From Earth orbit to the Moon and Mars, explore the world of human spaceflight with NASA each week on the official podcast of the Johnson Space Center in Houston, Texas. Listen to in-depth conversations with the astronauts, scientists and engineers who make it possible.

On Episode 235, Les Johnson and Julie Castillo-Rogez describe an experiment that will be deployed from the Artemis I mission to explore a near-Earth asteroid. This episode was recorded on February 16th, 2022.

Transcript

Gary Jordan (Host): Houston, we have a podcast! Welcome to the official podcast of the NASA Johnson Space Center, Episode 235, “Scouting an Asteroid.” I’m Gary Jordan and I’ll be your host today. On this podcast, we bring in the experts, scientists, engineers, and astronauts all to let you know what’s going on in the world of human spaceflight – and more. In our last episode we talked about NASA’s mega-rocket, the Space Launch System or SLS slated to launch soon on the first Artemis mission, Artemis I. The primary objective of the first Artemis mission, Artemis I, is testing the Orion spacecraft and the SLS on a mission around the Moon and returning safely to Earth for the first time. But this highly-anticipated launch is also jam-packed with secondary objectives and payloads — payloads being the stuff we’re bringing on the rocket. A few episodes back, we talked about BioSentinel, a microbiology experiment in deep space; there are many other secondary payloads like this aboard Artemis I. So we’re going to highlight another one. This one has its sights set on an asteroid. The experiment is called Near-Earth Asteroid Scout, or NEA Scout. Like BioSentinel, it’s another one of those shoebox-sized spacecrafts called a CubeSat that will be deployed from the SLS, but this one will unfold a massive 925 square foot solar sail as a means to navigate to a near-Earth asteroid and gather data that can’t be collected by observations from the Earth. How the solar sail works and what kinds of science we’re going to do, that’s what we find out on this episode. We’re bringing in the experts on the solar sail and on the asteroid science. For the NEA Scout sail, we have Principal Investigator Les Johnson, and for the NEA Scout science, Principal Investigator Julie Castillo-Rogez. Come sail away with me, to an asteroid with Les Johnson and Julie Castillo-Rogez. Enjoy!

[Music]

Host: Les and Julie, thank you so much for coming on Houston We Have a Podcast. What an exciting experiment, lots of cool stuff happening on Artemis I. This one just caught my eye because, you know, visually, if I’m thinking about as things are released, or deployed from the SLS, this is one that I think visually jumps out, because it’s got a giant sail and of course it’s going to a very cool place. I want to talk about this experiment, go deep into what this is all about and how it works. Lucky to have both of you on this, because we’re going to tackle that cool sail, and we’re also going to tackle the purpose of the experiment itself, the science, where we’re going and what we’re doing. Les, I want to start with you, you’re the lead principal investigator for, for the sails, which is a very cool technology; quick background and just what, what career path you took to get you to this position as the principal investigator for such a cool experiment?

Les Johnson: Well, quick background is I am a physicist by education, I’ve been at NASA for about 30 years, and I have always been a futurist and in my career I’ve focused on advanced propulsion, new ways to move spacecraft around in space. I became interested in solar sails, which is a way to fly once you get out of a planetary environment and you’re in deep space, away from the air and all the disturbances we have on the ground; it’s a very efficient way to get from place to place, and to go to distant destinations we need more efficient spacecraft propulsion. So this was, one that was a particular interest to me. And, and how I got here is a long story: it’s 20 years of ground-based technology development, writing mission proposals, and working with some really smart people to move the technology forward.

Host: Very interesting. I am very excited to get into this particular technology. It’s good to know what, what got you there. And then of course, we’re going to explore what that is. Julie, over to you: quick background, I know you, you, have an interest in planetary science and you’ve done a number of different things in that world. What led you to this particular experiment, NEA Scout?

Julie Castillo-Rogez: Yeah. So as you can get from my accent, I did all my studies in France and the laboratory I was involved in had a stake in the Cassini-Huygens mission. So that was in the mid-90s. And so I was very lucky one day to meet a principal investigator on the Cassini mission, and she invited me to come to JPL (Jet Propulsion Laboratory) and to do post-doctoral research at JPL. And so I came about twenty years ago, next month, and it’s been a fantastic experience working at JPL. So I’ve been on the Cassini mission and several other missions and especially one that played a big role in my career is the Dawn mission, that explored the dwarf planet Ceres in the asteroid belt, and I was the project scientist for that mission, during its extended, its extended period. So Ceres is the biggest object in the asteroid belt; NEA Scout is going to target the smallest asteroid ever visited by spacecraft, and so that is very exciting to me, to, to have been able to explore both the largest and the smallest asteroid in the solar system. And, and so I got involved in NEA Scout in 2013, I was part of a group at JPL pioneering deep space CubeSats. And we had already been working on a mission called INSPIRE (Interplanetary NanoSpacecraft Pathfinder In Relevant Environment), which did not fly but basically was the first step into the introduction of a new class of radios that allow us to communicate with CubeSats. And so we proposed to an opportunity from the Human Exploration and Operations Directorate and, and we got selected and, and then it was great because Les proposed a similar idea and, you know, I think the sponsor offered, oh, wow, you have these two groups of people, they really want to do the same thing so it must be very important. And that’s how we got started.

