# How far can we go with chemical rockets?

An episode of [Daniel and Kelly’s Extraordinary Universe](https://invisiblenetwork.ai/podcast/1436616330), 11 Aug 2026, 49 min.

Audio: https://podtrac.com/pts/redirect.mp3/traffic.omny.fm/d/clips/e73c998e-6e60-432f-8610-ae210140c5b1/f5d5fac6-77be-47e6-9aee-ae32006cd8c3/da75024b-31db-4171-9aab-b4a30008aadf/audio.mp3?utm_source=Podcast&in_playlist=b26cbbeb-86eb-4b97-9b34-ae32006cd8d6

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 for privacy information.

## Transcript

[0:07] Speaker 1: How long are we going to be stuck on this
[0:10] Speaker 1: tiny little rock? And by we I mean humanity, not
[0:14] Speaker 1: me or Kelly or even you. I'm plenty happy to
[0:18] Speaker 1: live out my life here on Earth, but humanity should
[0:21] Speaker 1: push beyond, should find other places to live, should walk
[0:25] Speaker 1: on alien planets, should see sunrises from different distance stars.
[0:31] Speaker 1: But what's gonna make that happen? For a long time,
[0:34] Speaker 1: humans have relied on chemistry. Blow stuff up, funnel the
[0:38] Speaker 1: output out the back of your ship, ship goes forward. Cool,
[0:42] Speaker 1: that works, But is that steampunk kind of plan going
[0:45] Speaker 1: to power our future? Can use that chemical technology to
[0:50] Speaker 1: explore the outer reaches of the Solar System and to
[0:53] Speaker 1: reach other stars. And if chemistry lets us down, what's
[0:57] Speaker 1: gonna come to the rescue? We know it's not. By today,
[1:01] Speaker 1: we'll dig into the issues with chemical rockets and show
[1:04] Speaker 1: you how physics is going to save the day. Welcome
[1:08] Speaker 1: to Daniel and Kelly's Extraordinary Propelle Universe.
[1:25] Speaker 2: Hello. I'm Kelly Waitersmith. I study parasites and space, and
[1:28] Speaker 2: it doesn't matter what method you use. I'm not going
[1:31] Speaker 2: to space.
[1:35] Speaker 1: I am Daniel. I'm a particle physicist who loves thinking
[1:37] Speaker 1: about aliens and I've never been on a rocket powered anything.
[1:43] Speaker 2: Yeah, I guess I haven't either.
[1:47] Speaker 1: But you have to think about it for a minute.
[1:49] Speaker 1: What were you considering where you're like, ooh with that
[1:51] Speaker 1: bicycle and I strapped a huge engine to rocket.
[1:55] Speaker 2: Powered or not? Well, I will embarrassingly admit that I
[1:59] Speaker 2: was thinking like, all right, how do planes go? How
[2:03] Speaker 2: do ships go? How do trains go? And then the
[2:06] Speaker 2: answer to all of them was not by rocket? So
[2:11] Speaker 2: you know, I just had to make sure I was
[2:13] Speaker 2: telling the truth.
[2:14] Speaker 1: All right, Well, let's close the gap in your answer.
[2:16] Speaker 1: You said you wouldn't go to space, but would you
[2:18] Speaker 1: get in a rocket powered something here on Earth?
[2:21] Speaker 2: Uh? No, I doubt It's hard for me to imagine
[2:25] Speaker 2: that I would do that.
[2:26] Speaker 1: What about you, Well, let's check it again after you
[2:29] Speaker 1: hear about ion thrusters, because maybe a gentle ion thrustered
[2:34] Speaker 1: powered bicycle is something you would ride, but.
[2:36] Speaker 2: That wouldn't be a rocket. Then right now we're in
[2:38] Speaker 2: a whole different zone. I do travel on vehicles, I'm
[2:42] Speaker 2: not anti getting around, but by rocket makes me a
[2:47] Speaker 2: little nervous. I think.
[2:48] Speaker 1: All right, Well, I hope on today's episode we're going
[2:50] Speaker 1: to expand your mental conception of what is and what
[2:54] Speaker 1: isn't a rocket. Oh, it's not all flames and exclusions.
[2:58] Speaker 2: Well, then, how do I know that I haven't been
[3:00] Speaker 2: on a rocket? If I clearly don't have a good
[3:03] Speaker 2: conception of what a rocket is and what it isn't.
[3:05] Speaker 2: All right, it's time to teach me. Let's jump in.
[3:09] Speaker 1: All right. Well, on today's episode, we're gonna be talking
[3:11] Speaker 1: about how it's possible to explore the rest of the
[3:14] Speaker 1: Solar System. Our current technology are ugly, nasty, dirty, expensive,
[3:20] Speaker 1: heavy chemical rockets. Can they take us to the edges
[3:24] Speaker 1: of the Solar system and beyond? That's the question we
[3:28] Speaker 1: put to our group of listeners who love to speculate
[3:31] Speaker 1: without a chance to google or do any research. If
[3:35] Speaker 1: that sounds fun to you, we'd love to hear your
[3:37] Speaker 1: thoughts on the topic of the day. Write to us
[3:39] Speaker 1: to questions at Daniel and Kelly dot org. In the meantime,
[3:43] Speaker 1: think about it for yourself for a moment. How far
[3:46] Speaker 1: do you think we can get with chemical rockets that
[3:49] Speaker 1: Kelly hates?
[3:50] Speaker 2: All right, hold on, hold on, hold on. Before we
[3:52] Speaker 2: hear from the extraordinaries, I want to a stick up
[3:56] Speaker 2: for myself and be stick up for rockets. You used
[3:58] Speaker 2: a bunch of really negative words to describe rockets. But
[4:01] Speaker 2: I watched a rocket launch to space once and I
[4:04] Speaker 2: thought it was very moving and beautiful. And I do
[4:09] Speaker 2: think it's amazing that we can use rockets to send
[4:12] Speaker 2: stuff to space. And though I don't want to be
[4:13] Speaker 2: absolutely i'd like to change the negative tone a little bit.
[4:17] Speaker 2: It's not perfect, they're not perfect, but they're pretty amazing.
[4:21] Speaker 1: They are pretty amazing, and I think it's awesome what
[4:24] Speaker 1: humans have built. Every time I see a project like
[4:26] Speaker 1: that literally get off the ground, I think, Wow, look
[4:30] Speaker 1: what we can do, and I'm odd the same way
[4:32] Speaker 1: I feel like when I see the Golden Gate Bridge
[4:34] Speaker 1: or any huge engineering project, like something that's so far
[4:38] Speaker 1: beyond the scope of an individual person, yet we can
[4:40] Speaker 1: do it collectively. It gives me hope for the future.
[4:43] Speaker 2: Yeah, i'd say it gives me goosebumps. I get very
[4:45] Speaker 2: excited seeing big projects. But so okay, But chemical rockets
[4:48] Speaker 2: have their limitations. They take something like six to nine
[4:51] Speaker 2: months if you're going to Mars. If we want to
[4:53] Speaker 2: explore the rest of the universe, it would be nice
[4:55] Speaker 2: if we could go faster.
[4:56] Speaker 1: For example, it would be nice if we could go further,
[4:59] Speaker 1: for sure. Yes, So we asked the extraordinaries how far
[5:03] Speaker 1: we can go with just chemical rockets. Here's what folks
[5:07] Speaker 1: had to say. The Moon and Mars, and even Mars
[5:11] Speaker 1: will be a stretch. We're going to need some new
[5:13] Speaker 1: form of propulsion to get further. We would need a
[5:18] Speaker 1: tank the size of Jupiter to get anywhere. Well, you
[5:22] Speaker 1: can get pretty far if you have patients, not too far.
[5:25] Speaker 1: I think our space pubes to date have got most
[5:29] Speaker 1: of their speed from gravitational slingshots, not chemical rockets. You
[5:34] Speaker 1: can explore as far as you want, as long as
[5:36] Speaker 1: you can keep finding chemicals. I imagine we could probably
[5:39] Speaker 1: go pretty far with chemical rockets if we could make
[5:42] Speaker 1: them big enough to carry a lot of chemical fuel.
[5:46] Speaker 2: We need nuclear fuel to go anywhere far away like Mars.
[5:50] Speaker 2: With our current chemical rocket technology, we're not able to
[5:54] Speaker 2: get even close to the speed of light.
[5:56] Speaker 1: I can imagine it would be very far. It's difficult
[5:59] Speaker 1: to even explore Mars.
[6:01] Speaker 2: That's kind of a hard question because I actually don't
[6:03] Speaker 2: know what a chemical rocket is to.
[6:05] Speaker 1: The asteroid belt. I think we need to switch to
[6:09] Speaker 1: solar sales or perhaps some kind of experimental ion drive.
[6:14] Speaker 2: I think we have a pretty space savvy audience.
[6:17] Speaker 1: Yeah, yeah, I think so. They've all read your book,
[6:20] Speaker 1: they thought about space.
