Centauri Dreams

When Les Johnson spoke to a session on sail technologies at the 100 Year Starship symposium in Houston last September, he startled some in the audience by going through a list of how many solar sail missions are now in the works. The European Space Agency’s Gossamer program accounts for one of these, which is already built and waiting for launch, but three are in the pipeline. The University of Surrey (UK) is a surprisingly active entrant, with three CubeSat sails set for flight in the next three years. We also have the Planetary Society’s LightSail to contend with, a CubeSat design with a 32 square meter sail when deployed.

There are other missions as well, with names like NEA [Near Earth Asteroid] Scout, Lunar Flashlight, and although it is now in limbo at least for several years, NASA’s Sunjammer. The surge in interest in CubeSats is hard to miss here. They’re cheap, small, and ideal for trying out sail experiments as we try to figure out how best to use this technology in space. Master it and we have the possibility of sail-driven CubeSat missions sent deep into the Solar System, carrying miniaturized payloads and perhaps flown in ‘swarm’ configuration. And of course the lessons we learn should scale to the larger sails we hope to fly as we build expertise.

Image: The European Space Agency’s Gossamer sail in an artist’s visualization. Credit: ESA/DLR.

Johnson is deputy manager for NASA’s Advanced Concepts Office at the Marshall Space Flight Center in Huntsville, AL. He’s also an active writer of science fiction, most recently portraying — in a novel called Rescue Mode — a Mars mission’s crew on a crippled ship locked in a struggle for survival. Somehow he also finds time to write non-fiction, bringing his formidable background in space technology to bear in Solar Sails: A Novel Approach to Interplanetary Travel, which I’m examining this week as part of a series on sailcraft and their uses. A look at his Amazon offerings illustrates just how prolific Johnson has been in the past decade. [Note: I’m not going to link to the Solar Sails book again until the new edition is available, which should be soon].

Missions in the Near Term

Creating public interest in space is crucial for beginning the gradual spread into the Solar System that some of us think will lead to an infrastructure that can support eventual interstellar missions. After all, we’re asking government, which that same public funds, to play a major role here, even as we also explore what commercial initiatives can achieve. So it’s helpful to keep in mind that technologies have their particular niches. As we talk about multi-modal technologies tomorrow, we’ll consider the fact in greater detail. We’ll never get off the ground with a solar sail — we need rockets for that — but there are missions that even the best rocket design cannot fly.

We know from Tsiolkovsky’s rocket equation that carrying more and more propellant gets to be a non-starter, because soon we’re adding propellant just to push more propellant. Here a solar sail can offer unique opportunities. Suppose, for example, that we want to observe what’s going on at the Sun’s poles. Missions we’ve launched Sunward have tended to stay in the ecliptic because that’s where Earth is, which means we have limited views even of the mid-latitude regions.

From the long-term perspective that Centauri Dreams takes, the solar wind might offer propulsive possibilities for various magnetic sail designs. But we have to reckon with the fact that we don’t understand it well. This stream of charged particles can flow outward from the Sun’s equatorial regions at 400 kilometers per second, but much faster streams, up to 800 kilometers per second, seem to originate in the mid to upper latitude regions. This is just one of the things we’d like to study in addition to the factors that lead to solar plasma outbursts.

A highly inclined orbit around the Sun is a tough challenge for chemical rockets, but it’s realistic to design a Solar Polar Imager sailcraft that can take advantage of the sharp increase in photon ‘push’ available to it at 0.5 AU. Remember, it’s an inverse square law, so that if we halve the distance between Earth and the Sun, we get four times the solar flux on the sail. One mission concept under study calls for a square, 3-axis stabilized sail about 150 meters on the side.

Many sail concepts involve close study of the Sun, including missions to warn of solar storms. Consider that coronal mass ejections (CMEs) can crank out accelerated particles with a speed of ejection up to 1000 kilometers per second, a serious ‘gust’ in the solar wind. Protecting our current infrastructure involves shielding the satellite assets we rely on, including the GPS system and the telecommunications satellites that drive cable TV systems. Adequate warning means these can be powered down in the event of a solar storm, and reoriented to put as much onboard spacecraft mass as possible between key systems and incoming radiation.

Image: Solar flares and CMEs are currently the biggest “explosions” in our solar system, roughly approaching the power in one billion hydrogen bombs. Fast CMEs occur more often near the peak of the 11-year solar cycle, and can trigger major disturbances in Earth’s magnetosphere. Credit: NASA/GSFC.

