Centauri Dreams

Several years ago in Italy Les Johnson told me that he had once had the coolest job description in NASA. And I remember those days in the early 2000s when I was just beginning to investigate interstellar issues, and Les, working at Marshall Space Flight Center in Huntsville, carried a NASA business card describing him as ‘Manager of Interstellar Propulsion Technology Research.’ These days Les’ title is not quite as exotic but he’s involved as ever with solar sails.

Les is also an author, and among his many books is one I recommend to anyone trying to learn more about solar sails. Written with Gregory Matloff and Giovanni Vulpetti, Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2008) reviews the history of the concept going back to James Clerk Maxwell, who pointed out in his monumental work on electric and magnetic fields that photons can impart force to an object they encounter. Konstantin Tsiolkovsky and the Latvian scientist Friderikh Arturovich Tsander went on in the early 20th Century to describe huge mirrors made of thin materials that used photons for propulsion.

Johnson pointed out to the audience at Starship Congress in Dallas that the effect of photons on spacecraft was readily apparent as early as 1960, when NASA launched ECHO 1, a communications satellite that was essentially a giant metal balloon off which signals could be bounced. A large reflective object like this responds to the momentum being exerted upon it by photons, an effect that could now be observed in space. But a lot of people aren’t aware of what Les went on to describe, the use of photon-imparted momentum to save Mariner 10.

Launched in 1973, the craft used a gravity assist around Venus to put it on a trajectory to study Mercury, entering a solar orbit that would allow several close passes by the planet. Flaking paint from Mariner 10’s high-gain antenna confused its navigation sensors, ultimately causing the spacecraft to roll, activating its gyroscopes and venting critical attitude control gas into space. The guidance and control team at JPL was able to solve the problem by tilting Mariner’s solar panels, allowing photon pressure to create enough torque to counter the aberrant motion.

Image: Les Johnson in a NASA photo. My inexpert camera work didn’t produce a good image during Les’ talk in Dallas, so I rely on Google’s image database.

Sunjammer and Its Contemporaries

The point could not be clearer: We know a lot about the physics here, and with solar sails, even though we have only two thus far successfully launched, we’re dealing with principles that have been understood for a long time. There was serious talk about a solar sail mission to Halley’s Comet in the 1980s, with a team at JPL developing several sail designs, among them a heliogyro with multiple blades and a sail that reached 640,000 square meters, a half mile to the side. The comet mission would have jump-started the sail business but NASA rejected the proposal as too risky. Lou Friedman, former director of the Planetary Society, has written about all this in another book you’ll want on your shelf. It’s Starsailing: Solar Sails and Interstellar Travel (John Wiley & Sons, 1988), a bit hard to find these days but well worth the effort.

I had heard about the Japanese IKAROS sail before it launched in 2010 but the reality of it didn’t really sink in until I watched the actual launch. I had a window up on my computer when my wife walked by and asked what I was looking at. “The Japanese are launching a solar sail,” I said, preoccupied as the countdown continued. She walked on out of the room and then came right back. “They’re launching what?!” She knew all about solar sails (the consequence of living with me), but she was startled that after all the NASA work, along with serious studies by DLR in Germany and the Russian Progress mirrors, it was Japan that actually took a sail into space.

Johnson told me after his talk that IKAROS had truly galvanized the sail community. NanoSail-D, launched late in 2010, came next, a three-unit CubeSat that had been built as a spare for an original sail that was lost in a 2008 launch attempt. I still remember Greg Matloff calling me up after the first launch failure to tell me there was a second NanoSail, and I told him he reminded me of S. R. Hadden, the billionaire in the film Contact who tells Ellie Arroway that there is a second stargate machine after the first one is destroyed. “Why build one when you can build two for twice the price?” says Hadden, memorably played by John Hurt.

The second NanoSail-D did the trick, deploying a 10 square meter sail after a rather dramatic delay. The sail spent 240 days in orbit before re-entering the atmosphere. Now, of course, we have Sunjammer to look forward to, a major step up from NanoSail-D. NASA is saying that Sunjammer will produce a maximum thrust of approximately 0.01 newton, the rough equivalent of the weight of one of those packets of artificial sweetener you see on cafe tables. That level of thrust makes the point that sails operate slowly but surely, with a constant acceleration that, over time, achieves velocities fully compatible with exploring the inner Solar System.

