we do as astronauts that really make you feel like an astronaut, and spacewalking is one of those things. I really just enjoy those days. I enjoy everything about the spacewalk.

UNIDENTIFIED PERSON #1: Shane (ph). Good morning to everyone in Houston. Great to be with you.

UNIDENTIFIED PERSON #2: Good morning, Chris and Luca. We are ready for you to head out over to the Z1 sites, and we'll get you set up and ready to go.

UNIDENTIFIED PERSON #1: Copy that, Shane.

CASSIDY: It can be a little frightening the first hour or so when you're out there and every now and then, even when you're experienced. You stop and take a look. It's a beautiful sight. When you see the Earth out on a spacewalk, it's usually between your toes, and you're seeing the Earth go by at 5 miles a second. And you know how far it is between cities and coasts, and you're seeing that distance happen within minutes. It really blows your mind. You have to actually tell yourself, hey, you're here to do a job and focus on it.

BRIGER: That's Chris Cassidy from the Disney+ documentary "Among The Stars." Chris Cassidy, welcome to FRESH AIR.

CASSIDY: Oh, thank you. Glad to be with you, Sam.

BRIGER: So the series starts out with you on a spacewalk with your fellow astronaut, Luca Parmitano. And this is - I think you're doing maintenance work on the space station. But during the spacewalk, there's a real problem. Parmitano's helmet starts filling up with water. Can you describe what was happening there?

CASSIDY: Yeah, you know, that was really interesting. In fact, that was the second spacewalk of two in a week. And so six days earlier, we had completed a spacewalk which had gone largely normal until the very end when we took our - we got inside and took our helmets off, and we noticed some amount of water on Luca's head. When talking with the engineers and us, we assumed it was one source of water, which is our drink bag, that had leaked in a way that got around his - the crown of his head. Fast-forward a week later - 45 minutes into the spacewalk, that same water he felt on his head, and this time it was - he could tell it was growing. The water was cold, which meant that - the only source of cold water is coming from the technical backpack - you know, the technical systems on the backpack. And that led us to the immediate conclusion that this is not normal, and we need to head in.

BRIGER: How dangerous was that situation? Like, could he have possibly drowned in his suit?

CASSIDY: So you definitely don't want water inside your helmet on a spacewalk. The outside - you know, everybody that's listening probably realizes it's a vacuum. There's no pressure outside. So the suit is your only source of pressure, which is therefore your only source of life. In order to get your helmet off, you must get back into an environment where you have pressure around you, and that exists only in the airlock, so as the water was accumulating on his head and covering - in short amount of time, covering his eyes, saturating his ears and is in his nose, all around his mouth, it became more and more apparent that we needed to really, in an expedited way, get his helmet off and get - make sure that he could have a pathway to air. And that was the process. This led us back to the airlock.

And then repressurizing the airlock is not a quick thing. I mean, it takes about 15 to 20 minutes to get it to a place where you can safely take the helmet off. And during that time, he and I were monitoring - I was doing it visually, he was doing it by could he get air? - but monitoring his ability to breathe and trying to race against the clock to when we would have enough pressure to take the helmet off and assure that he could breathe.

BRIGER: Right, because obviously you're in space. There's no gravity, so the water can just sort of move about and just go into his ears, his nose and just stop him from breathing, right?

CASSIDY: Exactly. And ironically, the water - the surface tension of the water really drives its behavior, and it'll stick to whatever it's stuck to, even if you - when it gets on your head, even if you shake your head, like, vigorously, back and forth, it really doesn't want to fling off. So once the water droplets stuck to his head, they were there and started to accumulate and spread around his head like a layer of frosting on a cake.

BRIGER: So the show "Among The Stars," you know, is a lot about this mission to repair this very expensive piece of equipment that's attached to the outside of the space station, and it's called the Alpha Magnetic Spectrometer, or AMS. First of all, can you just tell us what it is?

