Planetary Radio • Jan 06, 2021

Solar Cruiser: A Giant Sail Prepares for Space

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Les Johnson

Principal Investigator for the NEA Scout and Solar Cruiser solar sails, NASA Marshall Space Flight Center

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Bruce Betts

Chief Scientist / LightSail Program Manager for The Planetary Society

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Mat Kaplan

Senior Communications Adviser and former Host of Planetary Radio for The Planetary Society

It is many times larger than any previous solar sail, and it will pave the way for even bigger spacecraft propelled by light. Solar Cruiser principal investigator Les Johnson tells us about his latest project and looks to humanity crossing the gulfs of interstellar space. Stellaris: People of the Stars is a collection of science fact and fiction co-edited by Les. Mat and Bruce offer a copy in the new What’s Up space trivia contest.

Solar Cruiser
Solar Cruiser This artist's concept shows Solar Cruiser and the Sun.Image: NASA
NEA Scout sail unfurled
NEA Scout sail unfurled Image: NASA / Emmett Given
NEA Scout artist's concept
NEA Scout artist's concept Image: NASA

Related Links

Trivia Contest

This week's prizes:

A copy of Stellaris: People of the Stars, edited by Robert E. Hampson and Les Johnson.

This week's question:

What is the approximate mass of the NEA Scout solar sail?

To submit your answer:

Complete the contest entry form at or write to us at [email protected] no later than Wednesday, January 13th at 8am Pacific Time. Be sure to include your name and mailing address.

Last week's question:

How many crewed launches to space were there in 2020?


The winner will be revealed next week.

Question from the 23 December space trivia contest:

What is the approximate ratio of the average density of Jupiter to the average density of Saturn? In other words, how many times denser is Jupiter than Saturn?


Jupiter is nearly twice as dense as Saturn.


Mat Kaplan: NASA green lights a gigantic solar sail, this week on Planetary Radio. Welcome and Happy New Year again, all. I'm Mat Kaplan of The Planetary Society with more of the human adventure across our solar system and beyond. I wish I could say it's as big as an American football field, but at nearly 1,700 square meters, solar cruiser would cover a third of that field, and it will fly before the middle of this decade according to Principal Investigator Les Johnson. Les has returned for a wonderful conversation about this sail and even bigger ones in our future along with much more.

Mat Kaplan: We'll give away Les' book Solaris in this week's What's Up segment with Bruce. It's a terrific collection of science fact and fiction about what it will mean to be human when we become an interstellar species. There are a couple of space headlines worth mentioning that didn't quite make it into the New Year's Day edition of The Downlink, our free weekly newsletter. Recent Planetary Radio Space Policy edition guest, Scott Pace, has returned to academia from his service as executive secretary of the U.S. National Space Council.

Mat Kaplan: It's still not known if the incoming Biden administration will retain the council. I suspect we'll talk about this in the January Space Policy edition, premiering on Friday, January 8. The Puerto Rican government is not giving up on the Arecibo Observatory. Outgoing governor Wanda Vazquez Garced has set aside $8 million for repair or replacement of the big radio telescope. Of course, much more than that amount will be needed before work could begin.

Mat Kaplan: Japan's Hayabusa2 returned a total of 5.4 grams or two tenths of an ounce of material from asteroid Ryugu, not bad considering the goal was just a 10th of a gram. There's much more about this mission and many others at Lastly, I read in The Downlink about the delicate balance required for the generation and evolution of life on earth and other earth-like planets. A new computer simulation that includes asteroid impact, solar flares and volcanic eruptions on thousands of simulated earth-like worlds indicates that we and all the other living things on this planet beat the odds, so good on us.

Mat Kaplan: That's just part of what you'll find in The Downlink at planetary org. We've also got a new collection of space art for you to explore. Les Johnson last joined us in 2018. We talked then about the Near Earth Asteroid or NEA Scout Mission, the nearly 100 square meter solar sail effort headed by Les at NASA's Marshall Space Flight Center in Alabama. As you'll hear, we are nearing the launch of this asteroid explorer. It was only days ago that the space agency gave Les and his team the go ahead for construction of solar cruiser, a sail with 17 times as much sail as NEA Scout.

Mat Kaplan: Les is also a noted writer of science fiction and nonfiction science books. If there's anything that unites his writing and the work he does for NASA, it may be optimism and cheerful passion. I think you'll hear them in the conversation we recorded as the new year began. Les Johnson, welcome back to planetary radio, and congratulations on all the good things, so the good news that you've gotten in the last few weeks and what's coming up later this year mostly. I'm very glad to have you back on the show.

Les Johnson: Well, it's good to be back. I have to admit, as rough as 2020 was, we did get some good news toward the end of it that made it a little bit brighter.

Mat Kaplan: As we've said before on this show, 2020 was a much better year once you got off the surface of the planet.

Les Johnson: Well, my staff hasn't gotten us off the planet, although I think my feet were in the clouds for a few days when we found out that the solar cruiser was selected.

Mat Kaplan: Oh, I bet. That's a major congratulations on that recent announcement by NASA that that big sale has been green lighted. We're going to talk more about that later. Let's start with this more general question. Where are we in the evolution of light-driven sails?

