How far can we go with chemical rockets?

Daniel and Kelly explain the limitations of chemical rockets and how physics can propel humans to the outer reaches of the solar system. See omnystudio.com/listener https://omnystudio.com/listener for privacy information.

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

• 0:00Daniel Whiteson?

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• 1:16Speaker 03

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• 3:17Daniel Whiteson?

  How long are we going to be stuck on this tiny little rock? And by we, I mean humanity, not me, or Kelly, or even you. I'm plenty happy to live out my life here on Earth, but humanity should push beyond, should find other places to live, should walk on alien planets, should see sunrises from different distant stars. But what's going to make that happen? For a long time, humans have relied on chemistry. Blow stuff up, funnel the output out the back of your ship, ship goes forward.

• 4:00Daniel Whiteson?

  Cool, that works, but is that steampunk kind of plan going to power our future? Can we use that chemical technology to explore the outer reaches of the solar system and to reach other stars? And if chemistry lets us down, what's going to come to the rescue? We know it's not biology. Today, we'll dig into the issues with chemical rockets and show you how physics is going to save the day. Welcome to Daniel and Kelly's extraordinary propelled universe.

• 4:31Kelly Weinersmith?

  Hello, I'm Kelly Wiener-Smith. I study parasites and space, and it doesn't matter what method you use, I'm not going to space.

• 4:51Daniel Whiteson?

  Hi, I'm Daniel. I'm a particle physicist who loves thinking about aliens, and I've never been on a rocket powered anything.

• 5:00Kelly Weinersmith?

  Yeah, I guess I haven't either.

• 5:05Daniel Whiteson?

  But you had to think about it for a minute. What were you considering? Were you like, ooh, was that bicycle that I strapped a huge engine to rocket powered or not?

• 5:15Kelly Weinersmith?

  Well, I will embarrassingly admit that I was thinking like, all right, how do planes go? How do ships go? How do trains go? And then the answer to all of them was not fire rocket. So, you know, I just had to make sure I was telling the truth.

• 5:32Daniel Whiteson?

  All right, well, let's close the gap in your answer. You said you wouldn't go to space, but would you get in a rocket-powered something here on Earth?

• 5:39Kelly Weinersmith?

  Uh, no, I doubt. It's hard for me to imagine that I would do that. What about you?

• 5:46Daniel Whiteson?

  Well, let's check in again after you hear about ion thrusters, because maybe a gentle ion-thrustered-powered bicycle is something you would ride.

• 5:54Kelly Weinersmith?

  But that wouldn't be a rocket then, right? Now we're in a whole different zone. I do travel on vehicles. I'm not anti-getting-around, but by rocket makes me a little nervous, I think.

• 6:07Daniel Whiteson?

  All right, well, I hope on today's episode, we're going to expand your mental conception of what is and what isn't a rocket. It's not all flames and explosions.

• 6:17Kelly Weinersmith?

  Well, then, how do I know that I haven't been on a rocket if I clearly don't have a good conception of what a rocket is and what it isn't? All right, it's time to teach me. Let's jump in.

• 6:26Daniel Whiteson?

  All right, well, on today's episode, we're going to be talking about how it's possible to explore the rest of the solar system. Our current technology are ugly, nasty, dirty, expensive, heavy chemical rockets. Can they take us to the edges of the solar system and beyond? That's the question we put to our group of listeners who love to speculate without a chance to Google or do any research. If that sounds fun to you, we'd love to hear your thoughts on the topic of the day. Write to us to [email protected]. In the meantime, think about it for yourself for a moment. How far do you think we can get with chemical rockets that Kelly hates?

• 7:09Kelly Weinersmith?

  All right, hold on, hold on, hold on. Before we hear from the extraordinaries, I want to, A, stick up for myself and B, stick up for rockets. You used a bunch of really negative words to describe rockets, but I watched a rocket launch to space once, and I thought it was very moving and beautiful. And I do think it's amazing that we can use rockets to send stuff to space. So I don't want to be, I'd like to change the negative tone a little bit. It's not perfect. They're not perfect, but they're pretty amazing.

• 7:40Daniel Whiteson?

  They are pretty amazing, and I think it's awesome what humans have built. Every time I see a project like that literally get off the ground, I think, wow, look what we can do. And I'm awed. The same way I feel like when I see the Golden Gate Bridge or any huge engineering project, like something that's so far beyond the scope of an individual person, yet we can do it collectively, it gives me hope for the future.

• 8:01Kelly Weinersmith?

  Yeah, I'd say it gives me goosebumps. I get very excited seeing big projects. But so, okay, but chemical rockets have their limitations. They take something like six to nine months if you're going to Mars. If we want to explore the rest of the universe, it would be nice if we could go faster, for example.

• 8:16Daniel Whiteson?

  It would be nice if we could go further, for sure. Yeah.

• 8:19Kelly Weinersmith?

  Yes.

• 8:20Daniel Whiteson?

  So we asked the extraordinaries how far we can go with just chemical rockets. Here's what folks had to say.

• 8:26Speaker 06

  The moon and Mars, and even Mars, will be a stretch. We're going to need some new form of propulsion to get further. We would need a tank the size of Jupiter to get anywhere.

• 8:41Speaker 01

  Get pretty far if you have patience. Not too far, I think. Our space probes to date have got most of their speed from gravitational slingshots, not chemical rockets. You could explore as far as you want, as long as you can keep finding chemicals, I imagine.

• 8:57Speaker 07

  We could probably go pretty far with chemical rockets if we could make them big enough to carry a lot of chemical fuel.

• 9:04Daniel Whiteson?

  We need nuclear fuel to go anywhere far away like Mars. With our current chemical rocket technology, we're not able to get even close to the speed of light.

• 9:15Speaker 03

  I can't imagine it would be very far. It's difficult to even explore Mars.

• 9:20Speaker 07

  That's kind of a hard question because I actually don't know what a chemical rocket is.

• 9:24Speaker 06

  To the asteroid belt.

• 9:26Daniel Whiteson?

  I think we need to switch to solar sails or perhaps some kind of experimental ion drive.

• 9:32Kelly Weinersmith?

  I think we have a pretty space savvy audience.

• 9:35Daniel Whiteson?

  Yeah.

• 9:36Kelly Weinersmith?

  Yeah.

• 9:37Daniel Whiteson?

  I think so. They've all read your book. They've thought about space.

• 9:40Kelly Weinersmith?

  They've been listening to you for years, talk about this kind of stuff and they're paying attention. They know what's up.

• 9:46Daniel Whiteson?

  They didn't know until now, though, that you are anti rocket.

