The Rest Is Science - How To Fall To Earth (Without Burning Up)

Episode Date: February 26, 2026

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

Welcome to the rest of 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. And we answer the questions that are troubling yours. 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. 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. Yeah, absolutely. And later on, I mean, there's not many spaces on the internet where I could use this as a hook and tease. But I think the rest of science is one because Michael, later on, I'm going to be showing you the coolest thermal insulator that I own, okay?

The coolest. Like, is it skateboard and wear sunglasses and I'm trying to think of another cool thing. I don't know. Neither of us know cool things, let's be honest. Does it have ripped genes and a bad attitude? Hey, maybe. You'll just have to tune in to find out. Or tune in, stay tuned, I guess.

This episode is brought to you by Cancer Research UK. The word cancer comes from the Greek carcinos, meaning crab. And Hippocrates use that word because tumours can spread out like crab's legs. 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. 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. 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. For more information about Cancer Research UK, their research and breakthroughs and how you can support them, visit cancurecuk.org slash the rest is science. Laura versus fruit flies. Swarming your fruit and terrorizing your kitchen,

these little freaks multiply at a rate that would make a rabbit say, yo. Chill. But Laura shopped on Amazon and saved on cleaning spray, countertop wipes, and fly traps. Hey, fruit flies, your baby boom ends here. Save the Everyday with Amazon. This episode is brought to you by DeVille. Defender. With a towing capacity of 3,500 kilograms and a weighting depth of 900 millimeters, the Defender 110 pushes what's possible. Learn more at landrover.ca.

All right. Well, look, our first discovery doesn't even come from Hannah or me. It comes from you. Certainly does, because Anna has sent us in a question. This one's for you, Michael. What was one landmark experiment you wish you'd been at? Oh, okay. 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 going to have to say I would like to have been present at the Little Albert experiments. Oh, these are some of the most controversial experiments in the history of science. 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. Oh, okay. We should probably tell people what they are for those that happen. 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, let's just assume you can and not try. But 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. It's horrible. The research was based on Ivan Pavlov's work with dogs. okay and and I don't know I'll go on and on about this because I love psychology but the point is we've heard of the 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 salivated the mouth so what he did is he rang a bell every time he

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. 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 a like a steam engine. There's no evolutionary reason why it should provoke that involuntary response. 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 Rainer, they found a nine-month-old child. 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.

Was Albert adopted or were his parents involved in the experiment team? Do we know that? 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, but that throws into question the validity of the results because this infant wasn't a completely, you know, healthy infant. 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. 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. 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 cottonball 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 like 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. And it's very sad. 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 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 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 described, we didn't actually gain any serious or interesting scientific knowledge from. 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 very interested in fear as something that could be learned. And so, of course, she thinks the kind of more kinder thought, which is, well, fine. 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 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 kids' 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 silver lining I can find in it. But I still think I'd go back and stop it because if it wasn't Mary Covered Jones, it would be someone else. Yeah, I mean, I think science was on that path, right? Of like learned behaviors. It sort of feels like 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 them. I think maybe I was pregnant at the time or had a very small baby when I first watched them. Oh, yeah. terrible time to do it. I saw them in high school in a psychology class and I was like, what the heck? Let's make adults scared of things. I did that in a Mindfield episode. I got

electrically shocked every time a purple square was shown on a screen. 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. As it's decided over time. Can you see purple squares now? Yeah, yeah. It evaporated very quickly. How pink squares. No, no. Look away, Michael. 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, I knew what I was getting into. Let's get into something else, though. Here's a question from Dr. Sunny Macon. 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.

Okay. You better believe I got out of my calculator, Michael. Great. Yeah. I've run some numbers, maybe. Okay. Right. Super strength. I mean, this is incredibly cheap. 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. Good. Mass times gravity, times height, okay? That comes out about 1.2 million jewels, which sounds like a lot. But in dietary terms, it's about 280 calories. You're talking one Snickers bar, essentially, half a milkshake.

Easy peasy. You can do that. It's no big deal. Like one can of full sugar soda. Yeah. I mean, first of all, I want to take a break. That's like how efficient the human body is. Right?

Yeah. Oh, you know, you drank one can of soda. you better go lift. What was this that we're lifting a... A tank. 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. Well, it's also 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, like, exhausted yourself and it's like 60 calories, you can have half a peanut. Yes. That's not the exact. 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 an equation in terms of your physical accession. 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.

Okay. 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 human inefficient helicopter, okay? So if you're going to hover, you've got to accelerate air downwards. And for a human size object, I worked that out to be about, let's say, 100 kilowattts of power. that's quite a lot actually that is quite a lot

you're going to have to eat please check my calculations by the way on this you can write to me telling me I've got it wrong but I think that's about 300 burgers per hour which is quite a lot

actually you've got to you've got to pump through quite a lot there 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 rubbish helicopter so that's kind of medium But by a long way, the thing that is going to bankrupt you, energy-wise, is super speed. Really? Super speed is off the scale. So because for starters, you've got kinetic energy. Kinetic energy, by the way, is the equation for it 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 going to scale very, very quickly. If you run at 1% of the speed of light, which is compared to like a cosmic ray, for example, it's nothing. Snails pace. It's not moving, yeah. You've also got air resistance here that you've got to consider.