CASSIDY: It's a device that's about physically the size of a minivan, and the guts of it analyze deep space particles to determine the origins of life. You'll have to watch the show to get the Reader's Digest version on how that exactly happens, but that's my level of understanding, is it's very important to the - you know, the larger physics community and therefore to understanding the universe we live in.

BRIGER: And so what was wrong with the AMS that - what had to be repaired on it?

CASSIDY: Now this part is up my alley. The system itself is very sensitive to temperature, and space is a harsh-temperature environment - really cold in the shade and really hot in the sun. So you need a robust temperature regulation system, and that requires coolant to be circulating around it at a precise temperature, and then in order to pump that circulating fluid, you need pumps. And because those pumps are so critical to the life - to the health and wellness of the experiment, they had four redundant pumps. And you only need one for it to work, but because of a manufacturing flaw, all of the pumps had or were about to fail. I think two of the four had completely failed, and we were limping it along in that last year - 2019 into early 2020 - to keep the pumps running at a complete failure that the machine would not work.

BRIGER: So basically, you have to, like, bring this new piece of equipment and attach it to the AMS so it's providing the appropriate amount of coolant to the spectrometer. And when I was just thinking about it - like, if you had to do this on Earth, I bet this would maybe take, you know, like, 30 minutes or an hour to do - right? - like, to just attach this new thing to this other piece of equipment. Just give us a sense of how long it took in terms of, like, planning this operation out and actually, like, the amount of time in space doing spacewalks to fix this piece of machinery, took four spacewalks of about seven hours long, so you're somewhere in the 28-hour range for total time to do the repair in flight.

The preparation on the ground was over the course of about two years, and that was largely to make sure that we had the right mechanism to connect the new pipes to the old pipes. And these little - I say pipes. It's really a tube about the size - a little skinnier than a No. 2 pencil. Yeah, about half the diameter of a No. 2 pencil. And there were - there's four pumps. Each pump needs an in and an out. So there's eight lines that you have to cut. And how do you meet two metal tubes together, pointy end to pointy end? That's what - it took a long time to really make sure we had this designed properly, and that mechanism was called a swage fitting. Those exist in application and industry on the ground but a little bit different in zero gravity and making sure that you can operate them with big, heavy gloves on. And how do you do this fine-tuned surgery in basically a ski parka and ski mittens? That's the hard part.

BRIGER: So yeah. So just talk about how hard it is to actually do things in space. Like, to use a wrench or to, you know, cut something, like, how much dexterity do you have?

CASSIDY: Well, the answer to that question is whether or not you're inside the space station or outside. Most of the six months I was in space, you're doing all of that inside the space station and you don't have the gravity. So the tools and the little nuts and bolts are all floating around, but you still have your normal hands and your dexterity and you have your mouth you can hold something. And we use duct tape in creative ways to help you. So there's a little bit of challenges working inside the space station, but it's complicated 10x or more even when you go outside and you're wearing a space suit and trying to manipulate those same small things with big gloves on. And you can't put something in your mouth and you don't have the use of duct tape. So it's a little bit trickier. And therefore we design the tools to help astronauts in that environment.

BRIGER: So when I was watching the film, I saw that there was all this - there's just all this stuff on the outside of your spacesuit. Like, there's all these metal tools attached to the chest piece and then you have - it looks like you have maybe a mirror on one of your sleeves and a notebook on the other. Like, what are all those things doing on your suit?

CASSIDY: Well, they're all there for a purpose. The, quote, "notebook" is really the emergency checklist. So if you have a problem with your spacesuit and you get a different alarm, you can flip to the correct page, and it'll tell you what your corrective actions are all the way down to go back inside as fast as you can.

BRIGER: (Laughter) Yeah, you don't want to get that one, right?

CASSIDY: You don't want to get that one, which is the one - which was the message basically for the water in the helmet. Like, get out of here, you got no hope. The mirrors are for your switches because a lot of the switches are on your belly, the belly of the suit. And you can't look down and see them. So they're - the switches are labeled in backwards writing and you hold your mirror in front of you and then you can read those switches. The rest of the stuff is - means to hold things like a tool belt that a carpenter would have. We use the similar thing. It's just mounted on your chest and all of your tools are fixed there.

