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Rendezvous with Mars

Season 1Episode 151Jul 3, 2020

Patrick Chai, aerospace engineer at NASA’s Langley Research Center in Virginia, covers the challenges and needs for getting humans to Mars and the options for propulsion, duration, time, staging, and more that will be considered on this third episode of our Mars Monthly series. HWHAP Episode 151.

Rendezvous with Mars

Rendezvous with Mars

If you’re fascinated by the idea of humans traveling through space and curious about how that all works, you’ve come to the right place.

“Houston We Have a Podcast” is the official podcast of the NASA Johnson Space Center from Houston, Texas, home for NASA’s astronauts and Mission Control Center. Listen to the brightest minds of America’s space agency – astronauts, engineers, scientists and program leaders – discuss exciting topics in engineering, science and technology, sharing their personal stories and expertise on every aspect of human spaceflight. Learn more about how the work being done will help send humans forward to the Moon and on to Mars in the Artemis program.

On Episode 151, Patrick Chai, aerospace engineer at NASA’s Langley Research Center in Virginia, covers the challenges and needs for getting humans to Mars and the options for propulsion, duration, time, staging, and more that will be considered on this third episode of our Mars Monthly series, where we drop a new episode about a human mission to Mars on the first Friday of every month. This episode was recorded on February 4, 2020.

Check out the Houston, We Have a Podcast Mars Page for more Mars Monthly episodes.

Houston, we have a podcast


Gary Jordan (Host): Houston, we have a podcast. Welcome to the official podcast of the NASA Johnson Space Center, Episode 151, “Rendezvous with Mars.” I’m Gary Jordan, and I’ll be your host today. On this podcast we bring in the experts, scientists, engineers, astronauts all to let you know what’s going on in the world of human spaceflight. We’re continuing our Mars Monthly series with Patrick Chai Ph.D, who humbly requested to be referred to as an aerospace engineer at Langley Research Center in Virginia, but most would call him a rocket scientist. Patrick hones in on the challenges and needs for getting to Mars. The journey from Earth to Mars will be long, somewhere on the order of nine months. And with that, comes a series of challenges, some complicated orbital mechanics, and a whole range of options that consider things like propulsion, duration, timing, staging, and more. Yes, thanks to probes and rovers of the past we’ve landed on Mars, but a human landing will be like nothing else before. The transport will have to be much larger. And with humans onboard, you’re limited on the kinds of routes you can take to get to Mars. Sounds like a simple consideration. Yes, of course there will be humans, but it makes things a lot harder. Buckle up everybody and all aboard to the red planet. Here we go. The challenges and range of options on how to rendezvous with Mars with Patrick Chai. Enjoy.

[ Music]

Host: Patrick, thanks so much for coming on Houston We Have a Podcast today.

Patrick Chai: Thanks for having me.

Host: This is a really interesting and intricate discussion we’re going to have about what it takes to actually get to Mars. So, I might go a little bit off track. You are going to have to steer me to Mars in the right way. Tell me about your background and what got you to the path where you’re thinking about Mars rendezvous?

Patrick Chai: Well, I haven’t been at NASA very long. I started in the fall of 2014. I was a graduate student at Langley as well, doing some system analysis for different refueling options. So, I was a grad student. So, I got my Ph.D. from Georgia Tech. And Georgia Tech has a program at Langley associated with the National Institute of Aerospace, where grad students can, you know, be on-site there at Langley doing research with folks. And so, kind of gravitated me towards doing system analysis. So that’s how I ended up applying for a job at the branch I am in, which is a space mission analysis branch. And once I started there, that’s back when there was a big effort, defining what then was called the Evolvable Mars Campaign as part of President Obama’s flexible path definition, you know, going to the asteroids and, you know, having multiple paths getting — humans to Mars. So, I started doing a lot of the trajectory work for — for the Evolvable Mars Campaign and doing the definitions of how, you know, all the different mission opportunities and how much propellant it would take if we have, you know, different payloads and things like that. So that kind of evolved into me kind of being the lead of the trajectory analysis team there for — for all the Mars stuff and now I’m leading the Mars Integration Group here at NASA Langley. But I still want to do trajectory work because that’s the most important. That’s – the exciting things I like to do. Leading team is great, but it’s like one of those curses, you know, you move up and become management and they don’t let you do exciting work anymore [laughter].

Host: You have to delegate, delegate —

Patrick Chai: Yeah, exactly. It’s hard man. Learning how to delegate is a hard thing to do.

Host: Yeah, especially when you want to do —

Patrick Chai: Exactly.

Host: You want to get your hands dirty.

Patrick Chai: Yeah.

Host: So, you mentioned, you’ve alluded to a few different things in your description of what you’re thinking about. You’re thinking about systems, you’re thinking about trajectories, what are these different elements that you’re considering when you’re thinking how do we get to Mars?

Patrick Chai: So, you know, for what are we thinking about here. It’s always, you know, sending crew to Mars right. So, we have — we have a propulsion element that, you know, you need some sort of propulsion element to get, you know, your crew to and from Mars right, and the payload that you need to deliver. So, typically, you have a, you know, very traditionally you have a chemical stage and so, you have, you know, kind of like a regular rocket. You have, you know, fuel and oxidizer and, you know, they come into a combustion chamber and they combust and then you exhaust that out of the nozzle and that’s how you push things. Right? And — for Mars, you either pushing the Deep Space Habitat where the crew lives in or a payload which is what we call cargo, which is the lander that you need to land things on — Mars surface.