I was wondering if you were going to factor that in, yeah. Here it is. Here it is. Air resistance 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 just say you're running at the speed of sound. You're not just burning calories. You are, you're generating heat from air compression. You know, that's enough to melt lead. And, you know, if you're trying to run at the speed of light, which I think is what Superman does, you know, I've seen, I've seen the videos of him circulating the earth.

I don't think it was documentary, but... But yeah, I mean, 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. 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. Yeah, you really are. All right. So we went from a Snickers bar to 300 hamburgers an hour to constant nuclear bomb explosions.

Exactly. This is a simple, simple scale. Simple linear, hop skiff 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? Oh, yeah. I remember. I remember. I think we've talked about this before. I mean, I was a well. of you because of your appearances on number file. Correct. I don't know how we got in touch, but we went to lunch. Why did we do that, though? Like, did I have your email address? Let's think about what year this was. This was probably, what do you reckon, 2014, maybe?

That year would have been 2015. 2015, there you go. You were working at YouTube in London. Yeah. And you had been running Vsource for a few years by then already. So I had obviously watched every single one of your videos. Yeah, of course. 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? That makes sense. Okay, so we just ran into each other at the YouTube office in London. Yeah, yeah. You were very cool and I was not. What do you mean cool?

I remember the entire conversation, Michael. You were like, I was like, oh my God, it's Bezos and didn't want to say anything. It stood in the corner. And then you were like, oh, hang on, you're Hannah. And I was like, oh, my God. 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. 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. work for YouTube, right? It's a kind of fun job to know. True. And then you said that you were working on a video for Banakhtarski and you needed a proper mathematician and I didn't know any of those. We had lunch and you were gracious enough to listen to me basically do the Bonoktarsky video live. Which was great. 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. Oh, that's fine.

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, oh, could you eat your own poop if you cooked it first to I'm going to 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 going to 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.

The Bannock-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 Banachowski, I think it's got something like tens and tens of millions of views, right? Yeah, something like that. It is wild to the entire mathematical world that you could take

something that's mind-bendie and bring it to totes to so many people. I know. 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 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 Indian YouTuber who's like, let's get

into it. And they draw out every single thing and what each formula means and it's just, it makes me feel so great for the human race that we want this kind of content. So, yeah, with Bonak 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. 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 all this extra stuff come from? But that's how we met. There you go. Oh, that was a really nice, that was a really lovely question. Thank you. Yeah, thank you very much, Owen. Should we take ourselves to a break after that?

Yeah. This episode is brought to you by Cancer Research UK. We often think of beating cancer as treatment. But imagine stopping it before it begins. After years of work, cancer research UK scientists are launching a clinical trial of lung Vax, the first vaccine designed to prevent lung cancer. It builds on Tracer X, the world's largest cancer evolution study, which tracked lung cancer cells over many years to uncover the disease's earliest warning signs.

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their research breakthroughs and how you can support them, visit cancerresearchuk.org forward slash the rest is science. RBC Training Ground has discovered potential in over 20,000 Canadian athletes and counting. Your story could be next. If you've got the drive, they'll help you find your path to the Olympics. Let's see what you've got. Sign up for free at rbc trainingground.ca. 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 same. 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, is that getting rockets it's out of the atmosphere.

Actually, that bit's kind of easy. I mean, maybe easy is a bit strong, but it's not too difficult. When you're coming back down, though, you are travelling 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. So back in the 1950s, there's a guy called Harvey Allen, and he's set the task of designing these re-entry 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. Like slice through the air. 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 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 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, you want some kind of, like a buffer, like a protective blanket that you can sort of wrap around the reentry vehicle and take some of that heat away. 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 re-entered 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, basically a cushion at the front of the vehicle. You get a much bigger shock wave. You get way more friction, but it happens away from the skin of the vehicle. So that then it's not in direct contact with the sort of human-made substances around the side,

but it's kind of directed around it, right? I love this idea. I love this idea of backwards thinking. And this is exactly what they ended up doing. So even, I mean, they started off with these sort of these round shapes. The Russians copied the ideas. They used spheres. The American 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, 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? Right. kind of comes in. So this was the general idea, but still, even still, the temperatures on the surface could still get up to 1,250 degrees centigrade, right, still vast, vastly higher than anything that would be,

you know, you can't have like a metal skin on the outside, for example. Because it would melt. Because 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 reentry vehicles. I just think there's something deeply amusing to me about there being such a difficult problem and people turning it upside down

and going for the most unerodynamic space possible. So I did this video about it. And then I was in California. And 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. 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. Yeah, we're looking at a tile of. it looks like white foam. It's about the size of a domino, or maybe two dominoes on top of each other, stacked hamburger style. But it's also striated on the sides, 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. 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. But it's these fibers that are laid in such a way that this block is 94% air. Okay. So it messes with your mind because you look at it, it sort of looks. like, I don't know, like a piece of chalk, perhaps. It's sort of white and it's got almost powdery. Yeah, I was going to say like ceramic or chalk.