BRIGER: So when you're out there, it looks like you're really holding on to - there's, like, handrails all over this - all over the outside of the space station. And you're, like, moving around, you know, moving your hands along these rails. Are you also always tethered to the station in some way?

CASSIDY: You're always tethered to a station. That's then - your hands are your first defense from floating away. But as a secondary and tertiary method, we have a long kind of, like, metal braided cable like a shark fishing line that retracts and reels out in the same way a dog leash does. And then once you get to where you're going, you take a small little two foot - two- to three-foot-long fixed rope and connect yourself so that you can let go and you know you're always going to be within arm's length. The yellow handrails that you see all over the space station are put just for that purpose to travel on with your hands and then to hook to once you're ready to be in one spot.

BRIGER: So you're always doing these walks in teams, right? But what would be the protocol if somehow someone, you know, God forbid, was untethered and started floating away from the station?

CASSIDY: Yeah. First and foremost, we train to great lengths to not have that happen, but there's a couple things besides the tethers that I just described. We do have a very small amount of compressed nitrogen gas that you can pull out a controller and manipulate those gas little spurts to stop your floating away and then transfer that velocity back towards the space station. Normally, though, the primary means would be that dog leash would slowly reel you back if you did slip away with your hands. We trained for all those scenarios because you basically have the rest of your life to figure it out if you fail to connect.

BRIGER: Which is probably not very long.

CASSIDY: Not very long, yeah.

BRIGER: Well, let's take a short break here, and we'll be back. If you're just joining us, I'm speaking with retired astronaut Chris Cassidy, who's featured in the Disney+ documentary series "Among The Stars." More after a break. This is FRESH AIR.

(SOUNDBITE OF THE ACORN'S "LOW GRAVITY")

BRIGER: If you're just joining us, our guest is retired astronaut Chris Cassidy, who's featured in the Disney documentary series "Among The Stars." As it turns out, you were preparing to do these repairs to this module, the AMS, but because of a malfunction of a Soyuz rocket, you weren't going to be up in space at the station in time, and other astronauts that were already scheduled to go to space would have to take over. So first of all, during this, like, period of the space program, NASA was buying tickets to the space station on Russian Soyuz rockets. Can you explain that?

CASSIDY: Yeah. The space shuttle retired in 2011, and until the SpaceX Dragon started flying people in 2020, that put us in this nine-year period where the only way to get to the space station was on the Russian Soyuz rocket, and therefore other countries - U.S., Canada, Japan, European countries - we would buy seats from the Russians. And that included the training leading up to the flight and the flight itself and the return back to Earth. So two of my missions were this way.

BRIGER: So Chris, NASA's now partnered with the company SpaceX, who is transporting astronauts to the space station. How has that changed the program in your mind?

CASSIDY: Well, you know, for one, it takes us out of the dependency on the Russians to get our people to and from the space station. Now we have an additional means to get there and launching folks from Florida, from U.S. soil, and that's all good stuff. Ultimately, I think that with the success of SpaceX and, hopefully, Boeing soon, these commercial companies can take care of all the low-Earth orbit type of missions and allow NASA's to focus on going to Mars, return to the moon and those more expensive missions that are harder for a commercial company to make money in.

BRIGER: Are - do you have any concerns about a commercial company taking over the lead here?

CASSIDY: No, no. I think it's a great partnership and makes American people excited. And therefore, I think it's all healthy for space exploration.

BRIGER: So we see one of these - we actually see the first SpaceX rocket launching to carry two American astronauts to the space station, and you were on the space station to greet them. And I have to say, checking out the SpaceX rocket compared to the Soyuz or just what it looks like on the space station, it's so sleek. And like, the astronauts' spacesuits look all futuristic compared to NASA's spacesuits. Did you notice that?