Host: Very interesting, Julie; largest asteroid, smallest asteroid, that is, that is awesome. A very interesting, very interesting world to explore, this world of asteroids, and that is what this is doing. I hope I’m saying it right. Les, is it, is it N-E-A Scout? I was saying NEA Scout, but I think it’s N-E-A Scout?

Les Johnson: Well, I think it’s, it’s a matter of choice, Near-Earth Asteroid Scout, NEA Scout, yeah, all of those will work.

Host: So what is it then? What is, what, taking what this CubeSat is and what it’s going do, what, what is this mission that we’re doing?

Les Johnson: Well, let, just, just to maybe start that out, first off let’s get a matter of size. I, I think it’s really important for people to understand the size of things that we’re dealing with here. This spacecraft is really small. It’s going on the biggest rocket NASA has launched in decades, but the whole spacecraft is only what’s called a 6U CubeSat. And to put that in perspective, it’s about the size of a boot box. OK, so it’s a pretty small spacecraft, and it’s, it’s a fully functional interplanetary spacecraft. There are a lot of these CubeSats that fly in Earth orbit and they do great things, but they’re close to home, they don’t have to have a lot of power, they don’t have to have particularly radiation- hard electronics to survive the deep space environment, and they certainly don’t deploy big solar sails that are almost a thousand square feet in area, which is what our sail will look like. So it was quite a, quite a challenge that the JPL team, who built the spacecraft, worked toward, and for us here at NASA Marshall [Space Flight Center] to put the, the solar sail in. So NEA Scout will be a secondary payload on the Space Launch System, after the Orion, which is the human crew capsule, the primary mission for the launch of Artemis I is on its way to the Moon. We’re one of the 13 small spacecraft that’ll then be deployed from the SLS rocket’s upper stage. And our little, tiny spacecraft will be ejected from a little box in there. The spacecraft will stabilize itself; it’ll deploy its solar panels. I’ll test the radio, call home and say, hello, I’m here, let’s make sure everything’s working. It’ll be on a path toward the Moon. And the spacecraft will have a, a small thruster, conventional little chemical thruster called a cold gas thruster, that’ll put it on a trajectory so that it doesn’t hit the Moon and goes around it. And after we’ve been in the lunar vicinity for a while is when we’ll deploy the solar sail, which will give us the primary propulsion to take the spacecraft and the camera that Julie has on board the spacecraft to the asteroid, which will take about two years, give or take, depends on when we actually launch, in order to do the science at that asteroid. And we will be controlling the solar sail, the way that it points, which determines our thrust and how we travel the whole time during that mission. And then when we get near the asteroid, Julie will take over and, as we approach, she’ll be using the onboard instrument to do the science at the asteroid.

Host: Awesome; perfect mission profile. Obviously, I, I, and Les, I want to get into the objectives of the solar sail itself, but before, before we do that let’s go over to Julie for a second. Julie, Les mentioned science, do the science. High level, what is the science that we’re doing?

Julie Castillo-Rogez: So, we are going to obtain observations that help prepare for a future crewed mission. So the science is really about understanding the physical properties of near-Earth asteroid. And these physical properties include getting the size and the morphology on regional and global scale. The rotation properties, so the spin period, the position of the pole, that is very important when planning for a future crewed mission to understand how this kind of object is behaving from a dynamical standpoint, in order to prepare for operation, especially because with smaller asteroids they can have a very fast spin rate, and so that could be dangerous for operating at the surface of these objects. And then, at closest approach, we are going to obtain high resolution imaging of the order of ten centimeter per pixel, and that will tell us about the structure of the surface, what we, we call the regolith, it will tell us if the surface is very rocky, or it is very porous. Again, this is critical information in order to prepare for a future crewed mission.