The Advanced Composition Explorer (ACE) spacecraft is located at the L1 Lagrange point some 1.5 million kilometers from Earth, where it monitors the Sun for dangerous events. But the lead time for any warning is short, which emphasizes the need for a sailcraft that can be positioned between the Sun and the Earth. The current mission concept is called Heliostorm. It would use a square sail 70 meters to the side. A more advanced version with a circular sail 230 meters in radius could orbit the Sun at 0.70 AU, maintaining a direct line between Sun and Earth. Our solar storm warning time would go from the current one hour to between 16 and 31 hours.

Or consider the Mars sample return. It’s a high-priority item for astrobiology as we continue to learn about the Red Planet’s interesting past, but the fuel required not only to get our spacecraft to Mars but also to land, launch and return the sample is a demanding barrier to overcome. Instead, we can consider a lander sent to Mars by conventional rocket, one that collects the needed samples and returns them to Mars orbit. There we rendezvous with a sailcraft above Mars, eliminating the propellant mass we’d have otherwise needed for the return.

The list could go on, from ‘pole sitters’ that use solar photons to maintain a position over one of the Earth’s poles — useful for weather and environmental monitoring — to near earth asteroid reconnaissance, where sail propulsion allows a spacecraft to visit multiple NEA’s. The latter is a technology that couples nicely with the growing use of cubesats and miniaturized components, helping us characterize nearby asteroids in large numbers with swarm missions. I should mention too that a mission called L-1 Diamond is being examined that would use four sailcraft to study the Sun, orbiting it in a triangular formation with the fourth looking down at the pole.

So the range of missions is wide, and it extends as we gain expertise with sailcraft into ‘Sundiver’ missions of the sort that Gregory Matloff began writing about in the 1980s (an idea originally hatched by David Brin and Gregory Benford), which could lead to missions to the gravitational lens (550 AU) and even the Oort Cloud. One of the Solar Sails authors, Giovanni Vulpetti, has explored fast outer system missions in a 2012 book called Fast Solar Sailing (Springer). As our infrastructure builds, sail ‘clippers’ to Mars and other closer destinations may begin to supply distant colonists with the supplies they need.

One day, too, a large solar sail could conceivably be deployed near an asteroid on a dangerous trajectory, changing its reflectivity enough to alter its orbit over the course of decades. It’s a thought that takes me back to the Russian Znamya missions, ostensibly tests of a space mirror that might light Siberian cities at night, but for all intents and purposes a deployment shakeout of sail technologies. The potential of sails for interstellar missions continues to intrigue me, and I’ll be talking about sails and multi-modal propulsion designs for starships tomorrow. But building up our near-Earth infrastructure may be most economically served by near-term sails that can return data and haul supplies to support a civilization with a very deep frontier in mind.

Although we can trace the growth of research into interstellar flight all the way back to the days of Konstantin Tsiolkovsky, the effort has often operated outside of government channels. Scientists and engineers whose day job might take in aspects of rocketry were hard pressed to find time for studying trips to the stars when the proximate needs were better communications satellites or improved designs for reaching low Earth orbit. Nonetheless, work continued, marked by the enthusiasm of the practitioners for what was clearly the ultimate mission. Official or unofficial, small groups hammering on ideas have continued to debate the core concepts.

When the Jet Propulsion Laboratory in Pasadena turned Aden and Marjorie Meinel loose on a mission concept aimed at reaching 1000 AU back in the 1970s, the duo looked at two propulsion options. As the new edition of Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2014) points out, the first of these was a nuclear-electric ion drive using xenon. But the Meinels also evaluated the use of a solar sail unfurled near the Sun. It’s interesting to read here that Chauncey Uphoff, senior analyst on the propulsion phase, was unable to publish the results of the sail study, which wound up circulating only as an internal memo within NASA. [Note: The link above goes to the first edition of this book. The new edition is scheduled for publication within the next few weeks. I advise waiting for it.]

Uphoff’s memo considered the solar sail as an alternative propulsion method, and even if this particular deep space sail concept remained out of view, the efforts of Gregory Matloff and Eugene Mallove soon brought interstellar missions using solar sails to the attention of the community. One of the three authors of Solar Sails, Matloff went to work on what might be done with a close solar pass and sail deployment (partial or complete) at perihelion. He and Mallove evaluated space-manufactured metal sails closing to within 0.04 AU of the Sun’s center, with cables approximating the tensile strength of industrial diamond.

