Aerospace Engineering: How To Fall To Earth (Without Burning Up)

Rockets are built to slice cleanly through the atmosphere on the way up. Coming home, it turns out, requires... not turning into a fireball before a bellyflop When Space Shuttles reenter Earth’s atmosphere at 17,000 miles per hour, they don’t dive nose first. Instead they turn broadside to the atmosphere, deliberately creating more drag, more friction, more heat. At those speeds, oncoming air compresses into a shockwave hotter than molten lava. In this episode of Field Notes, Professor Hannah Fry and Michael Stevens explore the strange physics of coming home. Why is leaving Earth easier than returning to it? And what does a small, almost empty black tile reveal about the problem of meeting the world at 17,000 miles per hour? Along the way, they revisit controversial experiments in human fear, calculate which superhero power would bankrupt you in calories, and reflect on the thin boundary between surface and survival. ------------------- For more information about Cancer Research UK, their research, breakthroughs and how you can support them, visit ⁠⁠https://cancerresearchuk.org/restisscience⁠⁠ Cancer Research UK is a registered charity in England and Wales (1089464), Scotland (SC041666), the Isle of Man (1103) and Jersey (247). A company limited by guarantee. Registered company in England and Wales (4325234) and the Isle of Man (5713F). Registered address: 2 Redman Place, London, E20 1JQ. ------------------- Find The Rest Is Science all over the internet by ⁠⁠clicking here.⁠⁠ ------------------- Video Producer: Adam Thornton + Oli Oakley Video & Social: Bex Tyrrell Assistant Producer: Imee Marriott Senior Producer: Lauren Armstrong-Carter Head Of Digital: Samuel Oakley Exec Producer: Neil Fearn Learn more about your ad choices. Visit podcastchoices.com/adchoices

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

• 0:01Hannah Fry?

  Welcome to The Rest Is Science. This is Field Notes, sort of podcast expedition, if you will, where Michael and I, we trade the strange and curious objects that are filling our shelves and occupying our minds.

• 0:13Michael Stevens?

  And we answer the questions that are troubling yours.

• 0:16Hannah Fry?

  Mm-hmm, mm-hmm. Every week, one of us is going to bring a sort of strange and spectacular object onto the show, and together, well, we're going to see what sort of uncharted territory it takes us to.

• 0:27Michael Stevens?

  And we want to hear your questions, your theories, your thought experiments. So send them in and stay tuned to see where we end up.

• 0:33Hannah Fry?

  Yeah, absolutely. And later on, I mean, there's not many, many spaces on the internet where I could use this as a hook and tease, but I think The Rest Is Science is one because, because, Michael, later on, I'm going to be, I'm going to be showing you the coolest thermal insulator that I own, okay?

• 0:49Michael Stevens?

  The coolest? Like, does it skateboard and wear sunglasses? And I'm trying to think of another cool thing. I don't know.

• 0:58Hannah Fry?

  Neither of us know cool things, let's be honest.

• 1:01Michael Stevens?

  Does it have ripped jeans and a bad attitude?

• 1:03Hannah Fry?

  Hey, maybe. You'll just have to tune in to find out. Or tune in, stay tuned, I guess.

• 1:16Michael Stevens?

  This episode is brought to you by Cancer Research UK.

• 1:19Hannah Fry?

  The word cancer comes from the Greek kakinos, meaning crab. And Hippocrates used that word because tumors can spread out like crabs' legs.

• 1:28Michael Stevens?

  For a long time, cancer was poorly understood. And so I think because of that, it was almost scarier and people didn't even say its name. But what science has done since is replace uncertainty with understanding.

• 1:41Hannah Fry?

  But that understanding is an instant, because cancer isn't just one disease. It's hundreds of different diseases, each behaving differently, depending on where it is and its genes. And that complexity is why progress in cancer research can feel like it's slow. But step by step, research is saving and improving lives.

• 2:01Michael Stevens?

  And that's why Cancer Research UK, the world's largest charitable funder of cancer research, supports work across more than 200 types of cancer, from the tiny changes inside cells that start the disease, to better ways to spot it earlier and treat it more precisely.

• 2:18Hannah Fry?

  For more information about Cancer Research UK, their research and breakthroughs, and how you can support them, visit cancerresearchuk.org/the rest is science.

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• 4:36Michael Stevens?

  All right. Well, look, our first discovery doesn't even come from Hannah or me. It comes from you.

• 4:41Hannah Fry?

  Certainly does, because Anna has sent us in a question. This one's for you, Michael. What's one landmark experiment you wish you'd been at?

• 4:50Michael Stevens?

  Oh, OK. I, I thought of a lot of experiments when I heard this question. I think that ultimately, there are a lot of results I would have loved to have witnessed. But I'm gonna have to say I would like to have been present at the Little Albert experiments.

• 5:11Hannah Fry?

  Ooh. These are some of the most controversial experiments in the history of science, Michael.

• 5:15Michael Stevens?

  Exactly. And I wouldn't have wanted to be there to be a part of it or to watch it happen. I would have just gone there to stop it.