• 9:49Kelly Weinersmith?

  I've clarified I'm not anti rocket.

• 9:51Daniel Whiteson?

  If you liked rockets, you would have ridden on one, Kelly. Look, put your money where your mouth is, okay?

• 9:55Kelly Weinersmith?

  Have you ridden on a rocket? I've jumped out of a plane.

• 9:58Daniel Whiteson?

  No, but I'm not pro rocket.

• 10:01Kelly Weinersmith?

  I don't, how many people have opportunities to ride rockets?

• 10:04Daniel Whiteson?

  That's a good question. That's fair. Yeah.

• 10:06Kelly Weinersmith?

  Yeah.

• 10:06Daniel Whiteson?

  You're saying you haven't said no to a rocket ride?

• 10:08Kelly Weinersmith?

  That's right. That's right.

• 10:09Daniel Whiteson?

  But you would say no.

• 10:10Kelly Weinersmith?

  Yeah, I would. Yeah, I would. I would. But that doesn't mean I'm not, you know, happy they exist.

• 10:15Daniel Whiteson?

  Yes, me too. I'm happy people go and explore space, even though I never want to do so myself.

• 10:19Kelly Weinersmith?

  Amen. All right. Tell me about rockets, Daniel. How do they work?

• 10:24Daniel Whiteson?

  Right. So rockets are a way to increase your momentum. What you need is energy and mass. And basically, the way a rocket works is it uses energy to push mass out the back. And when you do so, you send momentum in one direction. And because of conservation of momentum, you go the other way. A basic example of a rocket is sitting in a rowboat, throwing rocks out the back of the rowboat. You're using energy to throw those rocks. The rocks have mass. And because they carry mass, they carry momentum. And so the rocks go one way and the boat goes the other way.

• 11:00Kelly Weinersmith?

  And the more mass, the more momentum, right? So if you threw a rock or a comparably sized chunk of Styrofoam, the rock would get you farther.

• 11:08Daniel Whiteson?

  That's right. Momentum, non-relativistically, is mass times velocity. And that's going to be absolutely crucial in today's conversation. So equivalent momentum by throwing a really massive thing very slowly, or by throwing a very low mass thing very, very fast, you get the same momentum. So those two options.

• 11:26Kelly Weinersmith?

  Okay.

• 11:26Daniel Whiteson?

  So what you got to do is transfer momentum out the back. Anything that does that basically is a rocket. You need energy and you need mass. And if anybody out there happens to be a space is not real denialist, yes, rockets can work in space. You don't need air for rockets. All right. The same principle, throwing mass out the back works just fine in an atmosphere or not in an atmosphere. These things are not climbing up the atmosphere and they're not pushing against the air. You're just throwing momentum out the back. So the rocket moves forward to balance the momentum and to maintain conservation of momentum.

• 12:03Kelly Weinersmith?

  So if you were on like an old fashioned paddle boat sort of thing, and the energy is your legs and you are spinning like a little paddle thing behind and it's pushing water out the back, is that a rocket? That's, why is that not a rocket?

• 12:22Daniel Whiteson?

  I see. You want to like reengineer the definition of rockets so that you have been on a rocket.

• 12:27Kelly Weinersmith?

  So you have, you have told me that the definition of rocket is broader than what I thought it was. And so now I'm trying to figure out what the bounds are.

• 12:36Daniel Whiteson?

  So unfortunately, I don't think a paddle boat qualifies as a rocket because a rocket needs to bring that mass with it, right? And on the paddle boat, you are pushing against the water. You're not like carrying water that you're then pushing out to the back of the paddle boat, right? So a rocket carries with it mass and throws that mass out the back. A paddle boat is not doing that. It's pushing against the water. So for example, a paddle boat would not operate in a vacuum, right? Whereas a rocket can.

• 13:04Kelly Weinersmith?

  I feel like you're moving the target. You didn't mention ahead of time that the mass has to go with you, but all right, all right.

• 13:09Daniel Whiteson?

  We're clarifying here. That's why we're having a conversation.

• 13:13Kelly Weinersmith?

  Okay.

• 13:13Daniel Whiteson?

  And in the future, every conversation I have, I will give complete exhaustive definitions of everything to future proof myself against any questions.

• 13:21Kelly Weinersmith?

  This is our last episode, folks. No one's going to want to listen after this. All right. Let's move forward. I'm get, we're carrying the mass with us.

• 13:28Daniel Whiteson?

  That's right. Okay. So you need energy and you need mass and you're going to carry the mass with you to throw it out the back. Cool. That's the basic principles of rockets. And the rockets we're familiar with are mostly what we call chemical rockets. And they solve this energy and mass problem by bringing fuel. Fuel stores energy, its chemical process releases it, and it provides mass that you can expel out the back. So it's like an all in one solution to the rocket problem. It's very cool, right? You burn the fuel and that gives you energy and it produces stuff that you can throw out the back. So that combustion reaction does everything you need.

• 14:04Kelly Weinersmith?

  Okay. And the difference between fuel and propellant, because the engineers, when we first wrote Soonish and we made the mistake of saying fuel or whatever, were very upset. And so the difference between fuel and propellant is that propellant, something gets thrown out the back? What is, what is the difference?

• 14:25Daniel Whiteson?

  Yeah. So propellant can be totally inert. It's just something you throw out the back.

• 14:28Kelly Weinersmith?

  Okay.

• 14:29Daniel Whiteson?

  So in the example of the rowboat, you're throwing rocks out the back, right? That's your propellant.

• 14:33Kelly Weinersmith?

  Okay.

• 14:33Daniel Whiteson?

  You're using it to propel yourself. But you wouldn't call rocks fuel. They don't store any energy. Fuel is something which stores energy and has propellant.

• 14:43Kelly Weinersmith?

  Okay.

• 14:43Daniel Whiteson?

  So it solves the whole rocket problem all in one.

• 14:46Kelly Weinersmith?

  Awesome. So one of the problems with this is that the stuff that you create, that you throw out the back is not necessarily great for the environment?

• 14:55Daniel Whiteson?

  Mm hmm.

• 14:56Kelly Weinersmith?

  Have we found a propellant that is less bad for the environment?

• 15:00Daniel Whiteson?

  Well, the basic process here is a chemical reaction combustion, right? Where you're turning, for example, like methane plus oxygen into CO2 and water and energy. So the core reaction is bad only in that it produces CO2, right? Which is not good for the environment.

• 15:16Kelly Weinersmith?

  Okay.

• 15:17Daniel Whiteson?