[6:21] Speaker 2: They've been listening to you for years talk about this
[6:24] Speaker 2: kind of stuff, and they're paying attention. They know what's up.
[6:27] Speaker 1: They didn't know until now, though, that you are anti rocket.
[6:30] Speaker 2: I di've clorified. I'm not anti rocket.
[6:33] Speaker 1: If you liked rockets, you would have written on one. Kelly, Look,
[6:35] Speaker 1: put your money where your mouth is.
[6:36] Speaker 2: Okay, have you written on a rocket? I've jumped out
[6:38] Speaker 2: of a plane.
[6:39] Speaker 1: But I'm not pro rocket.
[6:42] Speaker 2: I don't How many people have opportunities to ride rockets.
[6:45] Speaker 1: That's a good question, that's fair. Yeah, you're you're saying
[6:48] Speaker 1: you haven't said no to a rocket ride.
[6:49] Speaker 2: That's right, that's right.
[6:50] Speaker 1: But you would say no?
[6:51] Speaker 2: Yeah I would, Yeah, I would. But that doesn't mean
[6:55] Speaker 2: I'm not, you know, happy they exist.
[6:57] Speaker 1: Yes, me too. I'm happy people go and explore space,
[6:59] Speaker 1: even though I never want to do so myself.
[7:01] Speaker 2: Amen. All right, tell me about rockets, Daniel. How do
[7:04] Speaker 2: they work?
[7:06] Speaker 1: Right? So, rockets are a way to increase your momentum.
[7:09] Speaker 1: What you need is energy and mass and basically the
[7:13] Speaker 1: way a rocket works is it uses energy to push
[7:16] Speaker 1: mass out the back, and when you do so, you
[7:19] Speaker 1: send momentum in one direction, and because of conservation of momentum,
[7:23] Speaker 1: you go the other way. A basic example of a
[7:26] Speaker 1: rocket is sitting in a rowboat throwing rocks out the
[7:29] Speaker 1: back of the rowboat. You're using energy to throw those rocks.
[7:33] Speaker 1: The rocks have mass, and because they carry mass, they
[7:36] Speaker 1: carry momentum, and so the rocks go one way and
[7:39] Speaker 1: the boat goes the other way.
[7:41] Speaker 2: And the more mass, the more momentum. Right, so if
[7:44] Speaker 2: you throw a rock or a comparably sized chunk of styrofoam,
[7:47] Speaker 2: the rock would get you farther.
[7:49] Speaker 1: That's right. Momentum and non relativistically is mass times velocity,
[7:54] Speaker 1: and that's going to be absolutely crucial in today's conversation.
[7:57] Speaker 1: So equivalent momentum. By throwing a really mass thing very
[8:00] Speaker 1: slowly or by throwing a very low mass thing very
[8:04] Speaker 1: very fast, you get the same momentum. So those two options, okay,
[8:08] Speaker 1: So what you got to do is transfer momentum out
[8:10] Speaker 1: the back. Anything that does that basically is a rocket.
[8:13] Speaker 1: So you need energy and you need mass, and if
[8:16] Speaker 1: anybody out there happens to be a space is not
[8:18] Speaker 1: real denialist. Yes, rockets can work in space. You don't
[8:23] Speaker 1: need air for rockets, all right. The same principle throwing
[8:27] Speaker 1: mass out the back works just fine in an atmosphere
[8:31] Speaker 1: or not. In an atmosphere. These things are not climbing
[8:34] Speaker 1: up the atmosphere and they're not pushing against the air.
[8:37] Speaker 1: You're just throwing momentum at the back. So the rocket
[8:40] Speaker 1: moves forward to balance the momentum and to maintain conservation
[8:44] Speaker 1: of momentum.
[8:45] Speaker 2: So if you were on like an old fashioned paddle
[8:48] Speaker 2: boat sort of thing, and the energy is your legs
[8:52] Speaker 2: and you are spinning like a little paddle thing behind
[8:56] Speaker 2: and it's pushing water out the back, is that a rocket?
[9:02] Speaker 2: Why is it not a rocket?
[9:04] Speaker 1: I see you want to like re engineer the definition
[9:07] Speaker 1: of rockets so that you have been on a rocket.
[9:09] Speaker 2: So you have told me that the definition of rocket
[9:12] Speaker 2: is broader than what I thought it was, and so
[9:14] Speaker 2: now I'm trying to figure out what the bounds are.
[9:17] Speaker 1: So unfortunately, I don't think a paddle boat qualifies as
[9:21] Speaker 1: a rocket because a rocket needs to bring that mass
[9:24] Speaker 1: with it, right, And on the paddle boat you are
[9:26] Speaker 1: pushing against the water. You're not like carrying water that
[9:30] Speaker 1: you're then pushing out the back of the paddle boat. Right,
[9:33] Speaker 1: So a rocket carries with it mass and throws that
[9:35] Speaker 1: mass out the back. A paddle boat is not doing that.
[9:38] Speaker 1: It's pushing against the water. So, for example, a paddle
[9:41] Speaker 1: boat would not operate in a vacuum, right, whereas a
[9:44] Speaker 1: rocket can.
[9:45] Speaker 2: I feel like you're moving the target. You didn't mention
[9:47] Speaker 2: ahead of time that the mass has to go with you.
[9:49] Speaker 2: But all right.
[9:50] Speaker 1: We're clarifying here. That's why we're having a conversation. Okay,
[9:55] Speaker 1: And in the future, every conversation I have, I will
[9:57] Speaker 1: give complete exhaustive definitions of everything to future proof myself
[10:01] Speaker 1: against any questions.
[10:02] Speaker 2: This is our last episode, folks. No one's gonna want
[10:05] Speaker 2: to listen after this. All right, let's move forward. We're
[10:08] Speaker 2: carrying the mass.
[10:09] Speaker 1: With us, that's right. Okay, So you need energy and
[10:12] Speaker 1: you need mass, and you got to carry the mass
[10:14] Speaker 1: with you to throw it out the back. Cool. That's
[10:16] Speaker 1: the basic principles of rockets, and the rockets we're familiar
[10:19] Speaker 1: with them are mostly what we call chemical rockets, and
[10:21] Speaker 1: they solve this energy and mass problem by bringing fuel.
[10:25] Speaker 1: Fuel stores energy, it's chemical process releases it, and it
[10:30] Speaker 1: provides mass that you can expel out the back. So
[10:33] Speaker 1: it's like an all in one solution to the rocket problem.
[10:35] Speaker 1: It's very cool. Right, you burn the fuel and that
[10:38] Speaker 1: gives you energy and it produces stuff that you can
[10:40] Speaker 1: throw out the back. So that combustion reaction does everything
[10:44] Speaker 1: you need.
[10:45] Speaker 2: Okay. And the difference between fuel and propellant, because the
[10:52] Speaker 2: engineers when we first wrote soonish and we made the
[10:55] Speaker 2: mistake of saying fuel or whatever, we're very upset, and
[10:59] Speaker 2: so the difference twin fuel and propellant is that propellant
[11:03] Speaker 2: something gets thrown out the back. What is the difference.
[11:06] Speaker 1: Yeah, so propellant can be totally inert. It's just something
[11:09] Speaker 1: you throw out the back. So in the example of
[11:11] Speaker 1: the rowboat, you're throwing rocks at the back, right, that's
[11:14] Speaker 1: your propellant. You're using it to propel yourself. But you
[11:17] Speaker 1: wouldn't call rocks fuel. They don't storing the energy. Fuel
[11:20] Speaker 1: is something which stores energy and has propellant. Okay, So
[11:25] Speaker 1: it solves the whole rocket problem all in.
[11:27] Speaker 2: One, awesome. So one of the problems with this is
[11:31] Speaker 2: that the stuff that you create with that you throw
[11:34] Speaker 2: out the back is not necessarily great for the environment.
[11:37] Speaker 2: Have we found a propellant that is less bad for
[11:41] Speaker 2: the environment.
[11:42] Speaker 1: Well, the basic process here is a chemical reaction combustion
[11:45] Speaker 1: right where you're turning, for example, like methane plus oxygen
[11:49] Speaker 1: into CO two and water and energy. So the core
[11:53] Speaker 1: reaction is bad only in that it produces CO two right,
[11:56] Speaker 1: which is not good for the environment, but it can
[11:59] Speaker 1: be fairly clean otherwise. But some of the fuels do
[12:02] Speaker 1: produce other nasty stuff. There's a whole range of various
[12:05] Speaker 1: chemical processes used here, and ranges of kinds of fuel
[12:09] Speaker 1: that humans have used to make their glorious launches into space,
[12:14] Speaker 1: ranging from like solid fuels, so the Space Shuttle boosters
[12:17] Speaker 1: had aluminum powder and ammonium perchlorate in them, which you
[12:21] Speaker 1: can use as fuel, which is very cool. The SLS
[12:24] Speaker 1: core has liquid hydrogen and liquid oxygen, so very basic,
[12:29] Speaker 1: very simple stuff. But again this is fuel because it
[12:33] Speaker 1: can combust so it releases energy and it produces propellant
[12:37] Speaker 1: that you throw at the back. SpaceX Falcon rockets use
[12:41] Speaker 1: something called rp one or caralox, which is basically kerosene
[12:46] Speaker 1: plus liquid oxygen, so all the components you need for combustion.