Host: All right. So that’s the science that we’re doing, Julie. And I think what, what I’d like to get into now is just the how, and get into the details both from, from the sail’s perspective, and then Julie we’ll go to you as well from the science perspective and talk about how we’re characterizing and what instruments we’re using and what, and what, what that data science collection period is going to look like. top, top to bottom. Les, we’ll start with you, the sails, right? You mentioned that the sails are going to be deployed around the Moon. Let’s get into what this is. You, you, you already gave us a little bit of a preview; you’re talking about a CubeSat that’s the size of a boot box, that’s going to deploy an, a nearly 1000 square foot sail. That is huge. Let’s talk about what that is. What are the materials used, how that’s going to work; what is that technology, Les, that is behind these sails?

Les Johnson: Well, here is solar sail technology 101, right? Your listeners are familiar with sunlight or any light, and when it reflects from you and someone sees you, the, the particles of light, the photons, are reflecting from you and going to someone’s eye. What, what you don’t feel when light reflects from you — it’s a beautiful sunny day here in Alabama; if I were to go outside and, and bask in the Sun, a lot of light’s reflecting from me, but it’s also pushing on me. Light, although it doesn’t have any rest mass, it does have a little bit of momentum. And so, if you think of a particle of light, a photon, as being like a little BB and it bounces off of you, as it bounces off of you it’s, it’s giving a little bit of its momentum to you and pushing on you. Now that push is very, very small. If you were to take all of the sunlight at noon falling on two football stadiums, it would be roughly the same force as you would feel in the palm of your hand if you put a quarter and a nickel in your hand; you’d feel it was there, but it isn’t much. And that little bit of force when you’re away from the gravity and the air and all these things, it will inevitably start pushing on whatever it’s reflecting from. And so if you have a, a large, very lightweight reflector, reflector — think of a sail on a sailing ship, but much lighter weight — reflecting visible light, so it’s coated with aluminum — and the material we use for our sail is a plastic, it’s, it’s got a, you know, exciting name: Clear Polyimide 1, but it’s coated with aluminum to make it reflective, to maximize the number of photons reflecting from it — that little push is constant because the Sun’s always shining. And when you deploy it in space, your spacecraft will start to respond to that, and it will move. And it’ll move faster and faster and faster and keep accelerating as long as there is sunlight falling on it. And that’s important because when you’re traveling to these deep space destinations you, you have to get a lot of velocity and you have a long trip time because you have to go millions of miles to get there. And you might as well do it as efficiently as you can for, for mass. And so if you take the middle third of our boot box, we, we figured out you could put a sail of 925 square feet, almost a thousand square feet, folded and rolled on a spool that would be deployed out with four metallic booms that a, a motor starts to, to push out of the spacecraft. And as the booms come out, the sail is attached to each one of the four booms, and it slowly comes off the spool until you have this flat sheet of sail lying on these booms, attached to the spacecraft that’s out in space. Once the sail is deployed, we orient it to the direction we want the sunlight to fall from it, and the spacecraft will slowly start to accelerate beyond whatever speed the rocket gave it. And we tip and tilt the sail to change the angle with which the light reflects from the sail, because that changes the direction of the momentum that the light gives the spacecraft to push it, so it allows us to steer. So it, it is completely analogous to, to how, the, before we had steam engines and all that, that the big schooners and the sailing ships would cross the oceans, they were subject to the wind and the particles of air reflecting from the sail. But instead of that, and it’s not solar wind, that’s a different thing; we’re reflecting sunlight to make our sail move. And it, if you, if you look at a comparison with our spacecraft, if you had taken a conventional rocket propulsion engine in the same volume, put as much prop, propellant, in there and fuel as you could and said, go, you wouldn’t get nearly the performance over a two-year mission that we get with a sail to get us where we’re going. We really have to have a sail to do this mission. So that’s kind of solar sailing 101.

Host: So what I, a couple things I pulled from that, that I found particularly interesting. One was it sounds like, and correct me if I’m wrong, it sounds like you have a little bit of direction but it sounds like the acceleration is small, but constant; is that a good readback? Is, is that what’s happening?

Les Johnson: Absolutely. And that’s why it’s, that’s why it’s so efficient. I, I like to do the tortoise and hare comparison. We’re all familiar with that child’s story, right? And if you were to take the so, the, a solar sail that we have on NEA Scout, and you deploy it and next to it, you have a small spacecraft that’s the same size but instead of the volume in the spacecraft where we have the sail you have that conventional propulsion system, and you’re having a race and you say go, the, the little rocket engine will fire and the small CubeSat will take off and it’ll be out of sight in a few seconds or minutes. And you’ll wonder if the solar sail is ever going to move; it’ll just sit there. But over days it will have started moving and it’ll start moving faster and faster, and because you can only put so much propellant in this other spacecraft, it won’t be too long before, since it has, it runs out of gas and so it can’t accelerate anymore, it, it keeps going at whatever speed it was going when it ran out of fuel; our solar sail will pass it and keep accelerating because of that constant, low thrust. So it, it is a much more efficient in the small spacecraft case — you have to have something pretty lightweight for this to work — much more efficient than just about any chemical system you could put in the same volume.