  But it can be fairly clean otherwise. But some of the fuels do produce other nasty stuff. There's a whole range of various chemical processes used here and ranges of kinds of fuel that humans have used to make their glorious launches into space.

• 15:32Kelly Weinersmith?

  Woo.

• 15:32Daniel Whiteson?

  Ranging from like solid fuel. So the space shuttle boosters had aluminum powder and ammonium perchlorate in them.

• 15:39Kelly Weinersmith?

  Wow.

• 15:39Daniel Whiteson?

  Which you can use as fuel, which is very cool.

• 15:42Kelly Weinersmith?

  Mm hmm.

• 15:42Daniel Whiteson?

  The SLS core has liquid hydrogen and liquid oxygen. So very basic, very simple stuff.

• 15:49Kelly Weinersmith?

  Nice.

• 15:49Daniel Whiteson?

  But again, this is fuel because it can combust. So it releases energy and it produces propellant that you throw out the back. SpaceX Falcon rockets use something called RP1 or Carolox, which is basically kerosene plus liquid oxygen. So all the components you need for combustion. And then there's methylox, which is basically methane and liquid oxygen. Methane is just CH4. So that's pretty clean.

• 16:16Kelly Weinersmith?

  Yeah. Okay. Great.

• 16:17Daniel Whiteson?

  So the good news about chemical rockets is they produce high velocity and they use a lot of volume very, very quickly. So you can burn through a huge amount of fuel very quickly and produce a huge thrust. So you can get like exhaust velocity of like three kilometers per second. Remember, we were talking about throwing propellant out the back. And what you need is high velocity and high mass to get high momentum. And so this is pretty high velocity, three kilometers per second. But most importantly, it is a large volume here. So for example, each one of the Falcon 9's Merlin rocket engines burn 250 kilograms of fuel per second.

• 16:56Kelly Weinersmith?

  Wow. Wow.

• 16:58Daniel Whiteson?

  Exactly. And so this provides a huge momentum change, right? Because it's an enormous amount of mass moving with high velocity. And so each engine has a gigawatt of power. And the Falcon 9 has nine of these things.

• 17:13Kelly Weinersmith?

  Wow.

• 17:13Daniel Whiteson?

  So the basic principle for chemical rockets is a short, hard burn with a huge amount of thrust, right? You're burning a lot of fuel. You're getting a lot of velocity. It's not just one push. There's an extended burn as you're watching this thing lift off. But chemical rockets don't tend to burn for very, very long times for a reason we'll talk about in a minute. It's mostly like a big, hard push at the beginning.

• 17:36Kelly Weinersmith?

  Well, all of that sounds amazing. So what's the problem, Daniel?

• 17:39Daniel Whiteson?

  The problem is that consuming a lot of mass means having a lot of mass, right? So you're burning all of this fuel. You got to have a lot of fuel. And the burning is not very efficient because I'm going to poop on chemistry. It's a chemical reaction, right?

• 17:53Kelly Weinersmith?

  Boo. Oh, that wasn't the bad part. Sorry.

• 17:57Daniel Whiteson?

  Well, the bad part is that, you know, you're changing the configurations of these atoms. So you're releasing some energy, but the energy was stored in chemical bonds. And chemical bonds can store energy, but it's like a tiny fraction of all of the energy stored in that fuel. If you, for example, combine that fuel with an antimatter equivalent of that fuel, you could convert all of that matter into energy. It would be an enormous amount of energy produced. But instead, you're just like moving the atoms around and clicking them into slightly lower energy configurations. So for a lot of mass, you're getting a fairly small amount of energy out. So there's a small amount of energy carry for every pound of fuel or every kilogram of fuel that you're bringing along.

• 18:38Kelly Weinersmith?

  So is your argument that the problem is that it's chemistry, not physics? Because you want things to be happening on an antimatter physics level?

• 18:46Daniel Whiteson?

  Well, physics can access the power of the atom, right? It can annihilate these things and convert all of that stored energy, if you could do that. I'm not saying that, you know, I know how to design an antimatter engine or anything. But that's one of the problems with chemical rockets is that you're releasing a small amount of energy per mass. So you need a lot of mass, right? Now, if you need a lot of mass, you're going to need a bigger push to get you up to some velocity. And to get that bigger push, what do you need? More mass. That mass means you now need a bigger push. And so very quickly to get anywhere far, to get a big thrust, you need fuel. And then you need fuel to carry that fuel. And you need fuel to push that fuel. And you need fuel to carry that fuel. And so eventually you're mostly fuel.

• 19:29Kelly Weinersmith?

  I think it's like a mortgage, but how about you explain it, Daniel?

• 19:33Daniel Whiteson?

  Well, in a mortgage, what you're doing is you're borrowing money to buy your house and then you're paying it back, but the interest is compounded, right? And so for the first long time, you're just basically paying off the interest before you're ever paying off the principal. So you're not actually making progress on your loan. And by the time you pay off your house, you've ended up paying like three or five times as much as you borrowed, right? You borrow a million dollars to buy a house, you end up paying the bank $5 million over all that time. And so in the same way, when we launch a rocket, most of the mass of that rocket is the fuel. We're mostly launching fuel, right?

• 20:07Kelly Weinersmith?

  Yeah.

• 20:08Daniel Whiteson?

  We have like a little capsule on top where we get to put something we want, but mostly we're launching fuel because we need to burn that fuel when we're halfway up.

• 20:15Kelly Weinersmith?

  Yeah, I think the payloads that get sent up on rocket, it's like 10% or less when you look at a rocket is stuff that you're actually sending to space. And the rest of that skyscraper shaped thing is just fuel that's going to get you there.

• 20:28Daniel Whiteson?

  Yeah. So most of a Mars bound rocket's mass is propellant. So it takes like, for example, a thousand tons of methylox fuel to get to Mars from Earth orbit.

• 20:38Kelly Weinersmith?

  Wow.

• 20:39Daniel Whiteson?

  So if you're already in Earth orbit, right? And we're going to talk about how to get there in a minute, then it takes still a thousand tons and you're going to burn that fuel in space. It's going to give you a big push and then you're mostly going to coast the rest of the way to Mars. That's the way a chemical rocket works. But then if you want to get a thousand tons of methylox fuel into Earth orbit, you've got to spend 25,000 tons just to get that fuel up there.

• 21:03Kelly Weinersmith?

  Wow.

• 21:03Daniel Whiteson?

  Right? So that's a factor of 25 just to get the fuel you need into orbit. So you can burn it to get to Mars.

• 21:10Kelly Weinersmith?

  Holy cow.

• 21:11Daniel Whiteson?