[12:50] Speaker 1: And then there's methylox which is basically methane and liquid oxygen.
[12:54] Speaker 1: Methane is just H four, so that's pretty clean.
[12:57] Speaker 2: Yeah, okay, great.
[12:59] Speaker 1: So the good news about come rockets is they produce
[13:01] Speaker 1: high velocity and they use a lot of volume very
[13:04] Speaker 1: very quickly, so you can burn through a huge amount
[13:07] Speaker 1: of fuel very quickly and produce a huge thrust. So
[13:10] Speaker 1: you can get like exhaust velocity of like three kilometers
[13:13] Speaker 1: per second. Remember we were talking about throwing propellant out
[13:17] Speaker 1: the back, and what you need is high velocity at
[13:20] Speaker 1: high mass to get high momentum. And so this is
[13:22] Speaker 1: pretty high velocity three kilometers per second. But most importantly,
[13:26] Speaker 1: it is a large volume here. So for example, each
[13:29] Speaker 1: one of the Falcon nine's Merlin rocket engines burn two
[13:33] Speaker 1: hundred and fifty kilograms of fuel per second. Wow. Wow, exactly,
[13:40] Speaker 1: And so this provides a huge momentum change, right because
[13:44] Speaker 1: it's an enormous amount of mass moving with high velocity.
[13:47] Speaker 1: And so each engine has a gigawat of power and
[13:51] Speaker 1: the Falcon nine has nine of these things. Wow. So
[13:54] Speaker 1: the basic principle for chemical rockets is a short, hard
[13:58] Speaker 1: burn with a huge amount of thrust. Right, you're burning
[14:02] Speaker 1: a lot of fuel, You're getting a lot of velocity.
[14:05] Speaker 1: It's not just one push as an extended burn as
[14:07] Speaker 1: you're watching this thing lift off. But chemical rockets don't
[14:10] Speaker 1: tend to burn for very very long times for the
[14:12] Speaker 1: reason we'll talk about in a minute. It's mostly like
[14:15] Speaker 1: a big hard push at the beginning.
[14:17] Speaker 2: Well, all of that sounds amazing, So what's the problem, Daniel.
[14:20] Speaker 1: The problem is that consuming a lot of mass means
[14:23] Speaker 1: having a lot of mass, right, so you're burning all
[14:26] Speaker 1: of this fuel. You got to have a lot of fuel,
[14:28] Speaker 1: and the burning is not very efficient because I'm gonna
[14:31] Speaker 1: poop on chemistry. It's a chemical reaction, right.
[14:34] Speaker 2: And so oh, that wasn't the bad part.
[14:38] Speaker 1: Sorry, Well, the bad part is that you know, you're
[14:40] Speaker 1: changing the configurations of these atoms, so you're releasing some energy,
[14:45] Speaker 1: but the energy was stored in chemical bonds, and chemical
[14:48] Speaker 1: bonds can store energy, but it's like a tiny fraction
[14:50] Speaker 1: of all of the energy stored in that fuel. If you,
[14:53] Speaker 1: for example, combine that fuel with an antimatter equivalent of
[14:56] Speaker 1: that fuel, you could convert all of that matter into energy,
[15:00] Speaker 1: enormous amount of energy produced. But instead you're just like
[15:03] Speaker 1: moving the atoms around and clicking them into slightly lower
[15:06] Speaker 1: energy configurations. So for a lot of mass, you're getting
[15:10] Speaker 1: a fairly small amount of energy out. So there's a
[15:13] Speaker 1: small amount of energy carry for every pound of fuel
[15:17] Speaker 1: or every kilogram of fuel that you're bringing along.
[15:19] Speaker 2: So is your argument that the problem is that it's
[15:21] Speaker 2: chemistry not physics because you want things to be happening
[15:24] Speaker 2: on an antimatter physics.
[15:26] Speaker 1: Level, Well, physics can access the power of the atom,
[15:30] Speaker 1: right it can annihilate these things and convert all of
[15:33] Speaker 1: that store energy if you could do that. I'm not
[15:35] Speaker 1: saying that you know, I know how to design an
[15:37] Speaker 1: antimatter engine or anything, but that's one of the problems
[15:41] Speaker 1: with chemical rockets is that you're releasing a small amount
[15:44] Speaker 1: of energy per mass. So you need a lot of
[15:47] Speaker 1: mass right now. If you need a lot of mass,
[15:49] Speaker 1: you're going to need a bigger push to get you
[15:51] Speaker 1: up to some velocity. And to get that bigger push,
[15:53] Speaker 1: what do you need more mass? That mass means you
[15:56] Speaker 1: now need a bigger push and so very quickly to
[15:59] Speaker 1: get anywhere or to get a big thrust you need fuel,
[16:02] Speaker 1: and then you need fuel to carry that fuel, and
[16:04] Speaker 1: you need fuel to push that fuel, and you need
[16:05] Speaker 1: fuel to carry that fuel, and so eventually you're mostly fuel.
[16:10] Speaker 2: I think it's like a mortgage, But how about you
[16:12] Speaker 2: explain it, Daniel.
[16:15] Speaker 1: Well, in a mortgage, what you're doing is you're borrowing
[16:17] Speaker 1: money to buy your house, and then you're paying it back,
[16:20] Speaker 1: but the interest is compounded, right, and so for the
[16:23] Speaker 1: first long time, you're just basically paying off the interest
[16:26] Speaker 1: before you're ever paying off the principle, So you're not
[16:29] Speaker 1: actually making progress on your loan. And by the time
[16:31] Speaker 1: you pay off your house, you've ended up paying like
[16:34] Speaker 1: three or five times as much as you borrowed. Right,
[16:37] Speaker 1: you borrow a million dollars to my house, you end
[16:39] Speaker 1: up paying the bank five million dollars overall that time.
[16:42] Speaker 1: And so in the same way, when we launch a rocket,
[16:44] Speaker 1: most of the mass of that rocket is the fuel.
[16:47] Speaker 1: We're mostly launching fuel. Right. We have like a little
[16:50] Speaker 1: capsule on top where we get to put something we want,
[16:52] Speaker 1: but mostly we're launching fuel because we need to burn
[16:55] Speaker 1: that fuel when we're halfway up.
[16:57] Speaker 2: I think the payloads that get sent up on rocket,
[16:59] Speaker 2: it's like one percent or less. When you look at
[17:01] Speaker 2: a rocket is stuff that you're actually sending the space,
[17:04] Speaker 2: and the rest of that skyscraper shaped thing is just
[17:08] Speaker 2: fuel that's gonna get you there.
[17:09] Speaker 1: Yeah. So most of a Mars bound rocket's mass is propellant.
[17:13] Speaker 1: So it takes like, for example, one thousand tons of
[17:16] Speaker 1: metalocks fuel to get two Mars from Earth orbit. Wow.
[17:20] Speaker 1: So if you're already in Earth orbit, right, and we're gonna
[17:23] Speaker 1: talk about how together in a minute, then it takes
[17:25] Speaker 1: still one thousand tons and you're gonna burn that fuel
[17:28] Speaker 1: in space. It's gonna give you a big push, and
[17:30] Speaker 1: then you're mostly gonna coast the rest of the way
[17:32] Speaker 1: to Mars. That's the way a chemical rocket works. But
[17:35] Speaker 1: then if you want to get a thousand tons of
[17:38] Speaker 1: metalocks fuel into Earth orbit, you got to spend twenty
[17:41] Speaker 1: five thousand tons just to get that fuel up there.
[17:44] Speaker 2: Wow.
[17:45] Speaker 1: Right, So that's a factor of twenty five just to
[17:48] Speaker 1: get the fuel you need into orbit so you can
[17:50] Speaker 1: burn it to get to Mars, and a thousand tons
[17:53] Speaker 1: of metholocks is what it costs at the cheapest transfer moment. Right.
[17:57] Speaker 1: Remember Earth and Mars don't go around the Sun at
[17:59] Speaker 1: the same speed. So sometimes on the opposite side of
[18:02] Speaker 1: the Sun really hard to get to Mars. Sometimes they're
[18:04] Speaker 1: on the same side of the Sun, and once every
[18:07] Speaker 1: twenty six months they're at their closest approach and it's
[18:10] Speaker 1: the best time to go to Mars. So this is
[18:13] Speaker 1: the cheapest it could ever cost to go to Mars
[18:16] Speaker 1: using methalocks fuel.