Les Johnson: Well, we are in a revolution in sails that I didn't really envision happening when I first started working on solar sails. The work of The Planetary Society, the Japanese, the Europeans, what we've been doing at NASA, there's been a lot of advancement on various fronts with flights over the last 10 years, actual demonstrations, technology work, the interest coming with taking sails and putting lasers on them with the Breakthrough Starshot is exciting.

Les Johnson: You put it all together, and it really bodes well for this kind of propulsion becoming more common over the next few years, and enabling scientists to do things that they just couldn't do before. I'm optimistic.

Mat Kaplan: You are sure a big part of that trend. We talked a lot, not surprisingly, about NEA Scout, Near Earth Asteroid Scout, in our last conversation a couple years ago. We'll put a link up to that on this week's show page as well at You must be thrilled to know that it just might be carried into space later this year. Is that still what you're hoping for?

Les Johnson: That is our plan. In fact, we've been told to prepare for a light 21 launch. We're doing final integration of the NEA Scout spacecraft really as we speak. They don't typically let the PI touch the final hardware because I might break something, but the engineers and technicians who are doing that last bit of integration are busy doing that. We anticipate shipping to the launch site probably within the next six weeks or so.

Mat Kaplan: That's fantastic. By the way, it reminds me of visiting Cal Poly San Luis Obispo, where I was encouraged to pull LightSail2 out of its little pea pod ejector. I said, "I don't want to touch that. Do you know what Bruce Betts will do to me if I break this?"

Les Johnson: Well, that's how the PII of this whole thing feels. I'm the idea person, the analysis person. I'm not the final touches on the hardware person. I'm glad there are people that do that, so I don't have to worry about those things.

Mat Kaplan: Talk a little bit more about your ride into space. I didn't realize that you're getting this ride along with a whole bunch of other spacecraft, CubeSat spacecraft as far as I know.

Les Johnson: That's correct. We are one of the 13 CubeSat secondary payloads that will be on the first flight of the Space Launch System, Artemis-I. After the Orion spacecraft goes off to do its job at the moon, those small CubeSats will be deployed at various points in the trajectory, the upper stage they're calling bus stops, which is cool. In each bus stop, different CubeSats get deployed, call home to the Deep Space Network or whatever ground system that they have to work on. Then they go about doing their mission.

Les Johnson: It'll be after that. We'll be on a trajectory toward the moon. We'll deploy our sail, go into an elliptical orbit around the moon. Then when the phasing is right, we will spiral out and go to our target asteroid, which will take about two years of light sailing to get there carrying a camera to do some asteroid science.

Mat Kaplan: Do you know yet which asteroid you'll be targeting?

Les Johnson: We have candidates. The nice thing about a solar sail, which is one of the reasons to fly a sail, is we are not as dependent on launch window. We can reach a whole host of different asteroids. It's just a matter of trip time. We have a list of candidates. Our original 1991 VG may or may not still be our primary. We're going to nail that down over the next couple of months depending on when we actually launch. In terms of NEAs, we're pretty flexible. There are several on the list.

Les Johnson: It's what's called the NHATS database. Those are asteroids of interest for human exploration in the future, which as long as we visit one of those, we're going to be good and having our scientist at JPL, Dr. Julie Castillo-Rogez, who's the PI for the camera on the science side of things to get her the data she needs. It's our job on the sail side to make sure she gets there in a timely way to get that data.

Mat Kaplan: I learned something else about the unique capabilities of sails for missions like the one that you're going to undertake for NEA Scout. That is that you have the shot at getting a second pass, I guess, depending on which of these asteroids you head for. That just seems marvelous.

Les Johnson: Well, that's one of the advantages of a sail, right? I mean, you're not going to run out of fuel. It depends on the incident sunlight and the amount of time you have on your spacecraft. Our goal is to do a slow flyby, just a few tens of meters per second, to take pictures of the asteroid. But if for some reason we don't get sufficient coverage, 80% or more of the asteroid surface, because as we fly by and it's rotating, then we have the option if the spacecraft is still functional. All indications are with a reasonable time of flight, it should be.

Les Johnson: We're really only limited by the total radiation dose that our electronics will get before we anticipate having a failure that we could take a few months, which is how long it would take to basically fly out, loop around, reverse course and fly by again. That's certainly an option. It's not part of the baseline plan, but it is an extended mission that we might want to put out there if there's a sufficient reason to do that.

Les Johnson: If the spacecraft is still very functional and looks like it has lots of life remaining, we might see if there's another target nearby that we could go visit. It really just depends. It'll be a mission of opportunity.

Mat Kaplan: That is just fantastic. I do want to thank you on behalf of The Planetary Society for regularly acknowledging our own LightSail spacecraft as you already have in this conversation. How has LightSail aided your work on NEA Scout and another sail that we're about to talk about?

Les Johnson: Well, first off, The Planetary Society did a great thing flying these in Earth orbit showing on LightSail2 that you can actually do real sailing to adjust your orbital parameters. I think what it did is it made the public and decision makers around the world realize that sails were no longer theoretical, and they're real. I mean, we flew NanoSail-D. The Japanese flew the Icarus back in 2010. Those were great demonstrations, but they didn't really, in my opinion, have the impact on people's thinking in terms of the reality of solar sailing the way that The Planetary Society missions did.