• 5:23Hannah Fry?

  Oh, OK. We should probably tell people what they are for those that haven't heard of them.

• 5:27Michael Stevens?

  Yeah, so the Little Albert experiments were done sort of early, mid, early 20th century. The results were published in 1920. The experiments were conducted at John Hopkins by John Watson and his grad student Rosalie Rayner. The goal of the experiment was to see if they could teach a new fear to an emotionally stable child. Like, already, it sounds like, hmm, let's just assume you can and not try. But they, they both did this. Rosalie is the woman that you see in the videos. If you really want to have a bad time, go to Wikipedia. Look up the Little Albert experiment and you can watch the videos and she's the one there with the child.

• 6:13Hannah Fry?

  It's horrible.

• 6:14Michael Stevens?

  The research was based on Ivan Pavlov's work with dogs. OK? And, and I don't know, I'll go on and on about this because I love psychology. But the point is, we've, we've heard of Pavlovian reactions. What that is about is called classical conditioning. Can I entrain a behavioral response to a originally neutral stimulus? And what that means, in the case of the most famous experiment Pavlov did, was when dogs are fed, when- They smell food, they salivate at the mouth. So what he did is he rang a bell every time he fed his dogs. And when I say his dogs, I mean dogs in a laboratory. And he did this for many days until eventually he could just ring the bell. And the dogs would start salivating even if he didn't feed them.

• 6:59Michael Stevens?

  And he's like, wow, look at this. So this is a huge part of what became behavioralism. The study of human behavior as essentially inputs and outputs. Like, we learn things and we can be programmed like a machine. We've got buttons and levers and a human is kind of just like a steam engine.

• 7:17Hannah Fry?

  There's no evolutionary reason why it should provoke that involuntary response.

• 7:21Michael Stevens?

  Exactly. And so going off of this ringing bell associated with food. We start to combine them and we salivate when we hear a bell. Suddenly that might explain why we're afraid of things, why we love each other, why we decide to do the things we do, why we're greedy, why we're kind. All of this became less popular, starting in the 50s. The cognitive revolution changed and now even today we focus more on how we think. Because ultimately, we know too much based on how many inputs we've received. Point is, let's get back to the experiment. Here's what they did. They somehow, they being Watson and Rayner. They found a nine-month-old child.

• 8:02Michael Stevens?

  We don't know how, we don't know the actual identity of the child. But in the paper, they called the child Little Albert. Wasn't its real name.

• 8:12Hannah Fry?

  Was Albert adopted or were his parents involved in the experiment too? Do we know that?

• 8:18Michael Stevens?

  We don't know, but all of the theories are that the mother was in the picture and aware of what was going on. The extent to which she really had a choice is unclear. She may have worked at the hospital and sort of felt like, I can't really say no because these people are like, you know, they have power over me in the hierarchy of John Hopkins' research, or they're my boss, you know? Or, we don't know. There are some guesses as to who the person might have been. The person, the baby, Little Albert, may have been, I think the most popular theory is that it was this individual who died at the age of six of a condition of the brain unrelated to the experiment.

• 9:02Michael Stevens?

  But that throws into question the validity of the results because this infant wasn't a completely, you know, healthy infant.

• 9:10Michael Stevens?

  Anyway, what did they do? Okay, so they took this child and they presented it with all kinds of different objects. Literally, you name it, they showed it to the baby and the baby wasn't scared of any of it. The baby was curious. And then they decided, okay, let's associate something scary with one of these. And the one they chose was a white lab rat. They allowed the child to play with this lab rat. But every time they introduced the rat, the white rat, they hit a steel bar behind the child with a hammer. Boom, really loud. And the child started crying and needed to be consoled and was frightened. And then they would present the child with a rubber duck or a shoe and they wouldn't hit the steel.

• 9:53Michael Stevens?

  Only when the white rat was there would the child be frightened. And sure enough, they were able to teach the child, little Albert, to be afraid of fuzzy, white, cute things.

• 10:06Hannah Fry?

  Mm hmm.

• 10:07Michael Stevens?

  They could present the child with a rabbit and it would get scared. They were even able to present the child with a Santa Claus mask that had a cotton ball beard and the child would freak out and cry and scream. And they were like, wow, great job, everybody. We've learned that the human body is a machine that can be programmed and can learn. And like, that's just every behavior we have is some kind of learned thing. Now, despite just being unethical by today's standards, it also didn't give us any good results. Like, literally, it was one subject with no control subjects and they didn't do follow ups. They didn't then see if they could unlearn the fear from the child. They were just like, excellent, that was cool.

• 10:50Hannah Fry?

  Yeah.

• 10:52Michael Stevens?

  And it's very sad.

• 10:53Hannah Fry?

  This is really the reason why this is considered such a sort of bad experiment in the history of science. Of course, there were, I mean, look, worse things have happened to people on earth, right? Than them being, than them being jump scared. But the key point about this was that they spent days after days after days deliberately frightening a child, right? Which is just already quite a horrible image. But they introduced this irrational fear in a human that- That they never undid, that they had no understanding of how that might go on to impact his life going forwards. And that exactly as you describe, we didn't actually gain any serious or interesting scientific knowledge from.

