Game Changer: Exploring the New Paradigm for Deep Space

The game changer for space exploration in coming decades will be self-assembly, enabling the growth of a new and invigorating paradigm in which multiple smallsat sailcraft launched as ‘rideshare’ payloads augment huge ‘flagship’ missions. Self-assembly allows formation-flying smallsats to emerge enroute as larger, fully capable craft carrying complex payloads to target. The case for this grows out of Slava Turyshev and team’s work at JPL as they refine the conceptual design for a mission to the solar gravitational lens at 550 AU and beyond. The advantages are patent, including lower cost, fast transit times and full capability at destination.

Aspects of this paradigm are beginning to be explored in the literature, as I’ve been reminded by Alex Tolley, who forwarded an interesting paper out of the University of Padua (Italy). Drawing on an international team, lead author Giovanni Santi explores the use of CubeSat-scale spacecraft driven by sail technologies, in this case ‘lightsails’ pushed by a laser array. Self-assembly does not figure into the discussion in this paper, but the focus on smallsats and sails fits nicely with the concept, and extends the discussion of how to maximize data return from distant targets in the Solar System.

The key to the Santi paper is swarm technologies, numerous small sailcraft placed into orbits that allow planetary exploration as well as observations of the heliosphere. We’re talking about payloads in the range of 1 kg each, and the intent of the paper is to explore onboard systems (telecommunications receives particular attention), the fabrication of the sail and its stability, and the applications such systems can offer. As you would imagine, the work draws for its laser concepts on the Starlight program pursued for NASA by Philip Lubin and the ongoing Breakthrough Starshot project.

Image: NASA’s Starling mission is one early step toward developing swarm capabilities. The mission will demonstrate technologies to enable multipoint science data collection by several small spacecraft flying in swarms. The six-month mission will use four CubeSats in low-Earth orbit to test four technologies that let spacecraft operate in a synchronized manner without resources from the ground. Credit: NASA Ames.

The authors argue that ground-based direct energy laser propulsion, with its benefits in terms of modularity and scalability, is the baseline technology needed to make small sailcraft exploration of the Solar System a reality. Thus there is a line of development which extends from early missions to targets like Mars, with accompanying reductions in the power needed (as opposed to interstellar missions like Breakthrough Starshot), and correspondingly, fewer demands on the laser array.

The paper specifically does not analyze close-pass perihelion maneuvers at the Sun of the sort examined by the JPL team, which assumes no need for a ground-based array. I think the ‘Sundiver’ maneuver is the missing piece in the puzzle, and will come back to it in a moment.

Breakthrough Starshot envisions a flyby of a planetary system like Proxima Centauri, but the missions contemplated here, much closer to home, must find a way to brake at destination in cases where extended planetary science is going to be performed. Thus we lose the benefit of purely sail-based propulsion (no propellant aboard) in favor of carrying enough propulsive mass to make the needed maneuvers at, say, Mars:

…the spacecraft could be ballistically captured in a highly irregular orbit, which requires at least an high thrust maneuver to stabilize the orbit itself and to reduce the eccentricity…The velocity budget has been estimated using GMAT suite to be ?v ? 900?1400 m s?1, depending on the desired final orbit eccentricity and altitude. A chemical thruster with about 3 N thrust would allow to perform a sufficiently fast maneuver. In this scenario, the mass of the nanosatellite is estimated to be increased by a wet mass of 5 kg; moreover, an increase of the mass of reaction wheels needs to be taken into account given the total mass increment.

Even so, swarms of nanosatellites allow a reduction of the payload mass of each individual spacecraft, with the added benefit of redundancy and the use of off-the-shelf components. The authors dwell on the lightsail itself, noting the basic requirement that it be thermally and mechanically stable during acceleration, no small matter when propelling a sail out of Earth orbit through a high-power laser beam. Although layered sails and sails using nanostructures, metamaterials that can optimize heat dissipation and promote stability, are an area of active research, this paper works with a thin film design that reduces complexity and offers lower costs.

We wind up with simulations involving a sail made of titanium dioxide with a radius of 1.8 m (i.e. a total area of 10 m2) and a thickness of 1 µm. The issue of turbulence in the atmosphere, a concern for Breakthrough Starshot’s ground-based laser array, is not considered in this paper, but the authors note the need to analyze the problem in the next iteration of their work along with close attention to laser alignment, which can cause problems of sail drifting and spinning or even destroy the sail.