Les Johnson: I think the technical work that The Planetary Society did, as well as the outreach and awareness and education component, has been really critical toward making this capability more real in the minds of decision makers and scientists and people interested in sailing. I think it's contributed in many fronts.

Mat Kaplan: Well, thank you for that. We're, of course, very proud of this. I am told by my friend and colleague, Bruce Betts, that there's been a quite a conversation going on between your team and ours.

Les Johnson: Well, that's right. There's been a back and forth since the NanoSail-D, which flew before you guys did, and in your flight, we had people looking at your deployment test, your day in the life, folks on your review team, because we were anxious to get your lessons learned, because it came at a critical time for NEA Scout. We were in development of our hardware while you were flying LightSail1, and preparing to fly LightSail2. Anytime you have somebody that you can roll up your sleeves and say, "We did this or you did that. How did it go?"

Les Johnson: "Well, we tried that, and it didn't work, so we did this other thing." I mean, that just benefits everybody. The collaborative papers that are being published, I know we have some folks working with yours, looking at the orbital dynamics and mission analysis. I think it just helps give us more confidence in our tools for predicting solar sail trajectories and flight to be able to benchmark them with real data that you obtain. It's useful all around, I think.

Mat Kaplan: Let's talk about the next big step. I do mean a big step. I'll just congratulate you again on that recent green lighting from NASA for solar cruiser. Talk about this ambitious new sail.

Les Johnson: I'll be glad to. I'm thrilled. It's really almost a quantum leap. It's definitely in orders of magnitude leap in sail capability over anything that's been demonstrated so far. Just to give your listeners an idea, NanoSail-D was 10-square meters. The LightSails were 32-square meters. The Near Earth Asteroid scout is going to be about 86 square meters, almost 100 square meters, but solar cruiser is a technology demonstration mission to show the capability of an integrated sail craft using a sail that's 1,600 square meters.

Mat Kaplan: Wow.

Les Johnson: For those that don't think metric, that's about 17,000 square feet, which is about a third the size of a football field or the floor area of your favorite big-box retailer that will be packaged on a spacecraft that will fly as a secondary payload in 2024 with the IMAT mission. This little spacecraft is about 100 kilograms in weight, and we'll be deploying four big composite booms, which is also a first. Instead of using metallic booms, we'll be using lighter weight composite booms.

Les Johnson: We will deploy this big sail, and demonstrate the capability of a solar sail to fly sunward of the earth's sun, Lagrange point, and create an artificial halo orbit. Basically, that's just a fancy way of saying that we're going to do station keeping to keep the sail on the earth's sun line as the earth goes around the sun. Usually, a spacecraft that's in that location is going to drift because it's in a different orbit. It has a different orbital period, but we'll be using the constant sunlight pressure to stay on the earth's sun line.

Les Johnson: That's exciting because there are a lot of missions for space weather and Heliophysics that need this kind of an observation and the ability to do sustained station keeping there to answer some questions about the sun. There are lots of other missions we enable too, but I don't want to go on and on.

Mat Kaplan: Keeping track of solar weather, this might just save people trillions of dollars some day back here on Earth, right, because we need to know what's coming at us from the sun.

Les Johnson: We got to be careful her. We are actually not a science mission. We are demonstrating a capability for science. In our detailed study, we originally were going to carry an instrument called a coronagraph.

Mat Kaplan: Oh, I saw that.

Les Johnson: We had to descope that, and we did that very painfully. I didn't want to do that, but because we started being concerned about the overall cost, we had a cost limit for what the demo could be. By including the development of this new instrument, it really put us over the cost cap, and so we had to remove our science instrument to really make sure we had enough margin in our cost and mass to be able to demonstrate the sail without too much risk.

Les Johnson: At this point, we aren't actually performing science, although we are in discussions that I can't go into a lot of detail about about maybe adding a small instrument onboard, but it would not be the coronagraph that we had originally wanted to fly.

Mat Kaplan: It certainly stands alone in its value as a technology demonstration. You said there were other possible missions. This is something I heard you talk about, a presentation that you gave at the 2019 Tennessee Valley Interstellar Workshop, where you said that after establishing this halo orbit, it's just possible that solar cruiser might leave the plane of the ecliptic, where all of us live and all the other planets live, and give us a much better look, almost a first ever look, not quite, at our star's north and south poles. Is that something you're hoping for?

Les Johnson: Well, the solar cruiser won't actually make it to the poles, but we're going to demonstrate exactly what you said. We're going to take the sail, and we're going to change the sun incidence angles so that instead of accelerating or decelerating in the direction we're already moving around the sun in the ecliptic plane, that we start raising ourselves out of the ecliptic plane. We're going to demonstrate the capability of a sail to change inclination at about seven degrees per year.

Les Johnson: We won't actually be doing that change at that magnitude, because our mission's not that long, but we will be demonstrating the capability. That's the important thing for Heliophysics and the people that are funding us. They want to know that a sail can do what it's claimed to do, and then they can propose follow-on missions that are instrumented to go do that. This high inclination solar imaging is really important, because I was really stunned to learn this a few years ago.

Les Johnson: As much as we know about the sun, there's a lot we don't know. One thing we don't know is we don't have high resolution images of the sun's poles over sustained periods of time. Therefore, it makes it really hard to understand space weather, because how can you do a weather forecast on earth if you don't know what's going on at the poles all the time, right? The space scientists are really eager to, in the future, have missions that allow continuous observations at high inclinations and over the sun's poles.