Yeah. But when you pick it up, it's way lighter than your brain thinks it's going to be. It's unbelievably light. It's essentially solid air. That's what they have created. Solid air. Solid air. I love that.

Trapped in a cage of silica. It was invented by Lockheed missiles and space company. this one specifically for the space shuttle. It's called L-I-900. The 900 in it stands for its density. It's nine pounds per cubic foot. Okay, so there you go, Americans. You can tell this is made in America.

Yeah, right. But for context, by the way, styrofoam is three pounds per cubic foot. So this is slightly heavier than styrofoam, but not by much. sort of 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 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. Yeah. 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, 2,000 degrees Fahrenheit. So it's glowing hot. 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 wild. It's wild. There's this video of people doing that. 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. Right. Because both of them are like, woven rock fibers made into a material. 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. Hey, come back in five years. Let's see. So I was probably put it back in a little box, shall I. I didn't know that's how asbestos work. It's asbestos, like, trapping loads of, just traps loads of air molecules in it, does it? Yeah, it's like, it's also. Also, like, it's rock, so it's flame-proof, fire-resistant, and it traps in air.

And so it insulates really well. It doesn't burn. It's really marvelous stuff, except for that one problem. That one tiny thing. There was asbestos in my house, actually. When we did up my house, there was asbestos everywhere. And I got really scared because I'd had my babies in this house sort of sleeping next to asbestos walls. Yeah. And they said that actually, until you disturb it, it's completely fine.

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 accelerates can't. It's not just like it clogs up your lungs. So, yeah. What is that, what is the disease called? Mesothelioma? Something like that. We should talk to our friends at Cancer Research UK. We should.

Yes. So here's the thing, okay, so the, this idea of L.I. 900, they knew that they'd solved it, that they'd found something. It was this unbelievably good thermal insulator. But these tiles are so incredibly fragile. You can't have them as big. They can't be large. They couldn't just, like, create these massive tiles, put them in and off you go. The space shuttle wasn't covered in this uniform blanket. It was covered in 2,400 tiny individual tiles. Oh, wow. And because the space shuttles, like, bend and flexed during flight, you can't just, like, glue all of these tiles directly to the metal skin underneath because 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 if you broke one of them, you can just go and grab one off the shelf. You've got to grab that shape.

That exact shape, that custom replacement for that specific coordinate. These were an engineering nightmare, these things, because the other issue is that, you know, these are 94% empty space, right? Like, 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 sitting on the launch pad, the tiles would absorb like hundreds of kilos of water. Right. They would become solid liquid water.

Solid liquid water, exactly. 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. A lot of extra weight suddenly you've got to launch, yeah. Right. So then they're having to like inject these tiles with this waterproofing agent. Basically, you know, Scotch garden on steroids essentially.

Byer a needle, they've got to do it in all of these gaps and all of these holes. But they, one of the ways around this to sort of seal the outside is, especially on the belly, is they have this glaze that they put on it, this reaction cured glass, this borosilicate. And that helps to seal it, but also maximizes the heat immissibility, 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. Yeah. Cool, huh?

Isn't that cool? 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. Yeah. Isn't that amazing, though? Like this complete paradoxical material, something that's like, fragile enough to be crushed by a toddler, but strong enough to survive this plunge through the

atmosphere at like over a thousand degrees. 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 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. I always thought that was the hardest bit. The whole thing about whether the moon landing was real or not. I always thought that the argument for me was that getting to the moon and landing on the moon actually, I mean difficult, but 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 space race, combined with, you know, launching missiles and nuclear weapons from space and all of the sort of defense side of things that came with that. But you also had incredibly 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 hard. So it always seemed to me that the story.

showy bit wasn't the hard bit, which makes me believe that they did the entire thing. 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. 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, if I remember rightly. Yeah, because he's talking about like hours and hours of footage. that is like streamed out onto televisions. And somehow it needs to 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. 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. Yeah. Oh, yeah. Okay, well, we're hearing from our producer that, L.I. 900 would need to exposure to 1,450 degrees centigrade for hours, then crushed to have any carcinogenic potential whatsoever. So, who, live to fight another day, Michael. Wow. You're safe. Okay.

Important that you recognize I was willing to put my life into your hands for the purposes. I know. I know. But now we know that you were safe all along. So I feel better. All along. All right. Well, I guess we'll leave it there for this week. As ever, if you have, have any questions, ideas, tiles off the space shuttle that you want to send us, you can email us. The Rest is Science at Gohanger.com. And you can join our newsletter at therestis.com slash science. We're going to be back next Thursday with another edition of field notes and on Tuesday with our normal episode. So until then, stay curious.