But does the laser have to be on the Earth’s surface? We’ve had this discussion before, noting the political problem of a high-power laser installation in Earth orbit, but the paper notes a third possibility, the surface of the Moon. A long-term prospect, to be sure, but one having the advantage of lack of atmosphere, and perhaps placement on the Moon’s far side could one day offer a politically acceptable solution. It’s an intriguing thought, but if we’re thinking of the near term, the fastest solution seems to be the Breakthrough Starshot choice of a ground-based facility on Earth.

What we have here, then, could be described as a scaled-down laser concept, a kind of Breakthrough Starshot ‘lite’ that focuses on lower levels of laser power, larger payloads (even though still in the nanosatellite range), and targets as close as Mars, where swarms of sail-driven spacecraft might construct the communications network for a colony on the surface. A larger target would be exploration of the heliosphere:

…in this last mission scenario the nanosatellites would be radially propelled without the need of further orbital maneuvers. To date, the interplanetary environment, and in particular the heliospheric plasma, is only partially known due to the few existing opportunities for carrying out in-situ measurements, basically linked to scientific exploration missions [76]. The composition and characteristics of the heliospheric plasma remain defined mainly through theoretical models only partially verified. Therefore, there is an urgent need to perform a more detailed mapping of the heliospheric environment especially due to the growth of the human activities in space.

Image: An artist’s concept of ESA’s Swarm mission being deployed. This image was taken from a 2015 workshop on formation flying satellites held at Technische Universiteit Delft in the Netherlands. Extending the swarm paradigm to smallsats and nanosatellites is one step toward future robotic self-assembly. Credit: TU Delft.

Spacecraft operating in swarms optimized for the study of the heliosphere offer tantalizing possibilities in terms of data return. But I think the point that emerges here is flexibility, the notion that coupling a beamed propulsion system to smallsats and nanosats offers a less expensive, modular way to explore targets previously within reach only by expensive flagship missions. I’ll also argue that a large, ground-based laser array is aspirational but not essential to push this paradigm forward.

Issues of self-assembly and sail design are under active study, as is the question of thermal survival for operations close to the Sun. We should continue to explore close solar passes and ‘sundiver’ maneuvers to shorten transit times to targets both relatively near or as far away as the Kuiper Belt. We need test missions to firm up sail materials and operations, even as we experiment with self-assembly of smallsats into larger craft capable of complex operations at target. The result is a modular fleet that can make fast flybys of distant targets or assemble for orbital operations where needed.

The paper is Santi et al., “Swarm of lightsail nanosatellites for Solar System exploration,” available as a preprint.


Building Smallsat Capabilities for the Outer System

‘LightCraft’ is the term used by Slava Turyshev’s team at JPL and elsewhere to identify the current design of the ambitious mission we looked at briefly in the previous post. This is a Technology Demonstrator Mission (TDM), which can be considered a precursor to what may become a mission to the solar gravitational lens. The mission concept is under active investigation, partly via a Phase III grant from NASA’s Innovative Advanced Concepts office. Reaching the focal region (for practical purposes, beyond 600 AU) in less than 25 years requires changes to our thinking in propulsion, not to mention payload size and the potential of robotic self-assembly en-route.

Hence the paper the researchers have just released, “Science opportunities with solar sailing smallsats,” which makes the case for leveraging our growing expertise in solar sail design and the highly successful move toward miniaturization in space systems, which the authors believe can be extended to include smallsats operating in the outer Solar System.

The TDM mission is conceived as a series of preparatory flights that allow the testing and validation of the technology and operational concepts involved in a mission to the focal region. The implications are hardly limited to the outer Solar System, for the smallsat/sail paradigm should be applicable to a wide range of missions in the inner system as well.

Let’s pause for a moment on the term ‘smallsat,’ which generally refers to a spacecraft that is both small and lightweight, usually less than 500 kilograms, and sometimes much less, as when we get into the realm of CubeSats. Frequently in the news as we explore their capabilities, CubeSats can get down to less than 2 kilograms. What the authors have in mind is a demonstrator design that is scalable, the initial payload in the 1-2 kilogram range, but capable of moving up to between 36 and 50 kilograms.