Host: So what’s, what are some of the pros and what are some of the cons with having a low acceleration, but it sounds like there’s no, there’s no way to stop accelerating if, if, if maybe that’s a good readback as well. Is that, is that, is there any issues to that or is that maybe a good thing?

Les Johnson: Well, it, it, the, or the, the way orbits work, help you and hurt you in this case. Yes, you’re correct, globally, in that you can’t turn off the Sun, so as long as you’re close to the Sun the sail will continue to give you some kind of, of acceleration. And that’s one of the reasons this is an asteroid flyby and not an asteroid rendezvous, and that we aren’t going to be able just to match velocity and, and stay right next to the asteroid. We’re going to slowly fly by the asteroid for Julie’s camera to do its work. And, but that’s not quite the same as not being able to slow down, because you can tilt the sail to give you acceleration in the direction you’re already moving, which makes you go faster, or you can tilt it opposite that to give you some acceleration in the opposite direction, which is decelerating, and you actually slow down. So you, you can use the sail to accelerate or decelerate during flight. You just can’t stop accelerating or decelerating, [laughter] If that makes sense.

Host: It makes a lot of sense. I didn’t think about turning it the other way, but, but absolutely it, it would just be slow, right, so, so there’s, I guess there’s limits to, to how you, how you work that, but the, but the possibility exists. And, and, and the idea that you’re doing a flyby not a rendezvous into an orbit makes a lot of sense for, for what this technology is used for and what its purpose is. The other question I was going to ask…oh, sorry, Les, go ahead.

Les Johnson: Yeah. You asked what the limitations were. I didn’t mean to interrupt you. The, the limitations of a sail are that it’s really primarily used for a small lightweight spacecraft. And the reason for that is that low acceleration. In Newton’s laws, force equals mass times acceleration, and the sunlight force is constant so in order to have a large acceleration that’s usable, you have to have a very low mass spacecraft. So a solar sail works really well for a spacecraft that might weigh, you know, 10 to, to 20 pounds, maybe up to a hundred pounds with a much larger sail. But if you’re talking about something for human-class mission, the sail would have to have, you know, immense area and be much lighter weight than we can ever build today — well, I won’t say ever; anything that we could build today to be useful. So one of the limitations are, this is really only useful with small spacecraft today, and those near the Sun. It’s not a one size fits all propulsion system.

Host: Understood, understood. Real, real quick on the, on the, materials, you mentioned, you mentioned it was like a, a sort of a thin plastic, it was a very interesting material, but, just to, to pull back and think high level about what, what the, what the idea is here, it sounds like one of the important things is maybe low on the mass side, you want, you want a lightweight material, but you also want something that, and, and I don’t know if I’m reading this back right, either, but what exactly about, what properties of that material allow the sunlight to do its job and provide that acceleration — something lightweight, and then, and then what other properties?

Les Johnson: Reflective, reflective of sunlight.

Host:Reflective. Got it.

Les Johnson: Reflective. And in fact, the, the substrate material, which gives the sail its strength is the plastic, but it’s clear. And therefore the light would pass through and not, not do much in the way of propulsion. So we have to, to essentially spray-coat a thin layer of aluminum on it, which reflects 90%, 92% of the visible light, to give it its reflective properties. So you want a reflective coating and you want the substrate, what that coating is on, to be lightweight and strong because you don’t want to tear or rip the sail while you’re handling it to fabricate it or packaging it or deploying it. So it has to have a mixture of being really lightweight, which means it might be flimsy and easily damaged, and reflective, but not much mass, and trying to optimize that was the challenge.

Host: Got it. Now the interesting art of folding this thing. This is, this is a big question I have, Les, is how do you get such a large sail to fit in such a small area? And then how is that deployed?

Les Johnson: Well, the, the sail material itself. first off, think, it’s really thin. It, it’s basically thinner than a human hair.

Host: Oh, wow.