CASSIDY: Oh, yeah. You know, if you ever seen a Tesla, a brand-new Tesla, that's the same feeling that you have inside the SpaceX Dragon crew capsule. It's a big glass screen with all touch button controls, very - a few switches that are, you know, mandatory for emergency things. But for the most part, you described it perfectly - just a very sleek, clean machine.

BRIGER: So your first trip to space was on the space shuttle in 2009, I think. And this - you know, this was after the Columbia disaster of 2003 and also after it was announced in 2004 that the shuttle was going to be retired. And I read that your launch was actually delayed, like, five times. Some of those were because of weather, but two times it was delayed because of hydrogen leaks, which I think is potentially a dangerous situation. So I'm just wondering in these final years of the shuttle, did you ever worry that getting onto the shuttle was sort of more of a risky venture than it should be?

CASSIDY: Well, you - every time you climb into a rocket and light it on fire, it's a risk. But we learned - as the shuttle program matured, we learned with each mishap and near-miss mishap what those risks are and how to deal with them and how to protect against them. And the more you know, the more you have to analyze. So I think that in the beginning part of the shuttle, we we're flying a lot of it on - I wouldn't say luck, but with a little bit of a a blind spot to what some of the risky points were or just how risky some of those places were. An example is the foam - the thermal protection foam that came off on the Columbia accident. We didn't know about that for many, many launches. Subsequent to that, there was a great deal of focus on, do we have all the foam intact, yes or no? And if it's all intact, then you can feel reasonably safe. So there was definite learning curve as the shuttle program progressed.

BRIGER: So at that point, when you're being launched from the Earth, you're strapped to this rocket, do you have to just accept a certain powerlessness of your situation? Like, something really bad could go wrong, but there's not really a lot that you can do at that point. Do you just have to give yourself up to the moment?

CASSIDY: Well, you could look at it that way, but we do have a lot of checklists, and we train to be working malfunctions and problems right until the - you know, all the way as far as you can. And so that's why we spend countless hours in the simulators doing exactly that. The instructor team will fail certain components in this - in the computer simulator, and we respond. And then they fail another thing, and we respond. And they fail another thing and another thing until some of the cases are, by design, really not survivable. But it teaches you the mindset to triage just like you - as an ER doctor would in the emergency room and deal with the biggest problems first and then kind of get yourself well one case at a time.

BRIGER: We're speaking with retired astronaut Chris Cassidy, who's featured in the Disney+ series "Among The Stars." We'll be back after a short break. This is FRESH AIR.

(SOUNDBITE OF SUN RA'S "I DREAM TOO MUCH")

BRIGER: This is FRESH AIR. I'm Sam Briger sitting in for Terry Gross. My guest is recently retired astronaut Chris Cassidy, who's featured in the six-part documentary series "Among The" Stars that's on Disney+. It's about NASA and its missions on the International Space Station. Cassidy was the 500th person in space, which he's visited five times. The last was a six-month stay on the space station as its commander.

So let's talk a little bit more about living on the International Space Station. First of all, like, how big is it? How many different areas are there?

CASSIDY: Well, I like to describe it in terms of school buses connected together. Imagine eight or so school buses connected in different directions.

BRIGER: And what are your sleeping quarters like? It sounds like you usually have your own place, although maybe right now someone's sharing because there's six sleeping quarters and there's seven people. But, like, you're usually sleeping by - alone. And, like, are you attached to the wall or something? Like, how do you sleep in space?

CASSIDY: Yeah. Each sleeping quarters is about the size of a refrigerator, and that's your private space. So you put your sleeping bag on the wall, you tie it to the wall. And you zip yourself into the bag when you sleep at night. And then you've got some bags of clothing and pictures on the wall of your family and a computer to do email and to make telephone calls or to watch movies. But for the most part, you just go in there to sleep or to change your clothes if you need privacy. But the rest of the time you're out and about, zipping around, working on experiments or fixing broken things or preparing for a spacewalk or whatever the case may be.

BRIGER: But is it hard to sleep on the space station?