[18:18] Speaker 2: And even then it's not You've gone through all of that,
[18:20] Speaker 2: you've paid all of that, you've burned all of that
[18:22] Speaker 2: into the atmosphere, and it still takes you six to
[18:24] Speaker 2: nine months to get there.
[18:26] Speaker 1: Yeah, well, we're not going to solve that problem with
[18:29] Speaker 1: ion thrusters today that we are going to make it
[18:31] Speaker 1: more flexible to get to Mars. Okay, but you know
[18:34] Speaker 1: that's how we can get to Mars, and that kind
[18:36] Speaker 1: of works. But say you wanted to go further, right,
[18:39] Speaker 1: you wanted to go to Jupiter. Now you need two
[18:42] Speaker 1: thousand tons of metalocks fuel, and then you've got to
[18:44] Speaker 1: get all that into orbit, So it becomes very expensive
[18:48] Speaker 1: very quickly. In the prospect of like going to Alpha
[18:50] Speaker 1: Centauri using chemical rockets, you know, you end up needing
[18:54] Speaker 1: fuel tanks. Like the size of the moon.
[18:56] Speaker 2: Holy cow. All right, but don't despair, because we are
[18:58] Speaker 2: going to take a break, and when we get Daniel
[19:00] Speaker 2: is going to tell us about other solutions that are
[19:03] Speaker 2: going to get us to Alpha Centauri.
[19:05] Speaker 1: That are still not paddle boats that are.
[19:07] Speaker 2: Still not which aren't rockets because you're not taking the
[19:10] Speaker 2: water with you.
[19:32] Speaker 1: Hey, Kelly, let's design a rocket powered version of a
[19:34] Speaker 1: paddle boat. What would that look like?
[19:36] Speaker 2: That would I would bring the water with me and
[19:39] Speaker 2: then I would throw it out the back.
[19:41] Speaker 1: Yeah. So if you had like a fire hose on
[19:43] Speaker 1: the back of your boat, and you had a huge
[19:45] Speaker 1: tank of water, and you had some way to have
[19:48] Speaker 1: energy so you could pump that water out the back
[19:51] Speaker 1: of your boat, then yes, that would be a rocket
[19:53] Speaker 1: powered boat. It wouldn't be paddled anymore unless Kelly is
[19:56] Speaker 1: a source of energy. There we go, we have a
[19:58] Speaker 1: huge tank of water, we have Kelly on on a
[20:00] Speaker 1: little exercycle, and she's pumping, pumping, pumping, pumping, pumping, pumping.
[20:03] Speaker 1: There you go, and it's pushing water out the back
[20:06] Speaker 1: out of the tank. That's a rocket powered paddle boat
[20:09] Speaker 1: that we could legitimately build. And then you could say, yes,
[20:12] Speaker 1: I've been on a rocket.
[20:13] Speaker 2: Okay, but this is like the least exciting rocket history
[20:16] Speaker 2: has ever created. Like boo boo to this rocket. Right.
[20:22] Speaker 1: Well, that's the principle though, behind the other technology we're
[20:25] Speaker 1: going to talk about today. Instead of chemical rockets, where
[20:29] Speaker 1: you have the fuel that gives you the energy, though
[20:31] Speaker 1: very inefficiently, and the propellant which requires huge, massive tanks
[20:36] Speaker 1: to go anywhere, we're going to talk about ion thrusters,
[20:39] Speaker 1: which separate this question of propellant, the mass you're throwing
[20:42] Speaker 1: out the back, and the energy source in the same
[20:45] Speaker 1: way that like Kelly pumping the water out the back
[20:48] Speaker 1: of the boat separates the energy source from the water.
[20:50] Speaker 1: You're not getting the energy from the water. The water's
[20:53] Speaker 1: totally inert, right, sure. So ion thruster is the approach
[20:56] Speaker 1: we're going to talk about today, and it solves the
[20:58] Speaker 1: energy and mass problems separately. First we're gonna talk about
[21:01] Speaker 1: the mass and then we'll talk about the energy. And
[21:04] Speaker 1: they're called ion thrusters because they use ions. An ion
[21:08] Speaker 1: is just an atom that's not neutral. So if you
[21:10] Speaker 1: take for example, xenon, it's got a bunch of electrons
[21:13] Speaker 1: in it which balance out the protons in the nucleus
[21:15] Speaker 1: and then you if you strip some of those electrons away,
[21:18] Speaker 1: then you have an ion. So now it has a
[21:20] Speaker 1: positive charge, and that's useful because you can then accelerate
[21:25] Speaker 1: it in electromagnetic fields. Right, So if you have a
[21:28] Speaker 1: bunch of xenon and you strip away some of the electrons,
[21:31] Speaker 1: now they have a charge in them. If you can
[21:33] Speaker 1: build an electromagnetic field, it will accelerate those ions. Shoot
[21:38] Speaker 1: those ions out the back of your ship, and it's
[21:41] Speaker 1: the ionic equivalent of Kelly's water rocket. Right, Just shoot
[21:46] Speaker 1: them at the back. They carry momentum with them because
[21:49] Speaker 1: you have high velocity. Now and that's a rocket.
[21:52] Speaker 2: Okay, that sounds cooler. You're bringing ionized gas. Does this
[21:57] Speaker 2: help you cut down on weight because gas weighs less
[22:02] Speaker 2: than propellant? Or not really because once you can press
[22:04] Speaker 2: the gas, it gets heavy.
[22:06] Speaker 1: What's the deal here, No, it really does help you
[22:08] Speaker 1: cut down on weight because you're can be more efficient
[22:11] Speaker 1: about the energy. One of the things about chemical rockets
[22:14] Speaker 1: is that they're trying to solve both problems at the
[22:15] Speaker 1: same time, energy and mass, and the energy extracted via
[22:19] Speaker 1: combustion is very inefficient. And so here if we can
[22:22] Speaker 1: let's ignore the energy question for now, and imagine we
[22:25] Speaker 1: have some energy source to fund our electromagnetic fields. Now,
[22:29] Speaker 1: if we have some energy source, we can very efficiently
[22:31] Speaker 1: use that to accelerate our ions to very very high speeds,
[22:36] Speaker 1: and so we can get much more push per unit
[22:40] Speaker 1: of propellant, right, because the mask can go to higher velocities.
[22:44] Speaker 1: So it sounded impressive earlier when we said like three
[22:47] Speaker 1: kilometers per second for the propellant from a chemical rocket.
[22:50] Speaker 1: That's cool, But ion thrusters can get much higher velocities,
[22:54] Speaker 1: and so for every pound of propellant that we bring along,
[22:57] Speaker 1: we can now get much more push. And this is
[23:01] Speaker 1: what rocket folks talk about as specific impulse. If you
[23:04] Speaker 1: start reading about rockets, get into all these weird terms,
[23:06] Speaker 1: but it's just a bunch of fancy terms to cloud
[23:09] Speaker 1: some basic physics, which is you want as much momentum
[23:12] Speaker 1: change per mass. So remember momentum is mass times velocity.
[23:17] Speaker 1: If you're bringing rocks or water or xenon or whatever
[23:20] Speaker 1: to throw at the back, you want as much velocity
[23:24] Speaker 1: in every kilogram we throw out the back, because momentum
[23:26] Speaker 1: is mass times velocity, and so for a fixed mass,
[23:29] Speaker 1: you want the most velocity to get the most momentum.
[23:32] Speaker 1: And so if you can get these things to higher speeds,
[23:34] Speaker 1: which ion thrusters can do better than chemical rockets can do,
[23:38] Speaker 1: then you can get more momentum per unit mass that
[23:41] Speaker 1: you're bringing.
[23:42] Speaker 2: My gut would not have guessed that pushing an eyeon
[23:46] Speaker 2: would get things out the back faster than exploding propellants.
[23:50] Speaker 1: Cool Well, physics is more powerful than chemistry, Kelly. And
[23:54] Speaker 1: we're using electromagnetic fields here, right, And so if you
[23:58] Speaker 1: have enough energy, and this whole thing hinges on having
[24:00] Speaker 1: enough energy, which we haven't talked about at all yet,
[24:03] Speaker 1: but if you have an energy source, right, then you
[24:06] Speaker 1: can accelerate these ions to very high speeds. Here on Earth,
[24:09] Speaker 1: we accelerate protons, which are just hydrogen ions, to nearly
[24:12] Speaker 1: the speed of light, right, enormous amounts of momentum in
[24:15] Speaker 1: the collisions at the Large Hadron Collider, for example. So
[24:18] Speaker 1: this all sounds great so far. We're talking about a
[24:20] Speaker 1: lot of momentum created for every ion, right. The issue
[24:24] Speaker 1: is that it's hard to scale up. So we have
[24:26] Speaker 1: a small amount of mass in general, which means a
[24:29] Speaker 1: small momentum change. So while for each ion we can
[24:33] Speaker 1: get each one to high velocity, which means that per
[24:36] Speaker 1: pound of xenon we're bringing, we're getting a lot more
[24:39] Speaker 1: momentum change than per pound of methyl locks or hydro
[24:43] Speaker 1: lockx or whatever fuel chemical rocket is burning. It's a
[24:47] Speaker 1: real challenge to get a lot of kilograms out the
[24:50] Speaker 1: back to get a huge thrust. And so instead of
[24:54] Speaker 1: like a big hot burn that gives you a lot
[24:55] Speaker 1: of thrust all at once, an ion rocket tends to
[24:59] Speaker 1: have a slow, gentle, long thrust. So, for example, it
[25:03] Speaker 1: might take like two days for an ion rocket to
[25:05] Speaker 1: get you up to even like highway speeds, whereas a
[25:09] Speaker 1: chemical rocket's going to be like kaboom, we're going pretty fast,
[25:12] Speaker 1: but then it's all spent.