Much of this work was published in the 1980s in the Journal of the British Interplanetary Society, focusing not on beamed laser sails (Robert Forward’s theme) but solar sails using solely the momentum imparted by solar photons. A sail like this, once outbound, could wind cable and sail around the habitat section to provide shielding against cosmic rays. Even a large payload might be accelerated to speeds allowing a trip to Alpha Centauri in 1000 years. Matloff’s 1984 paper “Solar Sail Starships – The Clipper Ships of the Galaxy” (JBIS 34, 371-380) is a classic that he and Mallove would soon update not only in optimization studies in JBIS but also in popular texts like The Starflight Handbook (1989).

The Italian Sail Effort

But back to the theme of small-scale collaborations, from which the interstellar community has for so long benefited. It was back in 1993 at the International Astronautical Congress in Graz, Austria that a small group of solar sail enthusiasts gathered to organize a study of the technology. The study group that emerged was dubbed the Aurora Collaboration, a nod to Greek mythology, in which Aurora was the younger sister of Helios, the god of the Sun. Matloff was one of the core seven behind this collaboration, as was Giovanni Vulpetti, who became team coordinator. Other names familiar to Centauri Dreams readers will be FOCAL mission advocate Claudio Maccone and engineer and author Giancarlo Genta.

Image: Artist’s conception of a solar sail in space. Credit: Rick Sternbach.

Excellent work can come out of highly motivated small groups like these, and I think the Aurora effort deserves greater attention than it has received. Fifteen published papers emerged from its labors, with three presentations to European space agencies and a workshop held at the University of Rome. Computer code for optimizing sail trajectories, experimental work on layered sail construction (a plastic substrate that can be detached once the sail has been constructed in space), and a number of deployment concepts resulted. The collaboration also studied telecommunications systems, analyzed aluminum sail optical properties, and optimized trajectories for potential missions to near interstellar space and the Sun’s gravitational lens.

The latter deserves a note: The gravitational well created by the Sun’s mass causes light to curve as it grazes the Sun from an object directly behind it (as seen by the observer). The resulting lensing effect is promising for observations at various wavelengths, which is where we get the idea of a FOCAL mission to the focus beginning at 550 AU. What the Aurora team did — with results presented at a meeting of the International Academy of Astronautics in Turin, Italy in 1996 — was to produce preliminary results for a less demanding mission, a sail to the heliopause. The team members presented a thin-film 250-meter square sail and analyzed ways of reducing the sail areal mass thickness, as well as exploring communications options and offering a structural analysis.

The Aurora team’s sail would act as a bridge between the Voyager probes ( the Aurora spacecraft would exit the system about three times faster than Voyager) and later deep space designs. Massing 150 kg, it would use a close solar pass for acceleration and its target was a more manageable (in the near term) 50 to 100 AU. Without benefit of press coverage or large amounts of funding, the Aurora Collaboration moved the ball forward through serious volunteer efforts of the kind the interstellar community has always relied on.

Beamed Sail Experiments

As we go through the papers groups like these create, it’s easy to think of the interstellar effort as being almost entirely theoretical. But laboratory work on some of these technologies goes back a long way, and we can trace early sail studies in the lab to the work of Russian physicist Peter Lebedev in 1899, who experimented with metal sheets of differing levels of reflectivity to measure the effect of the exchange of momentum from photons. I mentioned above the Aurora Collaboration’s experimental work on layered sail construction, and in the early years of the 21st Century, Gregory and James Benford studied beamed sail technologies in a JPL lab.

Their findings are important as we move from straightforward solar sailing to the beamed variant that Robert Forward studied both for microwave and laser designs. As president of Microwave Sciences in Lafayette, CA, James Benford’s experience with microwaves led him to join with brother Gregory, a physicist and well-known science fiction writer, to use advances in materials technologies to attempt these laboratory experiments. Temperature was a key here: Working on Earth’s surface, a sail would have to overcome gravity, and to do that, the sail materials would need to be heated to temperatures higher than 1500 degrees Celsius.

Aluminum can’t handle that kind of punishment (its melting temperature is 660 degrees Celsius), and the problem persists with many potential metal sails. But carbon undergoes sublimation at temperatures above 3000 degrees Celsius, making the emergence of lightweight carbon structures as potential sail experiments the key. The Benfords used a 10-square centimeter sail in a vacuum chamber, demonstrating acceleration under a 10-kilowatt, 7 GHz microwave beam. Their sails remained intact after experiencing temperatures up to 1725 degrees Celsius.