CASSIDY: It's a little hard to sleep you because you're - the first week or so you're just getting used to not having a pillow. And you're not feeling the pressure on your whole body as you lie down on a mattress, as you would here. And so that new sensation and kind of telling yourself that it's time to relax, it takes a little bit of getting used to.

BRIGER: What are some of the difficult things that we take for granted here on Earth that just are harder in space?

CASSIDY: Well, it's easy to move heavy things. You know, like a thing a size of a refrigerator, you can move it around all by yourself and turn corners and manipulate it and all that. Something that is small and light, like a bag of washers or nuts and bolts, those are incredibly hard to deal with because they just float in a million different directions and quickly get us - get out of your view or get out of control. So it's very easy to lose small things. It doesn't - things don't fall to your feet like they do or you're accustomed to on the ground. They get sucked up in the air conditioning vents. And ultimately, you'll find things three or four days later in the air conditioner return filters if you do lose something.

BRIGER: And how long is a day in space on the space station?

CASSIDY: Well, we keep it kind of the same. Let's back up from sleep. We always protect for eight hours of sleep for crewmembers, and the rest of the day is kind of broken up into workday from roughly 7:30 in the morning to about 7:00, 7:30 in the evening. That 12 hours includes a break for lunch and about two hours for exercise, including cleaning up from exercise. And then the rest is your work day. And then the other time outside of that from 7:30 in the evening till bedtime is what I call the - what we call it now - prep for sleep, which is equivalent to you're done with work here on Earth. You're driving home. You stop at the grocery store, you get milk and eggs. And then you go home and you fix dinner. And then you watch the news and read a newspaper and go to bed. Kind of all that same stuff.

BRIGER: And how many sunsets do you see in the span of one space day?

CASSIDY: In one 24-hour period, you see 16 sunrises and sunsets.

BRIGER: Wow (laughter).

CASSIDY: We're not necessarily by the window every single one of those, so it's more like you float by and you look out and go, oh, it's daytime out here or, oh, it's nighttime out. It really doesn't affect your circadian rhythm or anything like that.

BRIGER: I have to ask you about peeing and pooping and waste management. I'm sure that's probably the most common questions you're asked, but...

CASSIDY: Yes. It's so common, in fact, that I made a video about it on YouTube. You can search Chris Cassidy space toilet, and it's very informative because I go through the whole soup to nuts, so to speak. But...

BRIGER: Spare us from from searching on YouTube if you could give us a quick summary here.

CASSIDY: No. 1 is pretty easy. You take - there's a tube, and you pull it off the wall. For both operations, you turn on a switch. It activates a fan. So there's a little bit of air flow. Your urine goes in this tube and actually goes into the water processing system. And that urine ultimately turns back into drinking water. We have filters and things that take out the nasty stuff, and we change out those filters periodically. That's actually a critical component for future space exploration. Reclaiming that urine into water saves tremendously on the amount of up-mass that we need to do. Sending water for a six-month mission, for example, would be crazy. But I digress.

Let me finish the thoughts on No. 2. No. 2, you go - the space toilet is pretty small. It's the circle that you - where the hole where you got to put your deposit is about 6 inches in diameter. It goes into a little plastic bag, including the paper to clean yourself, and then that plastic bag then falls into this bucket that's about the size of five-gallon, you know, five-gallon bucket. And then it lasts maybe a week, and then you change out to a new bucket.

BRIGER: So then is the waste eventually carried back to the planet?

CASSIDY: It's - we put it in a cargo ship and that will ultimately take away all of our trash. And these cargo ships burn up in the atmosphere. So that's where the poop goes.

BRIGER: So we don't have to look up...

CASSIDY: We don't have to look up and worry. Yeah. It burns up.

BRIGER: You know, a lot of your time up there, you're doing these experiments. Can you describe one of the experiments that you did?

CASSIDY: Well, there's so many. But there's a category of biological ones where we, the astronauts, are the subject, varying things - food, diet, exercise. And we give samples of all our fluids. And other things that are living animals - insects, lettuce, tomatoes, radishes, that kind of thing. Those are kind of cool ones to see because particularly things that are growing, you can follow it along. And the crew, everyone on the crew likes to check, you know, how's the radish doing today? There's other ones I participated in - with fire safety. You know, we had a control box where we burned different things to see how the combustion of fire-retardant materials varied in space versus home.