The goal is a demonstrator mission that will perform a one to two-year test flight using a solar sail and a sundiver maneuver to achieve speeds greater than 5 AU per year. The figure works out to something on the order of 23.6 kilometers per second, an impressive feat given that Voyager 1, our current record holder, is moving at 17.1 kps. With the TDM demonstrating the capabilities of the sail’s vane structure and the needed control for perihelion passage, the full solar gravitational lens mission contemplates still higher velocities, reaching 20 AU per year (roughly 95 kilometers per second).

The SGL mission concept is being built around in-flight cruise assembly of the full spacecraft through modules separately delivered as 20 kilogram or less smallsats. Given that overall design, you can see the need for the demonstrator mission to shake out both sail and sundiver concepts. Thus, while the TDM payload includes science instruments, the real focus here is on demonstrating the method: Use smallsat technologies with a highly maneuverable sailcraft to enable the fast travel times that will make reaching the focal region feasible. This is not the place to get into exoplanet imaging; we’ve discussed what a full-scale SGL mission could accomplish in these pages before. See, for example, A Mission Architecture for the Solar Gravity Lens.

So let’s focus on the sail and the sundiver maneuver. In the last post I mentioned the unusual design of the sail, which grows out of work at JPL in conjunction with L’Garde, further refined by space services company Xplore. The sail design, pictured below, draws on square panels aligned along a truss to provide the cumulative sail area needed for the mission. It’s a striking object, not the conventional image of a solar sail – I did a double take when I first encountered it in 2020. L’Garde has put together an eye-catching 1:3 scale model that hangs at the Xplore facility in Washington state.

Image: This is Figure 3 from the paper. Caption: TDM vehicle configurations (PDR: July 18, 2022). Credit: Turyshev at al.

The LightCraft TDM is envisioned as a 3-axis controlled spacecraft capable of the attitude control crucial for the Sundiver maneuver it will perform to reach cruise speed. Here are a few relevant details from the paper. Note the remark at paragraph close:

Each sail element, or vane, can also be articulated to provide fine control to both the resultant thrust from solar radiation pressure and the vehicle’s attitude. Each dynamic vane element is also a multifunctional structure hosting photovoltaics and communication elements with the requisite degrees of freedom to meet competing operational and mission requirements. The current TDM design total vane area is 120 m2 and the mass of the integrated TDM vehicle is 5.45 kg, resulting in an area-to-mass ratio of A/m = 22 m2/kg, or nearly 3 times the performance of other existing and planned sailcraft.

The mission concept relies on placing the sailcraft in a trajectory that takes it to solar perihelion – head first for the Sun, then leave it at high velocity, using the momentum of solar photons to push the craft, and again using the precise attitude control available through the SunVane design to adjust subsequent trajectory as needed. What this trajectory demands, then, is sail materials that can withstand a perihelion in the range of 15 to 20 solar radii, which the Phase III study research indicates will be available within the present decade.

This proof-of-concept demonstrator mission would aim at deployment through a rideshare launch, sharply reducing the cost in comparison with larger payloads, with checkout in a ‘super-synchronous’ orbit (meaning higher than geostationary orbit and moving faster than Earth’s rotation). The paper describes an ‘outspiral’ into interplanetary space following the checkout phase, with a pivot at perihelion (listed here as 0.24 AU) to harvest the solar momentum needed to reach cruise velocity. The SunVane design allows the necessary maneuvering, as follows:

The trajectory is achieved with three simple control laws to maneuver the vehicle from geosynchronous orbit to perihelion and then egress: 1) maximum acceleration: align vanes perpendicular to the Sun to increase velocity; 2) no acceleration: align vanes edge-on to the Sun; and 3) maximum deceleration: align vanes so that the resultant force is opposite to the heliocentric velocity vector, to decrease orbital kinetic energy.

Image: This is Figure 2 from the paper. Caption: Common TDM mission phases and systems engineering objectives. Trajectory plot shown is for the SGL mission. Credit: Turyshev et al.

You would think the diciest part of the mission would be at perihelion (and of course it’s crucial), but I was interested to see that the authors consider the most dynamic phase for the sailcraft is during the exit from Earth, where the vehicle alternates between acceleration and no-acceleration (factoring in eclipse periods). Reaching interplanetary space, the sail decelerates inward toward the Sun. The sail vanes are re-oriented at perihelion, with six degrees of freedom to ensure responsiveness to error.

