EPISODE 74: SOLAR SAILS

Transcript

Les Johnson: We’re flying on the first flight of the Space Launch System — Artemis I — as one of 13 small spacecraft that will reach Earth escape and go into Deep Space on a flight kind of directed toward the Moon when we launch early next year.

The technology keeps advancing to allow us to build larger and larger, lighter weight sails.

I think we’ll use these eventually to go to the stars.

Deana Nunley (Host): Welcome to Small Steps, Giant Leaps, a NASA APPEL Knowledge Services podcast where we tap into project experiences to share best practices, lessons learned and novel ideas.

I’m Deana Nunley.

Sailing on Sunlight is highly efficient, allowing spacecraft to achieve remarkable speeds and travel indefinitely in space – enabling rapid exploration of the outer solar system. NASA is developing new technologies for solar sail propulsion systems destined for future, low-cost Deep Space missions, and also getting ready to use solar sail propulsion early next year for a robotic reconnaissance mission to fly by a near-Earth asteroid.

NASA Marshall Space Flight Center’s Les Johnson is the Principal Investigator on the Near-Earth Asteroid Scout and Solar Cruiser missions. Les, thank you for joining us on the podcast.

Johnson: Well, I’m glad to be here. I’m always excited to talk about what I’m working on.

Host: Well, let’s start with a quick explanation of solar sail propulsion, especially as it pertains to the work that you’re doing.

Johnson: Well, the easiest way to think about – well, I guess there’s not really an easy way to think about it because it’s kind of counterintuitive. If you go outside on a Sunny day and the Sunlight’s reflecting from you, there are little particles of light called photons that are in there. And if you think of these like little BBs, they’re bouncing off of you. And as the light’s reflected from you. They’re pushing on you. You don’t feel it because it’s an extremely small push and the Earth’s gravity, the air blowing, all the other forces are just many, many, many times stronger.

But when you get out into space and you’re out of the Earth’s gravity and out of the atmosphere and you unfurl something that’s very reflective and lightweight, it will move as the light reflects from it. Just think of it as — best analogy I can give is you go back to the sailing ships of the of the nineteenth and eighteenth centuries. They put up the big sails and they reflected wind. And as the wind bounced off the sail, it pushed the sail when they dragged the ship along with it. So, with a solar sail it’s much the same. We have a large, lightweight reflector and as the Sunlight falls on it — not the solar wind, that’s something very different — as the Sunlight falls on it, it pushes it and it’s a means of propulsion without fuel.

Host: And propulsion without fuel is one of the main advantages of solar sails, right?

Johnson: Well, that’s the big one. And the reason that it’s a big one is you imagine in this day and age getting in your car and wanting to take a road trip across the country and never having to stop for gas, and that’s the big advantage. What happens with a spacecraft that uses a solar sail is you don’t have to carry all that weight of propulsion system and all that propellant. And a solar sail is really useful for keeping a spacecraft at some location or some region where you’re constantly having to thrust to stay there. And a conventional propulsion system — whether it be a chemical rocket or even an advanced electric propulsion system — will eventually run out of fuel and they can’t continue their mission.

So, the real advantage is almost exclusively in that continuous thrust. Now I do want to point out that solar sails, right now because we’re limited in the size of sail we can build and deploy, really only apply to small spacecraft. So, for the next several years or decades, we’re not talking about things with people. We’re talking about small robotic spacecraft. If we ever want to do sailing with people, we’ll have to build sails that are much, much larger than anything we can build today.

Host: So, what types of space missions are good candidates for solar sail propulsion?

Johnson: Well, I’ll start by eliminating what’s not. First off, solar sails won’t get you off the planet and into space because to get out of Earth’s gravity, you don’t just need a small constant push. You need a lot of thrust to overcome the weight that you have when you’re in the gravity field. So, for the foreseeable future, we will rely on rockets to get off of planets or away from big, massive objects like moons and otherwise.

But once you’re out in space, if you have missions for instance — I’ll give you the example of Near- Earth Asteroid Scout. We have a very small spacecraft. It’s about the size of a boot box. It’s got a camera on board. And the Science Principal Investigator is Dr. Julie Castillo-Rogez at JPL. And she’s responsible for the science on the mission. I’m the propulsion person.