Image: NASA engineers look at a 20-meter solar sail and boom system, developed by L’Garde Inc. of Tustin, Calif., after it is fully deployed during testing at NASA Glenn Research Center’s Plum Brook facility in Sandusky, Ohio. Red and blue lights help illuminate the four triangular sail quadrants as they lie outstretched in Plum Brook’s Space Power Facility — the world’s largest space environment simulation chamber. The sail material is supported by a series of inflatable booms that become rigid in the space environment. Credit: NASA/MSFC.

A Clear Roadmap for Sails

For interstellar purposes, of course, we want to do more than move supplies around the Solar System, and that is why Robert Forward started thinking in the early 1960s about pushing a huge solar sail with lasers. We might call this a ‘beamed sail’ or perhaps a ‘lightsail,’ though I like ‘beamed sail’ better because microwaves may be an even better way to push a sail and ‘lightsail’ implies lasers. In any case, the beaming idea is what counts, and Forward was soon joined by George Marx in exploring the idea. Marx wrote up a laser-driven sail concept in Nature in 1966, and we’ve been taking it apart and putting it back together ever since.

The path of sail development, though, is clear, as Johnson explained to the Starship Congress audience. You get to beamed sailcraft by first developing expertise with conventional solar sails. When the Planetary Society was attempting to launch its Cosmos sail in 2005, the plan had been to deploy and test the first operational sail, and as a part of that process a beamed microwave experiment was planned using the Deep Space Network’s Goldstone facilities. We would have learned much about the forces on the sail and how to maneuver it using both solar photons and microwaves if the craft had not been destroyed in a failed launch attempt.

We must hope Sunjammer has a better fate in its 2014 launch. Johnson said the design uses inflatable booms that are malleable and thus deployable, after which they become rigidized by the cold. The plan is to demonstrate attitude control and test the stability of the craft while performing maneuvers and moving close to the L1 Lagrange point to make scientific observations for NOAA (National Oceanic and Atmospheric Administration), which is one of the sponsors of the flight. There’s already a NOAA satellite at L1 that allows a ten to fifteen minute warning of solar storms. The beautiful thing about a sail is that it can move closer to the Sun than L1 and essentially ‘hover’ there, doubling the warning time for potential flare-induced power outages.

Of course, there are many things you can do with a solar sail, and Johnson’s comments ran the gamut. A geosynchronous satellite circling the Earth above the equator is able to stay over the same location on Earth, but you can’t accomplish that trick if you want to hover over the poles for scientific studies. Not unless you use a sail in a ‘polesitter’ orbit that relies on photon pressure to maintain its position. Robert Forward took out a patent on this concept, which he called a ‘statite,’ meaning a spacecraft that, although not orbiting, essentially hovers in place.

Forward wrote the concept up in a 1990 article for Analog and later in a story he called “Race to the Pole.” He describes how the sail would be positioned over the polar regions:

“.. . with the sail tilted so the light pressure from the sunlight reflecting off the lightsail is exactly equal and opposite to the gravity pull of the Earth. With the gravity pull nullified, the spacecraft will just hover over the polar region, while the Earth spins around underneath it.”

You can see what’s happening here: You’re balancing the Earth’s gravity and that of the Sun along with the centrifugal force of the Earth’s orbit around the Sun. A polesitter like this would be stable at about 250 Earth radii, a lot farther out than most communications satellites (about 6 Earth radii), but while this would be problematic for voice communications, it would be no problem for broadcasts into polar regions, and would allow useful meteorological studies.

Image: Dallas as viewed from my hotel room after the first day of Starship Congress.

I’ve hardly exhausted the sail mission concepts that have been studied, so we’ll continue in the same vein tomorrow. From a longer-term perspective, though, I want to emphasize that sails are a technology that can pay off at every level of development. As we look at practical uses for these craft in the near future, we can also contemplate developing them as interplanetary vehicles, perhaps as carriers of supplies to bases on Mars or the asteroids. And we can begin testing how sails behave under beamed power, knowing that building this expertise may one day lead us to space-based beamers that can drive a sailcraft out of our Solar System on an interstellar trajectory.

The evening after Jim Benford’s Starship Congress talk on his solar sail lab work at the Jet Propulsion Laboratory, a small group of sail advocates joined him in the Hilton Anatole’s top floor restaurant to talk over the issues. Benford is organizing Project Forward, named after the legendary Robert Forward, as an Icarus Interstellar effort to further refine the interstellar beamed sail concept. Asked to name the biggest problem areas for sails, the group came up with several, but at the top of the list was deceleration. How do you slow a beamed sail down when it arrives at its target?