All of this, the authors report, is well within the capabilities of the kind of onboard inertial sensors we already use in space operations. With the vanes used for propulsion, attitude determination and control are handled by reaction wheels, gyro, star tracker, sun sensors and accelerometers for yaw, pitch and roll. The preliminary studies reported in this paper show a sail area on the order of 100–144 m2, with the overall spacecraft mass coming in between 4.2 and 6.4 kg. Note that the demonstrator would use photovoltaic elements on the sail vanes for power. Future missions to the outer system will also demand radioisotope power.

I’ll turn you to the paper for further details about how the smallsat/sail concept can scale the TDM into future missions, such as sail material (currently Kapton but with other choices emerging), insulation for perihelion, and the various investigations re communications, batteries and the development of small radioisotope power sources.

So how likely is a Technology Demonstrator Mission to fly? The next steps are cited in the paper:

The 2020 NIAC Phase III study concluded with a TDM Preliminary Design Review (PDR) on July 18, 2022 [7]. Next is pre-project mission development, which includes final design, hardware development, full-scale prototype construction, as well as hardware and software testing… Should funding be available, the TDM Critical Design Review (CDR) may be conducted in November 2023, when flight project commitment is expected, including a firm costing of the TDM. The total project cost will depend on the selected mission objectives, science payload, and experiments, and is expected to be in the range of $17–20M.

It’s compelling to learn that a lightweight sundiver mission may be built at a cost of tens of millions (the authors cite $30-75 million), which is quite a contrast to the $2 to $5 billion cost of the typical flagship mission to deep space. Developing such technologies pushes us forward on the miniaturization of scientific sensors that will benefit all classes of future missions to deep space. But numerous opportunities would also open up for targets closer to home in the Solar System. We’ll look at some of those next time.

The paper is Turyshev et al., “Science opportunities with solar sailing smallsats,” available as a preprint.


The Emerging Sail/Cubesat Paradigm for Deep Space

We need to get to the ice giants. We have limited enough experience with our system’s larger gas giants, although orbital operations at both Jupiter and Saturn have been highly successful. But about the ice giants, their formation, their interiors, their moons (and even the possibility of internal oceans on these objects), we draw on only a single mission, Voyager II. Which is why the April 2022 decadal study (“Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032”) recommended a Uranus mission, complete with orbiter, to be launched in the late 2030s.

Can we do this under our existing paradigm for space exploration? A new paper titled “Science opportunities with solar sailing smallsats,” written by the Jet Propulsion Laboratory’s Slava Turyshev and co-authored by major proponents of solar sail technologies, makes the case for coupling our abundant advances in miniaturization with our growing experience in solar sails to achieve missions at significantly lower cost and substantial savings in time. Because staying within the traditional game plan, we are constrained by slow chemical propulsion (or low-readiness nuclear methods) as well as decades of mission planning, not to mention cruise times in the range of 15 years to reach Uranus. These are numbers that can and should be improved, and greatly so.

Fortunately, solar sailing is moving beyond the range of experiment toward practical missions that will build on each other to advance a new paradigm – smaller and faster. Much smaller and much faster. Consider: The Japanese IKAROS sail has already demonstrated the interplanetary possibilities of sails, while the success of The Planetary Society’s LightSail-2 helped to energize the NEA-Scout mission NASA launched in 2022. Concept studies continue. Japan developed OKEANOS, a hybrid sail/ion engine design as an outer planet mission as a follow-on to IKAROS (the mission was a finalist for funding but lost out to a space telescope called LiteBIRD).

But sail technology must be wed with practical payloads, and spacecraft acceleration is proportional to the sail area divided by the spacecraft mass, which means that miniaturization and the use of smallsats win on efficiency. Here we’re reminded of the recent success of the Mars Cube One (MarCO) smallsats, which worked in conjunction with the InSight Lander and demonstrated the practicality of the highly modular and integrated CubeSat format for missions well beyond Earth orbit (see MarCO: Taking CubeSat Technologies Interplanetary). Let’s remember too the advantage of smallsat launches as ‘rideshare’ payloads, significantly reducing the outlay needed.

Image: The first image captured by one of NASA’s Mars Cube One (MarCO) CubeSats. The image, which shows both the CubeSat’s unfolded high-gain antenna at right and the Earth and its moon in the center, was acquired by MarCO-B on May 9, 2018. Credit: NASA/JPL-Caltech.