She wants to go study an asteroid. Well, most missions to asteroids are fairly large spacecraft and half of it is propulsion system. Well, we aren’t that big. We’re very, very small and so what we’ve done is we’ve found a way to get all the thrust that we need to navigate to that asteroid without having to carry all the fuel, which means we can make a much smaller spacecraft. Now that’s for Near-Earth Asteroid Scout, which is the mission that’s slated to fly in early 2022.

And in 2025 we’re flying a bigger solar sail that’ll be doing a demonstration of how you can use solar sails to study the Sun, which is of great interest to solar physicists and I’ll give you a mission that there’s really very few other ways to do other than a solar sail and that would be to take our orbit of a spacecraft out of the ecliptic plane which is essentially orbiting the Sun’s equator as all the planets roughly do and taking a spacecraft to orbit over the north and south pole of the Sun. And in order to do that you require a lot of Delta-V, lots of change in velocity in order to take a spacecraft and completely change its orientation. And there is no propulsion system we have currently that can carry enough fuel to do that standalone. Period. End of story.

We’ve sent missions to look at the Sun’s poles in the past. There was one mission, they had to go all the way out to Jupiter, do a gravity assist around Jupiter, spend years in Deep Space. And come back and they got a couple of months’ views and that was it. So, what the heliophysicists want to do is send a spacecraft out that will slowly crank its inclination out of the ecliptic plane to orbit the north and south pole of the Sun. And you can do that with a continuous low thrust of a solar sail. Kind of a long-winded answer but there are lots of applications and those are two of the nearest term.

Host: Very interesting. What sparked your interest in solar sails?

Johnson: Wow. You’d have to go way back. I read a novel — I’m a science fiction fan and have been since I was a kid. I watched Star Trek, read science fiction. That’s how I decided I wanted to be a physicist and work for NASA. But there was a science fiction novel I read back in high school called “The Mote – M-O-T-E — in God’s Eye.” It was by Larry Niven and Jerry Pournelle, and it was a story of alien invasion, of aliens coming to the Earth and their spacecraft were propelled by these enormous sails pushed by sunlight and by high energy lasers to give them an extra push. And that’s how they were coming here. And so that was fascinating to me. It was just incredible. It was eye opening. And I kind of filed that away because I had to get my degree and little details like that. (laughter)

And when I finally started working at NASA it was a few years before I was given the opportunity to look at some advanced propulsion technologies — this would have been the late ‘90s, early 2000s — and among them a portfolio of many advanced propulsion technologies I was asked to examine for actually using for an interstellar mission someday in the future. And solar sails were there, and I remembered reading that book. And I thought, “Well, I wonder what the state of the art of solar sails is.” So, I started delving into it and realized that the technologies were coming together quickly to make this a potential reality and move it from the stuff of science fiction to reality. And I essentially raised my hand and said, “I’d like to work on this.” And my management said “go,” and so that’s how I got started.

Host: Wow, now that is a fascinating story. So, you’ve spent much of your career then working these concepts. Could you walk us through the progress you’ve seen across the years?

Johnson: Oh, my goodness, yes. Wow, where to even begin. Solar sails, in order to be effective, they have to be very, very lightweight. And I don’t want to throw a lot of math at the audience, but force equals mass times acceleration, right. And if the sunlight force is constant, which it is, the Sun puts out a constant amount of light all the time, roughly constant, then in order to get the acceleration you need to fly a spacecraft, you have to have low mass. And until 20 years ago spacecraft were super heavy.

And then in the late 2000s, early twenty teens, these new type of spacecraft became available in large part through the miniaturization of electronics. I mean, just look at your cell phone. 25 years ago, this would have been a computer that weighed as much as a big desk in a room, or bigger. And now you’ve got it in your pocket on your cell phone. Well, that revolution came to spacecraft design with the development of these things called CubeSats, which are essentially fully functioning spacecraft the size of a loaf of bread.

And when it became apparent we could build spacecraft that small and that lightweight, and then the advances in material science that were happening concurrently with that with lightweight reflective films and strong fabrics that could survive the space environment, it was kind of the moment where there was an epiphany that things were just coming together to enable us to do a demo and start thinking seriously about building solar sails.

So that led to the 2010 flight of the NanoSail-D, which was a 10-square-meter space solar sail that we flew out of NASA Marshall and Ames Research Center built the spacecraft. We built the solar sail. And it flew in Earth orbit and demonstrated the deployment of a sail. And then the Japanese built one and flew it in Deep Space. And The Planetary Society collaborated with us, looked at our design of NanoSail-D and did it one better and made a 32-square-meter sail out of a small spacecraft and flew that. And that ultimately led to the Near-Earth Asteroid Scout.