Les Johnson: The only way to do that really, that we know of, or the best way to do it efficiently is a solar sail.

Mat Kaplan: I'm thinking back to the Ulysses spacecraft, which gave us those earlier looks at the sun's poles, pretty much the only ones we've had. Didn't it have to go all the way out to Jupiter to be able to leave the plane of the ecliptic, and then make its way back in toward the inner portions of our solar system?

Les Johnson: It did, and it took a long time. They had to design the spacecraft to operate in deep space, which is a lot more expensive and a lot harder than just a sunward of earth. They didn't get that long view of the poles. It was a matter of weeks or months. A solar sail would let you go in toward the sun, and do that inclination change and then observe the whole time, and so you don't have to wait until you flown back from Jupiter to do that.

Mat Kaplan: That's physicist, Les Johnson, principal investigator for both the NEA Scout and solar cruiser sails. He and I will be right back with more great stuff. This is Planetary Radio.

Mat Kaplan: You've already addressed this somewhat, but say a little bit more, if you will, about why sails are the ideal vehicles for missions of discovery like the ones that we've just been talking about, NEA Scout and solar cruiser, or at least the potential that they offer for missions well into the future.

Les Johnson: I have to give a caveat, because I don't want to ever claim that there's one propulsion system that meets everybody's needs. There are different kinds of propulsion that are good for different kinds of missions. It turns out that there are a lot of robotic science missions that don't require very heavy payloads like tons, right, that we can build a sail of size to support to give them propulsion today. Now, within that scope, there are a lot of destinations that scientists want to go to that are very propulsive intense.

Les Johnson: They require very efficient propulsion and lots of propellant, which translates into pounds and pounds of fuel, and you still eventually run out of that fuel. I've mentioned one, which is the sustain station keeping on the sun-earth line. Another is changing heliocentric inclination, getting out of the ecliptic plane. That takes a lot of propellant. The reason for that is unlike when you launch a rocket into space on a mission that's in the ecliptic plane, which you launch, you automatically get the earth's velocity added to it because the earth's moving around the sun, right?

Les Johnson: You've already got a lot of velocity before you even get off the ground. Then you add to it with a rocket to get into space and escape the earth's gravity will. You're moving pretty fast, and you're already in orbit around the sun with some velocity that's much greater than zero. But when you try to get out of the ecliptic plane, you're essentially starting with zero velocity in that direction, and have to build it all up. It takes a lot of propellant to move these things to get them going fast.

Les Johnson: The sail lets you do that without fuel, so you don't have to run out of gas. There are other vantage points. Scientists in the Heliophysics community want to put spacecraft orbiting the sun, both leading and trailing the earth and have them stay in a fixed position and not drift. They've done missions like that before, a mission called Stereo. But over time, the spacecraft drifted because they couldn't carry enough propellant to remain on station. A sail would let them remain on station.

Les Johnson: It would also let them dynamically reposition. If for some reason they decide, "Well, we're getting good imaging here at this angle, but it'd be great if we move ahead two degrees." Well, with a sail, you just fly there and do it. It's just a matter of taking a little time to do that, and then there are planetary missions. I've been contacted by some folks that are interested in sample-return missions for Mercury. They're really limited on payload mass, and the sail gives you a lightweight return stage that you might be able to bring small samples back from Mercury using a sail spacecraft.

Les Johnson: There are innovative things that you can embed in the sail, thin film dust sensors to characterize interplanetary dust at various inclinations in the solar system, which is not well characterized. There are lots of interesting science questions to be answered by a relatively small spacecraft that just require a lot of fuel, and a sail let you get around that limitation.

Mat Kaplan: Well, thanks for keeping me out of trouble with friends like Marc Rayman at JPL, who of course, has had great success with another form of propulsion, ion propulsion, electric propulsion and many others, of course. There is one other mission profile for a sail that I've heard you talk about. That would be to hover not over the poles of the sun or to orbit the poles of the sun, but to hover over one of the poles on our own planet.

Les Johnson: Well, that would be a capability for a sail that would be substantially larger than solar cruiser, and one that we could potentially scale the technology up to do. I want to mention that we designed solar cruiser, and it's 1,600 square meters to be this point design. But in principle, there's no reason that you can't make the boons longer, and add more sail material and build a sail with our technology out to about 7,000 square meters or almost 10,000 square meters. Beyond that, you really need to start thinking about different technologies for the sail.

Les Johnson: These Earth pole-sitters, which you're describing, would be a way to take a sail craft, and instead of orbiting the Earth, you angle it such that you reflect the light to give you a net lift thrust to counteract the Earth's gravitational pull at the polls, and rather than orbit, you hover, balancing the force of gravity with the upward force of sunlight at the angled sail.

Mat Kaplan: Wow.

Les Johnson: Now, in order to do that, you have to have a much lighter weight, much larger sail than solar cruisers, so we aren't going to be able to demonstrate that, but we are the next step toward enabling that capability. I really believe that that is a capability that will be within reach by the time we fly solar cruiser in early 2025. That won't be something we'll demonstrate then, but I think by the time we've learned all the nuances of building this hardware, we'll have a pretty good idea of how to extend that sail size up beyond that seven to 10,000 square meters.

