Ion engines are a mainstay of science fiction, featured in both Star Trek and Wars. But this is a very real technology, successfully used on several missions out there in the Solar System right now. How do they work and what are the limits?
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Electrically powered spacecraft propulsion
Dawn Mission spacecraft
Ion drive meets drone, as small plane flies with no moving parts
Air-breathing electric propulsion
Experimental Study of Air Breathing Ion Engine Using Laser Detonation Beam Source
This new ‘ion drive’ airplane flew straight out of science fiction
Transcription services provided by: GMR Transcription
Fraser: Astronomy Cast, episode 507: Ion Engines. Welcome to Astronomy Cast, a weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, a senior scientist for the Planetary Science Institute, and the director of Cosmo Quest. Hey, Pam, how you doing?
Pamela: You know, I have to admit things are kinda ‘meh’ right now.
Pamela: So, I have this great, brand new job, and it’s one of these things, where for years, Cosmo Quest, Astronomy Cast, we’ve been trying to have a major impact while working from small institutions that are more focused on education than research, and it turns out sometimes you’re better off when you’re with the – well, the big publishers at the big research institutions. And so, I looked for a job and looked to move my entire program over to the Planetary Science Institute after happily getting hired. And NASA isn’t letting us cancel, or isn’t letting us transfer our grant and is in fact canceling our grant, so no one gets to keep the grant.
And, Merry Christmas, unless there is a miracle, I have to fire my entire staff, including myself because in astronomy, you can have a job and no salary. So, yeah, that happened.
Fraser: Oh, ouch.
Pamela: So, we’re trying –
Fraser: And of course, I mean, that – this grant – So, can you just explain briefly what this grant has been covering and was expected to cover?
Pamela: So, this grant started in 2015. We’re one of 31 projects selected when NASA did a massive reorganization, and brought all of their education and Citizen Science Programs together in one portfolio. Since then we have been working to launch new Citizen Science projects, this has been funding a whole bunch of student programmers in early career, not actually a student programmer, myself, and several educators, researchers, working on the science all over the nation. So, this is a number of scientists, a whole lot of students, a post-doc; all of us. No salary starting in January 1.
So, I know I can get more grant funding, I know I can keep the project going. We’ve been doing this since 2012, but as we’ve talked about in past episodes of Astronomy Cast, it takes significant time to apply for new funding. And we’d started to get some hints in August that this might be coming. We were warned the James Webb Space Telescope cost overruns would lead to the cancellation of some projects. And when someone takes the time to tell you that, you know you’re in jeopardy.
We also were told that there were new policy changes on Citizen Science coming that may cause funding difficulties as well. But August to now, I’ve been traveling the whole time, so yeah. I’m going into overdrive writing grants, proposals, and begging on the Internet, which is why we’re bringing this up during this episode.
Fraser: Right. Yeah. While –
Pamela: We –
Fraser: – Pamela writes all of the new grant proposals to fill in the funding, to make sure that all of these projects we’re working on continue along, grant need – I think you require some bridge financing.
Pamela: Yeah. So, I’m trying to raise funding, so that I don’t have to fire my staff. And I, personally (because I have a husband who can pay the bills) – I’m only going to pay myself off of any grants that I find. So, what we’re trying to do is raise money by lifting every rock we can. So, we have four specific asks for today: 1.) Go support Patreon. That will keep Suzie going, that will – (who has a college daughter) that will keep Chad going. Go – If you don’t wanna support us on Patreon, if you just wanted to a one-time drop of funding, Cosmo Quest X on Twitch is linked to our PayPal account.
We are in this weird situation where NASA cut our funds, but we’re not allowed to ask our funds on our website. So, I’m going to send you to Twitch, and donate through Patreon. The third thing you can do is give science for Christmas. You can go to Cosmo Quest X’s red bubble store, and buy all sorts of amazing artwork by Austin Hinterlander and Chris Spangler. They have done amazing space apparel, phone covers, the whole nine yards. Give everyone you know something like that.