Les Johnson: So the way I like to describe it is if you go to your kitchen and you get out Saran Wrap, imagine Saran Wrap that’s thinner than the Saran Wrap in your kitchen and not sticky. And that’s kind of what I, I think it, it feels like. Aluminum foil would be way too heavy, and aluminum foil more easily rips, so I don’t give that as the example. It’s a little harder to tear, you know, the plastic wrap that’s in your kitchen. So just imagine a lightweight plastic wrap with aluminum on it. That’s the material. And for, for this mission, we deploy a single large sail, and there are different approaches to, to building solar sail, but we settled on that for NEA Scout. And the best way to think about it is if you lay a piece of paper on, on, and you, you’re folding it, you, you, you fold it back and forth into a long, thin strip. So you do what’s called a z-fold. You, you take it, you, you fold the paper along its edge to whatever thickness you want, and then you just back and forth fold it until it kind of makes an accordion shape. And then you roll it, like roll it around your finger; we’re rolling it onto a spool. And that’s how it’s packaged for deployment in, in the spacecraft. And then below that is where we have these four metallic booms, which are like tape measures but instead of a curved and kind of a U-shape when they’re deployed, like if you take your tape measure, a metallic tape measure, and deploy it, it stays for, for a distance it’ll stay vertical or extended because it’s kind of a U-shape to it that keeps it extended and not just falling to the ground in gravity; ours, instead of a U it’s got kind of a V-shape to it after it’s off the spool. So just imagine a, tape measures, four tape measure spools, where we have a motor that instead of pulling the tape measure out, we push it out, and as the tip of the tape measure goes out it’s attached with a little spring to the four corners of the sail and it just pulls it off the spool. So as the booms are going out the spool is slowly rotating and the sail is coming off, until the booms are at full length and the sail is a flat plane. I hope that comes across on a podcast. [Laughter]

Host:[Laughter] I know it be, it would be so much better on a, on a, on a visual side, but, but that description was very artful, Les. I think it was good. I, I’m assuming you’re going to be monitoring the deployment and everything and, and that’s really the base of my next question, is on the, the objectives here: you’re the principal investigator for the sails on NEA Scout, what are your main objectives for, for this experiment?

Les Johnson: Well, there have been numerous smaller solar sails deployed in Earth orbit as deployment demonstrations. There was one that was in 2010 that NASA flew, it’s called NanoSail-D; The Planetary Society, a privately-funded group, has flown two that were three times larger than the NanoSail-D. This will be the first sail this large that the U.S. has flown. The Japanese flew one in 2010 called IKAROS, I-K-A-R-O-S (Interplanetary Kite-craft Accelerated by Radiation Of the Sun), and it was a deployment demo, it didn’t go anywhere to do science; very different design, too. But this will be the largest sail that we’ve ever flown, and it’s not just a deployment demonstration, we’re actually going to demonstrate that you can deploy it, you can control the direction of the thrust, and you can use it to perform a real science mission. So the objectives of the solar sail propulsion system are, first and foremost, to demonstrate that we can deploy it and control and navigate with the sail, and the second major requirement is that we get Julie’s camera where it needs to be so she can do her science. So those are the, those are the simplest ways to state our objectives. It’s a technology demonstration, and an enabler for science.

Host: Understood. So, so Julie, from your perspective, lot of pressure that you got to put on Les and his team to make sure that he gets you where you need to go.

Julie Castillo-Rogez: Yeah. [Laughter] So, thank you, Les. Good luck. So, the thing is that we are going to demonstrate a lot of the capability we want to accomplish at the asteroid, the capability we are also going to demonstrate it in cislunar space after deployment. We want to check that the camera is working properly and is meeting performance requirements. And hopefully we will be able to image the Moon, because we are getting close to it multiple times, and that would be such a great reward for this mission to get pictures of the Moon. And so, we are looking forward to that. But yeah, the, the bulk of the science that we want to accomplish takes place about two, two years after launch. And so we need to get to the asteroid three months before the encounter, we will start getting observations of the target in terms of, you know, getting its position; we want to be able to locate it very fast, and very well in order to use a technique called optical navigation to get close to it. And it’s thanks to optical navigation that we can target a very close, low altitude flyby. So we are planning to fly by less than one kilometer in altitude. And our camera is very performant, it’s the smallest camera that is used for science and used on a CubeSat. So it’s very performant, and we hope to get these images with a very good quality and also with high resolution of 10 centimeter per pixel.

Host: So from the ground you can observe this experiment, or not experiment, you can observe this asteroid, right, but it can’t tell you everything. So what are the gaps that this NEA Scout mission is going to, is going to fill, getting nice and close, and, and what, and exactly what kinds of images are you going to take that allow you to fill those gaps of what you can’t observe from the ground?

Julie Castillo-Rogez: Yeah, that’s a great question because we are capable with ground-based observatories to find a lot of asteroids, and in fact about every week there is an announcement of, I mean the discovery of a near-Earth asteroid coming very close to Earth. And so, we can detect these objects, but we can characterize them very well from the ground, unless they are relatively large, you know, a few hundred-meter across, or they are coming very close to Earth. And so, our targets, we have had multiple targets for NEA Scout in, in the course of the, of the year, our targets are very small, they are about ten meters across. And so from the ground we can only pinpoint them in the sky, but we are not able to tell anything about their properties, even the, we can’t, for example, get their rotation properties from the ground, and so for this we need to get very close to the target.