Host: OK, so — so you’re thinking about the energy, the type of energy it takes to actually get to Mars —

Patrick Chai: Yeah.

Host: And what you’re going to attach to that energy.

Patrick Chai: Yeah, and so, you know, you — for the integrated stack, you have your propulsion elements, and you have a payload and you have to be able to get that — all those things put together in orbit, and then you send that to Mars, and not only you have to send it to Mars, it has to have enough energy for you to come back too. So, you think about a big rocket, you know, as a reference, like a big rocket you’re launching from Cape Canaveral, you know, that is very short distance that they — it has to do comparatively to where we’re needing to go to Mars. So, you can imagine the energy that it would need to go all the way to Mars, it’s pretty significant.

Host: Yeah. So, what’s the difference when you’re thinking — we’re talking about the system level now, we’re talking about what you need to actually build to get to Mars. What differs when you add the human element?

Patrick Chai: So, when you add the human element, it’s kind of different, right? So, if you think about, you know, things we’ve sent to Mars, all the rovers and things like that, you know, those systems have a lifetime, right. But you know, it’s electronics, it’s — powered by battery or solar arrays or what not, they can take a much slower route to Mars which’s much more energy efficient, right. When you have a crew onboard, you have other considerations because, you know, if you have a longer duration in space, that means you have to bring more food, that means you have to bring — you have to have a bigger space for the crew, you have to have exercise equipment for the crew. So, what — what that actually translates to is you have more payload, you have more mass, you have to push — you’re pushing more things and that — we call it the tyranny of the rocket equation. It’s an exponential relationship. So, you add one, you know, you add one kilogram or one pound of payload. It’s not equivalent to one pound of propellant. It’s, you know, a much, much bigger amount. So, it can add up really quick. So, for a human mission and it’ll be a little bit for the — for the robotic too, you know, it’s all about minimizing that payload mass as much as possible. And these rovers that, you know, that [Jet Propulsion Laboratory] JPL is sending to Mars, I mean, they are very slimmed down, even though they’re very beefy and they’re very reliable. But they are always looking for different ways to do, you know, manufacturing and fabrication of these components to minimize the weight because weight is everything in space.

Host: Yeah. Yeah, so I guess, does — maybe this might be a ridiculous question, but does thinking about having larger systems to get to Mars change your trajectory or your possibilities of how to get there?

Patrick Chai: Um, it doesn’t really change the trajectory.

Host: OK.

Patrick Chai: Maybe a little bit, but it changes the way you think about how to get to that trajectory. So, everything we’ve sent to Mars so far, you’re thinking about these, basically, your traditional rocket, you know. And these are, you know, we’re getting much, much more efficient with them, but they’re very, I won’t say rudimentary, but these are, you know, systems that we’ve — we’ve been using for decades, right. And they have their limits in terms of how energy efficient, how much, you know, what we call specific impulse — impulse that it can deliver. And specifically, we’ll measure kind of the — how much — energy you can get out of the same amount of propellant. So, the higher you — higher that explosive impulse is, the better your system at being efficient. So, your traditional rocket you’re looking at maybe, you know, 300 to 400 seconds with a specific impulse and there’s a whole, you know, derivation equations. I don’t exactly know why they — use seconds as the — as the unit, but it falls out of the equation. But for a traditional chemical, you’re looking at 300 to 400 seconds of specific impulse. And, you know, with starting to think about some of these other, you know, kind of lower thrust and higher energy propulsion systems like the solar electric propulsion or a nuclear electric propulsion systems that can get you up to, you know, 3000 to 4,000 seconds worth of specific impulse — that increase tenfold. The drawback of that is you ended up having to stretch your in-space duration longer because it, you know, it naturally is a lower thrust propulsion system. So, that’s always the battle, you have — you have a low thrust and a high thrust system. A high thrust system kind of, you know, is a really big kick to get you out and you can do a little — another kick to kind of slow you down at Mars. And that’s relatively fast. Low thrust, you kind of — just like putzing along, you know, for a much longer duration, but ultimately it saves you a lot of propellant. And — it’s a much more robust too, if you increase your payload mass by a significant amount. It’s much more robust to be able to absorb some of that growth, so.

Host: Oh, OK, so there’s a lot of different factors your considering.

Patrick Chai: Yeah. And — what actually we have been working on in the last three, four years is actually kind of marrying the two. We’ve been working on what we call a hybrid propulsion system. And it’s kind of — it has both components. It has a high thrust and a low thrust system. So, you use the high thrust when you’re near — planets where they are actually most effective. And — you know, because you’re — you have to, you know, no matter what, you — have a long duration transit out to Mars and back anyway. So, you might as well use some of that time to do some thrusting and do some maneuvering. And we’ve — we found that the system that we’ve kind of, you know, been analyzing has some advantages and some disadvantages, but that’s how all this — and that’s important, I think, from, you know, what we — what we always harp on from our — branch, the system analysis — analysis perspective. We have to be able to understand the behavior of the system. Especially under uncertainty and under changing requirements in order to understand what you’re actually selecting. You can’t just say, oh, because we have this very, very tiny optimized point design, we’re going to select this — this propulsion option. Well, what if your design changes a little bit, you know, how sensitive you are — your system is to that change, and we need to have a better understanding of all that before we make — before we’re able to make really informed decisions.