  And a thousand tons of methylox is what it costs at the cheapest transfer moment, right? Remember, Earth and Mars don't go around the sun at the same speed. So sometimes they're on the opposite side of the sun, really hard to get to Mars. Sometimes they're on the same side of the sun. And once every 26 months, they're at their closest approach and it's the best time to go to Mars. So this is the cheapest it could ever cost to go to Mars using methylox fuel.

• 21:36Kelly Weinersmith?

  And even then, it's not, you've gone through all of that, you've paid all of that, you've burned all of that into the atmosphere and it still takes you six to nine months to get there.

• 21:45Daniel Whiteson?

  Yeah. Well, we're not going to solve that problem with ion thrusters today, though we are going to make it more flexible to get to Mars.

• 21:52Kelly Weinersmith?

  Okay.

• 21:52Daniel Whiteson?

  But you know, that's how we can get to Mars and that kind of works. But say you wanted to go further, right? You wanted to go to Jupiter. Now you need 2,000 tons of methylox fuel. And then you've got to get all of that into orbit. So it becomes very expensive very quickly and the prospect of like going to Alpha Centauri using chemical rockets, you know, you end up needing fuel tanks like the size of the moon.

• 22:15Kelly Weinersmith?

  Holy cow. All right. But don't despair because we're going to take a break. And when we get back, Daniel is going to tell us about other solutions that are going to get us to Alpha Centauri.

• 22:23Daniel Whiteson?

  That are still not paddle boats.

• 22:25Kelly Weinersmith?

  That are still not paddle boats, which aren't rockets because you're not taking the water with you.

• 22:41Speaker 03

  Hey, it's Niall Rogers, host of Before the Moment, a Lincoln original series. I'm driving across the country, meeting some of today's most fascinating artists. We're exploring the stories before the moment that changed everything. This week, I'm joined by my friend, John Ledge. What still makes you just sit down at the piano? If you sat down at 9:00 AM and then at noon, you had a full song that you recorded and putting your voice recorder or you recorded at your studio. You've just created something brand new that didn't exist when you woke up. Yes. What a beautiful feeling it is. I'm always excited to write something new. I've been writing for Broadway, which has been a new challenge with new collaborators and

• 23:26Speaker 03

  these amazing performers who are triple threats, who can sing and dance and act and do all these amazing things. And I want to keep feeling that. And so that means not resting on my laurels, going out there and continuing to create something new. Experience Before the Moment, a Lincoln original series on the iHeartRadio app, YouTube, or wherever you get your podcasts.

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• 24:49Kelly Weinersmith?

  Down the sideline touchdown.

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• 25:35Daniel Whiteson?

  Hey Kelly, let's design a rocket powered version of a paddle boat. What would that look like?

• 25:39Kelly Weinersmith?

  That would, I would bring the water with me and then I would throw it out the back.

• 25:44Daniel Whiteson?

  Yeah. So if you had like a fire hose on the back of your boat and you had a huge tank of water and you had some way to have energy, so you could pump that water out the back of your boat, then yes, that would be a rocket powered boat. It wouldn't be paddle anymore unless Kelly is the source of energy. There we go. We have a huge tank of water. We have Kelly on top on a little extra cycle and she's pumping, pumping, pumping, pumping, pumping, pumping. There you go. And it's pushing water out the back, out of the tank. That's a rocket powered paddle boat that we could legitimately build. And then you could say, yes, I've been on a rocket.

• 26:17Kelly Weinersmith?

  Okay. But this is like the least exciting rocket history has ever created. Like boo, boo to this rocket.

• 26:26Daniel Whiteson?

  Well, that's the principle though, behind the other technology we're going to talk about today. Instead of chemical rockets, where you have the fuel that gives you the energy, though very inefficiently, and the propellant, which requires huge massive tanks to go anywhere. We're going to talk about ion thrusters, which separate this question of propellant, the mass you're throwing out the back, and the energy source.

• 26:48Kelly Weinersmith?

  Okay.

• 26:48Daniel Whiteson?

  In the same way that like Kelly pumping the water out the back of the boat separates the energy source from the water. You're not getting the energy from the water. The water is totally inert, right?

• 26:58Kelly Weinersmith?

  Sure.

• 26:59Daniel Whiteson?

  So ion thrusters, the approach we're going to talk about today, and it solves the energy and mass problems separately. First, we're going to talk about the mass and then we'll talk about the energy. And they're called ion thrusters because they use ions. An ion is just an atom that's not neutral. So if you take, for example, xenon, it's got a bunch of electrons in it, which balance out the protons in the nucleus. And then you, if you strip some of those electrons away, then you have an ion. So now it has a positive charge. And that's useful because you can then accelerate it in electromagnetic fields, right? So if you have a bunch of xenon and you strip away some of the electrons, now they have a charge in them. If you can build an electromagnetic field, it will accelerate

• 27:40Daniel Whiteson?

  those ions, shoot those ions out the back of your ship and it's the ionic equivalent of Kelly's water rocket, right? Just shoot them at the back. They carry momentum with them because you have high velocity now and that's a rocket.

• 27:55Kelly Weinersmith?

  Okay, that sounds cooler. If you're bringing ionized gas, does this help you cut down on weight because gas weighs less than propellant or not really? Because once you compress the gas, it gets heavy. What's, what's the deal here?

• 28:10Daniel Whiteson?

  No, it really does help you cut down on weight because you can be more efficient about the energy. One of the things about chemical rockets is that they're trying to solve both problems at the same time, energy and mass. And the energy extracted via combustion is very inefficient. And so here, if we can, let's ignore the energy question for now and imagine we have some energy source to fund our electromagnetic fields. Now, if we have some energy source, we can very efficiently use that to accelerate our ions to very, very high speeds. And so we can get much more push per unit of propellant, right? Because the mass can go to higher velocities. So it sounded impressive earlier when we said like three kilometers per second for the propellant from a chemical rocket.

• 28:53Daniel Whiteson?

  That's cool. But ion thrusters can get much higher velocities. And so for every pound of propellant that we bring along, we can now get much more push. And this is what rocket folks talk about as specific impulse. If you start reading about rockets, you get into all these weird terms, but it's just a bunch of fancy terms to cloud some basic physics, which is you want as much momentum change per mass. So remember momentum is mass times velocity. If you're bringing rocks or water or xenon or whatever to throw at the back, you want as much velocity in every kilogram you throw out the back because momentum is mass times velocity. And so for a fixed mass, You want the most velocity to get the most momentum.

• 29:35Kelly Weinersmith?

  Okay.

• 29:35Daniel Whiteson?