And the fourth thing is, you can sponsor individual days of 365 Days of Astronomy, we have links for that as well, and next Tuesday – I’m giving Tuesday, I’m going to be sitting on Twitch, and recording all of your sponsorship messages. So, for $45 a day, if you go to 365daysofastronomy.org, look for the tiny words in the right hand side bar, where we’ve kind of hidden it, you can sponsor a day, use it to promote your organization, commemorate a past loved one, celebrate an anniversary, dedicate a day to whatever, and give me something cool to read. And I’m gonna record all of this on Tuesday. So, that’s it. Those are the things you can do. Please, don’t make me fire everyone for Christmas.
Fraser: Oh. I believe they call it laying off. But yeah. And, so I mean, just be clear, you don’t take a salary from what we do at Astronomy Cast, I don’t take a salary. You know, in 13 years, neither of us have taken any money from Astronomy Cast –
Pamela: We’ve paid for travel.
Fraser: We’ve paid – Right.
Pamela: We paid for a bunch of travel.
Fraser: Of course. But it’s the team.
Fraser: And we wanna be able to cover the costs of the people that help us with the projects that we do, and to be able to bring science. So, those of you who can support us, you know, and if you’ve been sitting on the sidelines, you’re, “I kinda like Astronomy Cast. I kinda like what gets done and I would really like that to continue and more of that. Citizen Science outreach to be out there in the world,” this is the time. We need your help. Please, follow some of the instructions. If people want one place where they can go and sort of see how they can participate, where would you recommend that they go? What’s the one place that then will lead them all of the paths?
Pamela: All of this is going to be up by the end of today on Twitch.TV/CosmoQuestX.
Fraser: Okay. Perfect. Then check that out. Before we begin the show, sorry we’re using so much time with the not show part. I just wanna do a quick shout out. Our good friend, Dr. Paul Sutter’s book, Our pl – You’re Place (sorry) in the Universe – Understanding Our Big, Messy Existence, just came out a couple of days ago, and I finally got my copy and I can’t wait to read it. So, congratulations, Paul. And of course, you can buy that wherever books are sold.
All right. Let’s move on to the show. Here we go. Ion engines are a mainstay of science fiction featured in both Star Trek, and Wars. But this is a very real technology successfully used on several missions out there in the solar system right now. How do they work, and what are the limits? Pamela, ion engines. Science fiction?
Pamela: Yeah. Tie fighters. Don’t you know? I – This was the original ion engine I learned about. It wasn’t the original ion drive, but when you see that blue glow on the back of a star destroyer, that is all the ionized particles coming out the back engines of this massive destroyer. It’s the same kind used on these little, tiny fighters, and they have the mass to acceleration ratio totally wrong, but they got the colors right. Maybe. Maybe.
Fraser: They totally look like actual – And that’s the part I love about ion engines when you see them in reality, is they totally look like science fiction. This diffuse blue glow that is streaming out behind the engine system, it’s fantastic. And so, finally we’ve got science fiction rea – is hitting our reality. So, what is the underlying science behind ion engines? How do they work?
Pamela: It is one of those rare cases where the name kinda says it all.
Fraser: They’re engines with ions.
Pamela: They take ions – Well, they’re engines that move ions. So, you have some sort of a electric field, or a magnetic field (or both) that are used to accelerate particles that you have ionized, or that the universe has ionized for you, and as you accelerate these particles, you shoot them out the back end of your vehicle, and, conservation of momentum, if something goes out the back, the space craft has to move towards the front.
Fraser: Why are ion engines so cool then?
Pamela: Well, with normal propellant, you can’t exactly get that burning Hydrogen, that burning – Well, it’s not burning Hydrogen. You can’t necessarily get that burning solid rocket fuel going at close to the speed of light as it escapes out the first or second stage of that space craft that you’re watching take off the planet Earth. And due to the tyranny of the rocket equation, you end up having to have –80-90% of your rocket is composed of fuel, just to get you off the planet.