Host: So, I mean, a hybrid approach sounds very reasonable because you’re considering the — flexibility, naturally when it comes to spaceflight with flexibility comes mass. Now you’re talking about two different propulsion systems.

Patrick Chai: Yeah, actually, it’s — it’s kind of — yeah, you know, it’s actually less in terms of mass. It’s more on the complexity side.

Host: Interesting.

Patrick Chai: In terms of mass, like, you know, given the same payload and the propulsion system, it depends on the trajectory you fly. The chemical is going to be the heaviest, and then the full [Electric Propulsion] EP up is going to be the lightest. But, you know, you trade time with that too, right? You know, the — the big rocket will get there — get you there the fastest, but the slow boat electric propulsion will get there, you know, much slower. So, we’re trying to, you know — so like, for instance, we have some planetary probes that are flying out there that uses electric propulsion, and then they’ve been very successful in doing that. But, you know, these things take years and years to get to wherever they’re going. And so, it’s great for the — for the planetary probe, you know, because they don’t really care about, you know, how long it takes to get there as long as they get there and do the science they need to do. But when you have crew onboard, you can’t really use that as consideration, right? Because you have to — you can’t just say, oh, we’re going to have, you know, we’re going to take three years just to get to Mars one way, right. It’s — you know, it’s a nonstarter because all the food and everything you have to bring. Unless we can figure out how to cryo-freeze people, I guess [laughter]. That would be a different ballgame [laughter].

Host: Wow. I mean, this — that’s one of the key elements here, we’re talking about sending humans to Mars. And that — with that you — one of the requirements is — is speed.

Patrick Chai: Yeah.

Host: You need to get there faster.

Patrick Chai: Yeah, you need to get there, you know, in a reasonable amount of time before all your consumables and things like that run out, right. Or you have to bring so much that you never even get started. Right.

Host: Yeah.

Patrick Chai: And so that’s always the challenge, right? Because you — you know, with exploring, I always use kind of the analogy of Christopher Columbus sailing across the Atlantic Ocean, right. When they leave, they had — there was — there was no expectation that they have any sort of, you know, stoppage and things like that on, you know, during the voyage, right.

Host: Yeah.

Patrick Chai: And so, they’re hoping that, you know, fingers crossed that they can get all the way to wherever they’re going and resupply, so they are planning however long they — were planning to sail. And, you know, at least — the good news for us is that at least we know where we’re — our destination instead of going sailing into the unknown. But to kind of, you know, give you some context, I mean, Columbus sailed in, you know, the late 1400s. And they got to the — I think the first trip they left the Spanish coast and got to, I think the Bahamas. That, you know, was like 6,500 kilometers worth of distance they traveled, you know, across the Atlantic Ocean in like 30 something days. If — if you use that same distance they traveled as, you know, if you — if you say, OK, that’s the same distance that you know, from Earth to Mars. The Moon, which is the furthest humans has ever been, is only two and a half miles off the coast.

Host: Oh.

Patrick Chai: So, give you some scale [laughter]. So, on all human existence, we’ve only gone two and a half miles off of the Spanish coast.

Host: Wow.

Patrick Chai: And we’re trying to get across the Atlantic Ocean to the new world.

Host: Yeah.

Patrick Chai: And trying to plan for that. And — and so that — that kind of gives you that scale. And I think people, you know, think oh yeah, we — you know, we — because we sent probes, you know, and things like that it should be easy, but it’s — it’s not at all because it’s — one of the most — I think fundamentally the most challenging things. That’s why we’ve been looking at it for — since the Apollo time, right. Because it is that challenging.

Host: Well, tell me a little bit more about what — what is so challenging about it when it comes to the orbital mechanics of it all that — how you’re limited with your opportunities, how, you know, you can’t really turn around.

Patrick Chai: Yeah, so we are, you know, very much governed by what, you know, the mechanics and the gravity and — I struggle a lot, you know, talking to just, you know, people, you know, like my family and things like that, like what I do, it’s not easy to like, have them understand that. So, I like to use the — the train analogy and, you know, imagine like, you have — you know, miniature train sets or big train sets — I don’t really want to know what you want to play with. But, you know, Earth and Mars are moving around the sun, like — like they are on train tracks, you know, circular or ellipsoid train tracks, right. So, it’s spinning around, but they’re moving at different speed. Right? So, your goal of getting from one to the other is you’re — you’re standing on one of the train track — on the trains, you know, moving with the train, and you’re trying to throw an egg across to — and have it land on the other train without it breaking. Right? And so — so it’s all — it’s not only do you have to like have enough energy to throw it. You also have to have enough energy to slow it down so that — when it lands — when it gets to the train, it doesn’t, you know, just splatter, right. That’s kind of like a basic analogy of how you, you know, what it takes. So, you can think about, you know, imagine like, if the trains are on, you know, opposite ends of the circle, right? You know, they’re not in sync, right?

Host: Yeah.

Patrick Chai: You’re not going to be able to, you know — you could, it would just take a lot more energy to throw and to — you know, and slow down, right. And so you — you’re — basically what we’re doing with Mars in terms of mission opportunities is that we’re waiting for the trains to kind of align itself to the proper, you know, proper orientations for us to kind of maximize the use of the velocity, the speed of our train, and maximize the speed, you know, on arrival of, you know, the Mars train kind of.