[25:13] Speaker 2: Okay, So does that mean ion thrusters can't get you
[25:16] Speaker 2: out of Earth's gravity?
[25:17] Speaker 1: Well, that's exactly right. Ion thrusters as currently designed can't
[25:21] Speaker 1: get you off of Earth because they're very very gentle thrust.
[25:25] Speaker 1: But they can burn for a very very long time,
[25:28] Speaker 1: and they're very efficient, and they can travel with a
[25:30] Speaker 1: small amount of mass. The amount of mass you need
[25:33] Speaker 1: to get to Mars, for example, is like thousands of
[25:36] Speaker 1: kilograms of xenon, which compares to like a thousand tons
[25:40] Speaker 1: wow of chemical fuel. So it's a huge difference. Your
[25:43] Speaker 1: whole ship is much lighter, and then that's not as
[25:46] Speaker 1: hard to get into orbit. Right, even if it's a
[25:48] Speaker 1: factor twenty five to get something into orbit from Earth,
[25:53] Speaker 1: you need to use chemical fuel to do that. Now
[25:55] Speaker 1: it's a factor of twenty five times a thousand kilograms
[25:58] Speaker 1: instead of a thousand tons, So it takes much less
[26:01] Speaker 1: chemical fuel to get an ion rocket into space with
[26:05] Speaker 1: the fuel it needs to get to Mars. Then chemical
[26:08] Speaker 1: fuel to get a chemical rocket into space.
[26:11] Speaker 2: Okay, so but we'd still have to use a chemical
[26:14] Speaker 2: rocket to get the ion rocket to space, and then
[26:16] Speaker 2: you use the ion rocket to get to where you're going.
[26:19] Speaker 1: Yeah, exactly, unless you can build these things in space
[26:22] Speaker 1: with Jeff Bezos's future space based economy from asteroids or whatever. Right,
[26:27] Speaker 1: So this isn't a complete solution. It's not like, you know,
[26:30] Speaker 1: we're building warp drives or teleportition portals to get to
[26:33] Speaker 1: Alpha Centauri you know, in your living room or something.
[26:36] Speaker 1: But this will allow us to get further out into
[26:39] Speaker 1: the Solar System and even past the Solar System once
[26:42] Speaker 1: we can get out of Earth's orbit.
[26:44] Speaker 2: Well, so you mentioned it takes two days to reach
[26:46] Speaker 2: highway speed. Do you eventually reach high enough speeds that
[26:49] Speaker 2: a trip to Mars wouldn't take like five billion years?
[26:53] Speaker 1: Yes? Absolutely, And NASA is building exactly this kind of rocket.
[26:56] Speaker 1: We're able to use it to go to Mars in
[26:58] Speaker 1: twenty twenty eight. It's called s Are Freedom. And I'll
[27:01] Speaker 1: tell you all about that trajectory once we're done talking
[27:04] Speaker 1: about the energy source of these things, which is really.
[27:06] Speaker 2: Important, all right, where do we get the energy?
[27:08] Speaker 1: So there's a long history actually of ion thrusters, which
[27:11] Speaker 1: I think is interesting and it traces sort of the
[27:13] Speaker 1: different sources of energy. People start talking about these things
[27:16] Speaker 1: back in like nineteen eleven, and there's a few different
[27:19] Speaker 1: approaches for how to build it and then how to
[27:21] Speaker 1: power it. The approach is for how to build it
[27:24] Speaker 1: are things like a gridded ion thruster. Basically you just
[27:27] Speaker 1: have like an anode and a cathode. You know, you
[27:29] Speaker 1: have one thing with a positive charge, one thing with
[27:31] Speaker 1: a negative charge. You put your charged particles in it.
[27:34] Speaker 1: They get attracted to the opposite side. They going fast,
[27:38] Speaker 1: they sum out the side. So it's a grid because
[27:40] Speaker 1: they're holes in it for the particles to go through.
[27:42] Speaker 1: So imagine like two chaining fences, one is positive, one
[27:45] Speaker 1: is negative. You drop a positive particle into their acceleer
[27:48] Speaker 1: sorts a negative one and then shoots through the gaps
[27:51] Speaker 1: and out the back. That's the basic principle, okay.
[27:54] Speaker 2: But so to make the anode in the cathode, do
[27:56] Speaker 2: you just like stick magnets on there or do you
[27:58] Speaker 2: need like a battery to make that happen.
[28:01] Speaker 1: You need energy, Yeah, you need electricity. Magnets can't do
[28:04] Speaker 1: that for you.
[28:04] Speaker 2: Okay.
[28:05] Speaker 1: Also, that design isn't great because it corrodes. You're shooting
[28:09] Speaker 1: particles at the chaining fence, which tends to eventually destroy
[28:12] Speaker 1: the chaining fence or it gets jammed up or something.
[28:15] Speaker 1: So these days to have even fancier ones to something
[28:17] Speaker 1: called a Hall effect thruster, which doesn't have the actual cathode.
[28:22] Speaker 1: As a virtual cathode, you have a magnetic field there
[28:25] Speaker 1: which traps electrons, so they move in circles and they
[28:29] Speaker 1: ionize the gas which they collide into. So there's sort
[28:31] Speaker 1: of like an electric mixing spoon that breaks up the
[28:34] Speaker 1: gas into ions, and then you don't actually have a
[28:38] Speaker 1: cathode there. The electrons there act as the cathode and
[28:41] Speaker 1: so there's nothing to get corroded or broken down. So
[28:45] Speaker 1: that's the technology. But in every case, the limit is
[28:47] Speaker 1: the energy source. If you want to crank up ion
[28:50] Speaker 1: thrusters to give you a real amount of thrust, then
[28:53] Speaker 1: you need a lot of energy. And so the first
[28:55] Speaker 1: example of an ion thruster I want to talk about
[28:57] Speaker 1: is called the sert Ones. This is back in ninth
[29:00] Speaker 1: teen sixty four. This was a test by NASA and
[29:03] Speaker 1: it was basically like, hey, does this work at all?
[29:06] Speaker 1: And it was up in space for thirty minutes and
[29:09] Speaker 1: it worked, and it was powered by chemical batteries, so
[29:12] Speaker 1: these were like zinc silver oxide batteries, and it wasn't
[29:16] Speaker 1: a whole lot of energy. It was like one point
[29:18] Speaker 1: seven kilowatts of energy, but you know, it didn't go
[29:22] Speaker 1: for very long, so they didn't even like try solar panels,
[29:25] Speaker 1: though a later version of it, SIR two, did have
[29:27] Speaker 1: solar panels. But this was like, hey, all the equations match,
[29:31] Speaker 1: does it actually work in practice? And it did, so
[29:34] Speaker 1: that's cool.
[29:35] Speaker 2: Very cool. Well i've never heard of it, which is
[29:37] Speaker 2: a bummer.
[29:38] Speaker 1: Oh yeah, it's pretty cute. It's just like a little ball,
[29:42] Speaker 1: and then people said, all right, well, let's try to
[29:44] Speaker 1: go for longer periods, and let's avoid using batteries and
[29:48] Speaker 1: let's use solar power. And this is really exciting because
[29:51] Speaker 1: now you're not carrying the energy with you at all.
[29:54] Speaker 2: And it's not a rocket.
[29:56] Speaker 1: No, it's a rocket if you're carrying the propellant, so
[29:58] Speaker 1: you still have to carry the mass you're throwing at
[30:00] Speaker 1: the back, but it's not a requirement that you bring
[30:02] Speaker 1: the energy with you, right, So if you can just
[30:06] Speaker 1: beam the energy to the rocket and you can use that,
[30:10] Speaker 1: then that's awesome. So they tried this. The first one
[30:13] Speaker 1: is called en Star, which has two point three kilowatts
[30:17] Speaker 1: of solar power. So these solar arrays generate the electrical
[30:21] Speaker 1: power which then you use to build electric fields to
[30:24] Speaker 1: shoot ionized xenon out the back at really high speeds.
[30:28] Speaker 2: Wow.