Host: In terms of this particular experiment, when it comes to near-Earth asteroids and what we know about them, is, from your perspective as a planetary scientist, has something like this been done before, have we been able to characterize small near-Earth asteroids in such a way that NEA Scout is doing right now, or is this sort of breaking new ground when it comes to understanding these smaller near-Earth asteroids?

Julie Castillo-Rogez: So there have been multiple missions to near-Earth asteroids, and of course there is ongoing OSIRIS-REx mission, from NASA, and the Japanese space agency is also running the Hayabusa2 mission. And they got, they received very recently their samples from an asteroid called Ryugu. And so, yeah, there have been missions to near-Earth asteroids but these are generally a few hundred meters in size. In our case, we are targeting these smaller objects and not necessarily driven by science requirements, but because the largest population of objects that we can reach with NEA Scout is with this class of asteroids, all on the order of 10 to 20 meters across. So the very nice thing is that it’s the first time that we will be able to observe one of these objects up close. And they’re interesting for multiple reasons. One is that they are potential targets for human exploration, and that is the original intent of NEA Scout is to provide reconnaissance of these objects for a future crewed mission. But they are also interested for science. We want to understand why we have this small objects wandering around; we think they are ejecta from asteroids in the main belt of asteroids at three astronomical units. But we don’t really understand their nature, and so what we will do with NEA Scout, you know, is of scientific value. And then the third very important thing that we are going to get out of this mission is information about a class of objects that is considered a potential issue for planetary defense. And, because there are relatively numerous, and they can create some damage when these objects, when these objects cross the atmosphere, they can create some, shockwave that can be harmful to people on the ground.

Host: Understood. Yeah. Defending the globe; not a bad, not a bad scientific objective to, to tackle here. Now, now when it comes to actually gathering this data, when it comes to, you said it’s going to be about two years until you actually get to the, get, get to the asteroid; how, what are, what are you doing, what is your team doing on the ground to prepare for that moment? How, how are you getting the data and mobilizing your team and receiving the data that you want, making sure that all of the systems are working; what are you doing in terms of the operations?

Julie Castillo-Rogez: We want to get very close to the asteroid, as I noted, we, we start doing optical navigation a few weeks before closest approach. And the key here is to be able to point the camera at the target throughout the duration of the flyby. And the flyby is about three hours long, because as Les noted earlier, I mean, a great advantage of the solar sail is that we can slow it down very efficiently. And so we are going to accomplish a very, very slow flyby, and I think it’s the slowest flyby of a planetary body ever accomplished by NASA. It’s going to be of the order of 10 to 20 meters per second. So, even if our object is very tiny, actually we can observe it for about three hours and, which is great duration because we can get a lot of images and we will be able to accomplish our objectives. But we need to be able to point the camera at the target throughout this duration, and that takes a lot of effort from a navigation standpoint, because the sail, it needs to be maneuvered, we need to understand its behavior in order to plan a trajectory that get us the science.

Host: Now, Les, from, from your team, sounds like one of, one of Julie’s, you know, hit, while her team is getting ready to, to take some of the photos, it’s up to you to, to get the spacecraft where it needs to be, to slow it down to the, to the anticipated speed. So, so what are you guys doing in terms of the operations side, from, from the solar sail, and then are you talking with, with Julie’s team, to, to make sure everything’s going smoothly?