Host: Yeah, what you’re trying to do is you’re trying to limit how, how hard you have to throw this egg.

Patrick Chai: Right, you — how hard you have to throw there, and you have to limit how hard you have to push back when — when you get to Mars. I remember seeing — it was a great video that Mythbusters did where they had a cannon strapped to the back of a truck and then they drove it at 50 miles an hour and they shot the cannon off the back and it kind of just fell on the ground.

Host: I saw that one. Yeah.

Patrick Chai: You’re basically doing that at Mars, right? And because you want to kind of just like be the same relative velocity. Because you’re moving fast compared to Mars, right? And you have to fire engines, you know, fire a rocket, your cannon, the other way to kind of slow down and drop in Mars. So, you want to minimize that as well, because that’s all, you know, we call it delta V, change in velocity. And that drives how much propellant you need. That drives how much your — your propulsion system has to be in terms of, you know, thrust and size, and all that and it kind of snowballs into this giant vehicle that you have to take to Mars. So then, you know, minimizing that is the goal, right? And that’s why we have very — these — these very what we call conjunction style trajectory that kind of minimizes that energy.

Host: OK, so how often do those opportunities come up?

Patrick Chai: Typically, the Earth/Mars synodic cycle is about 26 months, so every 26-ish month, you get one of those opportunities. That’s why, you know, for JPL, their rovers, they launch every two-ish years or so. So, this one, we have Mars 2020 now coming up this year, the last one was launched in 2018. So that’s kind of the cycle that we have. Every 26 months or so we have these kind of conjunction opportunities.

Host: Now, is there a factor you have to add into this whenever you’re looking at these opportunities for the fact that humans are going to be on this vehicle and that you’re going to have to return at some point?

Patrick Chai: Well, yeah, that’s always — that’s always the trick, right? People always think, you know, oh, one way, right. No, we have the, you know, it’s — it’s a round trip. So, back to the train analogy. You know, once you get to Mars, you, you know, in order to minimize the energy coming home, you have to kind of again, wait for the train to kind of get back into the proper alignment for you to come home. So typically, a conjunction style full round-trip mission, you’re, on average about 180 to 260 days, maybe up to 300 days, you know, out to Mars transit time. And you stay on the surface or stay at Mars for 300 to 500 days waiting for the planet to realign and then you come home on the same low minimum energy trajectory. So yeah, so you have a whole round trip about three years is typical, very, very typical. Now with different technology, high and low thrusts, those duration do fluctuate here and there a little bit. But typically, you’re looking at 900 to, you know, 1100 days of total, what we call time away from Earth, because that’s, you know, the days start counting when you push the button to leave Earth.

Host: Yeah.

Patrick Chai: There might be more time the crew has to spend near Earth to, you know, they have to launch, they have to get into the spacecraft. And get everything checked out before they can even do that. So, you have to kind of back that out even further to, you know, to get all the total crew time in space, which may be a little — little more maybe 100 days more than — then when the total Mars mission. Some opportunities, mission opportunities are a little shorter, some are longer because the Earth and Mars orbit are not perfectly circular around the sun. So, they do have variations in the — in the distance. So, some take a little longer, some a little faster — sometimes a little faster. So yeah, a lot — a lot to kind of, there’s just a lot of variability and — and we have to — we do have to take that into consideration. Because we’ve — we’ve had — we’ve gone into some, you know, bad rabbit holes and oh, you know, this mission opportunity is really, really easy. Let’s design to that. Well, if you do that, then if you miss it, you can’t go ever again, or you can’t go for 20 years.

Host: You have to have some flexibility —

Patrick Chai: Right. Exactly.

Host: Design your system is to meet these different —

Patrick Chai: Yeah. And there’s one of the things that we harp is, is we need to be robust with — with our design, because we understand that whatever we plan, there’s always a risk of slippage, like schedule slippage.

Host: Yeah.

Patrick Chai: And — we need to be robust to that. And that’s a challenge, right? Because you’re already designing — designing a system that is, you know, at the hairy edge of the performance, and you’re trying to get everything out of the system, you — you already can, and if you try to build in robust into that, you know, it’s — it’s a challenge, because it ends up being really big and bigger than it needs to be for — you know, it’s always the, you know, in every system analysis, and even aerospace — actually not just aerospace, but engineering in general, there’s this debate of robustness versus optimization, right. You know, optimizing, you know, onto the very, very, you know, you know — being able to optimize to something versus being robust to changes, and then that — battle is always been an ongoing, so.

Host: Yeah, because optimization is equal to efficiency, but robustness is equal to reliability.

Patrick Chai: Right. Yeah, exactly. So, you know — and there’s a balance to that. You can’t, you know, go full bore on one way or the other. Right. And so, yeah, you know, you could theoretically design a vehicle to fly every single mission opportunity. But that’s not realistic, right? Because you wouldn’t end up — you will be so unoptimized for every single opportunity that yeah, that you end up with a system that doesn’t necessarily closes from other programmatic perspective.

Host: What a challenge. Oh my gosh.

Patrick Chai: Yeah.

Host: Now, what about launch opportunities, you know, launch slips happen all the time.

Patrick Chai: Right.

Host: It’s just a regular part of spaceflight.