Now, with ion engines, you can accelerate those ions to close to speed of light. And this means that for a much smaller amount of mass, you can get a much larger (we call it delta v) change in velocity of your spacecraft. So, something like the Dawn Spacecraft is going to go through ounces of fuel in its ion engine and be able to accomplish 10 km/hour changes in velocity.
Fraser: I mean, there’s two parts to this that are really important. The first one is, when you look at the momentum equations when you consider the – when you run your equations with physics, you really have two things at play: you have mass, and you have velocity when you’re considering the overall momentum that’s coming through. And which of those matters more?
Pamela: It’s really the momentum is a combination of these.
Fraser: Right. Right.
Pamela: So –
Fraser: But velocity –
Pamela: – it –
Fraser: – is a big factor.
Pamela: It’s a big factor, and if I had the ability to hurdle microscopic black holes out the back end of a spacecraft, these have massive mass.
Fraser: Yeah. So –
Pamela: But I probably couldn’t hurdle them that fast.
Fraser: Right. Right.
Pamela: And so, there the huge amount of mass going at a not very large velocity would work, but except when you’re dealing with these degenerate matter and black hole matter, mass takes up space. So, if you don’t wanna take up too much space, you really wanna go faster.
Fraser: Right. You won’t – So, that’s – And that’s what I was driving at, which is that, you know, if you wanna carry a bunch of – Like, if you’re gonna try to impart momentum to your vehicle, you can either throw heavy objects out the back, which were very expensive to launch from Earth, or you can throw very light objects out the back of your spacecraft at incredibly fast velocities. So, that’s the first thing is that you’re – instead of having to throw big cinderblocks of the back of your spacecraft to impart velocity, you are – to impart momentum and velocity, you are throwing individual – literally individual ions out the back of your spacecraft at (as you say) enormous velocities. But the other part –
Pamela: I now really want to run calculations for an ion drive that uses microscopic black holes that are somehow charged.
Fraser: Or cinderblocks. But, sure. But then the second part to this is the fact that an ion drive can use electricity to accelerate the ions out the back of the engine. And so with a traditional, again, chemical rocket system, you need to have your oxidizers. You need to carry the fuel, and then you also need to carry the oxidizer for the fuel to be able to run the – to be able to use your chemical engine, and then you don’t get a very high, as you said, velocity of the back of the space craft. With an ion engine, you use electricity to accelerate them, and so you don’t need to carry that oxidizer. And the amounts of fuel that these engines require is tremendously less than a traditional chemical rocket.
Pamela: Yeah. The trickiest part (at least to me, looking at this) is making sure that you don’t inadvertently attract yourself back to your fuel. You are shooting ionized particles out the engines. And the way they do this is they excite the particles, Xenon gas, or something that’s commonly used for like the Dawn Spacecraft, and you’re stripping the electrons off of the Xenon. And they use Xenon, because it’s really easy to strip the electrons off and get a charged particle. But if you don’t also shoot the electrons back into the mix as it exits the spacecraft, to make that a neutral gas, then you’re gonna have the charged particles attracting you back to your spacecraft.
Pamela: So, it’s really cool to figure out how they’re able to do all of this. Take the electrons one way, take the charged particles the other, toss them all out so that they mix together, and off we go.
Fraser: Yeah. You’re getting rid of – You’re taking this plasma, you’re stripping off your – the negative charges and you’re dumping them, so that, as you said, you don’t get with your own – You know, and then you are accelerating these ions out the back of your engine and trying to make the whole thing balance. And the – So, what’s the big downside? Why isn’t it all just ion engines all the time?
Pamela: It’s slow. The tie fighters lied to us with their twin ion engines. They may be able to zoom around to the Millennian Falcon, but, let’s face it, that universe is bad with their units.