[30:28] Speaker 1: And this thing ran for sixteen thousand hours and used
[30:32] Speaker 1: only eighty kilograms of xenon propellant to get pretty good
[30:37] Speaker 1: delta ve it's like four point three kilometers per second.
[30:40] Speaker 1: So it was a good demonstration of these things running
[30:43] Speaker 1: for a long time, which is what we need to, like,
[30:45] Speaker 1: you know, get to alvia Centauri or get even to
[30:48] Speaker 1: the edges of the Solar System. The next step up
[30:51] Speaker 1: is the Dart mission. I don't know if people remember this.
[30:54] Speaker 1: This is the one that went and visited an asteroid,
[30:56] Speaker 1: and I think it dropped like a washing machine sized
[30:59] Speaker 1: block of metal on the asteroid, just like see, like
[31:02] Speaker 1: what happens. Yeah, Well, and.
[31:05] Speaker 2: It wasn't it also to see I mean, it was
[31:06] Speaker 2: to see what happens, but it was to see what
[31:08] Speaker 2: happens in the hopes that if there's ever an asteroid
[31:10] Speaker 2: coming towards us, we would have some information about how
[31:13] Speaker 2: hard you'd have to like hit it to try to
[31:14] Speaker 2: deflect it from the Earth or something. Right, it was
[31:16] Speaker 2: more than just see what happens.
[31:19] Speaker 1: Oh no, you're right. I'm actually thinking of the Hyabusa
[31:22] Speaker 1: mission that dropped a huge metallic paylude as an impactor
[31:26] Speaker 1: onto the asteroid. You go, But Dart is different, You're right.
[31:30] Speaker 1: Dart was there to like, let practice deflecting an asteroid,
[31:34] Speaker 1: so it does something else. So Dart's main propulsion was
[31:37] Speaker 1: a chemical rocket, but it also had an ion thruster
[31:39] Speaker 1: for testing, which is very cool.
[31:41] Speaker 2: Well, and didn't Hyabusa bring back a sample from the asteroid,
[31:45] Speaker 2: didn't it like it made an explosion to collect something.
[31:48] Speaker 2: I feel like you're being uncharacteristically negative, and I'm being
[31:51] Speaker 2: uncharacteristically positive this time.
[31:53] Speaker 1: No, No, that's totally positive. I love the idea of
[31:56] Speaker 1: dropping a washing machine sized piece of metal on an
[31:59] Speaker 1: asteroid just to see what happens. Like, that's fantastic. I'm
[32:02] Speaker 1: super curious, Like I love when that mission went up.
[32:05] Speaker 1: Total supporter. My tax dollars should definitely be spent on
[32:08] Speaker 1: missions like that.
[32:09] Speaker 2: Okay, yes, might as well. Okay, so we have gotten
[32:12] Speaker 2: to the asteroids with ion thrusters.
[32:15] Speaker 1: Yeah, exactly, And so this one also used sunlight electricity
[32:18] Speaker 1: from solar panels was used to accelerate zine on atoms.
[32:21] Speaker 1: The ion engine was not the primary propulsion system for
[32:24] Speaker 1: the impact. It was mainly a flight demonstration of this
[32:27] Speaker 1: next technology for future deep space missions. And ion thrusters
[32:32] Speaker 1: are all over the place, Like China's space station has
[32:34] Speaker 1: ion thrusters which it uses to adjust and maintain orbit.
[32:39] Speaker 1: Starlink uses ion thrusters that have Krypton or orgon in them.
[32:42] Speaker 1: They're good for like very gentle little pushes to like
[32:46] Speaker 1: maintain where you are, not like a quick like ooh,
[32:48] Speaker 1: I'm going to hit something. I really got to get
[32:50] Speaker 1: out of here. You know. Also stuff out in space
[32:53] Speaker 1: tries to avoid using thrusters whenever possible because it uses
[32:56] Speaker 1: up propellant. You know, for example, like Hubble or the
[32:59] Speaker 1: space telescopes, they try to do all of their adjustments
[33:02] Speaker 1: using reaction wheels. Like you want to turn the thing,
[33:05] Speaker 1: you have internal reaction wheels that store angular momentum, so
[33:08] Speaker 1: you can turn without any thrusters. We can talk about
[33:11] Speaker 1: that on another episode. Because every time you're using a thruster,
[33:14] Speaker 1: it's a rocket. You have propellent, you're using it up
[33:17] Speaker 1: and so eventually it will run out. But the real
[33:20] Speaker 1: question is like what is the best way to get energy.
[33:23] Speaker 1: We've talked about chemical batteries. That was always just going
[33:26] Speaker 1: to be a test. Solar power is cool, right because
[33:29] Speaker 1: it's up, it's free, it's out there. You don't even
[33:32] Speaker 1: need to bring the energy with you. That's very cool.
[33:34] Speaker 1: The problem is with solar power is that it drops
[33:37] Speaker 1: off really hard with distance. The intensity of sunlight goes
[33:40] Speaker 1: like one over the distance squared, So if you're twice
[33:44] Speaker 1: as far from the sun, you get one fourth the energy,
[33:47] Speaker 1: and space is big, and so by the time you're
[33:49] Speaker 1: like out by Jupiter, there's almost no usable solar power,
[33:54] Speaker 1: especially to power a rocket. So we need something else.
[33:57] Speaker 1: We need a low mass source of energy.
[34:01] Speaker 2: Physics is what we need. And when we get back,
[34:03] Speaker 2: we're going to hear about how physics is going to
[34:06] Speaker 2: get us out to Jupiter. Yeah, all right, Daniel, how
[34:29] Speaker 2: is physics going to get us to Jupiter?
[34:32] Speaker 1: Yeah, physics is absolutely the solution here. And so of
[34:36] Speaker 1: course the better way to make energy is not by
[34:39] Speaker 1: burning coal or burning methane, or even by solar power,
[34:43] Speaker 1: because that's not available out in the outer reaches of
[34:45] Speaker 1: the Solar system. It's nuclear power. Right, We already know
[34:49] Speaker 1: that the amount of energy stored in like a kilogram
[34:51] Speaker 1: of uranium is huge compared to what you could get
[34:55] Speaker 1: from a kilogram of methane. Right, Random Monroe has this
[34:59] Speaker 1: amazing He's in comic where he shows like the energy
[35:01] Speaker 1: density of like uranium versus coal, versus wood or whatever,
[35:05] Speaker 1: and it's just off the charts, you know, It's like
[35:08] Speaker 1: even on a log scale, it's just insane. And so
[35:12] Speaker 1: if you want a bunch of energy and you don't
[35:13] Speaker 1: want to carry a bunch of mass, around. This is
[35:16] Speaker 1: the way to go. Have a fission reactor. So bring
[35:19] Speaker 1: a small amount of uranium, use that to power your
[35:23] Speaker 1: nuclear reactor. Your nuclear reactor produces electricity. You feed that
[35:27] Speaker 1: into your ion thruster, and now you have propellent, which
[35:30] Speaker 1: is very effective because you're pushing it up to very
[35:32] Speaker 1: high velocities, and you have fuel uranium, which is very
[35:36] Speaker 1: low mass. You're gettin a lot of energy out of
[35:38] Speaker 1: it per kilogram.
[35:40] Speaker 2: I'm gonna look past the fact that you mentioned the
[35:42] Speaker 2: competition on this show. You know, I'm just kidding. We
[35:46] Speaker 2: love XKCD.
[35:47] Speaker 1: It's not a competition. It's good for everybody. And anyway,
[35:51] Speaker 1: nobody's in competition with Randall because he wins.
[35:54] Speaker 2: He does win, he does, and he's and he's a
[35:57] Speaker 2: lovely human being.
[35:58] Speaker 1: Also, yeah, he really is genuinely in that.
[36:00] Speaker 2: Yeah. Okay, all right, so we've decided that nuclear powered
[36:04] Speaker 2: ion thrusters are the way to go. Have we had
[36:07] Speaker 2: any nuclear powered ion thrusters in space? I know we've
[36:10] Speaker 2: had little nuclear reactors in space to power satellites.
[36:15] Speaker 1: So it depends on what you mean by we. The
[36:17] Speaker 1: United States hasn't.
[36:18] Speaker 2: By we, I mean humanity.
[36:21] Speaker 1: Well, then the Soviets have done that. The Soviets in
[36:24] Speaker 1: nineteen eighty seven had a nuclear powered plasma thruster in orbit,
[36:29] Speaker 1: so this has definitely happened. It's worked before, and the
[36:32] Speaker 1: US says, I'm sure, you know has operated nuclear fission
[36:35] Speaker 1: reactors in space. So all the pieces are there. We
[36:38] Speaker 1: know ion thrusters work in space when they have an
[36:40] Speaker 1: electrical energy source. We've powered nuclear fission reactors in space,
[36:45] Speaker 1: and the Soviets even put them together in eighty seven
[36:48] Speaker 1: to use nuclear powered ion thrusters.