Patrick Chai: Yeah. So — so that — that is the other challenge that we talked to, you know, we talked a little bit about the additional time that you need to account for, right. So, you know, so if you know your — your launch — your Earth departure date, right, is certain, you know, to maximize your mission opportunity, you’re going to plan for the launch slippage, right? So instead of saying, OK, the crew is launching three days before that Earth departure date, you’re going to have to launch — you’re going to have to plan to launch your crew a month, two months before that, that way if this slips you can have that — and we have a — we have some folks in the agency who’s done some fantastic work on — launch reliability, and they’ve done, you know, some great analysis to show, you know, in order to get to a 99% probability of actually getting you to this particular date you have to backtrack and figure out how much — how many days you need to plan to — to launch. The launch window, basically.

Host: Yeah.

Patrick Chai: So, it’s — it’s a little different than like saying going to the ISS or even back then the Apollo program where, you know, a little slippage, you end up — you can — you know, the lunar cycle is only 28 days, right? And so, if you miss that magical opportunity, it’s — you wait a month, and it’s —

Host: You wait a month?

Patrick Chai: It’s not you wait 28 months, or 26 months, right? So, it’s, yeah, you have to be robust in that. But there’s a limit to that, right? And because you don’t want to say, OK, we’re launching our crew six months in advance, and they sit around in orbit waiting for their opportunity to go.

Host: Right.

Patrick Chai: Right. And that’s — that’s unrealistic. So, it’s — it’s, again, back to optimize versus robust, this is the risk versus, you know, how risky you want to — want to be right. Is your — is a 99th percentile probability good enough? Is 90th? Is 70th? I don’t know, right? That’s something that, you know, it’s — a challenge for both engineers and the program managers to understand the risk and understand where we can absorb the risk and where we can’t.

Host: Yeah. So, what you’re doing is you’re — you’re thinking about this — this wide spectrum of possibilities. You’re thinking about how early do we have to launch? And how — how long is it reasonable for the crew to hang out in low-Earth orbit until they take that Mars injection burn and go to Mars?

Patrick Chai: Right.

Host: What’s reasonable there? What are the propulsion technologies that we can consider and what factors into that kind of design? You’re just thinking about this whole thing.

Patrick Chai: Yeah, and it’s a — it’s an integrated system and campaigns look, right. And that’s — that’s more so than just doing like the individual design for systems. And the other, you know, throw you another wrench [laughter]. You know, the longer you wait in orbit, the more your systems degrade, you — you might have propellant which are cryogenically — or cryogenic temperatures so they might be boiling off. So you have to top it off, and there’s a whole host of discussions and — on that and that’s why there’s, you know, there’s some discussion about different orbits where you do your — do your aggregation, different orbits where you do your rendezvous and things like that so that it’s — it’s not — LEO, the lower — LEO lower-Earth orbit is great because you can throw a lot of stuff into it from a launch vehicle perspective because it’s lower. But it’s a terrible environment for a spacecraft to hang out long term, especially big spacecraft. The thermal environment is pretty harsh from all of the — the radiation from coming — actually reflecting off the Earth from a thermal perspective is not — not great, but it’s also a lot of orbital debris because all the stuff that’s up there, so there’s a higher chance of that. The benefit is that you’re still in the radiation — in the radiation belt so you’re shielded from that. So there’s — there’s a trade, you know, if you want to go higher orbit, which is sometimes our preference because you can get out of the thermal environment, you can see out of the — the overall debris environment, but — but you take the hit on the radiation for some of your system. But the benefit is that since we’re designing a system to go to Mars, all of that system has to be radiation rated and protected anyway, so it’s not a huge issue with the exception of solar panels, which do degrade in radiation environment. So, you don’t want that to sit in the radiation environment for too long or else it degrades so that you can’t generate enough power.

Host: So, where are some of those higher orbits that we’re considering?

Patrick Chai: So, we’ve been talking a lot about orbits around the Moon — yeah, orbits around the Moon. The Honors Program is you know, with the — with the Gateway had — is out there in a — in a near rectilinear Halo orbit around the Moon. We have these Lagrangian points that a lot of folks are very interested in understanding and — and what — what those points really are is you think about from gravity, you know, pulling each other at the Earth and the Moon are in a gravitational pull system. So, the Lagrangian points are just points in the Earth/Moon system that are — that the gravity between the Earth and the Moon is relatively equal. So, you’re not really being pulled by the Earth, you’re — you’re being pulled by Earth and the Moon roughly equally. And so those are — those are points of interest because it’s a — you know, once you get into those points, you know, it’s easy to kind of get out of them. And so, you know, if you have a bunch of stuff stacked there, it doesn’t cost a lot of energy for it to go — to leave that orbit again to go somewhere else. And so it’s great for our aggregating a lot of — a lot of things and so, for, you know, what we’ve been looking at last couple years, with the assumption is always that we would launch all of our elements and components into that, you know, kind of like, we call it — we just call it cislunar orbit, lunar orbit, just to be generic because we haven’t really picked a particular orbit that we want to be in. It could be the same one that the Gateway is at that way we can have some synergy with that program.

Host: Right.

Patrick Chai: But we aggregate all that stuff there. But before Earth departure, you don’t really want to depart from that particular orbit. Because you do —

Host: Lagrangian linear —

Patrick Chai: Yeah.

Host: — Orbit? OK.