  And so if you can get these things to higher speeds, which ion thrusters can do better than chemical rockets can do, then you can get more momentum per unit mass that you're bringing.

• 29:45Kelly Weinersmith?

  My gut would not have guessed that pushing an ion would get things out the back faster than exploding propellants. Cool.

• 29:54Daniel Whiteson?

  Well, physics is more powerful than chemistry, Kelly. And we're using electromagnetic fields here, right? And so if you have enough energy and this whole thing hinges on having enough energy, which we haven't talked about at all yet.

• 30:06Kelly Weinersmith?

  Oh, okay.

• 30:07Daniel Whiteson?

  But if you have an energy source, right, then you can accelerate these ions to very high speeds. Here on Earth, we accelerate protons, which are just hydrogen ions, to nearly the speed of light, right? Enormous amounts of momentum in the collisions at the Large Hadron Collider, for example. So this all sounds great so far. We're talking about a lot of momentum created for every ion, right? The issue is that it's hard to scale up.

• 30:29Kelly Weinersmith?

  Oh.

• 30:29Daniel Whiteson?

  So we have a small amount of mass in general, which means a small momentum change. So while for each ion, we can get each one to high velocity, which means that per pound of xenon we're bringing, we're getting a lot more momentum change than per pound of methylox or hydrolox or whatever fuel a chemical rocket is burning. It's a real challenge to get a lot of kilograms out the back to get a huge thrust.

• 30:56Kelly Weinersmith?

  Okay.

• 30:56Daniel Whiteson?

  And so instead of like a big hot burn that gives you a lot of thrust all at once, an ion rocket tends to have a slow, gentle, long thrust. So if, for example, it might take like two days for an ion rocket to get you up to even like highway speeds. Whereas a chemical rocket's gonna be like kaboom, we're going pretty fast, but then it's all spent.

• 31:17Kelly Weinersmith?

  Okay, but so does that mean ion thrusters can't get you out of Earth's gravity well?

• 31:21Daniel Whiteson?

  That's exactly right. Ion thrusters, as currently designed, can't get you off of Earth because they're very, very gentle thrust. But they can burn for a very, very long time and they're very efficient and they can travel with a small amount of mass. The amount of mass you need to get to Mars, for example, is like thousands of kilograms of xenon, which compares to like a thousand tons

• 31:43Kelly Weinersmith?

  Wow.

• 31:44Daniel Whiteson?

  of chemical fuel. So it's a huge difference. Your whole ship is much lighter. And then that's not as hard to get into orbit, right? Even if it's a factor of 25 to get something into orbit from Earth, you need to use chemical fuel to do that. Now it's a factor of 25 times 1,000 kilograms instead of 1,000 tons. So it takes much less chemical fuel to get an ion rocket into space with the fuel it needs to get to Mars than chemical fuel to get a chemical rocket into space.

• 32:14Kelly Weinersmith?

  Okay, but so, but we'd still have to use a chemical rocket to get the ion rocket to space. And then you use the ion rocket to get to where you're going.

• 32:22Daniel Whiteson?

  Yeah, exactly. Unless you can build these things in space with Jeff Bezos's future space-based economy from asteroids or whatever. Right? So this isn't the complete solution. It's not like, you know, we're building warp drives or teleportation portals to get to Alpha Centauri, you know, in your living room or something. But this will allow us to get further out into the solar system and even past the solar system once we can get out of Earth's orbit.

• 32:47Kelly Weinersmith?

  Well, so, so you mentioned it takes two days to reach highway speed. Do you eventually reach high enough speeds that a trip to Mars wouldn't take like five billion years?

• 32:56Daniel Whiteson?

  Yes, absolutely. And NASA is building exactly this kind of rocket. We're hoping to use it to go to Mars in 2028. It's called SR Freedom. And I'll tell you all about that trajectory once we're done talking about the energy source of these things, which is really important.

• 33:10Kelly Weinersmith?

  All right, where do we get the energy?

• 33:12Daniel Whiteson?

  So there's a long history, actually, of ion thrusters, which I think is interesting. And it traces sort of the different sources of energy. People start talking about these things back in like 1911. And there's a few different approaches for how to build it and then how to power it. The approaches for how to build it are things like a gridded ion thruster. Basically, you just have like an anode and a cathode. You know, you have one thing with a positive charge, one thing with a negative charge. You put your charged particles in it, they get attracted to the opposite side, they go fast, they zoom out the side. So it's a grid because there are holes in it for the particles to go through. So imagine like two chain link fences, one is positive, one is negative. You drop a positive particle into there, accelerates towards the negative one, and then shoots through the gaps and out the back. That's the basic principle.

• 33:57Kelly Weinersmith?

  Okay, but so to make the anode and the cathode, do you just like stick magnets on there? Or do you need like a battery to make that happen?

• 34:04Daniel Whiteson?

  You need energy. Yeah, you need electricity. Magnets can't do that for you.

• 34:08Kelly Weinersmith?

  Okay.

• 34:08Daniel Whiteson?

  Also, that design isn't great because it corrodes. You're shooting particles at the chain link fence, which tends to eventually destroy the chain link fence or it gets jammed up or something. So these days they have even fancier ones, something called a hall effect thruster, which doesn't have the actual cathode, has a virtual cathode. You have a magnetic field there which traps electrons, so they move in circles. And they ionize the gas which they collide into. So there's sort of like a electric mixing spoon that breaks up the gas into ions. And then you don't actually have a cathode there. The electrons there act as the cathode. And so there's nothing to get like corroded or broken down. So that's the technology. But in every case, the limit is the energy source.

• 34:52Daniel Whiteson?

  If you want to crank up ion thrusters to give you a real amount of thrust, then you need a lot of energy. And so the first example of an ion thruster I want to talk about is called the CERT 1. This is back in 1964. This was a test by NASA and it was basically like, hey, does this work at all? And it was up in space for 30 minutes and it worked. And it was powered by chemical batteries. So these were like zinc silver oxide batteries. And it wasn't a whole lot of energy. It was like 1.7 kilowatts of energy. But, you know, it didn't go for very long, so they didn't even like try solar panels. Though a later version of a CERT 2 did have solar panels. But this was like, hey, all the equations match. Does it actually work in practice?

• 35:37Daniel Whiteson?

  And it did, so that's cool.

• 35:38Kelly Weinersmith?

  Very cool. Well, I've never heard of it, which is a bummer.

• 35:42Daniel Whiteson?

  Oh, yeah. It's pretty cute. It's just like a little ball. And then people said, all right, well, let's try to go for longer periods and let's avoid using batteries and let's use solar power. And this is really exciting because now you're not carrying the energy with you at all.