[36:51] Speaker 2: Wow. Yeah, and all the pieces are still there because
[36:53] Speaker 2: I think our attempt is still in orbit somewhere because
[36:57] Speaker 2: it didn't quite go according to plan exactly.
[36:59] Speaker 1: And so Naslam was already building fission reactors to operate
[37:04] Speaker 1: in space. It's part of the Lunar Gateway space station,
[37:07] Speaker 1: which was then canceled. But they had a power and
[37:10] Speaker 1: propulsion element being built by Lentis Space Systems, and they
[37:14] Speaker 1: were already building those, and so we're going to take
[37:16] Speaker 1: those and put them at the end of a very
[37:19] Speaker 1: very long trust to keep the radiation away from like
[37:22] Speaker 1: the electronics of the rest of the ship. And that's
[37:25] Speaker 1: going to provide power for the new SR one freedom,
[37:28] Speaker 1: which is going to power a mission to Mars in
[37:31] Speaker 1: twenty twenty eight, nuclear powered ion thrusters to Mars.
[37:36] Speaker 2: WHOA, okay, right, I feel like you're still going to
[37:38] Speaker 2: need the chemical rocket to leave Earth, but now you're
[37:42] Speaker 2: carrying with you a nuclear reactor. Yes, what is our
[37:46] Speaker 2: plan for making sure that everything stays safe?
[37:51] Speaker 1: Cross our fingers? Rely on engineers. There's not a great plan,
[37:56] Speaker 1: Like this is an issue. It's always going to be
[37:58] Speaker 1: an issue. Things go boom when you launch them, and
[38:02] Speaker 1: so if you're launching radioactive fuel into space, it's always
[38:05] Speaker 1: a danger.
[38:06] Speaker 2: There's not some way that you can like keep it
[38:08] Speaker 2: clad in something that's less likely to go boom and
[38:10] Speaker 2: then open it up and let the reactions start happening
[38:13] Speaker 2: once you're in space and away from Earth. There's no way.
[38:17] Speaker 2: I feel like there's got to be ways to make
[38:18] Speaker 2: this less scary.
[38:22] Speaker 1: So there are smart folks working on this, and of
[38:24] Speaker 1: course they're taking whatever precaution they can. One thing you
[38:26] Speaker 1: can do is don't turn on the reactor until you
[38:30] Speaker 1: get to space, for example, So there's still a risk
[38:33] Speaker 1: because you have uranium fuel in there, which is radioactive,
[38:36] Speaker 1: but you don't have like dangerous nuclear byproducts being produced
[38:40] Speaker 1: or already present on the rocket, so that if it
[38:42] Speaker 1: does go boom, you're spreading some uranium fuel, which is
[38:46] Speaker 1: not good, right, but it's not as bad as spreading
[38:49] Speaker 1: like really toxic radioactive nuclear byproducts.
[38:52] Speaker 2: Okay.
[38:53] Speaker 1: In addition, like you know, they're putting this thing inside
[38:56] Speaker 1: highly durable, heat resistant containment materials designed survive extreme launch
[39:01] Speaker 1: vehicle explosions, and it's supposed to be like subcritical no
[39:05] Speaker 1: matter what, even if the launch fails. So they're doing
[39:08] Speaker 1: what they can. And I have not dug into the
[39:09] Speaker 1: details here, but I trust the engineers are doing their
[39:12] Speaker 1: best to solve this problem. And I totally acknowledge that
[39:14] Speaker 1: it's very dangerous to be launching nasty, toxic stuff into
[39:18] Speaker 1: space because it might not get there.
[39:19] Speaker 2: All right, thank you for giving me my daily dose
[39:21] Speaker 2: of Kelly being able to like worry and be a
[39:23] Speaker 2: wet blanket. And all right, what do we need to
[39:26] Speaker 2: think about next?
[39:27] Speaker 1: So, the way this thing works is it's a fission reactor,
[39:30] Speaker 1: which means you're taking uranium and you're splitting it, and
[39:34] Speaker 1: that causes a chain reaction because it produces neutrons, and
[39:37] Speaker 1: those neutrons split other uranium atoms, and along the way
[39:40] Speaker 1: it produces heat, and then how do you turn that
[39:42] Speaker 1: heat into electricity? While on Earth we often do something
[39:45] Speaker 1: like boil water, right and use the steam to spin
[39:48] Speaker 1: a turbine, and so up in space we're doing something
[39:51] Speaker 1: only a little bit more sophisticated. Instead of water, we're
[39:55] Speaker 1: using a helium neon mixture, which still spins a turbine
[39:59] Speaker 1: and creates electricity. So the basic process is the same,
[40:03] Speaker 1: and then we can use that electricity to accelerate the ions.
[40:07] Speaker 1: The cool thing is the fuel is very very low mass.
[40:11] Speaker 1: Remember the propellant it's like thousands of kilograms of xenon,
[40:13] Speaker 1: which is still really low mass compared to the thousands
[40:16] Speaker 1: of tons you need for chemical rockets. But the uranium
[40:20] Speaker 1: fuel to give you the energy to push that out
[40:22] Speaker 1: the back is just like ten kilograms. It's like a
[40:25] Speaker 1: rounding error. That's how powerful uranium is. It's really amazing.
[40:30] Speaker 1: So it's almost as free as sunlight. But you can
[40:34] Speaker 1: keep using it when you get past Jupiter.
[40:36] Speaker 2: Why haven't we done this already? Then? This seems pretty
[40:40] Speaker 2: obviously better, is it? Because of concerns about like where
[40:43] Speaker 2: you're going to get the uranium. Yeah, why haven't we
[40:45] Speaker 2: done this already?
[40:46] Speaker 1: Well, you know, the Soviets have been doing it since
[40:49] Speaker 1: the eighties. But chemical rockets are easier, right, there's no
[40:52] Speaker 1: nuclear power issues and they can get off of Earth.
[40:55] Speaker 1: So this is really only useful for exploring in places
[40:58] Speaker 1: where you're not fighting a big gravity Well, okay, got it.
[41:02] Speaker 1: And it gives a pretty different signature for your mission.
[41:05] Speaker 1: Like for a chemical rocket, you want to get to Mars,
[41:08] Speaker 1: you burn for like ten minutes, and then you travel
[41:10] Speaker 1: to Mars for six to nine months, and you're going
[41:13] Speaker 1: to like peak velocity almost all the time, like thirty
[41:16] Speaker 1: three kilometers per second. An ion rocket is going to
[41:19] Speaker 1: give you like a year of continuous burn, and it's
[41:22] Speaker 1: going to start very very slow. It's going to eventually
[41:24] Speaker 1: get up to the same peak velocity thirty thirty five
[41:27] Speaker 1: kilometers per second. But it's not in freefall, right, It's
[41:30] Speaker 1: a continuous thrust and it can be adjusted in flight,
[41:34] Speaker 1: and so it's a very different kind of trajectory and
[41:37] Speaker 1: that makes it less sensitive to the timing. So you
[41:41] Speaker 1: can't get to Mars when it's on the other side
[41:43] Speaker 1: of the Sun. But because you're not just in freefall,
[41:46] Speaker 1: you can do all sorts of different kinds of trajectories,
[41:49] Speaker 1: so you don't have as narrow a window, so it
[41:52] Speaker 1: takes a little longer to get there, like a year
[41:54] Speaker 1: instead of six to nine months, but you have more
[41:57] Speaker 1: flexibility about when you go.
[41:59] Speaker 2: I don't think I understand the you're not in freefall thing.
[42:03] Speaker 2: Why would a chemical rocket versus an ion thing? If
[42:07] Speaker 2: you had enough chemicals for your chemical rocket, couldn't you
[42:11] Speaker 2: have the same flexibility.
[42:13] Speaker 1: In principle if you brought enough fuel along that you're
[42:16] Speaker 1: burning it during the flight, then yes, you could do
[42:19] Speaker 1: the same thing with the chemical rocket, but then you
[42:21] Speaker 1: need even more fuel. Member. Chemical rockets are very very inefficient, Okay,
[42:25] Speaker 1: that's why they like to burn it all the very
[42:26] Speaker 1: very beginning. And it's basically it's almost like a cannonball launch. Right.
[42:30] Speaker 1: A cannon ball, you give it one push and then
[42:32] Speaker 1: it just flies. Okay, chemical rocket, it's like ten minutes
[42:35] Speaker 1: of pushing and then it's just a free fall trajectory.
[42:39] Speaker 1: And so you're very limited in how you go from
[42:40] Speaker 1: one place to the other if you can't burn along
[42:42] Speaker 1: the way. So the ion thruster can do that, it
[42:45] Speaker 1: takes longer to get up to high speed, so the
[42:47] Speaker 1: whole trip is going to be a year. The plus
[42:49] Speaker 1: side is you have a little bit more flexibility, so
[42:51] Speaker 1: your window is a bit broader. It's still not like
[42:54] Speaker 1: we can go to Mars any time, and you can't
[42:56] Speaker 1: get to Mars faster, but there's a little bit more
[42:59] Speaker 1: flexibility there.
