In a month or two, NASA will launch its massive Space Launch System rocket from the Kennedy Space Center. While the spacecraft atop it will travel around the moon—the farthest from Earth a crew-capable craft will have ever gone—the rocket will also deploy a bunch of little CubeSats, including one called NEA Scout that will be propelled by a solar sail toward a nearby asteroid.
That project has come to fruition thanks to Les Johnson, head of that mission’s technology team at NASA’s Marshall Space Flight Center in Huntsville, Alabama. It’s a milestone for Johnson, who has been working on solar sails and other advanced propulsion systems for years.
Outside his day job at NASA, Johnson also writes nonfiction and science fiction books for popular audiences, many of which envision future interstellar voyages. His latest, A Traveler’s Guide to the Stars, explores the kinds of propulsion systems that could one day make these deep-space expeditions a reality.
This conversation has been edited for length and clarity.
WIRED: What inspired you to study space propulsion systems?
Johnson: Star Trek, if you go way back. I’ve been a science fiction fan and an advocate for space exploration and space travel since I was in elementary school. I was 7 years old when I watched Neil Armstrong walk on the moon. I was asleep probably, and I was in footie pajamas, and my parents woke me up to come watch this. And later, my older sister allowed me to stay up with her late to watch Star Trek reruns, and Lost in Space, so I was kind of hooked.
I decided at that age that I wanted to study physics and be a scientist. I always had bad vision and had been a scrawny kid, so I knew I wouldn’t be an astronaut—but I wanted to work for NASA.
One of the first projects I was assigned was to work on something called a space tether. Those are long wires that are deployed on spacecraft, and they can be used for scientific measurements. But there was a secondary effect in test flights: You could actually get propulsion in low Earth orbit using these wires, without electricity orfuel. So I got really excited: “Hey, this is a way to travel through space, at least in Earth orbit, where you may not ever run out of gas.”
So that’s what got me interested in advanced propulsion. From there it spread out to solar sails, and to nuclear propulsion. As a result of that, I got involved with some groups outside of NASA, people thinking about how we might go to the stars. They’d ask me, “What’s a viable method to go to Proxima Centauri?” So things kind of snowballed from there.
How does a solar sail work?
It’s not the solar wind—that’s an unfortunate naming problem. A solar sail is propelled only by light. Light is made up of photons, and those photons don’t have mass. But they do have momentum, like a molecule of air in the wind. And just like a sailboat on a lake or the ocean, when the wind blows against the sail, some of the momentum of the air particles is absorbed by the sail, which causes it to recoil, which is pushing on the sail. And through the mast, it pulls the boat with it.
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Out in space, as photons of light reflect from the sail, the light gives up a little of its energy and momentum, and that momentum goes into the motion of the sail and it pushes it.
How far from the sun can you go while still getting a significant amount of energy from it?
This is why solar sails are really cool, and this is why I like them for interstellar travel. Let’s go out the Earth’s distance from the sun, 1 AU, 93 million miles. When you unfurl a sail of any size, say it’s 100 square meters, the sunlight falling on it pushes on it. As you move away from the sun, the intensity of sunlight falls off pretty rapidly, and so does the thrust. But if you deploy a sail closer to the sun, the thrust level goes up dramatically.
If you have a light enough sail, you can get a really big acceleration. If you get well inside the orbit of Mercury and you have a sail that only weighs 1 or 2 grams per square meter—which is about 20 times better than we can do today—and you have a sail that’s like a square kilometer, if you add a laser to boost it, you can get enough thrust to go out of the solar system at a significant fraction of the speed of light, like 10 percent. It’s unbelievable. That’s where you can get a trip that will get you to Alpha Centauri in hundreds of years, as opposed to thousands or tens of thousands with chemical rockets.
When I first saw these numbers, I thought, “That’s great, but we have no material that can stand those loads that’s that lightweight. That material is ‘unobtainium.’” That was pure science fiction. Then in 2004, graphene was found. The discoverers of that got a Nobel Prize for it in 2010. That’s a single layer of carbon. It has all the thermal and mechanical properties you need to build this huge sail; you just have to put something on it to make it reflective, like a layer of aluminum. And suddenly, this looks possible.
We don’t know how to engineer anything that big yet. But we’ve gone from a material that doesn’t exist to one that does exist in the last two decades. And if you augment that with a high-power laser, like the folks at the Breakthrough Starshot want to do, it’s like a lot more suns falling on it, which means you can accelerate it to much higher speeds, potentially up to 5, 10, 20 percent the speed of light. And all of this without violating the laws of physics. The only laws you’re violating are known engineering. Nobody knows how to build these things, but we will! We’ll figure it out.
How did you get involved with the NEA Scout’s solar sail?