• 11:33Michael Stevens?

  Yeah, it was not a scientific method experiment. It was a let's poke around and see what happens. Let's poke it and see if it bleeds and then we'll move on and go catch a movie. I want to say the only sort of silver lining is going to come from that thought of like sometimes out of trauma, growth can occur. Or at least like sometimes it takes despair for there to be triumph. And Watson, after these experiments, did a lot of weekend lectures. And he did one, and in the audience was a woman named Mary Cover Jones. And she was

• 12:15Michael Stevens?

  very interested in fear as something that could be learned. And so of course, she thinks the kind of more kinder thought, which is we'll find, but we should also maybe focus on how fears can be unlearned. And she happened to know a child that had been brought to her because the child was afraid of fuzzy white things. Strangely, it was not little Albert. This was like, I think, a six year old. So she says, well, I'm going to do the opposite of little Albert. I'm going to teach a child to not be afraid of furry white things. And she developed a method that involved something that I think a lot of us are familiar with

• 12:55Michael Stevens?

  today, gradual exposure. All right. So just slowly introducing fuzzy things to the child, and also having the child hang out with other children who were not afraid of furry things. And she was able to essentially cure the child of this irrational fear that was making the kid's life worse. And to this day, she's considered the mother of behavioral therapy. And so I don't know, maybe little Albert's sacrifice brought us more quickly to behavioral therapy that helped people's lives. That's the only

• 13:32Michael Stevens?

  Silver lining I can find in it, but I still think I'd go back and stop it because if it wasn't Mary Cover Jones, it would be someone else.

• 13:38Hannah Fry?

  Yeah, I mean, I think science was on that path, right, of like learned behaviors. Sort of feels like it's only the adjacent possible to that is unlearning behaviors. Great answer, though. Yeah, great answer. I haven't thought about Little Albert for a while. I'm not going on the Wikipedia to watch the videos. I remember watching them and finding, I think maybe I was pregnant at the time or had a very small baby when I first watched them.

• 13:58Michael Stevens?

  Oh, yeah. Terrible time to do it. I saw them in high school in a psychology class and I was like, what the heck?

• 14:06Michael Stevens?

  Let's, let's make adults scared of things. I did that in a Minefield episode. I got electrically shocked every time a purple square was shown on a screen.

• 14:14Hannah Fry?

  Oh, yeah.

• 14:14Michael Stevens?

  And it did the same thing, like, like, just, it created an involuntary panic reaction whenever I saw purple squares, but I consented to it.

• 14:23Hannah Fry?

  Has it subsided over time? Can you see, can you see purple squares now?

• 14:26Michael Stevens?

  Yeah, yeah, it evaporated very quickly.

• 14:29Hannah Fry?

  How pink squares?

• 14:31Michael Stevens?

  No, no.

• 14:31Hannah Fry?

  Look away, Michael.

• 14:34Michael Stevens?

  No, I'm fine. I'm not a nine-month-old baby. So I knew what I was in and I was prepared to accept a fear of purple squares for life. So, you know.

• 14:44Hannah Fry?

  Yeah.

• 14:45Michael Stevens?

  I knew what I was getting into. Let's get into something else, though. Here's a question from Dr. Sunny Makin. If superhero powers obeyed basic physics and conservation of energy, which powers are actually the most expensive in energy terms? I'm thinking of things like super strength, flight, and super speed.

• 15:02Hannah Fry?

  Okay, you better, you better believe I got out my calculator, Michael.

• 15:06Michael Stevens?

  Great, yeah.

• 15:07Hannah Fry?

  I've run some numbers, baby. Okay, right, super strength. I mean, this is incredibly cheap. There's just, there's almost no energy involved in this. So let's say you're trying to lift up a tank, okay? Tank about 60 tons, all right? The work done, right, I'm going back to A-level physics here.

• 15:25Michael Stevens?

  Good.

• 15:25Hannah Fry?

  Mass times gravity times height, okay? That comes out about 1.2 million joules, which sounds like a lot, but in dietary terms, it's, it's about 280 calories. You're talking, you're talking one Snickers bar, essentially, half a milkshake. Easy peasy. You can, you can do that. It's no big deal.

• 15:47Michael Stevens?

  Like one can of full sugar soda. Yeah. I mean, first of all, I wanted to take a break. That's a- How efficient the human body is.

• 15:54Hannah Fry?

  Right.

• 15:54Michael Stevens?

  That's like, oh, you know, you, you drank one can of soda, you better go lift. What, what was this that we're lifting a?

• 16:01Hannah Fry?

  A tank.

• 16:03Michael Stevens?

  A tank. Go lift a tank. And then you can earn yourself another, I don't want to say earn. Basically, let me just leave it at that. The human body is really efficient.

• 16:13Hannah Fry?

  Well, it's also, it's also um, the wrong way to think about it. Like, you know, if you go on a treadmill and you're counting calories on your treadmill and you finish and you've, you've like exhausted yourself. And it's like 60 calories, you can have half a peanut.