Patrick Chai: You don’t want to — you could depart from those, you know, those Lagrangian orbits but you have — you get a much more benefit from the Earth’s gravity if you’re leaving from a, you know, Earth orbiting gravity or Earth orbiting orbit. So, basically, what you want to do is you want to come back and kind of swing by the Earth on your way out, that would be much more efficient to be able to do that. So, the assumption is always we come back to a high-Earth orbit. We call lunar distance high-Earth orbit. So, it’s a big, big orbit that comes by the Earth and it comes — when it goes out — so orbits are always defined by perigee and apogee, a perigee that is closest to the main body and apogee is the distance furthest away. So, your perigee is close to the Earth, but your apogee is actually at lunar distance. So, a lot of our trajectory we — we’ve actually been doing a lot of analysis and building a lot of these very intricate trajectory for — for how we do Earth departure. To try to minimize the energy we need. Right? Typically, you know, when you do your — what we call the Trans-Mars injection burn so burning for Mars, you come by Earth and you fire on your — your rocket engine and you burn really hard and you — you kick out there, which is great and it’s fast, but it takes a lot of energy, takes a lot of propellant. So, we’re — you know, when designing our hybrid low and high thrust combination type of propulsion system, we’re trying to minimize how much of that chemical burn that we have to do. So, we would design up these pretty intricate trajectories in which we come back into this higher Earth orbit, where the apogee is at lunar distance, and we phase it so that after the crew gets onboard the spacecraft, we target the Moon, we do a lunar flyby and — and that kicks us out to heliocentric space.

Host: Oh interesting.

Patrick Chai: And then that gives us enough energy to go out. And that — this only really works for the — hybrid because technically once you get flung out by the Moon, you don’t have enough energy to get to Mars yet. But with the low thrust you can then use the time you have in space to thrust and to kind of keep pushing and pushing harder, and harder, harder for you to catch up to Mars.

Host: OK, so this Moon slingshot thing works only if you have the high — the solar electric propulsion.

Patrick Chai: Yeah, electric propulsion.

Host: Give you the extra boost.

Patrick Chai: Yeah. So, and because — yeah, or else, you know, this orbit, if you just slingshot around the Moon you don’t have any more propulsion, you eventually fall back into Earth in little like — like maybe like a year later you catch back on Earth again. But so, you — you go on a little tour of the — of the Earth orbits area but you don’t — yeah, you don’t — you’re not really going anywhere particularly — in particular, so.

Host: Yeah, so — so that brings up a good point. So, let’s say for whatever reason you do that Mars or that Moon slingshot, but your solar electric propulsion doesn’t kick in. Now, you got to swing back. You’re hanging out in space for a year.

Patrick Chai: Yeah. Yeah, that’s always the challenge.

Host: Right.

Patrick Chai: Well, you know, there is this quote unquote, “abort.” I think people, you know, like, you know, not their fault. Like, when people say, abort, you know, everybody have a good understanding what that is, right? Because we have all these abort scenarios, like, you know, oh, you’re coming back home pretty quickly, and things like that. But yeah, man, when you’re in heliocentric space it’s — it’s not that simple. It’s — and it’s — it takes a lot of energy to come — to turn around and burn home. For your — kind of like a low thrust, you just do a lunar gravity assist. Let’s say your scenario, your electric propulsion system just craps out and doesn’t do anything. The benefit is that you still have your chemical propulsion system. So, you could theoretically — and you’re not — you’re — in terms of your overall energy, you’re not that far from Earth. We haven’t really done the analysis to understand how quickly you can get back. That’s really, really dependent on the orbit of the particular date you’re looking at, and things like that. But we do have some analysis done on the high thrust side of things, you know, after you do your big burn can you — you know, I guess in those scenarios is like, oh, if a crew has some issues and need to come home, how — what are the options, right? And, honestly, it’s pretty limited, even with like, you know, this is one of the benefits of — people have touted for the nuclear thermal propulsion system which has is — it’s kind of like a — it’s kind of like the chemical but instead of combustion in the — using the propellant, you’re using a nuclear reactor to, you know, heat up the fuel and expand it out. So, it has a higher specific impulse. But you have to carry a nuclear reactor around which has its own challenges. But they talk about these abort scenarios because — because it has the efficiency that — that the chemical system doesn’t have, but that’s still, you know, once you kick out and especially those high thrust trajectory, you’re much, much higher energy. So, in order for you to turn around and burn back this — burn back towards Earth, it’s not a straight shot, you know, it’s not like me, you know, doing a U turn and coming home. There’s — there’s no — U turn really to be done. You almost have to burn enough so that you fly by — you do a really close swing by of the sun and come back and catch back up on Earth — with Earth again, and that — that could take you know, a year or so, or six months. Depending on when that burn happens, so the challenge we’ve always had when — when these type of questions come up is, in what scenario do you have a crew, I guess, crew health situation where, you know, the full, you know, Mars mission three years round trip is not acceptable, but a six months, one year return is acceptable, right? What — what scenario are you thinking about? Because, you know, I always say, I don’t know what the scenario is, right.

Host: Yeah.

Patrick Chai: And so that’s — that’s the I think that — and it’s important to have that discussion. Because we need to understand where these questions are coming from, and the people’s concerns about — about all this and understanding, like the scenarios in which they want to protect for and this is, again, back to the risk thing, right? If we want to protect for certain scenarios, we can run the analysis to show what it would cost, right and then it’s up to the decision maker to, you know, weigh the cost benefit analysis. Say do we want to protect for this one or not? What are the probability of this actually happening? And — we from a system analysis perspective need to show, you know, OK, if you want to protect for this risk here are the implications and how that — you know, how that system change impacts the rest of the other systems that are related, right.