A number of possibilities suggest themselves and at this point all of them are completely theoretical. Forward himself wrote up a ‘staged sail’ concept, in which the outer ring of the sail detaches as the star is approached, moving ahead of the inner ring and attached payload. The Earth-based beamer bounces the laser off the larger sail ring, which reflects it back to the smaller sail and slows it for orbital insertion. The maneuver is described in Forward’s 1984 novel Rocheworld, where a three-part sail is used to allow for crew return.

Here’s Forward’s description of a mission to Barnard’s Star on the sailship Prometheus as the sail staging has begun. The inner sail (and payload module) is being turned around so as to receive laser light from the outer ring segment, called the ring-sail:

As the central sail was almost halfway around, the ring-sail readjusted again and started to bring the rotation of the central sail and Prometheus to a halt. The teamwork of the four computers was perfect. The rotation stopped at the same instant the central sail was exactly one hundred and eighty degrees around. The central sail now had its back to the light coming from the solar system while it faced the focused energy coming from the ring-sail. Since the ring-sail had ten tines the surface area of the central sail, there was ten times as much light pressure coming from the ring-sail than from the solar system. The acceleration on the humans built up again, stronger than before, but now it was a deceleration that would ultimately bring them to a stop at Barnard.

Have a look at the diagram below to see the basic method. If the image looks familiar, it’s because this is one of the few I’ve found illustrating Forward’s idea, taken from his original paper on a mission to Epsilon Eridani. Remember that Forward was assuming a huge lens in the outer Solar System which would be used to keep the beam tightly collimated. You can see how complicated this is, and why a magsail also suggests itself as a somewhat more direct option, though braking against a star’s stellar wind brings up numerous problems of its own.

Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus.

If you look at the Forward scheme, though, you can see why the second big problem for beamed sails is jitter. No matter how accurate your beamer, you’ve got huge issues trying to deliver a laser beam to a sail ten light years away, a kind of accuracy that’s just as breathtaking as the power requirements for driving the sail in the first place — Forward’s Epsilon Eridani mission calculation cited a power requirement of 7.2 terawatts. In fact, in a 2003 paper, Travis Taylor and Gregory Matloff took note of the jitter issue and how far we are away from solving it:

The analysis given here, which did not take into account pointing jitter, suggests a minimum of about 15 km in radius for the collector. If pointing error is considered, it appears that the current state-of-the-art of jitter control is many orders of magnitude from enabling a laser sailing mission. Beam control is the largest obstacle for laser sailing.

Building Large Structures in Space

But it’s interesting to see that useful work is being done on the matter of building and deploying large structures in space. Forward’s company Tethers Unlimited, which he founded in 1994 in partnership with Robert Hoyt, has just been awarded a $100,000 grant to develop its SpiderFab project, which will use 3D printing methods in orbit to create the kind of structures we need. You can see how critical this is: Right now a major part of the cost of engineering and launching space systems revolves around the demands of surviving the launch phase, not to mention the cost of the launch itself. Tethers Unlimited plans to find ways around the problem, as described in a report Hoyt wrote for NASA:

We propose to develop a process for automated on-orbit construction of very large structures and multifunctional components. The foundation of this process is a novel additive manufacturing technique called ‘SpiderFab’, which combines the techniques of fused deposition modeling (FDM) with methods derived from automated composite layup to enable rapid construction of very large, very high-strength-per-mass, lattice-like structures combining both compressive and tensile elements. This technique can integrate both high-strength structural materials and conducting materials to enable construction of multifunctional space system components such as antennas.

Image: SpiderFab combines techniques evolved from terrestrial additive manufacturing and composite layup with robotic assembly to enable on-orbit construction of large spacecraft components optimized for the zero-g environment. Credit: Tethers Unlimited/NASA.

Rob Adams (NASA MSFC) described current work on 3D printing in his session at Starship Congress, a technique that would make it possible to create parts on the fly, and one that could even be used for printing out various kinds of foods — NASA has been looking at 3D treatments of pizza (I kid you not) as an experiment in lowering food waste and varying what astronauts eat to go beyond the typical pre-packaged fare. SpiderFab is a step further, the kind of robotic technique that could one day be used not only to build kilometer-scale sails but also key parts of the beamer.