• 16:26Michael Stevens?

  Yes.

• 16:27Hannah Fry?

  Um, that's not the exact, it's, it's extremely, extremely depressing. But the key point is that just being alive, just breathing, just being upright, just existing and thinking uses up a lot of calories. So you shouldn't ever think of calories as a sort of um, an equation in terms of your, your physical exertion. But I'm doing it purely for the purposes of this question. And super strength is very cheap and easy. Okay. Flight, I'm going to go medium level for flight. Okay.

• 16:54Michael Stevens?

  Okay.

• 16:55Hannah Fry?

  Um, so if you are, I'm going to just, you know, ignore the aerodynamics and just, you know, and just think of you pushing against gravity, keeping yourself up. That essentially means that you need to be a very, a, a human inefficient helicopter. Okay.

• 17:10Michael Stevens?

  Hmm.

• 17:10Hannah Fry?

  Um, so you're, if you're going to hover, you've got to accelerate air downwards. Um, and for a human sized objects, I, I worked that out to be about, let's say a hundred kilowatts of power. Well, that's quite a lot, actually. That is quite a lot. You're going to have to, to eat.

• 17:28Hannah Fry?

  Please check my calculations, by the way, on this. You can, you can write to me, tell me that I've got it wrong. But I think that's about 300 burgers per hour.

• 17:35Michael Stevens?

  Wow.

• 17:36Hannah Fry?

  Which is quite a lot, actually. You've got to, you've got to, you've got to pump through quite a lot. Um, there, I'm not, because I'm not using aerodynamics. If you were going forwards, if you were gliding, it would be more efficient. But I'm just, if you can just move yourself upwards like a, like a rubbish helicopter. Um, so that's kind of medium, but uh, by a long way, the thing that is going to bankrupt you energy wise is super speed.

• 18:00Michael Stevens?

  Really?

• 18:00Hannah Fry?

  Super speed is uh, Uh. Super speed is off the scale. So um because for starters, you've got kinetic energy. Um kinetic energy, by the way, is the the equation for is half mv squared. Notice that v squared. So the faster you get, if you run twice as fast, you need four times the amount of energy. So super speed is it's going to scale very, very quickly. If you run at one percent of the speed of light, which is compared to like a cosmic ray, for example.

• 18:30Michael Stevens?

  It's nothing.

• 18:31Hannah Fry?

  Snails pace.

• 18:32Michael Stevens?

  It's not moving. Yeah.

• 18:34Hannah Fry?

  You've also got air resistance here that you've got to consider.

• 18:36Michael Stevens?

  I was wondering if you were going to factor that in. Yeah.

• 18:39Hannah Fry?

  Here it is. Here it is. Air resistance um scales with v cubed, my friend. It's the cube law. Oh, lordy. So if you're going to run twice as fast through air, you need eight times the power to push the air out of the way. So, you know, let's make it slower. Let's let's just say you're running at the speed of sound. Um. You're not just burning calories. You are you're you're generating heat from air compression. You you you know that that's enough to to melt lead. Um. And, you know, if you're trying to run at the speed of light, which I think is what Superman does. Um. You know, I've seen I've seen the videos of him circulating the earth. I don't think it was documentary, but um.

• 19:23Hannah Fry?

  But yeah, I mean, you're you're you're you're going to be causing nuclear explosions all over the place. Obviously, every time I do one of these calculations, it ends in nuclear explosions.

• 19:33Michael Stevens?

  As it should, as it should. So super speed. I mean, yeah, listening to you explain it, it makes sense. Super speed also requires super strength. Because you're having to push an enormous amount of air out of the way very quickly.

• 19:47Hannah Fry?

  Yeah, you really are.

• 19:49Michael Stevens?

  All right. So we went from a Snickers bar to three hundred hamburgers an hour to constant nuclear bomb explosions.

• 19:59Hannah Fry?

  Exactly. This is a simple, simple scale. Simple linear hop, skip and a jump from one to the other. Okay, all right, here's another question. Oh, okay, this is nice. This is from Owen, who wants to know when we first met, Michael.

• 20:12Michael Stevens?

  Oh, yeah.

• 20:13Hannah Fry?

  I remember. Do you remember?

• 20:15Michael Stevens?

  I think I remember. I think we've talked about this before. I mean, I was aware of you because of your appearances on Number File.

• 20:23Hannah Fry?

  Correct.

• 20:24Michael Stevens?

  I don't know how we got in touch, but we went to lunch. Why, why did we do that, though? Like, did I have your email address?

• 20:36Hannah Fry?

  Let's think about what year this was. This was probably, what do you reckon, 2014, maybe?

• 20:40Michael Stevens?

  That year would have been 2015.

• 20:43Hannah Fry?

  2015, there you go. You were working at YouTube in London.

• 20:46Michael Stevens?

  Yeah.

• 20:48Hannah Fry?

  And you had been running Vsauce for a few years by then already. So I had obviously watched every single one of your videos.

• 20:55Michael Stevens?

  Yeah, of course.