Mat Kaplan: Are there other innovations that will be incorporated with solar cruiser that bode well for the future?

Les Johnson: Absolutely. One of them that I'm pretty excited about is we will be flying embedded thin film photovoltaic solar panels, but instead of being rigid, thick, heavy panels, these are essentially the terrestrial thin film, low cost solar cells that you'll find on calculators and you'll find on various devices that are being deployed all over the world. These thin films are what you would expect. I mean, they're as thin as... They're thinner than aluminum foil or like saran wrap, and they have decent efficiency of converting sunlight to electrical power.

Les Johnson: We've demonstrated you can embed those into the sail, and only increase its mass slightly, and generate pretty significant power. We're going to fly some coupons of this on solar cruiser to demonstrate that. There's also a flight in 2022 that I'm the Co-investigator on that is funded by a different part of NASA called the lightweight integrated solar array, which will demonstrate this capability in Earth orbit using a CubeSat. Sound familiar?

Mat Kaplan: Yes.

Les Johnson: Demonstrate the CubeSat. Why this is exciting is because if you were to take a solar sail the size of the NEA Scout, not even solar cruiser class, 86-square meter NEA Scout, and you were to cover it with these thin film photovoltaics, you could get enough power for a spacecraft well beyond Jupiter to Saturn, perhaps out to Uranus, and power a spacecraft out there with sunlight, not requiring a nuclear power source.

Mat Kaplan: Wow.

Les Johnson: What's exciting for me about this evolution of sail technology is that you wouldn't necessarily be solar sailing, but you'd be taking the same fundamental technology, and solving one or the other persnickety problems for deep space missions, which is power. It might be a more affordable way to do outer planetary missions than having to go the route of including a radioisotope power source on board.

Mat Kaplan: I think of what they had to go to, the lengths they had to go to with more standard solar arrays to get Juno out to Jupiter. That's pretty exciting in itself. How will you be controlling? How will solar cruiser orient itself?

Les Johnson: It's complex. One of the things having worked on solar sails for almost 20 years off and on is funding allowed. I used to think it was all about making it big and deploying the sail. All the effort went into, "How do we make it bigger, lighter? How do we do deployment? What's the big risk with the deployment?" What Planetary Society has shown and we've shown on our test for NEA Scout, and what we did on NanoSail-D is deployment works. We know how to deploy a sail. We can do this.

Les Johnson: The challenge is managing the momentum. That constant sunlight pressure means that any asymmetries between your center of pressure of the light and your center of mass of the spacecraft results in a torque, a push on one edge of the sail that is more on one side than the other side. Unless you can control that momentum, that little push over time, you'll lose control of the sail and not be able to fly anywhere. We really believe now that the biggest challenge we're going to face is controlling this momentum during the flight, and so the technologies we're going to do that with our your traditional spacecraft technologies of reaction wheels, et cetera.

Les Johnson: But we're also including an active mass translator, which is basically a slider in the X and Y that allows you to slide sideways and up and forward and back your spacecraft relative to the center of the sail to try to balance that center of mass and center of pressure. It'll be motorized, but even that's not enough. There is another thin film we're embedding in the sail, which is a liquid crystal that when you pass a little current through it, it changes its transmissivity, and you can change the sail from being reflective to mostly absorptive.

Les Johnson: What that does is it changes the amount of pressure you get by the light hitting it wherever you're changing that, which allows you to adjust the light pressure as you want on the various edges of the sail to help control that momentum. I hope that all made sense.

Mat Kaplan: Absolutely. Now, the use of these LCD panels, is that the work that was pioneered by the Japanese with IKAROS?

Les Johnson: They did that. We tried to license their technology, and they wouldn't license it to us.

Mat Kaplan: No kidding.

Les Johnson: We had to go initially to the University of Maryland, and a researcher there who's now moved to UC Davis, Dr. Jeremy Munday, to do some pioneering work in establishing a domestic capability to do this. Now, he's working with one of our subcontractors, NeXolve, who also provides the fabric of the sail through Roccor. Roccor is a small business. It's our primary contractor for the sail hardware. I want to give a shout out to them because they're the primary small business that we're working with, but they have a subcontractor, NeXolve, who is taking these formulations, and embedding them in the sale material itself.

Les Johnson: The Japanese pioneered and said it could be done, but we had to learn how to do it ourselves.

Mat Kaplan: Let me change gears slightly here. I mentioned the Tennessee Valley Interstellar workshop, which you seem to be a regular at that annual gathering, though I imagine it didn't happen in 2020, or at least it didn't happen in person. We will put a link up to your excellent 2019 presentation on this week's show page also at Do you agree with our co-founder, Lew Friedman, and the people behind Breakthrough Starshot that you mentioned? We've talked with them on the show, that sails remain the most or maybe even the only practical way for us to reach the stars, at least with technologies that we currently have within our reach?

Les Johnson: Off and on throughout my career, since I had the privilege of having probably the coolest job title that anyone's ever had at NASA, which was manager of interstellar propulsion technology research. That was back in the early 2000s. I did that for two years before the funding dwindled. That's where I got interested in sails. We looked at all the different options for how you might reach the star someday, looked at fusion propulsion, anti-matter, laser sails, solar sails, microwave sails, project Orion. You go down the list of all these far out ideas that physics says are possible, but we don't know how to engineer yet.