Fraser: All right.
Pamela: It turns out that in general, you just can’t toss enough particles out fast enough to be able to do anything rapidly. I remember the day that they did the hasta la Vesta celebration with the Dawn Spacecraft leaving Vesta to head off towards Ceres, and they kind of had to estimate when the spacecraft would be leaving because it’s such a slow acceleration that they weren’t entirely sure when they’d hit escape velocity. It’s not like you just turn it on and off you go.
Pamela: No. No. Not so much.
Fraser: Yeah. And I think it was (and I forget the exact number, and I apologize) – it’s something like 0 to 60 in weeks.
Fraser: Right? So, you turn on the engine and it’s been described that you take – that the actual force that’s coming off from the back of the ion engine is about the same as you take a piece of paper, hold it on your hand, feel the force of gravity of the piece of paper (which you can’t), that’s the amount of force this thing applied to the back of your entire thousands of kilogram spaceship. And so, it is a constant acceleration, but it is incredibly weak. But, upside, you can run it for a long time. And that’s what they did. And so, they’ll run these ion engines for days, months, weeks. There are test ion engines here, on Earth, that have been run for, I believe, years.
Pamela: And they’re starting to develop drones with no moving parts that use ion drives and ionized air.
Fraser: Yeah. Now, that’s – There’s a bunch of super cool technologies and you skipped right to the end.
Pamela: Of course, I did.
Fraser: Yeah, but this came out like two days ago. So, this is brand –
Fraser: – new science. Did you –
Fraser: – read into this at all? Have you learned what’s going on here?
Pamela: Yeah. So, it – words. Overly excited about this really cool new technology. So, it had long been known, because of early testing, this is technology that was first prognosticated as, “Well, we should be able to do this,” all the way back in the early 1900s. And when I say the early 1900s, I’m talking like pre-Goddard. This was figured out by a Russian scientist in 1911, Konstantin Tsiolkovsky, and then Goddard built on what Tsiolkovsky had predicted, and these two early pioneers of rocketry basically said, “Okay. So, we know that our atmosphere has charged particles in them. What if we sent up a ion drive that can accelerate those few charged particles? They’re gonna hit other particles, they’re gonna –”
And this whole process just drives going faster and faster and faster. Which is super cool, but wasn’t practical because creating the energy field necessary required significant weight until very recently. So, they did tests to see if they could drive air using ions. A have to admit, I actually the first time I saw a Dyson heater, I was like, “Oh, I wonder if that’s ion air.” No. It’s not as near as I can tell.
Fraser: It turns out it’s not. It’s just a traditional fan and some kind of plastic casing that makes it look like it’s doing that.
Pamela: I was so sad.
Fraser: I know. I know.
Pamela: But, so, people had been testing ion air accelerators to get air moving without fans and things that can do harm. Well, it worked, it just required a lot of electricity. Well, nowadays we figured out how to handle all the parts at a significantly lower energy cost, and with significantly less weight. Because everything, except for the thing making ions is now much more energy efficient. And things like Lithium ion batteries significantly lower the weight.
Fraser: Yeah. And so, there have been some tests of an airplane that was flying around – I think at MIT –
Fraser: – where there are no moving parts. It’s flying around just through electricity, ionizing particles in the atmosphere and using those for thrust. Absolutely fascinating. Possibly not practical in any way, shape or form, that it won’t scale up and you can’t turn it into anything that will be super useful, but at the same time possibly the way to make – Oh, you know the thing from the Avengers? The big, floating – You know.
Pamela: So, the thing here is, the amount of air thrust that they can create should double if you double the amount of thrusters that you have, but mass in general goes as the cube of the size because of that whole volume problem. So, the mass of your drone is growing faster than your thrust is growing, but this starts to get – at least people like me thinking about what about combining [inaudible] [00:21:17] and ion thrusters?