• 20:56Hannah Fry?

  Of course, as most of the internet had. And I was in YouTube, recording a video. And then, as I was leaving, you happened to be in the lift, right?

• 21:08Michael Stevens?

  That makes sense. Okay, so we just ran into each other at the YouTube office in London.

• 21:13Hannah Fry?

  Yeah, yeah, you were very cool and I was not.

• 21:17Michael Stevens?

  What do you mean, cool?

• 21:18Hannah Fry?

  I remember the entire conversation, Michael. You were like, I was like, oh, my God, it's Vsauce. And didn't want to say anything and stood in the corner. And then you were like, oh, hang on, you're Hannah. And I was like, oh, my God.

• 21:34Michael Stevens?

  No, I had, really? Because I was really nervous. Because you were like an actual smart person. I was just a guy who was like, look at this cool thing.

• 21:43Hannah Fry?

  And then I was like, oh, my God, you know who I am? And you were like, well, yeah, it's kind of my job to know. I work for YouTube, right? It's kind of my job to know.

• 21:50Michael Stevens?

  True.

• 21:51Hannah Fry?

  And then you said that you were working on a video for Banach Taski. And you needed a proper mathematician. And I didn't know any of those.

• 21:59Michael Stevens?

  We had lunch and you were gracious enough to listen to me basically do the Banach-Tarski video live.

• 22:05Hannah Fry?

  Which was great.

• 22:06Michael Stevens?

  And I just needed someone to tell me, does this make sense? Can it be followed? Are there any egregious mistakes? And your interest in it and also the fact that you weren't immediately like, okay, no, this is obviously wrong, gave me so much confidence.

• 22:28Hannah Fry?

  Oh, that's fine.

• 22:29Michael Stevens?

  That I made the video. And if you look at the history of Vsauce, that's a turning point in the channel where I went from, ugh, could you eat your own poop if you cooked it first? To, I'm gonna take a long time to really dive into something, to explain something that I can't understand. That hasn't been taught, I think, just in the right way yet. And I'm not gonna do it right either, but I'll at least add a different voice to it. And so every video after that is like more than 20 minutes long. Every video before that one is like eight minutes long. So, yeah, you played this like pivotal role in the evolution of the channel and myself. So that's how we met.

• 23:12Hannah Fry?

  The Banach-Tarski video, by the way, I should tell everybody, is talking about a extremely obscure and mind-bending mathematical theorem where when you take the surface of a sphere and rearrange it in a particular way, you can end up with two of the original object without adding or deleting anything. It's extremely mind-bending. And I think up until that point, really, nobody had delved into anything that complicated in a public forum, in a sort of, in a, something that was to be consumed by everybody. And those of you who haven't seen Michael's video on Banach-Tarski, I think it's got something like tens and tens of millions of views, right?

• 23:57Michael Stevens?

  Yeah, something like that.

• 23:58Hannah Fry?

  It is wild to the entire mathematical world that you could take something that's mind-bending and bring it to totes to so many people.

• 24:06Michael Stevens?

  I know, there's, there's an appetite for it. Online. And I think ever since then, not that I started it, but I knew these things would be popular. And now thankfully, we've got three blue, one brown. We've got all these channels that are like, no, let's do it. Let's, let's get into this. And now it's like, no matter how deep of a question I have, I'm like, what exactly is energy? Or why did chimneys break in the middle when they fall? There's always some like Indian YouTuber who's like, let's get into it. And they draw out every single thing and what each, each formula means. And it's just, it makes me feel so great for the human race.

• 24:47Hannah Fry?

  Yeah.

• 24:48Michael Stevens?

  That we want this kind of content. So yeah, with Bonnok Tarski, there's like one thing I wish I had done differently, which is the way I describe choosing the first set of points from the sphere I could have been more clear about.

• 25:02Michael Stevens?

  So one of these days I'll revisit it. But yeah, the point is that this very paradoxical dissection puzzle requires removing points in very precise ways and then putting them back together and like, oh crap, I have two now. Wait, where did they all, where did all this extra stuff come from?

• 25:20Michael Stevens?

  But that's how we met.

• 25:21Hannah Fry?

  There you go. Oh, that was a really nice, that was a really lovely question. Thank you. That's, um, yeah, thank you very much, Owen. Should we take ourselves to a break after that?

• 25:30Michael Stevens?

  Yeah.

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• 27:10Hannah Fry?

  Welcome back, everybody. I promised you some thermal insulation and I'm going to deliver. Oh, I'm going to deliver on the most exciting thermal insulation that there is. Not just in the world, but I'm going to say in the solar system. Okay, that's where I'm going with this because I'm talking space here. I want to bring you back, Michael, to the 1950s when the space race was in full force and there was an incredibly difficult problem that people were trying to solve, which is how on earth do you get vehicles from space back to Earth? The re-entry vehicles for NASA. Now, the thing is that getting rockets out of the atmosphere, actually, that bit's kind of easy.

• 27:53Hannah Fry?