Solar sails are fast and, using the momentum of solar photons, require no onboard propellant, as both chemical and electrical methods do. Wedding sail propulsion to miniaturization in smallsats opens the way for spacecraft sent on ‘sundiver’ trajectories to harvest momentum from solar photons for the push to the outer Solar System. Here we’re taking advantage of a sail’s ability to change orbit by adjusting its attitude, another obvious plus. The authors believe that sailcraft built along these lines can achieve speeds of 33 kilometers per second, which works out to roughly 7 AU per year.

All of this leads to particular types of mission. From the paper:

As the solar sailing smallsats will be placed on very fast trajectories, placing Sundivers in orbit around a solar system body will be challenging. However they naturally yield several mission types including fast flybys, impactors, formation flights, and swarms. As the weight of the system is constrained, any instruments on board need to be small, lightweight, and low-power. Given the ongoing efforts in miniaturization of many instruments and subsystems, these challenges will be met by our industry partners who are already engaged in related technology developments.

As we continue to refine sail materials and advance deployment strategies, we are also learning how to harden smallsat computers for deep space while modularizing their components. Jupiter will be reachable with cruise times of two years, Saturn with three. What looms now is further development in the form of a technology demonstration mission (TDM) that has grown out of Turyshev and team’s Phase III study for NASA’s Innovative Advanced Concepts Office based on a sailcraft design that may one day reach the Sun’s gravity lens, which for effective science begins at 550 AU and extends outward.

The TDM would further develop solar sail technologies with an eye toward the kind of ‘sundiver’ maneuver that would make such fast missions possible. It will be enabled by a series of preparatory solar sail flights that will validate the final TDM vehicle.

Coming back briefly to an ice giant mission, designing and building the kind of craft envisioned in the 2022 Decadal is at least a decade’s work, and the cost of sending an orbiter to Uranus likely pushes beyond $4 billion. We’re contemplating this at the same time that the Decadal Survey is recommending, as its second highest priority for the upcoming decade, an Enceladus orbiter/lander flagship mission. NASA’s budget would be strained to the maximum to get even the Uranus mission off in the 2030s, which would push our next encounter with the ice giants back yet another decade.

The authors argue that we need to get realistic about what we can do with fast flybys not just to the ice giants but to numerous destinations in the Solar System. In the next several posts, I want to explore the TDM mission as presented in the new paper, considering the mission concept and implications before moving on to look at the kind of destinations the combination of sails and smallsats will enable us to reach.

Image: This is from the paper’s Figure 1, showing sailcraft design evolution during the period of 2016-2022. We’ll talk about this unusual configuration in the next post.

The paper is Turyshev et al., “Science opportunities with solar sailing smallsats,” available as a preprint.


Solar Sailing: The Beauties of Diffraction

Knowing of Grover Swartzlander’s pioneering work on diffractive solar sails, I was not surprised to learn that Amber Dubill, who now takes the idea into a Phase III study for NIAC, worked under Swartzlander at the Rochester Institute of Technology. The Diffractive Solar Sailing project involves an infusion of $2 million over the next two years, with Dubill (JHU/APL) heading up a team that includes experts in traditional solar sailing as well as optics and metamaterials. A potential mission to place sails into a polar orbit around the Sun is one possible outcome.

[Addendum: The original article stated that the Phase III award was for $3 million. The correct amount is $2 million, as changed above].

But let’s fall back to that phrase ‘traditional solar sailing,’ which made me wince even as I wrote it. Solar sailing relies on the fact that while solar photons have no mass, they do impart momentum, enough to nudge a sail with a force that over time results in useful acceleration. Among those of us who follow interstellar concepts, such sails are well established in the catalog of propulsion possibilities, but to the general public, the idea retains its novelty. Sails fire the imagination: I’ve found that audiences love the idea of space missions with analogies to the magnificent clipper ships of old.

We know the method works, as missions like Japan’s IKAROS and NASA’s NanoSail-D2 as well as the Planetary Society’s LightSail 2 have all demonstrated. Various sail missions – NEA Scout and Solar Cruiser stand out here – are in planning to push the technology forward. These designs are all reflective and depend upon the direction of sunlight, with sail designs that are large and as thin as possible. What the new NIAC work will examine is not reflection but diffraction, which involves how light bends or spreads as it encounters obstacles. Thus a sail can be built with small gratings embedded within the thin film of its structure, and the case Swartzlander has been making for some time now is that such sails would be more efficient.

A diffractive sail can work with incoming light at a variety of angles using new metamaterials, in this case ‘metafilms,’ that are man-made structures with properties unlike those of naturally occurring materials. Sails made of them can be essentially transparent, meaning they will not absorb large amounts of heat from the Sun, which could compromise sail substrates.