Les Johnson: Oh, I can’t imagine not talking with Julie’s team. [Laughter] Yeah, our team, and it’s a pretty extensive team, we have, we have some, some really talented people working on this, doing something that no one’s ever done before, which is controlling a solar sail in flight from, from essentially point A to point B. And that’s a lot more challenging than it might appear, because not only do you have to be concerned that the sail deploys intact — doesn’t tear or get stuck during deployment or rip or something like that — once it’s deployed, if there’s any, inhomogeneity in the sail, if there, if there’s some, if, if one part of the sail doesn’t look essentially like another part of the sail, then you get uneven reflection of sunlight. And that can cause a torque, which can cause your sail to start, well, just imagine if you, if you deploy something and you push harder on one side of it than you do the other because of some small hole in the sail or, or something that didn’t deploy quite right, you eventually start spinning or lose control, because you’re, you’re not pushing on the sail equally. If it’s perfectly flat and your spacecraft’s right in the middle, and you’re pointed directly at the Sun and the forces are even on a whole sail and it’s pretty simple, but, but life isn’t like that. There, there, we’re not going to be pointing straight at the Sun all the time, there’s bound to be a little billowing, maybe a small tear here or there in the sail. So you’re going to have differences in the sunlight pressure across the surface. And we have to detect that and balance the forces on the sail actively using something called an active mass translator, which is, is a fancy way of saying that we, we have the bulk of the spacecraft attached to the sail on a table that can go back and forth and up and down, and so, we, we maneuver the, the spacecraft mass to even out its location compared to where we might have uneven forces pushing on the sail, which could cause us to lose control. We call that our momentum management. And so when we deploy this sail, we have to learn its characteristics. We’ve got a new propulsion system; how, how’s this going to fly? So we have to get our feet wet first and foremost, thinking in terms of, wow, we’ve got a little bit more force on this corner of the sail than we anticipated, we have to compensate by moving things around a little bit. And, and as we navigate, we have to be aware of this, you know, this problem and, and take that into account as we point the sail. And then as the mission goes along and we fly those characteristics, hopefully won’t change a lot, but they could. For instance, if a small micro-meteorite puts a tiny hole in the sail or something else happens, and, and we get a different orientation, we have to detect that and compensate for it to continue to control the sail. So when, when I look at this, for years people developing sails, and I was guilty of this too, I used to think that the hardest part was building a big lightweight sail and getting it to deploy in space. And that is a challenge, but we’ve done big sails and we’ve deployed big ones on the ground and we think we know how to do that. Turns out, I think the biggest challenge we’re going to have is that learning to fly, and managing that momentum and making sure that we can keep control of the sail with sufficient accuracy to target the asteroid and get where we need to be when we need to be there. So our operations initially are going to be pretty intense, trying to figure out how is this thing flying. And then once we get used to it, we can get into a cadence of, of not having to actively fly it as frequently. The, the nice thing about a sail is that nothing happens quickly. This is a small amount of acceleration. So if there is a problem and the sail starts to tip or tilt and give us some control problems, it’s not going to happen rapidly, we’ll have time to detect that with the instruments, talk to it through the deep space radio network and, and give it the commands we need to, to keep the control and, and keep flying in the direction we want to go. So that, that’s kind of how the operations are going to be. It’ll be very intense the first couple of months, and then we expect it’ll get a little bit more relaxed and a little less intense until we get to the asteroid rendezvous phase, when it’ll ramp up again.

Host: So what’s interesting here is we’re talking about this timeline, right, of, of post-deployment from the Space Launch System on Artemis I, you got, you said, it’s going to be a very dynamic a couple of months, and then you can slow down. The rendezvous time being, you know, maybe two, two years I think is what you guys are estimating, down the line until you actually get to the target asteroid. What’s interesting from your team though, Les is after you meet that objective and, and you, you get to the asteroid at the anticipated rendezvous site at the anticipated speed, Julie can do, Julie and her team can do her theme, thing, what are you guys going to do after that? Where is this thing going to sail to? Can you continue on and just keep testing?

Les Johnson: Well, that’s the beauty of a solar sail, is if the mechanical systems are all still working and the computer system hasn’t been basically damaged by some solar radiation event, which is a risk over this time period, then in theory we can do something else. And so we will be looking during the mission at what those something elses are. And we’ve already started thinking about it. One option would be if, if Julie and her team come back and say, you know, we, we got 80% of the surface covered and we’ve looked at this, and if we go back by the asteroid, we can get a hundred percent, say — I’m just making these numbers up, Julie can correct them — there are trajectories we could do with the sail, where we fly out, turn around and come back and do another flyby. Might take a few months, but we can do that. Another option would be, if everything’s still functional, we will have been tracking what other NEAs could be in range and all the flight operations constraints — we, we can’t go too far away from the Earth because we’ll lose radio contact, so there are a lot of constraints that kind of confine where we can fly — it could be we go visit another NEA and give Julie’s team something else to look at. Then, then there’s always the possibility that we, we, that from a technology point of view, one of the things I’d like to do is, is test the limits of our ability to control the spacecraft, and basically start changing the angle with regard to the Sun to higher and higher incidence angles to see how well we can control it so that we learn the limits of control of the system. Now the, the downside to that is, is once you exceed the limit of control, you’ve lost control and the mission is over. But we would learn a lot from that, and it would help us design the next missions. So there, there are lots of different things we could do.

Host: Very interesting, future science and pushing the limits of the technology. Now, Julie, from your end, I’m, I’m assuming you are also planning for this, right? You have your science objectives, and you’re, you’re going to be doing your best to get as much as you can on the first try, but beyond that what are you thinking about, not only in terms of analyzing that, that, data that you’re getting on the initial run, but thinking about future opportunities as well?