• 35:57Kelly Weinersmith?

  Then it's not a rocket.

• 35:59Daniel Whiteson?

  No, it's a rocket if you're carrying the propellant. So you still have to carry the mass you're throwing at the back, but it's not a requirement that you bring the energy with you, right?

• 36:07Kelly Weinersmith?

  All right, all right.

• 36:08Daniel Whiteson?

  So if you can just beam the energy to the rocket and you can use that, then that's awesome. So they tried this. The first one is called NSTAR, which has 2.3 kilowatts of solar power. So these solar arrays generate the electrical power, which then you use to build electric fields to shoot ionized xenon out the back at really high speeds.

• 36:32Kelly Weinersmith?

  Wow.

• 36:32Daniel Whiteson?

  And this thing ran for 16,000 hours and used only 80 kilograms of xenon propellant to get pretty good delta V. It was like 4.3 kilometers per second. So it was a good demonstration of these things running for a long time, which is what we need to like, you know, get to Alpha Centauri or get even to the edges of the solar system. The next step up is the DART mission. I don't know if people remember this. This is the one that went and visited an asteroid and I think it dropped like a washing machine sized block of metal on the asteroid, just like see like what happens.

• 37:07Kelly Weinersmith?

  Yeah. Well, you know, wasn't it also to see, I mean, it was to see what happens, but it was to see what happens in the hopes that if there's ever an asteroid coming towards us, we would have some information about how hard you'd have to like hit it to try to deflect it from the Earth or something, right? It was more than just see what happens.

• 37:23Daniel Whiteson?

  Oh no, you're right. I'm actually thinking of the Hayabusa mission. That dropped a huge metallic payload as an impactor onto the asteroid Ryugu. But DART is different. You're right, DART was there to like, let's practice deflecting an asteroid, so it does something else. So DART's main propulsion was a chemical rocket, but it also had an ion thruster for testing, which is very cool.

• 37:45Kelly Weinersmith?

  Well, and didn't Hayabusa bring back a sample from the asteroid? Didn't it like it made an explosion to collect something? I feel like you're being uncharacteristically negative and I'm being uncharacteristically positive this time.

• 37:57Daniel Whiteson?

  No, no, that's totally positive. I love the idea of dropping a washing machine sized piece of metal on an asteroid just to see what happens. Like, that's fantastic. I'm super curious. Like, I love when that mission went up. Total supporter. My tax dollars should definitely be spent on missions like that.

• 38:13Kelly Weinersmith?

  Okay, yes, might as well. Okay, so we have gotten to the asteroids with ion thrusters.

• 38:18Daniel Whiteson?

  Yeah, exactly. And so this one also used sunlight, electricity from solar panels, was used to accelerate xenon atoms. The ion engine was not the primary propulsion system for the impact. It was mainly a flight demonstration of this next technology for future deep space missions. And ion thrusters are all over the place, like China's space station has ion thrusters, which it uses to adjust and maintain orbit. Starlink uses ion thrusters that have krypton or argon in them. They're good for like very gentle little pushes to like maintain where you are. Not like a quick like, ooh, I'm gonna hit something. I really gotta get out of here, you know? Also, stuff out in space tries to avoid using thrusters whenever possible because it uses up propellant.

• 39:00Daniel Whiteson?

  You know, for example, like Hubble or the space telescopes, they try to do all of their adjustments using reaction wheels. Like you want to turn the thing, you have internal reaction wheels that store angular momentum. So you can turn without any thrusters. We can talk about that on another episode. Because every time you're using a thruster, it's a rocket, you have propellant, you're using it up. And so eventually it will run out. But the real question is like, what is the best way to get energy? We've talked about chemical batteries. That was always just gonna be a test. Solar power is cool, right? Because it's up, it's free, it's out there. You don't even need to bring the energy with you. That's very cool. The problem is with solar power is that it drops off really hard with distance.

• 39:42Daniel Whiteson?

  The intensity of sunlight goes like one over the distance squared. So if you're twice as far from the sun, you get one fourth of the energy. And space is big. And so by the time you're like out by Jupiter, there's almost no usable solar power, especially to power a rocket. So we need something else. We need a low mass source of energy.

• 40:04Kelly Weinersmith?

  Physics is what we need. And when we get back, we're gonna hear about how physics is gonna get us out to Jupiter.

• 40:10Daniel Whiteson?

  Yeah.

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• 43:13Kelly Weinersmith?

  All right, Daniel, how is physics going to get us to Jupiter?

• 43:17Daniel Whiteson?

  Yeah, physics is absolutely the solution here. And so, of course, the better way to make energy is not by burning coal or burning methane or even by solar power, because that's not available out in the outer reaches of the solar system. It's nuclear power, right? We already know that the amount of energy stored in like a kilogram of uranium is huge compared to what you could get from a kilogram of methane, right? Randall Monroe has this amazing comic where he shows like the energy density of like uranium versus coal versus wood or whatever, and it's just off the charts. You know, it's like even on a log scale, it's just insane. And so if you want a bunch of energy and you don't want to carry a bunch of mass around, this is the way to go.

• 44:02Daniel Whiteson?

  Have a fission reactor. So bring a small amount of uranium, use that to power your nuclear reactor. Your nuclear reactor produces electricity. You feed that into your ion thruster, and now you have propellant, which is very effective because you're pushing it up to very high velocities. And you have fuel, uranium, which is very low mass. You're getting a lot of energy out of it per kilogram.

• 44:25Kelly Weinersmith?

  I'm gonna look past the fact that you mentioned the competition on this show. No, I'm just kidding. We love X K C D.

• 44:32Daniel Whiteson?

  It's not a competition. It's good for everybody. And anyway, nobody's in competition with Randall because he wins.

• 44:39Kelly Weinersmith?

  He does win. He does. And he's a lovely human being also.

• 44:43Daniel Whiteson?

  Yeah, he really is genuinely a nice guy.

• 44:45Kelly Weinersmith?

  Yeah, okay. All right, so we've decided that nuclear-powered ion thrusters are the way to go. Have we had any nuclear-powered ion thrusters in space? I know we've had little nuclear reactors in space to power satellites.

• 45:00Daniel Whiteson?

  So it depends on what you mean by we. The United States hasn't.

• 45:03Kelly Weinersmith?

  By we, I mean humanity.

• 45:06Daniel Whiteson?

  Well, then the Soviets have done that. The Soviets in 1987 had a nuclear-powered plasma thruster in orbit. So this has definitely happened. It's worked before. And the US, as I'm sure you know, has operated nuclear fission reactors in space. So all the pieces are there. We know ion thrusters work in space when they have an electrical energy source. We've powered nuclear fission reactors in space, and the Soviets even put them together in 87 to use nuclear-powered ion thrusters.