Host: Yeah. This goes back to what you’re thinking about, which is this wide range of possibilities. Thinking about propulsion technology, when you’re thinking about possible trajectories or in the orbital mechanics of it all, how to design a mission profile, however long it may be. That’s a, you know, they’re all worthy discussions to have. But I think we’re — really what it comes down to is a mission to Mars is a risk, you know, like —

Patrick Chai: Yeah, absolutely.

Host: You can’t make it risk free.

Patrick Chai: Yeah, absolutely.

Host: That would be amazing, but it’s just not the way it is.

Patrick Chai: Yeah. It’s like going skydiving, right.

Host: Yeah.

Patrick Chai: There’s some inherent risk in that and there’s nothing you can do to remove all the risk, right? And so, we can sit here and talk about, you know — and engineering design system to be as risk free as possible, but it will not be zero percent risk, right? I mean, inherently, I mean, just getting on the rocket and going into space is probably the riskiest part of you know — could be one of the riskiest part of the whole endeavor. Right? And so, at some point, we are — we will — we have to be willing to take risk, right. And I think one of the challenges we have as an agency, especially when it comes to the Mars program, is that number one, it’s — it’s so challenging, right, we’ve kind of established how difficult it is. So, we need to have these hard discussions about risk, and cost, and schedule, and programmatics, and things like that. But — but it’s — it’s also double challenging because it’s so far away always it seems like.

Host: Yeah.

Patrick Chai: Right? It’s hard to make decisions now, the hard decisions that might not have impact till 10, 15 years down the road.

Host: Let’s talk about the Moon to Mars. What we’re — what we’re striving for right now.

Patrick Chai: Yeah.

Host: You already mentioned a little bit about the Artemis program and what we can learn about the near rectilinear Halo orbit and some of the technologies needed for Gateway. What will that help inform when it comes to some of these propulsion technologies and some of the ways that we’re thinking about how the Moon can help us get to Mars?

Patrick Chai: Yeah, from the propulsions perspective, I mean, that’s one of the benefit, right, what — you know, for the — you know, what we — what we’ve been looking at the — the sub cam hybrid is that the electric propulsion thrusters that we are, you know, that we’re planning on potentially using for Mars transit is — being planned for the Gateway as well. Right. And so, there are some synergy there. And there’s some, you know, obviously, there’s — will be some challenges with the development cycle because the power level is totally different, right? But there’s, you know, you kind of draw a little family tree back up to that — that particular design. I think the big thing for the Artemis program is kind of how the private — private public partnership is kind of, you know, unfolding in front of our eyes. I think getting kind of the framework for this — for this private public partnership as part of the Artemis program, understanding how we can leverage a lot of things that are happening in the — in the industry — in the private sector, can really inform and really drive how we think about designing the mission and think about how we either procure or leverage all the things that are happening and, you know, obviously the private industry have quite different objectives compared to what the government and NASA wants to do. We want to explore, we want to, you know, expand our knowledge and — really drive and push the technological boundaries. And so that part is — has to be the forefront of what we — try to plan for. And I think we are. But I think the industry in terms of their push for innovation in areas that the government might not be interested in is also very beneficial because they are very interested in streamlining the manufacturing process, being more efficient with their design cycle and — how robust their systems are in the way they do their testing.

Host: So, a lot we can learn from the commercial way of operating things —

Patrick Chai: Yeah.

Host: Implementing commercial partnership.

Patrick Chai: Yeah, you know, just under — having synergy with a lot of things they’re doing and leveraging some of the stuff they’re — leverage — having just — just dialogue and just understanding how they’re doing their, you know, business model, if you want to call it and how we can learn from that, right, and change the way we’re doing testing and doing developments and things like that. You know, in all industry and in tech in particular, you know, you need to have these kind of breakthrough and these industry kind of breaking, you know, moments, right, and to kind of drive us out of these kinds of complacency that we’re in. And, you know, at some point, we got to decide, yeah, we got — we want to go to Mars, and we want to be able to do certain things, and we just have to, you know, push for it.

Host: Yeah. So, it’s that there is a balance I feel between innovation, you know, trying new things and doing things a certain way and maybe the tradition does have a certain benefit of precedence.

Patrick Chai: Yeah.

Host: It informs what is possible because —

Patrick Chai: As long as we learn from our history, and we, you know, we learn from the mistakes we’ve — made, right. You know, I struggle with that every day, you know, make sure that I think this way, is it because I’m being informed by data and — and I’m being informed by — by good sound analysis, or am I, you know, leaning this way because of my, you know, inherent bias or inherent perception or preconceived notion of what the system is. Right?

Host: Yeah.

Patrick Chai: And that struggle is — is, you know, is — real at all levels. And that’s something that we have to be kind of very cognizant of.

Host: Yeah. So when you’re thinking about solving these problems with team members, finding a way to think efficiently and making sure that you’re considering all of these different things, but not getting too bogged down by these decisions, that — that’s a balance just in and of itself, not just — not just designing these trajectories and thinking about these systems, but coming to a consensus on how to proceed.