I have been working on solar sails since the early 2000s. It was one technology of many, in a portfolio of advanced propulsion that I was working on at my day job at NASA. It involved electric propulsion, nuclear propulsion, sail propulsion, some chemical work, and solar sails were a part of that. That was about the time little CubeSats were being flown, small, bread-loaf-sized spacecraft that a lot of universities now fly in low Earth orbit. NASA was trying to figure out, “Hey, can we do useful things with these? Does anybody have a payload?” We said, “We have some solar sail hardware. Let’s test a sail deployment in Earth orbit.”
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So in 2010, we flew a 10-square-meter sail called Nanosail-D. And that was successful. Then the Space Launch System was starting to move forward, and someone at NASA said, “This rocket’s going into deep space. It will have extra payload capability, we can take some of these CubeSats.” So I led a team and we wrote the proposal for NEA Scout using a scaled-up version of the Nanosail-D.
Tell me about some speculative propulsions you’ve explored, such as pulsed fusion and antimatter.
Oh, it’s all cool! I could talk for hours! I’ll start with the things I think are possible within the known laws of physics. I don’t want to be arrogant here: Scientists throughout history have made the mistake of saying, “Oh, that’s impossible,” and then 50 years later somebody proves them wrong.
There are a few ways to get to the stars. One is sails—light sails, solar sails. Chemical rockets just don’t have the energy density to do it. Nuclear-thermal rockets basically use a small version of the reactor that produces electrical power in a power station near you. You miniaturize it and put it on a rocket and use fuel, and it’s superheated by the nuclear reactor. That’s an improvement in performance over a chemical rocket, and it’s something I think we ought to be doing for the exploration of our solar system, but it won’t take you to the stars. You can’t carry enough fuel in the mass you have available to make it work.
Its descendant, fusion, which people are working on to try to have a cleaner source of power on Earth, is: Instead of splitting atoms, you’re combining them, like the way the sun produces energy. You’re squeezing hydrogen atoms so tightly until they become helium, and then they give off energy. If you can do that in a controlled reaction, you get a lot more energy out than you put in. You could use that as a propulsion system to build a rocket. It would have to be a really big rocket, because you’d have to carry a lot of fuel: Think of a rocket bigger than the Empire State Building. But it would work. You could get to the nearest few stars, like maybe Proxima Centauri, but not Ross 248, which is 10 light-years away.
One of my favorites after that is antimatter. People hear that and think, “That’s out of Star Trek.” Which it was. But it’s real. In high-energy reactions, like at the CERN collider in Europe and other particle accelerators, when we smash atoms together at high speed, lots of things break apart and fly off. But a curious thing people discovered is that there are things that look like a proton, have the mass of a proton, but have a negative charge. And then they discovered these lighter-weight things that look like electrons, but they have a positive charge. So scientists have taken these antiprotons, combined them with positrons, and made anti-hydrogen. That’s in small quantities, because when these anti-particles encounter their normal matter counterparts, they undergo—in physics terms—annihilation. That mass gets turned into energy. They explode and give off gamma rays, all kinds of secondary particles—it’s a very energetic explosion. A tablespoon of antimatter would basically destroy a city—that’s how much energy is packed into antimatter.
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You could take a lot of this antimatter, store it in a perfect vacuum, and then as you need it for your reaction mass to propel your spaceship, you have a stream of it that goes in and annihilates with normal matter and you use that energy. We don’t know how to do that, but nature says it’s possible. Now, I don’t think I want to build this on Earth, because you’re going to need tons of antimatter. If you lost control of it, that would be a disaster.
Buried in there is another pretty interesting idea that is not as good as antimatter or fusion, but it’s really close. That’s something called a fission pulse. You may have heard of Project Orion. That was a really cool project in the Cold War, in the late ’50s and into the ’60s, where some scientists including the late Freeman Dyson said, “Maybe instead of using a rocket to put a spacecraft into space, what would happen if we used a series of controlled explosions under a big steel plate?”
It’s like, if you put a rock on top of a firecracker, the rock gets launched, right? Imagine a series of explosions under a steel plate. It’ll start getting off the ground—“Boom, boom, boom!”—to higher and higher speeds as you keep detonating these explosions. You could potentially get this plate or whatever’s on it—a spacecraft—moving to really high speeds. These scientists figured out, if you have a spacecraft the size of an aircraft carrier and you put extremely large plates under it, that are big enough to shield it from the radiation from the bomb going off, and you started exploding atomic bombs every three seconds under it, you could get tremendous speeds and you could use this to send a spacecraft, with a trip time of a few hundred years, to the nearest star. Of course you destroy the ecosystem while you’re launching it. But in theory, yeah, that ought to work!