Les Johnson: I came to the conclusion that is aligned with what you just said, with Lew Friedman and a lot of the folks at Breakthrough Starshot, that the technology that looks like it's most in reach, even though we don't have it yet, is going to be some kind of a solar sail or its cousin, a laser beam energy sail to take our first spacecraft to another star. I want to emphasize this is my opinion. NASA doesn't endorse any of these projects, of course, but based on my analysis and what I've looked at, that's one of the reasons I chose to work on solar sails is because I believe that the work I'm doing on it is going to help lay the foundation for my technological descendants someday to send that mission to another star.

Les Johnson: I firmly believe that's the case.

Mat Kaplan: You clearly have kept your eyes focused on interstellar distances and reaching the stars. I'm thinking of the book that you edited, only, what about, a little over a year ago, now you're in a quarter or so, called Solaris that you wrote with Robert Hampson, a collection of science fiction stories and fact-based speculative essays. I just bought it. I think that we may have a nice announcement about that book in this week's What's Up. What were you setting out to do with Solaris?

Les Johnson: Well, first thing I have to do is let your listeners know that my book writing has nothing to do with my day job at NASA. I do that totally on my own time. NASA doesn't endorse it. I really don't try to mix the two whole lot, but I can answer questions about it. If you get an author talking, the never want to shut up, so you better be careful.

Mat Kaplan: That's all right with me.

Les Johnson: No, it's an outgrowth of meetings of the Tennessee Valley Interstellar workshop, where we talked about the issues associated with eventually going to the stars. It really became clear that sending a robotic probe is one thing, but if we ever want to send people, the challenges are just enormous. There's going to be a question of, "Do we modify where we're going to suit Earth life and Earth people, or do we modify people to suit that environment to which they go, and what are the issues associated with these squishy people having to take voyages that might take centuries to get to another star and to give that a serious treatment?"

Les Johnson: The book idea and the collection of stories with essays came about as a result of that, because we wanted to basically take the public people who are interested in the topic on a ride that would inspire them with the science fiction stories, which are all based on real science that we couldn't invent just speculative faster than light drives for people to zip, hither and yon, but within the nut laws of physics, there's interesting stories to tell. Then intersperse that with essays by different experts on human spaceflight and what happens to the body for long duration of spaceflight.

Les Johnson: Got an essay in there for Martin Rees talking about the future of intelligent life in the cosmos, which is absolutely inspiring. I heard him give that talk at a breakthrough initiatives meeting, and asked him to turn it into an essay for the book. The goal is just to get people to realize that interstellar travel may be possible, how tough it will be, and to entertain as we think about it, which is one of the things that got me interested in science and studying physics was reading science fiction, just like what we tried to do in Solaris.

Mat Kaplan: You and me both. There's another book that you and chemist Joe Meany wrote not too long ago about that fascinating and very promising material called... Well, I've always called it Graphene, but I note that you pronounce it graphene, emphasis on the second syllable. It said on the cover of the book, do you still believe it will revolutionize the world?

Les Johnson: I do. I think we're seeing more and more graphene coming into products. Oh my goodness, we're going to be seeing it in things called super capacitors to replace chemical batteries. I got interested in graphene, because I read a paper by a longtime friend and collaborator and someone I'm sure who's familiar to you, Dr. Gregory Matloff, who wrote a paper shortly after the discovery of graphene about how you could make a huge solar sail from it to go to the stars.

Les Johnson: What was exciting to me about that is that when we first looked at these really big sails back in the day, where I mentioned I managed Interstellar propulsion research, and we concluded that you might need to sail the size of Texas that's one atom thick made from a material at the time, in the early 2000s, that we called unobtainium. It was pure science fiction in terms of the material. The science was good, but the materials or the material science was fiction. But when graphene was isolated in 2004, and its discoverers got the Nobel Prize in 2010, and we looked at the properties of graphene, it is a one atom layer thick material substrate that exactly fits what we need to build these big sails.

Les Johnson: Now, we don't know how to build a sail that big. I would never claim that we do right now, but someday we will, and someday, we might be able to make graphene sheets that large. The exciting thing is that when we first looked at it, it was theoretically possible, but the material didn't exist. In two short decades, the material now exists, and it's a question of how do you engineer it? That's exciting.

Mat Kaplan: That is both exciting, and I love your optimism. I look forward to seeing those even bigger sails. Les, before we go, another part of your life, non-NASA life, is your own science fiction. Your next novel comes out what? This summer, I think. I'm looking forward to taking a look at Saving Proxima when it appears, but I also want to note that your very first science fiction novel you co-wrote with a great author and space advocate, Ben Bova, back in 2014. Rescue Mode was about the first human journey to Mars.

Mat Kaplan: As you know, Bova passed away barely a month ago. When I first met him many years ago, he was still president of the National Space Society. He gave me some pivotal words of encouragement. Would you like to say something about Ben?