Patrick Chai: Yeah, absolutely. And, you know, it’s great that we, you know, our branch in particular have had, you know, every, you know, year or so, we get one or two new hires, and we have a bunch of interns that come in. So, it’s always great when they come in with fresh perspective. They have not been in the mud, you know, making these trajectories or analysis and things like that. So, they come in and say, “wait, why did you do it that way?” And we go, “yeah, why did we do it that way?” And we need that, right? Because, you know, you don’t want to end up in a — in a kind of an environment in which new people coming in, you know, are conforming, you know, you — they’re expected to conform to whatever standard you have. You want them to come in with a fresh mind, with critical view and to provide us with a different perspective. And that’s, I think, the most valuable thing we have, especially here at NASA because of, you know, our notoriety and people want to come to work at NASA and it’s a great thing. And we need to keep hiring, we need to keep these people coming in and young minds and — and really get them engaged, not just in school, but you know, before. So, I think a lot of the public outreach things that we do is extremely important. I think NASA is still one of the most sought-after thing. People say, oh, you work in NASA? That’s amazing. And — and it is. You know, I’m thrilled to go to work every day. And — and we need that kind of enthusiasm, you know, to continue for — and people coming in are, you know, they’re brilliant. And you need to be — make sure that we are doing the right thing and doing — doing things that, you know, to make them feel like they’re empowered to change the status quo.

Host: Yeah. So thinking about what we’ve discussed today, thinking about the difficulties of rendezvousing with Mars, and with the different technologies that we’re considering, reaching out to those people who want to come work at NASA and — and fight to innovate and fight to change the way that things are doing, what are — what is the message that you want to send to them?

Patrick Chai: It’s — you know, it is a very exciting time, right, because — because we have this whole Moon/Mars enterprise now, the Honors Program is spinning up and we are going to, you know, go to the surface of the Moon to — to do some very exciting science and with the goal on the horizon of always going to Mars. So, these systems that we are developing and partnering with, you know, industry partners to develop are going to set — lay the foundation for what we do at Mars, right. And so, it’s very important for us to not only do the mission to the Moon, but also be very, very cognizant of that these are laying the stepping stones. So, we need to learn as much as we can, as we do these things, to inform all the things that we’re going to be planning on doing when we, you know, build a spacecraft to go Mars, right. And so, I think it’s — it’s a very exciting time for NASA.

Host: Well, it’s an exciting time. And the future is — is exciting, because I think every year is just going to be — is another step closer.

Patrick Chai: Yeah. And — and, you know, like I said, I think throughout this podcast is it’s a challenging problem and it is probably one of the — one of humanity’s greatest challenge, you know, and I think if there’s a will we, you know, I don’t doubt that we can achieve it. But it is a challenge and — and we have some of the brightest people on the planet trying to solve it. And, you know, it’s a great thing, and it’s — it’s something that, you know, I think it’s going to be so rewarding for all of humanity. It’s one of those, you know, you think about what people talk about, you know, well what does NASA do besides sending people to space? We do these innovations that we have to develop so that we can send people to Mars. They will have a long-lasting effect on our society. And we might not know what that is till years later, but it will. And that’s kind of the challenge from a “PR perspective”, right? You know what, you know, costing X amount of dollars to send people to Mars. So, what does that, you know, we could spend that money here on Earth, but it’s like, you don’t know what you’re going to get out of it. But we know that it’s going to be impactful because we have — we have a track record of — of all the great things that have spun off from NASA technologies that we’ve, you know, came out of the lunar program and the space station program. Right? And the Mars program will be an exponential in terms of how challenging it is, right? We’re only two and a half miles off the coast of Spain right now. We’re going all the way to America, right? And so, they will — whatever we come up with in terms of solutions and technology advancement will be, I think societal changing.

Host: Yeah. Well, I’m glad you’re on the team thinking about this [laughter]. And looking forward to bringing on others to continue it. It’s going – it’s been great work. And this has been a fascinating conversation. Patrick, thanks so much for coming on.

Patrick Chai: Thank you very much for having me. Yeah, it’s been great.

Host: Awesome.

[ Music]

Host:Hey, thanks for sticking around. Really fascinating conversation we had with rocket scientist extraordinaire, Patrick Chai. I hope you’ve been enjoying some of these Mars Monthly episodes. We started with Michelle Rucker, and just the general concept and really the outline of a mission to Mars. Then we explored some “Concepts Near Science Fiction” with Jason Derleth. Patrick Chai takes us into the third month. I hope you’re sticking around, we got a lot more episodes coming your way. Again, we’re going to do it on the first Fridays of every month. So, stay tuned for some of the upcoming episodes. If you liked this episode, or many of the other Mars Monthly episodes of Houston We Have a Podcast, you can find them all at, you can also check out some of our other episodes, you really don’t have to listen to them in any particular order. That’s all at as much — as well as the many other podcasts that we have across the whole agency. If you want to talk to us at Houston We Have A Podcast, you can find us at the NASA Johnson Space Center pages of Facebook, Twitter and Instagram. Just use the hashtag #AskNASA on your favorite platform to submit your idea for the show. And make sure to mention us at Houston We Have a Podcast. This episode was recorded on February 4th, 2020. Thanks to Alex Perryman, Pat Ryan, Norah Moran, Belinda Pulido, Jennifer Hernandez, and Michelle Rucker. Thanks again to Patrick Chai for taking the time to come on the show. Give us a rating and some feedback on whatever platform you’re listening to us on and tell us how we did. We’ll be back next week.