According to a figure in your book, it looks like it’s hard to strike a balance to achieve both efficiency and thrust—and to also not have something cost a gazillion dollars.
Unfortunately, if we’re talking about building something at the scale to send a reasonably sized spacecraft to the nearest star, it’s going to be—with today’s capabilities—a really expensive endeavor. But over time, the capability evolves.
That curve you’re talking about limits rockets. It applies to any rockets that have fuel on board: chemical rockets, electric rockets, nuclear-thermal, fusion, and even antimatter. You’ve got the mass of your spacecraft, and to get it moving, it requires a certain amount of fuel at a certain thrust level. To keep it going faster, you have to load more fuel on it, which increases the weight, which means you need more fuel to move it initially. Eventually it gets to a point where you get diminishing returns.
That’s why I like sails, where the energy is not on the ship; it comes from somewhere else, so you don’t have to worry about that efficiency curve getting you. That’s a beautiful way to get around that problem.
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For very long interstellar trips—things that are farther than the closest star—continuous fusion, antimatter, and sails are the only thing that will let you get there. But the better the thrust performance, the worse the efficiency it has, with every system we’ve looked at.
What motivated you to write this book, A Traveler’s Guide to the Stars?
I go back to what motivated me to study science: It was our achievements in space, going to the moon. It was the dreamers, science fiction writers, and television shows, and this notion that in this big universe, as we look out and we discover exoplanets and we find that some of these exoplanets live in regions around their star where there might be liquid water, there might be a place where life could go and exist.
I am a believer that life is good and that it’s a morally good thing to try to preserve and protect and spread life. We as a species, as humans, should strive to use space resources to make life better on Earth and expand our presence in the solar system, and eventually start sending our children to spread life into the rest of the universe, which sure looks like it’s a cold, dead universe. If it is, then let’s go fill it up with people who have hopes, dreams, aspirations, to create art and be human.
How long will it take humanity to design and send a robotic probe to another star system?
Part of that’s going to be a function of how hard we try. If we keep going on the path we’re going—which isn’t a bad path, but it’s taking longer than we thought it would to get the costs of launch down—I think it’ll be 300 years.
But if someone were to come along and say, “Here’s a blank check. Let’s go figure this out,” we could do it probably in less than 100 years. It’s a challenge limited by engineering knowledge, but interest, enthusiasm, and funding could accelerate it.
Now if it’s the public purse, politicians have to balance that with all the other things: health care, police. I’m just thankful our society places a value on science and exploration at any level. So it’s a balance of priorities.
What might a crewed space journey to another star system look like?
Let’s assume we’re not going to fundamentally change our own biology through genetic engineering, that 100 years from now, people are still people as we’d recognize them today, but maybe living longer, maybe with better health care. I think it would be a voyage of hundreds of years, in a ship where there would be generations that are born and die, before you ever reach the nearest star. It would be a concept like in the movie Passengers, but not with suspended animation, because I’m really skeptical of that.
Now if we have breakthroughs in medical research that allow us to engineer ourselves to be adapted to spaceflight, perhaps engineer ourselves to be like bears, where we could go into hibernation, and then you combine that with rocket science and propulsion science, a voyage of hundreds of years might still be the case, but wouldn’t necessarily be generations. It might open the possibility of the people who get on the ship being the ones who get off the ship. But that’s two levels of revolutionary breakthroughs.
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What are your thoughts about sending robots versus people into space? That seems to be the eternal debate—with the moon, asteroids, and Mars?
It’s going to be both. I think that’s what history has shown. Before we sent people into space, we sent Sputnik and Explorer 1 and other robotic spacecraft. Before we went to the moon, there were the Surveyor missions that we sent, and the Soviets sent spacecraft, and then we sent people. For decades we’ve been sending robotic spacecraft to Mars. I think we will send people to Mars. I’m hoping that will be in my lifetime.
When I look at that debate, I think it’s a false dichotomy. And I’ve got a story in the book: I went to a meeting probably eight to 10 years ago on new strategies for exploring Mars. There was a debate going on there, with panelists on stage, about whether we should send people to Mars. Is it really worth it? There was this reserved chair in the first row that was empty. And then in walks Buzz Aldrin. Buzz, the second man to walk on the moon, makes his entrance, and sits down. And he’s there for like five minutes. He stands up, and raises his hand. He looked at all of us and said, “OK, let’s suppose we had a way to do this tomorrow. How many of you would sign up for a one-way trip to Mars?” I was stunned. I want to go as a tourist, but I want to go back home. But it was over half the people, and a lot of them who raised their hands were those who had been arguing we should only send robots. But as soon as they were given the thought, “Oh, we could send people—then of course I’d go.” That moment crystallized in my head that if the capability exists, we’re going to do both. It will first be the robots, then we’ll send people.