Les Johnson: Well, Ben will be sorely missed. I first discovered his work when I was in high school, and he was editing Analog Magazine and then Omni Magazine. He wrote the kind of science fiction that really inspired me. I mean, it was believable stuff, real people and believable situations that I could see happening in my lifetime for the most part. When I first started working in the field, I was asked by the local chapter here in Huntsville, Alabama, to bring in a keynote speaker for a conference, and had a little bit of a budget to pay travel expenses.

Les Johnson: I thought, "Why not? I'll ask Ben Bova." To my great surprise, he agreed. This was in the late '80s. I was green behind the years working in my first aerospace job, didn't know much about anything at all, but he came to Huntsville, had him over to my home. We talked. I guess it was early '90s, had a great connection made. Our paths crossed over the years, and I was given the opportunity to write a book with him. He actually wrote my publisher after he read some of my stuff and said, "Hey, do you think Les would want to co author a book with me?"

Mat Kaplan: Wow.

Les Johnson: That's one of those questions, "How could you say no?" He was a great mentor, collaborator. I learned a great deal from him. One of the things I do want to say about Ben is he was one of the most sincere, nicest people I've ever met. He never seemed to have a negative word to say about anyone. He was always giving constructive, thoughtful advice, and just seemed to be a genuinely nice person. I was very impressed by that. He is going to be missed I know by the community of science and science fiction and his family.

Les Johnson: I think it was a loss for the world. He was a true gentleman.

Mat Kaplan: Well, that description of Ben Bova sure fits my own very limited experience with him. I only regret that I never asked him to join us on Planetary Radio. You want to leave us with a little bit of a tease for that next book that, I guess, might just be taking us out to that nearest star where we now know there is a world that just possibly might support [inaudible 00:39:18] capable of supporting life?

Les Johnson: Well, the only teaser I can give you is that we hope that the people who read it will not only be entertained, but give some serious thought about what it means to be human, because this is actually the first book in a three book series. We are going to be exploring a lot of questions about, "Are we alone in the universe? If somebody else is out there, where are they? Why?" Then we have a lot of fun on the techno geek side playing around with time dilation and what happens when spacecraft are traveling close to the speed of light.

Les Johnson: The experience of time passing is different for people on board versus the society, the left behind or where they're going. We've got a hopefully an entertaining story of discovery that will also make folks think a little bit about on those starry nights when you're out looking at the conjunction of Mars and Saturn, and you see all the stars up there, and you wonder, "Are we alone? Who else is out there, and are we ever going to go?" That's what we're trying to convey in this book is that sense of wonder, which really motivates me in everything I do.

Les Johnson: It motivates my day job working on solar sails. I am thrilled to be working on hardware that will probably be flying in space a million years after I'm dead, something that I've been involved with, which is a neat, personal, I don't know, gratification of doing this beyond the science that it's going to do. You mentioned already I'm an optimist, and I like my work to convey that, and I like my fiction to convey that. I think tomorrow can be better than today, and it's really up to us, and I believe that.

Mat Kaplan: Thank you, Les. I look forward to reading Saving Proxima in a few months, and not long after that, the launch of NEA Scout, and a few years down the line, that much bigger sail solar cruiser. So much to look forward to.

Les Johnson: Well, thanks for having me. It's a lot of fun. I always enjoy our chats.

Mat Kaplan: Physicist and author Les Johnson is at NASA's Marshall Space Flight Center in Huntsville, Alabama. Stay with us for a chance to win Solaris, his collection of science fiction and fact.

Kate Howells: Hi, I'm Kate from The Planetary Society. For all its troubles, 2020 has still seen some terrific space accomplishments. We asked our members and supporters to vote for their 2020 favorites. You can see the results at We're talking about the best solar system image, the most exciting moment in planetary science and much more. That's Happy holidays from The Planetary Society.

Mat Kaplan: It's time for the first What's Up of 2021, so we welcome back the chief scientist of The Planetary Society, Bruce Betts, whose name was heard, taken in vain a couple of times in that conversation with Les Johnson. Now, you'll have to listen to it. I know you haven't heard it yet.

Bruce Betts: Hi.

Mat Kaplan: It's okay. It's all good. It's all good. We say nice stuff.

Bruce Betts: I'd pay you to delete those things.

Mat Kaplan: You would not want me to. You'd pay me to keep these in this time. In fact, that's a racket I should have thought of a long time ago.

Bruce Betts: Oh no. No. Let's talk about the night sky, and hope you forget this.

Mat Kaplan: I'll settle for that.

Bruce Betts: Mar's up high looking reddish, fairly bright in the south in the early evening. You might catch Jupiter and Saturn low in the West shortly after sunset, Jupiter looking much brighter. They are getting lower and lower, and then we've got in the pre-dawn still Venus getting lower and lower in the east. In the early evening, keep checking out fabulous Orion coming up in the East in the early evening. That's what's up in the sky, comets. [inaudible 00:42:58] what's up in the sky. There's a lot more, but instead, we're going to go on to this week in space history.

Bruce Betts: 1968, the last surveyor mission, the robotic soft landers of the U.S. program, surveyor seven landed on the moon.

Mat Kaplan: What a spectacular robotic program. I mean, just so much more successful than it probably had any right to be.

Bruce Betts: Yeah and often, of course, overshadowed legitimately by Apollo, but it was quite the successful program in the '60s.

Mat Kaplan: Do you remember which Apollo mission they landed on purpose close enough to one of the surveyors that they went over, and took it apart a little bit?