Moreover, these optical films allow for lower-mass sails that are steered by electro-optic methods as opposed to bulky mechanical systems. They can maintain more efficient positions while facing the Sun, which also makes them ideal for the use of embedded photovoltaic cells and the collection of solar power. Reflective sails need to be tilted to achieve best performance, but the inability to fly face-on in relation to the Sun reduces the solar flux upon the sail.

The Phase III work for NIAC will take Dubill and team all the way from further analyzing the properties of diffractive sails into development of an actual mission concept involving multiple spacecraft that can collectively monitor solar activity, while also demonstrating and fine-tuning the sail strategy. The description of this work on the NIAC site explains the idea:

The innovative use of diffracted rather than reflected sunlight affords a higher efficiency sun-facing sail with multiplier effects: smaller sail, less complex guidance, navigation, and attitude control schemes, reduced power, and non-spinning bus. Further, propulsion enhancements are possible by the reduction of sailcraft mass via the combined use of passive and active (e.g., switchable) diffractive elements. We propose circumnavigating the sun with a constellation of diffractive solar sails to provide full 4? (e.g., high inclination) measurements of the solar corona and surface magnetic fields. Mission data will significantly advance heliophysics science, and moreover, lengthen space weather forecast times, safeguarding world and space economies from solar anomalies.

Delightfully, a sail like this would not present the shiny silver surface of the popular imagination but would instead create a holographic effect that Dubill’s team likens to the rainbow appearance of a CD held up to the Sun. And they need not be limited to solar power. Metamaterials are under active study by Breakthrough Starshot because they can be adapted for laser-based propulsion, which Starshot wants to use to reach nearby stars through a fleet of small sails and tiny payloads. The choice of sail materials that can survive the intense beam of a ground-based laser installation and the huge acceleration involved is crucial.

The diffractive sail concept has already been through several iterations at NIAC, with the testing of different types of sail materials. Grover Swartzlander received a Phase I grant in 2018, followed by a Phase II in 2019 to pursue the work, a needed infusion of funding given that before 2017, few papers on diffractive space sails existed in the literature. In a 2021 paper, Dubill and Swartzlander went into detail on the idea of a constellation of sails monitoring solar activity. From the paper:

We have proposed launching a constellation of satellites throughout the year to build up a full-coverage solar observatory system. For example a constellation of 12 satellites could be positioned at 0.32 AU and at various inclinations about the sun within 6 years: Eight at various orbits inclined by 60 and four distributed about the solar ecliptic. We know of no conventionally powered spacecraft that can readily achieve this type of orbit in such a short time frame. Based on our analysis, we find that diffractive solar sails provide a rapid and cost-effective multi-view option for investigating heliophysics.

Image: The new Diffractive Solar Sailing concept uses light diffraction to more efficiently take advantage of sunlight for propulsion without sacrificing maneuverability. Incidentally, this approach also produces an iridescent visual effect. Credit: RIT/?MacKenzi Martin.

Dubill thinks an early mission involving diffractive sails can quickly prove their value:

“While this technology can improve a multitude of mission architectures, it is poised to significantly impact the heliophysics community’s need for unique solar observation capabilities. Through expanding the diffractive sail design and developing the overall sailcraft concept, the goal is to lay the groundwork for a future demonstration mission using diffractive lightsail technology.”

A useful backgrounder on diffractive sails and their potential use in missions to the Sun is Amber Dubill’s thesis at RIT, “Attitude Control for Circumnavigating the Sun with Diffractive Solar Sails” (2020), available through RIT Scholar Works. See also Dubill & Swartzlander, “Circumnavigating the sun with diffractive solar sails,” Acta Astronautica
Volume 187 (October 2021), pp. 190-195 (full text). Grover Swartzlander’s presentation “Diffractive Light Sails and Beam Riders,” is available on YouTube.


Interstellar Implications of the Electric Sail

Not long ago we looked at Greg Matloff’s paper on von Neumann probes, which made the case that even if self-reproducing probes were sent out only once every half million years (when a close stellar encounter occurs), there would be close to 70 billion systems occupied by such probes within a scant 18 million years. Matloff now considers interstellar migration in a different direction in a new paper addressing how M-dwarf civilizations might expand, and why electric sails could be their method.