• 45:36Kelly Weinersmith?

  Wow, yeah, and all the pieces are still there because I think our attempt is still in orbit somewhere because it didn't quite go according to plan.

• 45:44Daniel Whiteson?

  Exactly. And so NASA was already building fission reactors to operate in space as part of the Lunar Gateway Space Station, which was then canceled. But they had a power and propulsion element being built by Lantares Space Systems, and they were already building those. And so we're going to take those and put them at the end of a very, very long truss to keep the radiation away from like the electronics of the rest of the ship. And that's going to provide power for the new SR1 Freedom, which is going to power a mission to Mars in 2028. Nuclear-powered ion thrusters to Mars.

• 46:21Kelly Weinersmith?

  Whoa, okay, all right. I feel like you're still gonna need the chemical rocket to leave Earth. But now you're carrying with you a nuclear reactor.

• 46:29Daniel Whiteson?

  Yes.

• 46:31Kelly Weinersmith?

  What is our plan for making sure that everything stays safe?

• 46:36Daniel Whiteson?

  Cross our fingers, rely on engineers. There's not a great plan. Like, this is an issue. It's always going to be an issue. Things go boom when you launch them. And so if you're launching radioactive fuel into space, it's always a danger.

• 46:51Kelly Weinersmith?

  There's not some way that you can like keep it clad in something that's less likely to go boom and then open it up and let the reaction start happening once you're in space and away from Earth? There's no way. I feel like there's got to be ways to make this less scary.

• 47:07Daniel Whiteson?

  So there are smart folks working on this, and of course, they're taking whatever precaution they can. One thing you can do is don't turn on the reactor until you get to space, for example. So there's still a risk because you have uranium fuel in there, which is radioactive, but you don't have like dangerous nuclear byproducts being produced or already present on the rocket so that if it does go boom, you're spreading some uranium fuel, which is not good, right? But it's not as bad as spreading like really toxic radioactive nuclear byproducts.

• 47:37Kelly Weinersmith?

  Okay.

• 47:38Daniel Whiteson?

  In addition, like, you know, they're putting this thing inside highly durable, heat-resistant containment materials designed to survive extreme launch vehicle explosions. And it's supposed to be like subcritical no matter what, even if the launch fails. So they're doing what they can, and I have not dug into the details here, but I trust the engineers are doing their best to solve this problem. And I totally acknowledge that it's very dangerous to be launching nasty toxic stuff into space because it might not get there.

• 48:05Kelly Weinersmith?

  All right, thank you for giving me my daily dose of Kelly being able to like worry and be a wet blanket. And all right, what do we need to think about next?

• 48:12Daniel Whiteson?

  So the way this thing works is it's a fission reactor, which means you're taking uranium and you're splitting it. And that causes a chain reaction because it produces neutrons, and those neutrons split other uranium atoms, and along the way it produces heat. And then how do you turn that heat into electricity? Well, on Earth, we often do something like boil water, right? And use the steam to spin a turbine. And so up in space, we're doing something only a little bit more sophisticated. Instead of water, we're using a helium-neon mixture, which still spins a turbine and creates electricity. So the basic process is the same. And then we can use that electricity to accelerate the ions. The cool thing is the fuel is very, very low mass.

• 48:56Daniel Whiteson?

  Remember, the propellant is like thousands of kilograms of xenon, which is still really low mass compared to the thousands of tons you need for chemical rockets. But the uranium fuel, to give you the energy to push that out the back, is just like 10 kilograms. It's like a rounding error. That's how powerful uranium is. It's really amazing. So it's almost as free as sunlight, but you can keep using it when you get past Jupiter.

• 49:21Kelly Weinersmith?

  Why haven't we done this already, then? This seems pretty obviously better. Is it because of concerns about like where are you going to get the uranium? Yeah, why haven't we done this already?

• 49:32Daniel Whiteson?

  Well, you know, the Soviets have been doing it since the 80s, but chemical rockets are easier, right? There's no nuclear power issues, and they can get off of Earth. So this is really only useful for exploring in places where you're not fighting a big gravity well.

• 49:46Kelly Weinersmith?

  Okay, got it.

• 49:47Daniel Whiteson?

  And it gives a pretty different signature for your mission. Like for a chemical rocket, you want to get to Mars, you burn for like 10 minutes, and then you travel to Mars for six to nine months, and you're going at like peak velocity almost all the time, like 33 kilometers per second. An ion rocket is going to give you like a year of continuous burn, and it's going to start very, very slow. It's going to eventually get up to the same peak velocity, 30, 35 kilometers per second, but it's not in free fall, right? It's a continuous thrust, and it can be adjusted in flight. And so it's a very different kind of trajectory, and that makes it less sensitive to the timing. So you can't get to Mars when it's on the other side of the sun, but because you're

• 50:30Daniel Whiteson?

  not just in free fall, you can do all sorts of different kinds of trajectories, so you don't have as narrow a window. So it takes a little longer to get there, like a year instead of six to nine months, but you have more flexibility about when you go.

• 50:44Kelly Weinersmith?

  I don't think I understand the you're not in free fall thing. Why would a chemical rocket versus an ion thing? If you had enough chemicals for your chemical rocket? Couldn't you have the same flexibility?

• 50:58Daniel Whiteson?

  In principle, if you brought enough fuel along that you're burning it during the flight, then yes, you could do the same thing with a chemical rocket, but then you'd need even more fuel. Remember, chemical rockets are very, very inefficient.

• 51:10Kelly Weinersmith?

  Okay.

• 51:10Daniel Whiteson?

  That's why they like to burn it all at the very, very beginning. And it's basically, it's almost like a cannonball launch, right? A cannonball, you give it one push and then it just flies.

• 51:18Kelly Weinersmith?

  Okay.

• 51:18Daniel Whiteson?

  Chemical rocket, it's like 10 minutes of pushing and then it's just a free fall trajectory.

• 51:23Kelly Weinersmith?

  Got it.

• 51:24Daniel Whiteson?

  And so you're very limited in how you go from one place to the other if you can't burn along the way. So the ion thruster can do that. It takes longer to get up to high speed. So the whole trip is going to be a year. The plus side is you have a little bit more flexibility. So your window is a bit broader. It's still not like we can go to Mars anytime and you can't get to Mars faster, but there's a little bit more flexibility there.

• 51:45Kelly Weinersmith?

  Okay. I mean, that, that's pretty cool because needing to wait two years

• 51:49Daniel Whiteson?