Julie Castillo-Rogez: Yeah. I’m for pushing the limits of technology, actually; I’d be very curious to see, you know, how far we can go with the sail. There is a unique opportunity there because, I mean the more we learn about navigating a solar sail, you know, the better we are prepared for future missions, using that kind of propulsion system for deep space exploration. So that would be my preference. Of course, I mean, we are responsive to another sponsor, and so it would depend also on what another wants to do with the spacecraft, but we would come with a bunch of options. And I think that pushing the boundaries of technology is very important.

Host: Julie, in this moment, we’re only, you know, we’re, we’re very close to actually launching this thing. How, how are you feeling? Do you feel prepared, do you feel excited that this is, this is right around the corner? We’re very, very close.

Julie Castillo-Rogez: Yeah. [Laughter] So, that is a fantastic question because we were supposed to launch in December 2017. And as you know the, Artemis I launch has been delayed. And that is, that has been very beneficial to us because we’ve had more time to prepare. And now, I mean, it’s, as you said, it’s right around the corner and we are getting a little bit excited about the imminent launch. We are in the process of closing, you know, some, our requirements and making sure that everything is covered, and we are going to have a big review with our, another sponsor to, to make sure that we are ready for operations. And, and as Les noted, I really want to emphasize that we have a fantastic team. The team is absolutely extraordinary. It’s been a privilege to work with that group and, you know, they’re really on top of things and making sure that we are going to have a successful mission. And so, yeah, we are excited for sure. And actually, I can’t wait for the launch sometime this spring year.

Host: Very exciting, and, and Les I’ll, I’ll end with this, the same question to you. You have been working very hard on this for a while, and you’ve already thought about, you know, what you have to do, and then even beyond; how are you feeling in this moment with, with launch right around the corner?

Les Johnson: Well, I’m extremely nervous. [Laughter] There’s a lot riding on this. We’ve spent years working on the sail and the technology. As Julie mentioned, we were originally supposed to fly much sooner, and this particular flight sail has been packaged onto its spool for several years now. And I’m, I get a bit nervous when things are in storage that long, right? And, and you’re going to do such a complex system, for the first time ever; there are going to be, what we call unknown unknowns, which, which are things that you had no clue were going to happen, that you have to in real time respond to and, and fix, and I expect that we will have a lot of surprises but we’ve got a really smart team and, a pretty robust little spacecraft that we ought to be able to deal with that. So I’m just nervous. We’ve got a lot to do. We’re doing something for the first time ever and I fully expect it will not go as planned, initially, and we’ll have to figure out how to solve problems that we haven’t even foreseen, but if any team can do it, this team can do it.

Host: Well. I certainly appreciate the confidence and, and I’m just wishing all your teams the best of luck coming up here and, because, you know, it might just, as you’re saying the unknown unknowns, it might take just a bit of that, but the preparation surely help, surely helps. And what you guys are doing is just so cool, talking about sailing the cosmos and studying asteroids — it just doesn’t get any better than that. Les and Julie, thank you so much for coming on Houston We Have a Podcast. I had such a blast talking with both of you, and I’m really excited for this launch coming up for you.

Les Johnson: Thanks for having us.

Julie Castillo-Rogez: Thanks very much.

[Music]

Host: Hey, thanks for sticking around. I had a great time talking with Les and with Julie today, learning about NEA Scout; very interesting secondary payload that’s going to be on Artemis I, coming right around the corner. Go to NASA.gov/Artemis to learn more about the mission. You could check out some of the other secondary payloads, there’s quite a few of them, and then also just about the mission itself. That’s again, NASA.gov/Artemis. We have a full Artemis collection of podcasts on our website, go to NASA.gov/podcasts: you can find us there, we’re called Houston We Have a Podcast, and then once you click on our podcast, go the left side and you can see our Artemis episodes collection. You can listen to them in no particular order. We’re trying to cover a lot of Artemis topics this year because it’s a very exciting mission. If you want to talk to us, ask a question or may be mention a topic, you can chat with us on NASA Johnson Space Center social pages, on Facebook, Twitter, and Instagram. Just use the hashtag #AskNASA and make sure to mention it for us at Houston We Have a Podcast. This episode was recorded on February 16th, 2022. Thanks to Alex Perryman, Pat Ryan, Heidi Lavelle, and Belinda Pulido for their role in making this podcast possible, and to Laura Rochon, Molly Porter and Ian O’Neill for helping us secure the guests. And of course, thanks again to Les Johnson and Julie Castillo-Rogez for taking the time to come on the show. Give us a rating and feedback on whatever platform you’re listening to us on and tell us what you think of our podcast. We’ll be back next week.