The carbon microtruss used in the JPL work, developed by San Diego’s Energy Science Research Laboratories and ten times thinner than a human hair, handled the heat requirement with ease. In fact, the Benfords were able to observe accelerations of several gravities in their tests. They also saw a phenomenon known as ‘desorption,’ in which the rapid heating from the microwave beam evaporates molecules — CO₂, hydrocarbons and hydrogen — incorporated into the sail during the manufacturing process, adding an interesting second source of acceleration to a sail. A carefully applied layer of compounds painted onto a sail thus creates a propulsive layer of its own.

Image: Carbon disk sail lifting off of truncated rectangular waveguide under 10 kW microwave power (four frames, 30 ms interval, first at top). Credit: James and Gregory Benford.

Solar Sails notes the desorption findings, but the Benfords produced a result that I consider far more valuable. Their experiments have demonstrated that the pressure of a microwave beam will keep a concave-shaped sail in tension. The beam is itself producing a sideways restoring force. The terminology here is ‘beam-riding,’ and in the case of future sail designs, it means that a properly shaped sail will be stable under the intense beam that drives it.

Moreover, it becomes clear from this laboratory work that the beam can carry angular momentum which it can communicate to the sail. We have, then, a mechanism for allowing ground-based controllers to stabilize a beamed sail against yaw and drift. This important finding grows out of comparatively inexpensive experiment and meshes with ongoing efforts to study the deployment and control of conventional solar sails. What we are seeing is a technology track that holds the promise for space missions on both an interplanetary and interstellar scale.

Tomorrow I’ll continue this series on solar sail and beamed sailcraft with a look at near-term sail concepts discussed in Solar Sails and the mission needs that drive them.

It was back in 2008 that Copernicus Books published an excellent introduction and reference to space sail technologies. Now the work of Gregory Matloff, Giovanni Vulpetti and Les Johnson, Solar Sails: A Novel Approach to Interplanetary Travel is about to be released in a new edition that I’ve been reviewing for the past month (Note: the 2nd edition is not yet up on the book sites, but publication is slated for later in November). The new version preserves the older edition’s structure but inserts three new chapters covering recent developments, one of which — the cancellation of the Sunjammer sail mission — is too current to have made it into the text. [Addendum: My mistake! Although the text I saw didn’t have the Sunjammer news, Les Johnson tells me that the authors were able to insert it into the final version].

So let’s start with that to get up to speed, and then I want to use Solar Sails as a guide through a series of posts covering not just sails themselves, their variants and their potential missions, but their relationship to an emerging interplanetary and even interstellar framework. The new edition is made to order for that, because in addition to providing the needed background to get any newcomer up to speed on how sails operate, it takes pains to contrast sail technologies with conventional rockets as well as other deep space concepts.

But first, Sunjammer, which you can read about in an article with the woeful title NASA Nixes Sunjammer Mission, Cites Integration, Schedule Risk in SpaceNews. Here we learn from writer Dan Leone that NASA has given up on flying the Sunjammer sail in 2015. The 1200 square-meter sail was under development at L’Garde in Tustin, CA, which according to the article will be laying off about half its employees in the near future. I won’t get into the details of Leone’s article, but the upshot is that we’ll likely see no Sunjammer launch before 2018.

SpaceNews quotes an email from congressman Dana Rohrabacher (R-California) to this effect:

“Obviously, I’m very disappointed that we won’t complete this… We never seem to be able to afford these small technology development projects that can have potentially huge impacts … but we can find billions and billions of dollars to build a massive launch vehicle with no payloads, and no missions,” he said, referring to NASA’s Space Launch System heavy-lift rocket.

Image: Artist’s conception of a solar sail above the Earth. This supple technology has numerous near-Earth benefits but scales well to missions to the outer Solar System and beyond. Credit: NASA.

Small Missions with Big Advantages

At this stage of the game, solar sails are indeed small technology projects with potentially big impacts, and a look through Solar Sails confirms the case that this technology is both ready to fly and possessed of certain key advantages over chemical rockets. We can’t launch them from Earth, so we need chemical rockets to get them into an orbit from which they can be deployed, but once there, sails need to carry no propellant themselves. We can leave behind not just the safety concerns about solid or liquid rocket boosters but also the sheer complexity of the engine, and the extreme situations it must be harnessed to overcome.