You can have completely silent transportation. Now, I admit, I’m pretty I’m some day gonna die by electric car because I don’t hear them coming. But the idea that we can have drones that don’t sound the way quad copters sound, I – If we’re gonna have a droney future, I want it to be one filled with ion drives.
Fraser: Yeah. This is a thing that people aren’t ready for. The people think we’re gonna have drones and we’re gonna fly around our drones, we’re all gonna have our personal drone, but these things make a lot of noise, and there’s –
Fraser: – no way, under the laws of physics, to prevent that noise from happening. And so, if you replace cars with drones, there would just be this racket all the time everywhere. So, yeah. Absolutely.
So, there’s a couple of interesting technologies. Have you heard about this idea of an air-breathing ion engine? An ion thruster? So, this was a test that was made by the European Space Agency and some groups working with them. And so, one of the ideas that they had was instead of taking Xenon fuel, putting that on your spacecraft and then using that as your fuel source (which will eventually run out), what they’re testing out is can you have a satellite that is flying low enough to the Earth be able to pull in particles from the air and turn those into fuel; turn those into thrust. And they were able to do this.
So, you can have an ion thruster, a solar powered ion thruster that sits on a satellite that is much lower to the Earth than the International Space Station, other things, where they’re starting to get buffeted by the atmosphere, but pull in enough particles from the atmosphere to be able to counteract the thru – the deceleration into the atmosphere, and essentially station-keep itself at a very low altitude forever without any fuel because it’s just pulling in particles from the atmosphere using solar energy to accelerate them and maintain itself. So, ion engines are the best.
Pamela: And when you start thinking about all the ways that you can combine them with Hydrogen scoops and other forms of scoop technology for interspace – interstellar travel, when you start thinking about combining them with solar sails, we’re looking at a very graceful and slow moving spacecraft future, where we are able to escape a lot of that, well, rocket equations problem that has killed so many space craft we loved.
Fraser: Yeah. That doesn’t help us to get this stuff off the surface of the Earth though. We’re still gonna need those –
Fraser: – chemical rockets to launch from planet Earth to get – Because again, the force from a piece of paper is not gonna give you enough thrust to get your spacecraft out into space. But far more powerful ion engines are in development, including a pretty interesting one at NASA. Have you seen their new hall thruster that they’re working on?
Pamela: No. I’ve heard that it’s coming, but I don’t know the latest details.
Fraser: Yeah. So, that – I mean, it is just like an enormous amount more electricity running through the system and they’re able to get the – I believe they’re able to get the – get up to one Newton.
Pamela: So, that is a unit of force. One Newton is the force of –
Fraser: It’s like 9. Newtons is –
Pamela: – it’s kilograms, meters per second. So, it’s one kilogram – I – it – I can’t.
Fraser: It’s far more – the crude ion thrusters are .5 Newtons – Oh, So, I think it’s more than that. So, they’re able to get it up by – I think by like a factor of 10. Anyway, so it’s significantly more force coming out of this ion thruster. But the problem is it’s that it’s gonna require an enormous amount of electricity. More than you can generate with solar panels. So, the idea is to match this up with some kind of nuclear fission reactor on –
Pamela: And this gets us right back to Star Wars –
Fraser: I know.
Pamela: – where they’re using fusion reactors –
Fraser: Fission. Just fission. Not fusion. Don’t be crazy.
Pamela: – to generate – Right. But in Star Wars.
Pamela: In Star Wars they’re using fusion reactors to generate charged particles that they’re accelerating out of their star destroyers, so that they can bring their triangular death upon the universe.