It’s an intriguing notion because M-dwarfs are by far the most numerous stars in the galaxy, and if we learn that they can support life, they might house vast numbers of civilizations with the capability of sending out interstellar craft. They’re also crippled in terms of electromagnetic flux when it comes to conventional solar sails, which is why the electric sail comes into play as a possible alternative, here analyzed in terms of feasibility and performance and its prospects for enabling interstellar migration.

The term ‘sail’ has to be qualified. By convention, I’ve used ‘solar sail,’ for example, to describe sails that use the momentum imparted by stellar photons – Matloff often calls these ‘photon sails,’ which is also descriptive, though to my mind, a ‘photon sail’ might describe both a beam-driven as well as a stellar photon-driven sail. Thus I prefer ‘lightsail’ for the beamed sail concept. In any case, we have to distinguish all these concepts from the electric sail, which operates on fundamentally different principles.

In our Solar System, a sail made of absorptive graphene deployed from 0.1 AU could achieve a Solar System escape velocity of 1000 kilometers per second, and perhaps better if the mission were entirely robotic and not dealing with fragile human crews. The figure seems high, but Matloff gave the calculations in a 2012 JBIS paper. The solar photon sail wins on acceleration, and we can use the sail material to provide extra cosmic ray shielding enroute. These are powerful advantages near our own Sun.

But the electric sail has advantages of its own. Rather than drawing on the momentum imparted by solar photons (or beamed energy), an electric sail rides the stellar wind emanating from a star. This stream of charged particles has been measured in our system (by the WIND spacecraft in 1995) as moving in the range of 300 to 800 kilometers per second at 1 AU, a powerful though extremely turbulent and variable force that can be applied to a spacecraft. Because an interstellar craft entering a destination system would also encounter a stellar wind, an electric sail can be deployed for deceleration, something both forms of sail have in common.

How to harness a stellar wind? Matloff first references a 2008 paper from Pekka Janhunen (Finnish Meteorological Institute) and team that described long tethers (perhaps reaching 20 kilometers in length) extended from the spacecraft, each maintaining a steady electric potential with the help of a solar-powered electron gun aboard the vehicle. As many as a hundred tethers — these are thinner than a human hair — could be deployed to achieve maximum effect. While the solar wind is far weaker than solar photon pressure, an electric sail of this configuration with tethers in place can create an effective solar wind sail area of several square kilometers.

We need to maintain the electric potential of the tethers because it would otherwise be compromised by solar wind electrons. The protons in the solar wind – again, note that we’re talking about protons, not photons – reflect off the tethers to drive us forward.

Image: Image of an electric sail, which consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of meters into the surrounding solar wind plasma. Therefore the solar wind ions “see” the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or “flying” the resulting spacecraft electrically. Credit: Artwork by Alexandre Szames. Caption via Pekka Janhunen/Kumpula Space Center.

For interstellar purposes, we look at much larger spacecraft, bearing in mind that once in deep space, we have to turn off the electron gun, because the interstellar medium can itself decelerate the sail. Operating from a Sun-like star, the electric sail generation ship Matloff considers is assumed to have a mass of 107 kg, assuming a constant solar wind within the heliosphere of 600 kilometers per second. The variability of the solar wind is acknowledged, but the approximations are used to simplify the kinematics. The paper then goes on to compare performance near the Sun with that near an M-dwarf star.

We wind up with some interesting conclusions. First of all, an interstellar mission from a G-class star like our own would be better off using a different method. We can probably reach an interstellar velocity of as high as 70 percent of this assumed constant solar wind velocity (Matloff’s calculations), but graphene solar sails can achieve better numbers. And if we add in the variability of the solar wind, we have to be ready to constantly alter the enormous radius of the electric field to maintain a constant acceleration. If we’re going to send generation ships from the Sun, we’re most likely to use solar sails or beamed lightsails.

But things get different when we swing the discussion around to red dwarf stars. In The Electric Sail and Its Uses, I described a paper from Avi Loeb and Manasvi Lingam in 2019 that studied electric sails using the stellar winds of M-dwarfs, with repeated encounters with other such stars to achieve progressively higher speeds. Matloff agrees that electric sails best photon sails in the red dwarf environment, but adds useful context.