All of this can get touchy when we’re operating on Earth — The Space Shuttle’s main engine, says NASA, “operates at greater temperature extremes than any mechanical system in common use today,” and indeed, we can look at the temperature contrasts in such an engine, from down to 20 degrees Kelvin (the temperature of liquid hydrogen fuel) all the way up to 3600 degrees K in the engine’s combustion chamber when the hydrogen burns with liquid oxygen.

But the deep space situation is likewise problematic. A probe to another planet has the same need to store, pump and mix liquid fuel with an oxidizer, but it must also rely on pumps and other internals that have been in a state of storage for up to years at a time. Let me quote the book on a telling case in point:

In 2004, the rocket engine used by the Cassini spacecraft to enter into Saturn’s orbit had to fire for more than 90 minutes after being mostly dormant since its launch 7 years previously. The engine performed as designed, but as Project Manager Bob Mitchell is quoted as saying before the engine was ignited: “We’re about to go through our second hair-graying event… Todd Barber, Cassini’s leader for the propulsion system, called that system “a plumber’s nightmare.” So complicated was the engine that a complete backup was launched onboard in case the primary were to fail… The mass required for the spare engine might have been used to accommodate more science instruments.

None of this is to downplay the need for basic rocketry to get us to Earth orbit, but a case for continued, and ramped up, experimentation with solar sails is certainly there. When we’re contemplating missions to the outer Solar System and beyond, we have to look at the rocket equation developed by Konstantin Tsiolkovsky, which has governed everything we do with these tools. It tells us that a rocket gains speed linearly as its starting mass of propellant rises exponentially. So if we keep adding more fuel to get where we want to go faster, we demand still more fuel, in exponential fashion, just to push the mass of the fuel we added in the first place.

The case space scientists will have to continue to make is that sails are not just exotic attempts to mimic the great sailing ships of old (although the analogy is delightful and often tapped by sail theorists). Instead, by carrying no propellant, the sail gets us around the rocket equation entirely. We wind up with tiny thrust delivered, in the case of solar sails, by the momentum imparted by photons from the Sun. Small thrust over time builds continuously, allowing the slowly-starting sail to gradually overtake the probe hurled along the same trajectory by a chemical rocket.

Sunlight drops sharply as we move toward the outer Solar System, and indeed, by the time we’ve reached the orbit of Jupiter, our sail is experiencing a severe shortage of solar photons. But if we turn our attention to deep space, we have the option of beaming energy to the sail through laser or microwave methods that can compensate for the loss. In fact, some of the designs for beamed sails offer us interstellar options, trips to nearby stars within decades, at the cost of building a Solar System-wide economy that can afford the needed power stations. Solar Sails explores these options in detail, as we’ll see later this week.

Enter the Multi-Modal Mission

Just how and where to deploy solar sails on interplanetary missions? We know that at the distance of the Earth from the Sun, the solar flux is on the order of 1.4 kilowatts per square meter, which works out to being nine orders of magnitude weaker than the force of the wind on the Earth’s surface. You can see why sails have to be both lightweight and large. We also know that the light impinging on the sail varies inversely by the square of the distance from the Sun. This is why the Japanese space agency JAXA, which pulled off the successful IKAROS sail mission, is looking at a Jupiter mission using not just a solar sail but ion propulsion. The sail gets you to interplanetary speeds but the ion engine will be fully efficient at 5 AU and beyond.

When I wrote Centauri Dreams (the book), I speculated that a true interstellar mission might be likewise reliant on more than one propulsion technology. A laser-beamed lightsail might, for example, deploy a magnetic sail using a lightweight but immense superconductor loop to brake against a destination star’s stellar wind upon arrival. The idea of multi-modal propulsion was hardly original with me. In fact, Giovanni Vulpetti had been talking about such an idea for some time, and in Solar Sails refers to it as ‘multiple propulsion mode.’ Film director James Cameron also picked up on the concept in 2009’s Avatar, in which the starship Venture Star uses both antimatter technologies as well as a laser-driven lightsail. Hollywood had never before shown such an interesting starship idea.

Tomorrow I want to look not just at interstellar sail theory but in particular at some of the private initiatives that have pushed sail design forward, in particular the Aurora Collaboration in Italy (both Vulpetti and author Gregory Matloff were key players here, with Vulpetti serving as team coordinator), and the laboratory work accomplished by James and Gregory Benford at the Jet Propulsion Laboratory in California, where core beamed sail ideas were put to the test.