And what’s really cool for me is even since we started NEA Scout, we’re using these metal booms to deploy our sail. If you look at a sailing ship, you’ve got the big wooden mast. Something has to support the sail. Well, in space you have to support your thin film sail. So, we were using these metallic booms that have flown in space before but metal’s heavy. And since we’ve started the work on Near-Earth Asteroid Scout carbon composite materials, which are lightweight composite materials, have gotten used in space.

And so now the next-generation solar sails are using composite booms which are even lighter weight. So, what’s happening is the technology keeps advancing to allow us to build larger and larger, lighter weight sails. So, we’re just making progress increasing sails size by a factor of 10 with every flight, which is a pretty good set of improvements.

Host: For sure. Could you tell us more about Near-Earth Asteroid Scout?

Johnson: Well, NEA Scout is a great little spacecraft that has taken a long time to get to fruition. It began as an idea of a much larger spacecraft to do multiple rendezvous at near-Earth asteroids, go fly from one to the next carrying instruments to do science. And it became apparent with the development of these CubeSats that we could make it much smaller and still do really good, good work. So, there was a proposal call that came out within NASA, I don’t know, 2013-2014. I led a team from NASA Marshall to do a mission called NEA Scout to take a science instrument and go survey an asteroid because at that time NASA was really thinking about sending people to visit NEAs as well as the Moon and Mars in the future. Well concurrent with that, NASA’s other, one of other research centers – JPL — submitted a proposal to do the exact same thing. And it had the exact same name – Near-Earth Asteroid Scout. So, when NASA Headquarters got these two proposals which had the same name and were essentially proposing to do the same thing, they contacted the leads of those two proposals, which was me and Julie, and said, “OK, we only want to do one of these. You two get together and figure out how you’re going to do this together.”

And so, we collaborated. And so, we both are PIs. I’m the Principal investigator for the propulsion system. She’s the Principal investigator for the science. The solar sail is built and developed here at NASA Marshall and the spacecraft is developed at JPL. And we’re flying on the first flight of the Space Launch System — Artemis I — as one of 13 small spacecraft that will reach Earth escape and go into Deep Space on a flight kind of directed toward the Moon when we launch early next year. So, it’s pretty exciting.

We’re going to have a two-year mission after we’re deployed using the sail propulsion system to take us out of the Earth Moon system and fly to a small asteroid where the science instrument will do high-resolution imagery and give us some science data on what that asteroid is made of. Is it one rock? Multiple rocks? What’s on the surface? Does it have a dust cloud around it? You know, answering some really good science questions about what’s in the neighborhood and, oh by the way, demonstrating the flexibility of a solar sail to navigate across interplanetary space and get within about a half a kilometer of this asteroid. So, it’s kind of a combined technology demonstration and science mission.

Host: What are the biggest challenges?

Johnson: Well, that’s Interesting you should say that. It’s a very different thing to develop a technology and test it on the ground than it is to fly it. And I have learned a lot as we’ve gone from idea to let’s build the hardware and then figure out how to fly it. And I used to think that the biggest technical challenge for solar sails was building really big, lightweight sails. I thought that would be the biggest, hardest challenge. And it was a challenge. But it turns out it’s not the most significant challenge to flying them.

The most significant challenge to flying this big lightweight structure in space is maintaining control over it so that it doesn’t start spinning or tumbling and become useless as a propulsion system. This constant sunlight pressure is how we derive our thrust. But if there’s a slight misalignment between where the center of mass is versus the exact center of all the sunlight pressure on the sail, then you get a push more on one side of the sail than the other, which makes it want to tip and tilt and go the wrong direction and make it harder to steer.

And so, we’ve had to come up with some pretty innovative engineering approaches to manage that torque, that asymmetric thrust where one side gets a little bit more pushed than the other or it might start spinning a little bit and you have to take that spin out of it. And that turns out to be the biggest challenge we’ve had is developing both the hardware and the software to control the systems that will enable us to keep control over the sail’s orientation as we fly. So yes, the deployment is a challenge. We think we’ve got that figured out. Once we deploy, keeping it going in the direction we want to go is going to be our hardest challenge.