[43:00] Speaker 2: Okay, I mean that that's pretty cool because needing to
[43:02] Speaker 2: wait two years, yeah for every launch window, Like yeah,
[43:07] Speaker 2: I mean you missed that. Now you're waiting two years.
[43:09] Speaker 2: Like that's that's a real pain in the rear ends
[43:11] Speaker 2: is how do we how much do ion frusters open
[43:13] Speaker 2: this window?
[43:14] Speaker 1: So chemical rockets have like a few weeks of a
[43:17] Speaker 1: window when they can effectively get to Mars, and ion
[43:19] Speaker 1: thrusters have like a few months. Nice, so it's a
[43:22] Speaker 1: meaningful change. It's again, it's not like totally solving the problem.
[43:25] Speaker 1: It's not like you can go to Mars on a whim,
[43:28] Speaker 1: but there is flexibility there.
[43:29] Speaker 2: Well, but you could imagine a few weeks of bad
[43:32] Speaker 2: weather totally messing up your Mars plans if that bad
[43:36] Speaker 2: weather happens during your window, but like probably not gonna
[43:38] Speaker 2: have a couple months of bad weather, So like that
[43:41] Speaker 2: makes your probability of getting to Mars. I don't know,
[43:43] Speaker 2: I think much.
[43:44] Speaker 1: Higher, Yeah, exactly, looknd maybe.
[43:46] Speaker 2: In a ray of sunshine.
[43:50] Speaker 1: And so for example, if your launch is like delayed
[43:52] Speaker 1: by ninety days, the spacecraft just like adjusts the thrust
[43:55] Speaker 1: profile burns more propellant to catch up to Mars rather
[43:58] Speaker 1: than having to scrub the entirely. And adding a little
[44:01] Speaker 1: bit more propellant is not disastrous because the propellant is
[44:05] Speaker 1: fairly low mass here. We're talking about like a few
[44:07] Speaker 1: more kilograms of xenon right, and the energy is very,
[44:11] Speaker 1: very dense, So we don't have this rocket equation trap
[44:14] Speaker 1: where we fall into where every time we bring a
[44:16] Speaker 1: pound more fuel, now we need thousands more pounds of
[44:19] Speaker 1: fuel to push that fuel, and we still got to
[44:21] Speaker 1: launch this thing up into space. But remember the fact
[44:23] Speaker 1: that it's lower mass means it's cheaper to launch it
[44:26] Speaker 1: into orbit in the beginning. And we're going to do
[44:29] Speaker 1: this in like twenty twenty eight. It's just around the corner.
[44:32] Speaker 1: SR one Freedom is going to go to Mars. It's
[44:34] Speaker 1: going to drop three helicopters that land themselves on Mars,
[44:39] Speaker 1: which is amazing given Mars has a very dilute atmosphere
[44:43] Speaker 1: and helicopters have a real challenge to navigate there. And
[44:46] Speaker 1: then it's going to keep going. It's just going to
[44:49] Speaker 1: keep going and explore. I love that when we just
[44:51] Speaker 1: like send something out into space and because it has
[44:54] Speaker 1: an ion thruster, it can go for a while.
[44:57] Speaker 2: Where's it going next?
[44:58] Speaker 1: So the final target is specified, but the idea is
[45:02] Speaker 1: explore the outer Solar System, including high powered missions beyond Jupiter.
[45:06] Speaker 2: Wow.
[45:06] Speaker 1: So they're still figuring it out, but we have flexibility
[45:10] Speaker 1: because we're not just limited to narrow launch windows. I
[45:13] Speaker 1: also want to give a shout out to the listener
[45:16] Speaker 1: who commented on gravitational slingshots, which are another way that
[45:20] Speaker 1: NASA has historically used to explore the Outer Solar System
[45:23] Speaker 1: without bringing a lot of propellant. You know, how do
[45:26] Speaker 1: you change your direction without any fuel? Gravitational slingshots are
[45:30] Speaker 1: super duper cool, but they also take forever. You need
[45:33] Speaker 1: like everything to be in the oriented and exactly the
[45:35] Speaker 1: right place. So sometimes you do like two or three
[45:38] Speaker 1: swings around things. It could take like ten years to
[45:41] Speaker 1: like gravitationally slingshot your way from one place to another.
[45:44] Speaker 1: So ion thrusters are much more direct way to get there.
[45:48] Speaker 2: I think Kim Stanley Robinson used gravity slingshots as a
[45:52] Speaker 2: really great plot point in Aurora, Like they were running
[45:54] Speaker 2: out of fuel, so everybody had to go into like
[45:56] Speaker 2: hibernation and they use gravity slingshots to get you there.
[45:59] Speaker 2: Like it takes a long time. Yeah, but if you
[46:02] Speaker 2: don't have fuel and you can use that that, if
[46:04] Speaker 2: you've got the time, it's a good way to go.
[46:06] Speaker 1: Yeah. It's also cool because it's physics again, right, which
[46:09] Speaker 1: inherently makes it awesome.
[46:11] Speaker 2: Yeah, better than chemistry. Notice, good biology, but it's a hierarchy,
[46:14] Speaker 2: it's okay.
[46:16] Speaker 1: And their other side benefits, like if we can get
[46:18] Speaker 1: reactors to work in space, maybe that's going to help
[46:20] Speaker 1: us figure out how to get reactors to work in
[46:23] Speaker 1: crazy environments like on the Moon, and people who are
[46:25] Speaker 1: talking about building moon bases are talking about powering those
[46:28] Speaker 1: things with nuclear reactors. Getting to Mars, it's going to
[46:31] Speaker 1: help us open up more launch windows. It's a good
[46:34] Speaker 1: way to send cargo to Mars, and it opens up
[46:37] Speaker 1: the outer Solar system. Remember, solar power is just not
[46:40] Speaker 1: useful past Jupiter, which gets like four percent of the
[46:44] Speaker 1: Earth's sunlight. So if we're going to explore the outer
[46:48] Speaker 1: reaches of the Solar System. It's not going to be
[46:50] Speaker 1: with chemical rockets. It's going to be with ion thrusters.
[46:53] Speaker 2: So if we want it to get like straight to
[46:55] Speaker 2: Jupiter really fast, we'd want ion thrusters powered with nuclear
[47:00] Speaker 2: reactors rather than gravity sling shots. Yeah, absolutely, okay cool?
[47:04] Speaker 2: And what a voyager you use? Voyager is a gravity
[47:06] Speaker 2: slingshot situation?
[47:08] Speaker 1: Right, yes, exactly?
[47:09] Speaker 2: Okay, Well, this is amazing. I can't wait to see
[47:11] Speaker 2: how those helicopters fare in the Martian atmosphere like the
[47:15] Speaker 2: helicopter that came before.
[47:17] Speaker 1: I can't wait to see if Kelly ever achieved her
[47:18] Speaker 1: lifelong dream of sitting on a rocket.
[47:21] Speaker 2: Yeah. Well, I hope you're sitting right behind me, Daniel,
[47:25] Speaker 2: and I hope it's a safe rocket like our paddle
[47:27] Speaker 2: boat rocket and not a chemical rocket.
[47:30] Speaker 1: Well, I look forward to sitting on that rocket with you, Kelly.
[47:33] Speaker 1: Until then, thank you everybody for taking this journey with
[47:36] Speaker 1: us through the history of rocket development and into the
[47:39] Speaker 1: future for how humans could explore the Solar System.
[47:42] Speaker 2: Yay space exploration. Until next time.
[47:51] Speaker 1: Thanks everybody for listening. Please go and do us a favor.
[47:54] Speaker 1: And rate the show on whatever podcast app you're using.
[47:57] Speaker 1: It really helps people find us.
[48:00] Speaker 2: Daniel and Kelly's Extraordinary Universe is edited by the amazing
[48:03] Speaker 2: Matt Kesselman.
[48:04] Speaker 1: He really is a wizard. You can also find us
[48:08] Speaker 1: online on Blue Sky, Instagram, and x D and K Universe.
[48:13] Speaker 1: Come engage with us.
[48:14] Speaker 2: You can email us at questions at Danielankelly dot org.
[48:18] Speaker 2: We really do want to hear from you, and you.
[48:20] Speaker 1: Can find our website www dot danieland Kelly dot org,
[48:24] Speaker 1: where you'll also find an invitation to join our discord
[48:27] Speaker 1: where everybody comes and talks about the amazing universe.
[48:31] Speaker 2: And we also have the most amazing moderators. This is
[48:35] Speaker 2: an iHeart podcast. Thanks for joining us.

Shows are listed from Apple Podcasts' charts and directory, and Podcast Index where it is used. Every show and episode belongs to its publisher and plays from the publisher's own feed; this site hosts none of them.