Fraser: Yeah. Yeah. So, you can imagine a future spacecraft is gonna be equipped with a NASA-built fission reactor that is using that fission power to power high thrust ion engines. You’re still not gonna be able to launch from say, Earth, or even the Moon, or Mars, but you could take a much larger spacecraft and do a lot more interesting missions. Do you remember the JIMO Mission? The Jupiter Icy Moons Orbiter Mission? This was a mission –
Fraser: – that was planned back, I think, when we started Astronomy Cast, and it would be an orbiter that would visit each of the four Galilean moons. And it was gonna be equipped with a fission reactor and powering all of these ion engines. And that’s the kind of ability to change velocity that you would need to be able to go to all of these different worlds. So, it got cancelled. But –
Pamela: Yeah. This is why Space Craft are dead to me until they launch, although apparently I’m dead to James Webb Space Telescopes over budget.
Fraser: Yeah. Woa woa. It’ll launch.
Fraser: And then it’ll thank you. But Hayabusa –
Fraser: – is using an ion engine.
Pamela: Yes. And this is really the way of asteroid exploration of the future. Just kind of carry on slow and steady, studying those rocks.
Fraser: Yeah. And so, right now, I mean, let’s run through all the missions that are operating with ion engines as we speak. Hayabusa –
Pamela: Oh, man.
Fraser: There’s the Dawn Mission. Those are the ones that –
Pamela: Dawn, Rest In Peace, has ceased operations and is simply orbiting Ceres without communications at this stage.
Fraser: Previous to that there was the European Space Agency, they had their Smart I spacecraft –
Fraser: – which was able to travel from the Earth to the Moon purely on ion power very slowly, very carefully. There was the Deep Space I spacecraft, which, I think was the first one. And that was really like a test bed of a –
Fraser: – pile of different technology.
Pamela: Sert I and II in the 1960s pre-dating Star Wars, were the first applications.
Fraser: Oh, really? Okay. All right. Yeah.
Fraser: So, it’s – and there’s sort of a funny history about the ion engines that led into them using it on the Deep Space I. They had to – they were too terrified. NASA was too scared to use a bunch of these technologies for actual missions. So, they gathered up like 15 different experimental technologies, put them all on this one mission Deep Space I, and then just had it test out to prove that all of these were feasible ideas. Including ion engines and automated – it had it’s own way of finding its location in the solar system, and it had a bunch of other technologies on board. So –
Pamela: RND money done right.
Fraser: Yeah. I think it was a great way to do it. Right? And so, you know, now we really know that these ion engines are so successful. They’re just stable and they just – they push, push, push, and I really hope that we’ll see more of them on more – Everything should be equipped with an ion engine all the time.
Pamela: Yeah. And we’re seeing this coming more and more out of Europe, which is what – Let me rephrase that. We’re seeing this more and more coming out of non-NASA. So, BepiColombo, which launched –
Pamela: – a few weeks ago. It’s another one of these. And the thing about that mission that really gets me is, it was named in 1999, which meant that people were already working on it long enough to know it was gonna be a thing when they named it in 1999. It just launched now in 2018, and it’s seven years before it will hit its scientific orbit. And here they are, flying with ion drives on a spacecraft that – heck, I had just gotten my Master’s degree when they finished building this thing. When they started naming this thing.
Fraser: And, of course, being able to go close to the Sun to be able to get more solar power, to be able to power your ion engines is better. They don’t work so well solar powered if you’re gonna try to go out to the outer solar system, where you don’t get a lot of energy. So, it’s really that sweet spot, a place that you have a lot of electricity, and you don’t nee – and you’re not trapped in a gravity well, then ion engines –
Fraser: – make perfect sense all day long. All right. Well, thanks, Pamela. We’ll see you next week.
Pamela: My pleasure.
Female Speaker: Thank you for listening to Astronomy Cast, a non-profit resource, provided by the Planetary Science Institute, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astronomy Cast. You can e-mail us at email@example.com, Tweet us at Astronomy Cast, like us on Facebook, and watch us on YouTube. We record our show live on YouTube every Friday at 3 p.m. Eastern, 12 p.m. Pacific, or 19:00 UTC. Our intro music was provided by Davis Joseph Wesley, the outro music is by Travis Cyril, and the show was edited by Suzie Murph.
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Duration: 32 minutes