Bruce Betts: I always thought it was an amazing coincidence that they ended up pretty close to it.

Mat Kaplan: Well, look over there.

Bruce Betts: That was Apollo 12 and surveyor three. It makes for a rather interesting, spectacular pictures on the moon of them going over to the robotic lander that preceded them.

Mat Kaplan: Figured you'd be on top of that.

Bruce Betts: Yeah. Now, I love those. Some of my favorite pictures from the surface of the moon are showing both the lunar module and the surveyor three spacecraft and astronaut.

Mat Kaplan: Humans and robots together.

Bruce Betts: Yeah, it's a thing. It's a good thing. We're going to see good things. We're going to go on to random space that.

Mat Kaplan: That was gentle.

Bruce Betts: It was gentle. Even though I haven't heard it yet, you talked solar sails. I thought I'd point out that as of the day this airs first, January 6, 2021, LightSail2, our solar sail spacecraft, has traveled over 350 million kilometers.

Mat Kaplan: Oh my gosh.

Bruce Betts: More than 200 million miles, doing more than 8,000 orbits of the earth.

Mat Kaplan: That's amazing. I'm really blown away. I had no idea it would be that much after this long.

Bruce Betts: I mean, it goes in circles, but it does more than that.

Mat Kaplan: If it had gone in a straight line, where could we have gone by now? Almost to Jupiter?

Bruce Betts: No, but you'd get out significantly past Mars. You'd be pushing the asteroid belt. Of course, that would be a different mission with different physics requirements to escape Earth, et cetera, but yeah, or you could... As long as we're in a fantasy world, you could fly to the sun and back.

Mat Kaplan: Sure, why not? We'll go at night.

Bruce Betts: I love that joke. All right, let us go on to the trivia contest. I asked you, "What is the approximate ratio of the average density of Jupiter to the average density of Saturn?" In other words, how many times denser is Jupiter than Saturn? How did we do, Mat?

Mat Kaplan: We got some wonderful responses. This one, now, you did say approximate, so he's able to squeak in. Most people gave us a very precise amount it came to like 1.93 times stances. Does that match up with what you had in mind?

Bruce Betts: Yeah, but I was just looking for approximate. I'd take an integer if you've got one.

Mat Kaplan: Mike Fowler has one for you. Mike Fowler of Texas, first time winner, congratulations, Mike. He said approximately two. You have won yourself the brand new Planetary Society baseball cap from the Chop Shop Planetary Society store. You can find it at of all things. Fascinating. A whole bunch of people talked about that whole thing I've heard before about how Saturn is so not dense, that it's less dense than water, and therefore would be able to float.

Mat Kaplan: We heard this from Robert Laporta, Laura Dodd, Bob Lee and some other folks, but I think you'll like this one. John Leyendecker in Colorado, he said, "Yeah, it would float if you had a bathtub big enough, but it might leave a ring."

Bruce Betts: I would be so scared of that bathtub if you find one that big.

Mat Kaplan: That much water too. Bob Clean in Arizona says, "It took me a long time to figure this out leading me to the conclusion that my own mean density is very high." Joseph Potre in New Jersey, "Must resist reference to a holiday fruitcake."

Bruce Betts: That's a much bigger difference sometimes.

Mat Kaplan: Here's that poem from Dave Fairchild. If you measure densities of planets in our [inaudible 00:47:27], Saturn is the least of them despite its gorgeous zin, a planet light enough to float upon an ocean wide, uncounted tons of rocket fuel. Of course, it's liquefied, and if we check on Jupiter when all is said is done, it beats its nearest neighbor by a score of two to one. But let me add a random fact for densities, you all, for every planet that we've got, the earth will rule them all.

Bruce Betts: One planet to rule them all.

Mat Kaplan: Does he mean what I think? We are the densest.

Bruce Betts: Are you referring to the people on this show right now? Yes. Are you referring to Earth as compared to other planets in our solar system? Yes.

Mat Kaplan: All of the above.

Bruce Betts: We are the densest, and we're proud.

Mat Kaplan: All right, take us on to another contest.

Bruce Betts: You heard about Near Earth Asteroid Scout, NEA Scout. What's the approximate mass of NEA Scout? Go to

Mat Kaplan: You have until the 13th. That would be Wednesday, January 13, 2021 to get us this particular answer. If you make it past, you might just win yourself a copy of one of those books we talked with Les Johnson about, Solaris, people of the stars that he edited with Robert Hampson, collection of science fiction and science fact, wonderful essays about how humanity may change as we become an interstellar species someday. Good stuff, I have it, and I'm enjoying the book.

Mat Kaplan: I'm done.

Bruce Betts: All right, everybody, go out there, look up the night sky, and think about the year 2021. 21 is one more than 20. Thank you. Good night.

Mat Kaplan: That's why I love doing this with you, because we can count on you to bring us these profound revelations. That's the chief scientist of The Planetary Society with us for another year, 2021, as we do What's Up on Planetary Radio. Planetary Radio is produced by The Planetary Society in Pasadena, California, and is made possible by its starry-eyed members who wish you smooth sailing in this new year. Mark Hilverda is our associate producer. Josh Doyle composed our theme, which is arranged and performed by Peter Schlosser. Ad astra.