BRIGER: That sounds like a pretty dangerous experiment.

CASSIDY: Well, it can be, but it - trust me. NASA doesn't do anything dangerous. So there's probably a belt and suspenders for every level of protection on that experiment. And one of the cool ones - I thought it was cool - was watching how water droplets behaved and experiment with a water sprayer. There's other things that have much more significant impact to life on Earth, like tissue growth. You know, growing a tissue on a petri dish on Earth, it's very two-dimensional and flat. In space, it can grow in three dimensions, which is a game changer. Let's see. DNA sequencing was a part of - there's lots and lots and lots, and the list goes on.

BRIGER: You say that a lot of things can kill you in space. What are some of the biggest concerns that you have while you're on the space station?

CASSIDY: You know, everybody sees the launch day and the reentry day as two high-profile days, and they are in terms of risk. But there's lots of debris up there in the space - in space, as evidenced by most recently I think a week or two ago. There was some Russian debris that was scattered around from a satellite that they shot.

BRIGER: Yeah, they were testing an anti-satellite weapon, I guess.

CASSIDY: Anti-ballistic - yeah, anti-satellite weapon. And that poses a great risk to the crew and to the vehicle. The space station is in good shape, but it's been up there for 20 years. And some of the metal has been exposed to extreme heat ranges and torques based on the rotation of the spacecraft for all these years. And I think that some of the metal could fatigue at some point.

BRIGER: So, Chris, just how dangerous is space debris or space junk? Like, is there so much of it that you have to be consciously looking out for it? Like, when you're in a space capsule, like, do you have to avoid it? What's the deal there?

CASSIDY: Well, it's a big sky. Like, you don't - you look out the window, you don't see space junk flying all around. And we talk in missed distances of kilometers is too close for comfort. So it's not like they're zinging by 10 meters away from you, and you think, oh, that was close. No, these are - we don't ever see them, and they're tracked by tracking stations on the ground. But that said, it is the biggest risk of a space mission is getting hit by a particle. And I've been out on spacewalks, and pretty much every spacewalk I've been on, you can see divots in the metal, the outside skin of the space station, where particles have hit. And they haven't had enough energy to poke a hole, but you can definitely tell it's getting hit.

BRIGER: If you're just joining us, I'm speaking with retired astronaut Chris Cassidy, who's featured in the Disney+ documentary series "Among The Stars." More after a break. This is FRESH AIR.

(SOUNDBITE OF OSCAR PETERSON'S "GOD REST YE MERRY GENTLEMEN")

BRIGER: If you're just joining us, our guest is retired astronaut Chris Cassidy, who's featured in the Disney documentary series "Among The Stars." So let's talk about the actual return from space. It sounds like it takes six hours. What are the dangers of reentry? In the documentary, you talk about how you can bounce off the atmosphere and just head off into space.

CASSIDY: Yeah. You know, you're - the atmosphere is a skinny little layer around the Earth, and you have to penetrate it to get back home. And it's way beyond my level of math, but there's a certain angle where you have to hit it just right. Like, if you're skipping a rock on a lake, if you do it too shallow, it skips and skips and skips, which is your goal when you're skipping rocks. But when you're returning to Earth, you don't want to just skip and skip and skip. You want to actually go into the atmosphere, which is akin to the lake. If you throw a rock straight down at the lake, it goes crashing down and doesn't skip at all, and it goes too hard, impacts into the rocks below. So we're trying to find that exact balance between just a nice angle and not too steep where it's too much on the people in the spacecraft. So that's the gist of it.

BRIGER: Tell us about the deorbit burn. How long does that take?