Host: Could you talk us through that? How do you control and maneuver the sail when it’s operational?

Johnson: Wow, a combination of many things. The sunlight reflecting from it gives you a net force that is dependent on the orientation of the sail. The easiest way to understand this is if you take a sheet of paper — if this were visual, I’d have a piece of sail in front of me and I’d be doing a visual aid — but you point the flat part of the sail toward the Sun and it’s easy to understand that it’s going to push you away, right, from the Sun. The light’s going to reflect and push you away. But everything in space is moving. The Earth’s orbiting the Sun. We go around once a year, coming up, you know, getting ready to officially start winter here not too long in the future. And it’s because the Earth’s axial tilt and where we are in our orbit and all that comes together.

So, if you tilt the sail, you can change the thrust direction from being just radially out from the Sun and it might be that you’re getting a little bit of thrust then to the right or the left by the angle with which you’re reflecting the light from the sail. And since we’re orbiting the Sun already if we tilt the sail so that we get a little push in the direction we’re already moving, we’ll accelerate and that spirals us away from the Sun. If you tilt the sail in the other direction so that you get a little bit of force in the direction opposite to that with which we’re already moving, you decelerate, and the Sun’s gravity starts pulling you in toward the Sun. And so, it’s actually easier to fly a sail toward the Sun than away from the Sun, which is kind of counterintuitive.

But how do we orient the sail? We have reaction wheels. We have a cold gas thruster which gives out little poofs of air. And we also have a table that moves the sail left and right and back and forth that changes the center of the sail relative to all the sunlight falling on it to get the push on the side we want so we can tip and tilt the sail. So, that was the complex answer. The simple answer to how do we control the sail is we’re going to tip and tilt it using onboard systems to get the light to push us in the direction we want the light to push us so that we can navigate to where we want to go.

Host: And are you doing this from a mission control center?

Johnson: The spacecraft will be largely autonomous. We do have an operations center for it here in Huntsville as well as at JPL, and we’re using NASA’s Deep Space Network to communicate with it. But we’re not constantly steering it. It’s not like a video game where we’re going to have somebody in the room and they’re constantly flying the sail. Fortunately, the forces are small. Things happen slowly. So, we can give commands to the spacecraft and if we don’t talk to it again for several hours or even days, we then see what its orientation is, and we issue corrections and commands to change that. And then there’s some autonomy built in the spacecraft that if it starts going in the wrong direction too fast that it goes into a safe mode until we can talk to it and get it oriented as we want it to be. So, we are flying it remotely. But again, it’s not like you would imagine in a video game. Things are happening much slower than that, and just sending a command every now and then is all we need to do to keep it on track.

Host: And then in addition to NEA Scout, you’re also the Principal Investigator on another solar sail mission. Could you fill us in on Solar Cruiser?

Johnson: Yeah, I sure can. NEA Scout is a fairly large sail. It’s just under a thousand square feet. And if you go outside and think about it, that’s about the length of a school bus. So, the sail is a school bus by a school bus, and it’s deployed from a boot box. So, it’s a very small spacecraft — this big sail.

Solar Cruiser takes it to the next level. We are replacing these heavy metallic booms with these lightweight carbon composite booms. We’re using the same sail material which is a plastic that’s coated with aluminum. It’s pretty robust, strong plastic. But instead of about a thousand square feet, it’s going to be over 17 thousand square feet, which is roughly the size of a third of a football field. I mean it’s big. It will fly in 2025.

NEA Scout is funded by Human Exploration under the Advanced Exploration Systems. Solar Cruiser is also funded by NASA but is funded by the Science Office, the Heliophysics Division, Science Mission Directorate, and we will be going toward the Sun to a region called L1 and a little bit closer to the Sun than that. We will have the sail deployed and we’re going to constantly thrust so that we stay in that relative location to the Earth as the Earth is orbiting the Sun. And it’s to demonstrate that we have the control and pointing to carry very precise science instruments to get to these destinations that the heliophysicists, the solar physicists, want to better understand our Sun, like the orbiting the Sun’s poles, looking at the Sun from different vantage points where spacecraft have to constantly fire their thrusters to remain where they need to be. We would eliminate the need for those thrusters and just use the sunlight itself as a thrust mechanism to keep us where we want to be. So, we’ll be flying in early 2025 with Solar Cruiser. It’s pretty exciting.