We’re in the early days of sail design but we’re making progress, both in the lab and in space through missions like IKAROS and NanoSail-D. I think too about the two sails that NASA deployed at its Plum Brook facility in Sandusky, Ohio in 2005. These were demonstrators built within a vacuum chamber that have paved the way for Sunjammer, a mission Les Johnson described to the Starship Congress audience. Sunjammer folds up into something the size of a dishwasher, but when deployed in space it will have spread to 1200 square meters, seven times the area of the IKAROS sail, while weighing a scant 32 kilograms (ten times less than IKAROS).

Launch of Sunjammer is currently planned for next year. The mission stirs fond memories of Arthur C. Clarke, whose story “Sunjammer” inspired its name. We should also acknowledge Poul Anderson, who confusingly enough published a story of the same name in Analog a month after Clarke’s story appeared in a 1964 issue of Boy’s Life. You’ll find the Clarke story retitled “The Wind from the Sun” in many anthologies, but whatever its name, the story of a solar sail race to the Moon firmly established the sail concept as a player in the minds of science fiction readers and helped its acceptance by the public.

Johnson, whose recent book Going Interstellar (co-edited with Jack McDevitt) should be on the shelf of any interstellar advocate, went on to describe the Sunjammer mission and a number of other sail concepts currently under development. I had intended to get to all of these today but in my enthusiasm I’ve run out of time, so we’ll continue with more of Les Johnson and talk of where sails are going tomorrow. I was glad to see that Starship Congress was heavy on sail technologies, including the uses of nanotechnology and an interesting idea for a beamed laser infrastructure that we’ll be examining as I continue sorting out my notes.

The Taylor and Matloff paper referred to above is “Space Based Energy Beaming Requirements for Interstellar Laser Sailing,” CP664, Beamed Energy Propulsion: First International Symposium on Beamed Energy Propulsion, ed. By A.V. Pakhomov (2003), American Institute of Physics 0-7354-0126-8.

Low clouds had descended upon Dallas when I landed at Love Field, and by the time I got to the Hilton Anatole for the Starship Congress being hosted by Icarus Interstellar, the city outside was swathed in mist. This was last Wednesday evening, and it was late enough by the time I had dinner that a quick stroll through the cavernous facility was about all I wanted to do before getting some sleep. The Anatole, though, was gorgeous, filled with paintings and sculpture, some of which (two statues of elephants) became helpful landmarks as I learned to navigate the place.

Image: An atrium at the Hotel Anatole at night, one of two, connected so that inveterate walkers like myself could make figure-eight circuits by following the room corridors on any floor.

Starship Congress was a roaring success, the kind of thing that happens when you put people with passionate interests in the same place who usually know each other only through email or by reputation. Saving the day for me on day one was my friend Pat Galea, an Icarus Interstellar fixture, who had brought one of those little MiFi ‘hotspots’ with him. For we learned that the hotel wireless didn’t extend into the meeting room, but Pat, who is to computer issues what Itzhak Perlman is to violins, soon had four of us up and running with a slow but adequate connection. That allowed me to send out the occasional tweet and check various things on Google.

I was glad that the Icarus Interstellar Kickstarter campaign had gone as well as it had, raising a good deal more than the initial goal, and thus funding the travel costs of a number of speakers. We’re all learning our way with crowdsourcing tools like Kickstarter, but they offer a level of public engagement that Richard Obousy, Icarus Interstellar’s president, acknowledged in his opening remarks. In the post-Apollo era, everyone is aware of the need to find ways to engage the public in the great issues of space exploration, and having a stake in an interstellar conference, even if the contributor can’t make the trip, is one way to get this done.

Riding the Sail

My conviction is that the first serious mission targeting another star will use sail technologies in one form or another. The first conference session was devoted to sails, leading off with Jim Benford’s keynote, followed by Les Johnson, who described current and near-term work. Right now the only propulsion method that will get us to interstellar velocities is the sail, and even then we’re talking no more than a couple of hundred kilometers per second, so it’s still a long trip. The Alpha Centauri crossing at 300 kilometers per second (perhaps realizable through a close pass by the Sun with deployment of the sail at perihelion) would still take 4300 years.