CASSIDY: The deorbit burn is about four minutes long, 4 1/2 minutes long. And it's a very, very, very tight calculation, and we're timing it down to the second. And if it - we have these - the procedures - emergency procedures has it - if you under burn or over burn, which is the same thing as are you going to skip or go in. If you under burn, you could skip. If you over burn, you could slow yourself down too much and dig in too hard and have too high Gs on coming back. So that deorbit burn is really critical to get to slow down to the exact speed you need to.

BRIGER: And it looks like your capsule is just in flames at that point.

CASSIDY: It's totally in flames. In the Soyuz, there's two windows on either - a window on either side. And all of a sudden, the whole capsule inside turns like this fire orange color. It's really freaky.

BRIGER: Well, I bet (laughter). So once you successfully land, you've been in space for six months not dealing with gravity. Can you actually walk? Like, it looks like when you came out of the capsule, like, people were carrying you.

CASSIDY: Yeah, right when we land - and again, I'm talking all Soyuz. It's slightly different for SpaceX and how they land in the water, and then they get put onto a ship. So the process is just a little different. But for the Soyuz, we land on land in the steps of Kazakhstan. The helicopters find you right away, particularly if the weather is clear, which it was in both my landings. You could possibly open the hatch yourself and get yourself out, but it's a lot of work. It's very - everything feels extremely heavy, including your head and your body.

So we just sit there and wait about five, 10 minutes for somebody to open the hatch and assist us and pull us out. And then just like you described, they carry us to some chairs. We sit in the chairs and get some early medical attention. And over the course of about that next 45 minutes to an hour, you're slowly getting used to it to the point of, after that long, you can take your suit off and not get too sick. And then in a gingerly way, walk to the helicopter with some assistance. Over the course of the next 24 hours, we make it back to Houston. And by then, I was able to walk off the airplane and be with my family.

BRIGER: But then you have a two-week quarantine period - right? - and rehabilitation.

CASSIDY: The two-week quarantine period was a new thing for me this time. And I got to be honest, Sam, that was the worst part of the being gone for six months was being 3 miles from my house and my wife and not being able to go home.

BRIGER: Yeah. I bet.

CASSIDY: Normally, in non-COVID times, you land back in Houston and, with the exception of a couple of hours of initial medical check, you can spend that first night at home in your own bed.

BRIGER: What do you think are the next steps for manned space travel and - like, so what do you think we should be considering doing next? And what are some of the main obstacles you think there are in terms of a deeper space travel?

CASSIDY: If you think about in 1969, when Neil and Buzz were about to step off the lander and walk - put footprints on the moon for the very first time, the world was following along with grainy black-and-white televisions and radios. And it was something that galvanized everybody. Now, when somebody does that on Mars, the whole world will be watching live on their internet, on their handheld device on their - that they're looking at, you know. We're all connected that way. So I think that it's hard to put - because I'm often asked, what's the value in spending money in sending people to Mars?

But I think it's that overall boost to humanity that - it's not going to be one country that goes to Mars. It's going to be a crew of four different countries, all of those people sharing in the excitement and therefore all those countries and continents and people feeling that excitement together. And that's where I think our - that's what our goal is as NASA and as a manned space community is to to put people on Mars. And sure, there's definite benefits and jobs and technology that will come from having several big programs that are required to get people there. But ultimately, it's that sense of camaraderie around the world that we'll all feel when we're watching it happen.

BRIGER: Would - if you were offered that job to go to Mars, would you want to take it?

CASSIDY: Oh, I'd do it in a heartbeat.

BRIGER: You would.

CASSIDY: Yeah. Yeah. It needs to be a round trip. This business of who is going to go for one-way, I think that's a bunch of malarkey. I would sign up for a round trip.

BRIGER: Well, Chris Cassidy, thanks so much for being here today.

CASSIDY: Thank you, Sam. It's been a real pleasure.

GROSS: Chris Cassidy is a recently retired astronaut who served as the U.S. commander of the International Space Station. He's featured in the six-part documentary series "Among The Stars" on Disney+. He spoke with fresh air producer Sam Briger. After we take a short break, our rock critic Ken Tucker will tell us what made his best-of list this year. This is FRESH AIR.

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