  I mean, maybe easy is a bit strong, but it's not too difficult. When you're coming back down, though, you are traveling at such an unimaginable speed that you have so much air that needs to get out of your way. You're squashing this air so rapidly with such a force that the temperatures that you start reaching are way higher than basically any human-made product could withstand.

• 28:25Hannah Fry?

  So so back in the nineteen fifty s, there's a guy called Harvey Allen and he's set the task of designing these these reentry vehicles for NASA. And the first ideas, everyone was like, well, we just need it to be like a rocket shape. Basically, we need to make it as streamlined as possible.

• 28:43Michael Stevens?

  Like sliced through the air.

• 28:45Hannah Fry?

  Yeah, like a needle, right? Pierce through it, get through the atmosphere quickly, minimize the drag, minimize the friction forces on the side, get in. But these temperatures, these speeds, I mean, they are so massive that it's just, it it just wasn't, nothing would work. So Harvey had this absolutely genius idea because he realized that when something is streamlined, all of that heat, it is going to happen right next to the skin of the vehicle, right? It's like literally on the surface. And really what you want, you want some kind of a, like a buffer, like a protective blanket that you can sort of wrap around the reentry vehicle and take some of that heat away.

• 29:27Hannah Fry?

  So his idea was what if, what if instead of making it streamlined, you just deliberately make it as unstreamlined as possible, right? What if we make the aerospace equivalent of a manhole cover, right? That's essentially what he was thinking. And the point about this is that if you reenter the atmosphere with like a massive flat wall, then the air that's getting crushed, it can't get out of the way quick enough. So it ends up forming this, this basically a cushion at the front of the vehicle. You get a much bigger shockwave, you get way more friction, but it happens away from from the skin of the vehicle so that then it's not in direct contact with the sort

• 30:12Hannah Fry?

  of human made substances around the side, but it's kind of directed around it, right?

• 30:16Michael Stevens?

  Right.

• 30:16Hannah Fry?

  I love this idea. I love this idea of backwards thinking. Um. And this is exactly what they ended up doing. So even, uh, I mean, they started off with these sort of, um, these, these round shapes. Um, the Russians copied the ideas, they used spheres. The Americans sort of settled on that cone shape, which would come in, it's sort of belly flopping into the atmosphere. And then even the space shuttle, uh, which looks more like a plane, right? But when it would come into the atmosphere, it wouldn't go nose first and dive through. It belly flops, right?

• 30:48Michael Stevens?

  Right.

• 30:49Hannah Fry?

  It kind of comes in. Um, so this was the, the general idea, but still, even still, the, uh, temperatures on the surface could still get up to one thousand two hundred and fifty degrees centigrade, right? Still vast, vast, vastly higher than anything that would be, you know, you can't, you can't have like a metal skin on the outside, for example.

• 31:12Michael Stevens?

  Cause it would melt.

• 31:13Hannah Fry?

  Cause it would melt, exactly. So I should tell you, actually, I did a video on this. I did a little social media video about this. Just, I really love the aerodynamics of, of reentry vehicles. I just think it's, there's something that deeply amusing to me about there being such a difficult problem and people turning it upside down and going for the, the most un-aerodynamic.

• 31:32Michael Stevens?

  Yep.

• 31:32Hannah Fry?

  Um, space possible. So I did this, did this video about it. And then I was in, um, in California and, uh, I was in Google, actually, and someone came up to me and they were like, oh, I knew you were going to be here today. Um, I just finished working for NASA and now I work for Google. And I have for you a tile of the particular material that was used on the space shuttle. This is the material that they came up with. That was the workaround for these absolutely insane temperatures on the space shuttle.

• 32:14Michael Stevens?

  Yeah, we're looking at a tile of, it looks like white foam. It's about the size of a domino or maybe two dominos on top of each other stacked hamburger style. But it's also striated on the side. Like a cake made of very thin layers all stacked up with black and white lines. It's a glossy black paint on one face, one of the large faces. But then it's white everywhere else, except for those thin black lines on the sides. And on the back, though, it looks like it's made almost differently of transversely organized sections.

• 32:53Hannah Fry?

  Basically, what you have is these many, many, many layers, both horizontally and vertically. And this is actually, it's pure silica. It's 100% silica.

• 33:04Michael Stevens?

  Huh.

• 33:05Hannah Fry?

  But it's these fibers that are laid in such a way that this block is 94% air. Okay, so it's, it messes with your mind because you look at it, it sort of looks like, um, I don't know, like a piece of chalk, perhaps. It's sort of white and it's got almost powdery.

• 33:22Michael Stevens?

  Yeah, I was gonna say like ceramic or chalk.

• 33:25Hannah Fry?

  Yeah, but when you pick it up, it's, it's way lighter than you would, than, than, than your brain thinks it's gonna be. It's unbelievably light. It's, it's essentially solid air. That's what they've created. Solid air.

• 33:36Michael Stevens?

  Solid air, I love that.

• 33:37Hannah Fry?

  In a cage of silica. It's, it was invented by Lockheed Missiles and Space Company. This one specifically for the space shuttle. It's called LI900. The, the 900 in it stands for its density. It's nine pounds per cubic foot. Okay, so there you go, Americans. This is, uh, you can tell this is made in America.