Host: That does sound exciting. Les, once you get a sail to space, what are the primary risks?

Johnson: Wow, you should be in our risk management meetings. When you start a project, those who aren’t familiar with the process — once you’re selected you have to figure out everything that can go wrong and figure out as a team how you’re going to deal with that if you can, or whether you just accept the risk if it’s a low probability. So, you get all the engineers and scientists in a room. And this is actually — it’s one of the scarier parts of planning a mission — but it’s also kind of fun because at our core, engineers and scientists also like to think, “Gee, what all can go wrong?” And so, you end up making this huge list of all the things that can fail and if you’re not careful, you look at it and say, “Oh, my goodness this is too hard.” Well, space is hard, and so we come up with engineering solutions to make most of these risks go away or the probability become pretty low.

So, I would say that our biggest risk moments for the flight are the deployment, but it’s not so much the deployment that’s a problem. It’s maintaining control of the spacecraft as the sail is deploying. You’re out there. There’s no gravity. You’ve got this structure. These booms are being pushed out by a motor. The sail is unfurling. If one side snags, if there’s something that causes it to not go out evenly in all directions, the spacecraft could tumble. So, it’s a real critical event moment to get the sail deployed.

The other risks are that we lose control of it. Like I mentioned earlier, what if something happens and the sail’s craft starts tumbling? How do we stop that and regain control of it?

A lot of people worry about micrometeorites. It’s a frequent question I get. The sail will be hit by little, teeny tiny dust grains traveling at 18,000 miles an hour or more. They won’t really do that much damage though because the sail is so thin. And these things are moving so fast that they don’t have time to interact with it and do much damage. They just poke a hole in it and fly right on through. So, we are expecting to lose a little bit of area of the sail over time from these micrometeorite impacts and that’s OK. We’ve designed in the thrust loss to take into account for that. So that’s a risk.

We think that the NEA Scout mission will ultimately end not with a failure of the sail but when the radiation dose of all the electronics gets too high. Because it’s a small spacecraft and a limited budget, we aren’t flying the most radiation-hard electronics, nor can we afford all the mass for shielding. So, the mission-ending event is not likely to be caused by the solar sail but by some of the electronics failing on the spacecraft.

Host: What do you see as the future of solar sails?

Johnson: Oh, now you’re getting to my core.

Host: Sweet spot here.

Johnson: It’s a sweet spot for me. I think we’ll use these eventually to go to the stars and I’ll just be very bold about that. It’s not a capability we have today. We’re not even close. But it’s the ultimate goal of what got me interested. I mentioned early in the interview that I was inspired by a science fiction novel about traveling aliens coming to Earth on big solar sails. Well, I think we’ll ultimately send spacecraft to the stars on solar sails.

And these sails will be very different than the ones we have today. They will be much larger. Instead of 17,000 square feet, they need to be the size of cities or states, hundreds of square miles, OK. They need to be 25 times lighter in weight than we have today and they’ll need to fly really close to the Sun to have a lot of solar pressure on them to get them started on their voyage. But all that I’ve just described to you is physically possible.

And when I started working solar sails 20 years ago — and I didn’t come up with these sail designs. They were dreamed up by the great physicist, the late Dr. Robert Forward whose work I really, really admire, and have been built on by others. The materials to build these big sails didn’t even exist 20 years ago. I’m pleased to report that the materials we need to build the sails now exist. It’s a material called graphene. It was discovered in 2004. The discoverers got the Nobel Prize for their discovery in 2010.

We don’t know how to build sheets of graphene that are the size of a city yet. I mean, we’re building coupons that are a few millimeters, smaller than an inch. But that’s OK. It exists. We know how to make it in small quantities. It’s now an engineering challenge of how to make it larger, right? So, for me, ultimately, I think we’ll see solar sails studying the Sun from all kinds of vantage points to advance science. I think we’ll be sending small robotic craft all over the solar system using sails. And I think in 200 years our descendants will be launching the first robotic probe to Alpha Centauri using a solar sail.

Host: Many thanks to Les for chatting with us on the podcast. You’ll find his bio and links to topics discussed during our conversation at APPEL.NASA.gov/podcast along with a show transcript.

If you want information about what else is happening at NASA, we encourage you to check out other NASA podcasts at nasa.gov/podcasts.

As always, thanks for listening to Small Steps, Giant Leaps.