This is always a bit of a mind-bender because Voyager 1, the fastest probe we have leaving the system right now, moves at a ‘mere’ 17 kilometers per second, and while New Horizons topped that briefly in the early part of its journey, it will ultimately pass Pluto/Charon at about 13.9 kilometers per second. We need to find ways to ramp these numbers up, and that search begins in the lab. What Benford described in the opening session was a series of experiments he and his brother Greg made on beamed propulsion back in the year 2000. The researchers were working with a carbon fiber mat shaped into a small sail with a thickness of less than 1 millimeter.

Carbon fiber is ideal for sail work because when you put a microwave beam on the sail the material absorbs energy and begins to heat. A sail made of aluminum would begin to melt as you reach about 900 K, limiting possible accelerations, but carbon fiber has a low areal density (about 8 grams per square meter in the material the Benfords used) and a microwave reflectivity approaching 90 percent. The material is actually a carbon-carbon microtruss, meaning a core of carbon fibers is fused to a textured outer surface. With carbon nanotubes woven into the material, this microtruss is capable of temperatures up to 3000 K, at which point it doesn’t melt but sublimes, going from solid to gas with no intervening liquid state.

Working with a sail in an Earth-bound laboratory means you have to achieve an acceleration of one g just to lift off, but the Benfords were able to get up to 10 gs in these experiments, using a wavelength of about 3 centimeters and a pulse duration of 0.2 seconds. Although the tiny experimental sails began to heat up at higher beam powers and bounced off the ceiling of the lab, they survived and remained undamaged after the flight.

All this is provocative because a normal solar sail — think Japan’s IKAROS, for example — is pushed solely by sunlight, the fact being that while photons have no mass they do impart momentum. That’s fine when working in the inner Solar System but the effect of sunlight drops drastically as you move outward, dropping off by the inverse square of the sail’s distance from the Sun. In other words, a sail at Jupiter’s 5 AU from the Sun receives only 4 percent of the sunlight it would in Earth orbit, so an outbound sail is going to need a microwave or laser push if we want to keep it under acceleration all the way to system’s edge and beyond.

Enter the Beam Riders

The laboratory work that has been done thus far is highly encouraging. Having learned that a sail can indeed be pushed to high accelerations by using a microwave beam, the Benfords were also able to show that a sail of the right shape — concave and something like a parachute — will be stable and stay centered in the beam. In fact, the beam induces a sideways restoring force so that even assuming a certain amount of ‘jitter’ in the beam itself, the sail is capable of riding the beam. We’re a long way from the laboratory to the gigantic sails envisioned by Robert Forward, but we’re getting good indications that once we have the expertise to build the right kind of ‘beamer’ in space, the physics will allow sail missions that can reach interstellar velocities.

Transmitting angular momentum to a sail through a beam of photons has also been demonstrated and is, in Benford’s words, ‘a trivial process,’ so that sail deployment might become a matter of putting a large sail into space and inducing a spin to unfold it. Whatever deployment method might be used, the fact that a beam can carry angular momentum means that controllers can stabilize the sail against yaw and drift once deployed. Keep in mind, too, that a number of sail mission concepts actually call for lower power densities than the Benfords needed in the lab — remember, they were pushing the sail from deep within a gravity well. So the experimental case for beamed sails is solid and we can look forward to space-based testing.

Image: Jim Benford discusses the experiments he and his brother performed on sail beaming at the Jet Propulsion Laboratory.

Half a million dollars produced the results Benford described, making the point that while we do have these initial results, we lack a large body of data to draw from not just with sails but most other interstellar propulsion proposals as well. “We have far too many concepts and far too little data,” Benford reminded the audience, adding that “Nature will produce the answers if we ask the correct questions.” Indeed. And these experiments tell us that useful work this early in the interstellar process can grow out of laboratory studies that aren’t hugely expensive. The interstellar community should be thinking about how it can support such ground-breaking work not only on sails but other proposed solutions to the interstellar propulsion conundrum.

One of the exciting thing about interstellar studies is the sheer number of questions it raises. With sails we are dealing with a subject that has moved out of the conceptual phase. We know the physics and are beginning to demonstrate sail methods here on Earth. We can now move in two directions, the first being to work on new materials that will be lighter and more responsive to the beam on the sail. The second step, an obvious one, is to continue with actual space deployments of sails and begin to do beaming experiments on them. More tomorrow as I look at other sail presentations from Starship Congress, beginning with Les Johnson’s overview on what we’ve accomplished so far in space and what we can expect in the next few years.