• 34:03Michael Stevens?

  Yeah.

• 34:04Hannah Fry?

  But for context, by the way, Styrofoam is three pounds per cubic foot. So this is slightly heavier than Styrofoam, but, but not by much. Sort of, that's, that, that's the sort of weight that it is. The thing about this stuff is that it is, because it is, you know, sort of solid air in a lot of ways. You could easily crush this. You could like, just. Put your hand through it. I mean, it sort of feels quite fragile under, um, under your fingers. Now, the thing is, all of these little pockets of air in here, air is this unbelievably good insulator, you know, heat doesn't really like travelling across air gaps.

• 34:44Michael Stevens?

  Yeah.

• 34:44Hannah Fry?

  So when you have, you know, an unimaginable number of tiny little caged air bubbles inside a material like this, it means that it is so poor at conducting heat that you can literally put this in a kiln, heat it to well over, you know, a thousand degrees centigrade, two thousand degrees Fahrenheit.

• 35:04Michael Stevens?

  Like, so it's glowing hot?

• 35:06Hannah Fry?

  Glowing hot, literally red hot. And yet you can still pick it up with your bare hands because it is so bad at conducting heat that it's not going to burn your skin, right? It's, it's, it's wild. It's wild. There's this video of people doing that.

• 35:23Michael Stevens?

  I've seen the video, but I never really knew what they were made of. But now hearing your description of these silica fibers makes me think that it's almost like a possibly safer version of asbestos.

• 35:36Hannah Fry?

  Right.

• 35:37Michael Stevens?

  Because both of them are like woven rock fibers made into a material. Um, they're both bad to get in your lungs. But I don't know, that silica stuff might be a little bit less carcinogenic. Only one way to find out, though.

• 35:52Hannah Fry?

  Hey, come back in five years.

• 35:55Michael Stevens?

  Let's see.

• 35:56Hannah Fry?

  So I was probably put it back in the little box, July. Um, I didn't know that's how asbestos works. Is asbestos like trapping loads of, um, just traps loads of air molecules in it, does it?

• 36:07Michael Stevens?

  Yeah, it's like, uh, it's also like, it's rock, so it's, it's, it's flameproof, fire resistant, and it traps in air. And, uh, so it insulates really well. Um, it.

• 36:19Michael Stevens?

  Yeah, it doesn't burn. Um, it's really marvelous stuff, except for that one problem.

• 36:26Hannah Fry?

  That one tiny thing. There was asbestos in my house, actually. When we um, when we did up my house, there was asbestos everywhere. And I got really scared because I'd had my babies in this house, um, sort of sleeping next to asbestos walls.

• 36:39Michael Stevens?

  Yeah.

• 36:39Hannah Fry?

  And uh, and they said that actually, until you disturb it, it's completely fine.

• 36:43Michael Stevens?

  It's fine. It's a rock. It's sitting there. But once you decide to like, remove it, that's when you've got to do a lot of safe precautions because it breaks into little tiny, little tiny like needles that float in the air and you breathe them in, they get stuck in your lungs. They don't dissolve or digest or anything because it's rock. And for reasons I'm not really clear of, it also um, uh, accelerates cancer. It's not just like it clogs up your lungs. Uh, so, yeah. What is that, what is the disease called? Mesothelioma?

• 37:16Hannah Fry?

  Hmm, something like that. We should talk to our friends at Cancer Research UK.

• 37:19Michael Stevens?

  We should, yes.

• 37:22Hannah Fry?

  Um, so here's the thing, okay. So, so the, the, this, this idea of um, LI nine hundred, they sort of, they knew that they'd solved it, that they'd, um, that they'd found something that was just unbelievably good thermal insulator. Um, but these tiles are so incredibly fragile, you can't have them as big. They can't be large. They couldn't just cut, like, create these massive tiles, put them in, and uh, and off you go. The, the space shuttle wasn't covered in this uniform blanket. It was covered in two thousand four hundred tiny individual tiles.

• 37:54Michael Stevens?

  Oh, wow.

• 37:55Hannah Fry?

  And because the space shuttle was like, bend and, and flexed during flight, you can't just like glue all of these tiles directly to the metal skin underneath because they, they would snap. So they had to, there was like this strain isolation pad, which was then glued to the ship. And then every single tile on top had a unique shape, right? Which means that if you broke one of them, you couldn't just go and grab one off the shelf.

• 38:21Michael Stevens?

  You've got to grab that shape.

• 38:23Hannah Fry?

  That exact shape, that custom replacement for that specific coordinate. These were an engineer. Nightmare these things because the other issue is that, you know, these are ninety four percent empty space, right? Like they're they're they're solid air as it were, which means it's incredibly porous. So if it's raining, if it's raining while the shuttle's sitting on the launch pad, the tiles would absorb like hundreds of kilos of water.

• 38:50Michael Stevens?

  Right, they would they would become solid liquid water.

• 38:54Hannah Fry?