  Yeah.

• 51:49Kelly Weinersmith?

  for every launch window, like, you know, I mean, you missed that. Now you're waiting two years. Like, that's, that's a real pain in the rear end. So how do we, how much do ion thrusters open this window?

• 51:59Daniel Whiteson?

  So chemical rockets have like a few weeks of a window when they can effectively get to Mars and ion thrusters have like a few months.

• 52:06Kelly Weinersmith?

  Nice.

• 52:07Daniel Whiteson?

  So it's a meaningful change. It's again, it's not like totally solving the problem. It's not like you can go to Mars on a whim, but there is flexibility there.

• 52:15Kelly Weinersmith?

  Well, but you could imagine a few weeks of bad weather totally messing up your Mars plans if that bad weather happens during your window, but like probably not going to have a couple of months of bad weather. So like, that makes your probability of getting to Mars, I don't know, I think much higher.

• 52:30Daniel Whiteson?

  Yeah, exactly.

• 52:31Kelly Weinersmith?

  Look at me being a ray of sunshine.

• 52:35Daniel Whiteson?

  And so for example, if your launch is like delayed by 90 days, the spacecraft just like adjusts the thrust profile, burns more propellant to catch up to Mars rather than having to scrub the mission entirely. And adding a little bit more propellant is not disastrous because the propellant is fairly low mass. Here we're talking about like a few more kilograms of xenon, right? And the energy is very, very dense. So we don't have this rocket equation trap where we fall into where every time we bring a pound more fuel, now we need thousands more pounds of fuel to push that fuel. And we still got to launch this thing up into space, but remember the fact that it's lower mass means it's cheaper to launch it into orbit in the beginning. And we're going to do this in like 2028. It's just around the corner.

• 53:17Kelly Weinersmith?

  Woo.

• 53:17Daniel Whiteson?

  SR1 Freedom is going to go to Mars. It's going to drop three helicopters that land themselves on Mars.

• 53:24Kelly Weinersmith?

  Amazing.

• 53:25Daniel Whiteson?

  Which is amazing, given Mars has a very dilute atmosphere and helicopters have a real challenge to navigate there. And then it's going to keep going. It's just going to keep going and explore. I love that when we just like send something out into space and because it has an ion thruster, it can go for a while.

• 53:42Kelly Weinersmith?

  What, where's it going next?

• 53:43Daniel Whiteson?

  So the final target isn't specified, but the idea is explore the outer solar system, including high powered missions beyond Jupiter.

• 53:51Kelly Weinersmith?

  Wow.

• 53:52Daniel Whiteson?

  So they're still figuring it out. But you know, we have flexibility because we're not just limited to narrow launch windows. I also want to give a shout out to the listener who commented on gravitational slingshots.

• 54:04Kelly Weinersmith?

  Oh.

• 54:04Daniel Whiteson?

  Which are another way that NASA has historically used to explore the outer solar system without bringing a lot of propellant. You know, how do you change your direction without any fuel? Gravitational slingshots are super duper cool, but they also take forever. You need like everything to be in the oriented and exactly the right place. So sometimes you do like two or three swings around things. It could take like 10 years to like gravitationally slingshot your way from one place to another. So ion thrusters are much more direct way to get there.

• 54:33Kelly Weinersmith?

  I think Kim Stanley Robinson used gravity slingshots as a really great plot point in Aurora where like they were running out of fuel. So everybody had to go into like hibernation and they used gravity slingshots to get you there. It's like, it takes a long time.

• 54:46Daniel Whiteson?

  Yeah.

• 54:47Kelly Weinersmith?

  But if you don't have fuel and you can use that, that if you've got the time, it's a good way to go.

• 54:51Daniel Whiteson?

  Yeah. It's also cool because it's physics again, right? Which inherently makes it awesome.

• 54:56Kelly Weinersmith?

  Yeah. Better than chemistry, not as good as biology, but it's a hierarchy. It's okay.

• 55:01Daniel Whiteson?

  And there are other side benefits, like if we can get reactors to work in space, maybe that's going to help us figure out how to get reactors to work in crazy environments like on the moon. And people who are talking about building moon bases are talking about powering those things with nuclear reactors. Getting to Mars, it's going to help us open up more launch windows. It's a good way to send cargo to Mars and it opens up the outer solar system. Remember, solar power is just not useful past Jupiter, which gets like 4% of the earth's sunlight. So if we're going to explore the outer reaches of the solar system, it's not going to be with chemical rockets. It's going to be with ion thrusters.

• 55:38Kelly Weinersmith?

  So if we wanted to get like straight to Jupiter really fast, we'd want ion thrusters powered with nuclear reactors rather than gravity slingshots.

• 55:47Daniel Whiteson?

  Yeah, absolutely.

• 55:48Kelly Weinersmith?

  Okay. Cool. And voyage, what did Voyager use? Voyager's a gravity slingshot situation, right?

• 55:53Daniel Whiteson?

  Yes, exactly.

• 55:54Kelly Weinersmith?

  Okay. Well, this is amazing. I can't wait to see how those helicopters fare in the Martian atmosphere like the helicopter that came before.

• 56:02Daniel Whiteson?

  And I can't wait to see if Kelly ever achieves her lifelong dream of sitting on a rocket.

• 56:07Kelly Weinersmith?

  Yeah. Well, I hope you're sitting right behind me, Daniel. And I hope it's a safe rocket like our paddle boat rocket and not a chemical rocket.

• 56:15Daniel Whiteson?

  Well, I look forward to sitting on that rocket with you, Kelly. Until then, thank you, everybody, for taking this journey with us through the history of rocket development and into the future for how humans could explore the solar system.

• 56:27Kelly Weinersmith?

  Yay, space exploration. Until next time.

• 56:37Daniel Whiteson?

  Thanks, everybody for listening. Please go and do us a favor and rate the show on whatever podcast app you're using. It really helps people find us.

• 56:45Kelly Weinersmith?

  Daniel and Kelly's Extraordinary Universe is edited by the amazing Matt Kesselman.

• 56:49Daniel Whiteson?

  He really is a wizard. You can also find us online on Blue Sky, Instagram and X, D and K Universe. Come engage with us.

• 56:59Kelly Weinersmith?

  You can email us at [email protected]. We really do want to hear from you.

• 57:05Daniel Whiteson?

  And you can find our website, www.danielandkelly.org, where you'll also find an invitation to join our Discord where everybody comes and talks about the amazing universe.

• 57:16Kelly Weinersmith?

  And we also have the most amazing moderators. This is an iHeart Podcast. Thanks for joining us.

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