  Solid liquid water, exactly. And then and then, I mean, then you've got problems of, you know, maybe that water could freeze, it could expand, it would shatter all the tiles, it would boil off in orbit, you know, it would blow them all apart.

• 39:06Michael Stevens?

  A lot of extra weight, suddenly you've got a launch, yeah.

• 39:09Hannah Fry?

  Right, so then they're having to like inject these tiles with this waterproofing agent. Basically, you know, Scotchgard has on steroids essentially. Via a needle, they've got to do it in all of these gaps and all of these holes. But they they, one of the ways around this to sort of seal the outside is, especially on the belly, is they have this this glaze that they put on it, this this reaction cured glass, this borosilicate. And that helps to seal it, but also maximizes the the heat emissibility, sort of sheds heat back into space. So these are on the bottom, and then the white tiles, these are the ones on the top.

• 39:48Michael Stevens?

  Yeah.

• 39:49Hannah Fry?

  Cool, huh? Isn't that cool?

• 39:50Michael Stevens?

  Yeah, because the bottom of the space shuttle is black. And so I was wondering, so is that what I'm looking at there? Yeah, the black side is the side that faces out during the belly flop.

• 40:01Hannah Fry?

  Yeah, isn't that amazing, though? Like this this complete paradoxical material, something that's like fragile enough to be crushed by a toddler, but but strong enough to survive this plunge through the atmosphere at like over a thousand degrees.

• 40:16Michael Stevens?

  Yeah, I think that describes a lot of people I know. I love it too, because we talk so much about to leave Earth, how hard it is to to to leave, to have the delta V required to like leave our cradle. And yet, coming back home is its own engineering problem. It's like the Earth doesn't want to accept us back readily.

• 40:40Hannah Fry?

  I always thought that was the hardest bit. The the whole thing about whether the moon landing was real or not. I always thought that the the argument for me was that getting to the moon and landing on the moon actually might be difficult, but not not anywhere near as difficult as the problem of getting back off the moon, getting back down to Earth and landing safely. And that was always the thing is that you had this moment in time, these, well, unbelievable geopolitical pressure for this for the for the space race, combined with, you know, launching missiles and nuclear weapons from space and and all of the the sort of defense side of things that that came with that. But you also had incredibly

• 41:22Hannah Fry?

  gung ho pilots. I don't think it would have been difficult to find somebody who was willing to do a suicide mission to the moon and just stay there. Like that bit wasn't wasn't hard. So it always seemed to me that the showy bit wasn't the hard bit, which makes me believe that they did the entire thing.

• 41:40Michael Stevens?

  Have you seen that video where the guy shows that in 1969, it would have been easier to have gone to the moon and come back than it would have been to create a broadcast of that length, like a pre-recorded film segment? Yeah. He's like, actually, that feed of the moon landing is so long that if you had done it in a studio beforehand on film, and then you had to play it back like it was live and not have a single piece of dust or a hair on it, not allow any mistake to show that you're just playing back a reel, like that would have been an even bigger technological miracle than actually going to the moon and back. It's a very fascinating video.

• 42:23Hannah Fry?

  Yeah, I think, you know, I sort of vaguely do remember he was talking about the size of the reels, like the physical size of the reel of tape, right, if I remember rightly.

• 42:32Michael Stevens?

  Yeah, because he's talking about like hours and hours of footage that is like streamed out onto televisions. And somehow it needs to, uh, never look that way. It needs to look like a live broadcast. So it probably was, because if they had solved that, that would have been more unbelievable and more of an achievement than the moon landing.

• 42:56Hannah Fry?

  The other argument that I find compelling is that how many thousands of people were involved, right? How many, the cleaners, the sort of the janitors, the like people who are painting the walls, all the way through to the engineers and the astronauts and the people in politics. Not one of them, you know, not one of them, however, many decades later slipped up and accidentally told everyone that it was a ruse. I mean, I just don't believe in humans that much. I just don't believe in humans' abilities to keep secrets that much.

• 43:28Michael Stevens?

  Yeah. Oh, yeah.

• 43:30Hannah Fry?

  Okay, well, we're hearing from our producer that LI900 would need exposure to 1,450 degrees centigrade for hours, then crushed to have any carcinogenic potential whatsoever. So, whoa, live to fight another day, Michael.

• 43:46Michael Stevens?

  Wow, you're safe. Okay.

• 43:48Hannah Fry?

  Important that you recognize I was willing to put my life into your hands for the purposes of this podcast.

• 43:53Michael Stevens?

  I know. But now we know that you were safe all along, so I feel better.

• 43:57Hannah Fry?

  All along. All right, well, I guess we'll leave it there for this week. As ever, if you have any questions, ideas, tiles off the space shuttle that you want to send us, you can email us, [email protected].

• 44:11Michael Stevens?

  And you can join our newsletter at therestis.com/science.

• 44:18Hannah Fry?

  We're going to be back next Thursday with another edition of Field Notes and on Tuesday with our normal episode.

• 44:23Michael Stevens?

  So until then, stay curious.

• 44:37Speaker 06

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• 45:17Michael Stevens?

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