Let’s think about generation ships departing from an M-dwarf. Whereas the electromagnetic flux from these stars is far below that of the Sun, the stellar wind has interesting properties. We learn that it most likely has a higher mass density (in terms of rate per unit area) than the Sun, and the average stellar wind velocity is 500 kilometers per second. Presumably a variable electric field aboard the craft could adjust to maintain acceleration as the vehicle moves outward from the star, although the paper doesn’t get into this. The author’s calculations show an acceleration, for a low-mass spacecraft about 1 AU from the Sun, of 7.6 × 10?3 m/s2 , or about 7.6 × 10?4 g. Matloff considers this a reasonable acceleration for a worldship.

So while low electromagnetic pressure makes photon sails far less effective at M-dwarfs as opposed to larger stars, electric sails remain in the mix for civilizations willing to contemplate generation ships that take thousands of years to reach their goal. In an earlier paper, the author considered close stellar encounters, pointing out that 70,000 years ago, the binary known as Scholz’s Star (it has a brown dwarf companion) passed within 52,000 AU of the Sun. We can expect another close pass (Gliese 710) in about 1.35 million years, this one closing to a perihelion of 13,365 AU. From the paper:

Bailer?Jones et al. have used a sample of 7.2 million stars in the second Gaia data release to further investigate the frequency of close stellar encounters. The results of this analysis indicate that seven stars in this sample are expected to approach within 0.5 parsecs of the Sun during the next 15 million years. Accounting for sample incompleteness, these authors estimate that about 20 stars per million years approach our solar system to within 1 parsec. It is, therefore, inferred that about 2.5 encounters within 0.5 parsecs will occur every million years. On average, 400,000 years will elapse between close stellar encounters, assuming the same star density as in the solar neighborhood.

If interstellar missions were only attempted during such close encounters, we still have a mechanism for a civilization to use worldships to expand into numerous nearby stellar systems. It would take no more than a few star-faring civilizations around the vast number of M-dwarfs to occupy a substantial fraction of the Milky Way, even without the benefits of von Neumann style self-reproduction. With the number of planetary systems occupied doubling every 500,000 years, and assuming a civilization only sends out a worldship during close stellar encounters, we get impressive results. In the clip below, n = the multiple of 500,000 years. The number of systems occupied is P:

At the start, n = 0 and P = 1. When 500,000 years have elapsed, the hypothetical spacefaring civilization makes the first transfer, n = 1 and P = 2. After one million years (n = 2), both the original and occupied stellar systems experience a close stellar encounter, migration occurs and P = 4. After a total elapsed time of 1.5 million years, n = 3 and they occupy eight planetary systems. When n = 5, 10 and 20 the hypothetical civilization has respectively occupied 32, 1024 and 1,048,576 planetary systems.

With M-dwarfs being such a common category of star, learning more about their systems’ potential habitability will have implications for the possible spread of technological societies, even assuming propulsion technologies conceivable to us today. What faster modes may eventually become available we cannot know.

The paper is Matloff, “The Solar?Electric Sail: Application to Interstellar Migration and Consequences for SETI,” Universe 8(5) (19 April 2022), 252 (full text). The Lingam and Loeb paper is “Electric sails are potentially more effective than light sails near most stars,” Acta Astronautica Volume 168 (March 2020), 146-154 (abstract).


Toward a Multilayer Interstellar Sail

Centauri Dreams tracks ongoing work on beamed sails out of the conviction that sail designs offer us the best hope of reaching another star system within this century, or at least, the next. No one knows how this will play out, of course, and a fusion breakthrough of spectacular nature could shift our thinking entirely – so, too, could advances in antimatter production, as Gerald Jackson’s work reminds us. But while we continue the effort on all alternative fronts, beamed sails currently have the edge.

On that score, take note of a soon to be available two-volume set from Philip Lubin (UC-Santa Barbara), which covers the work he and his team have been doing under the name Project Starlight and DEEP-IN for some years now. This is laser-beamed propulsion to a lightsail, an idea picked up by Breakthrough Starshot and central to its planning. The Path to Transformational Space Exploration pulls together Lubin and team’s work for NASA’s Innovative Advanced Concepts office, as well as work funded by donors and private foundations, for a deep dive into where we stand today. The set is expensive, lengthy (over 700 pages) and quite technical, definitely not for casual reading, but those of you with a nearby research library should be aware of it.

Just out in the journal Communications Materials is another contribution, this one examining the structure and materials needed for a lightsail that would fly such a mission. Giovanni Santi (CNR/IFN, Italy) and team are particularly interested in the question of layering the sail in an optimum way, not only to ensure the best balance between efficiency and weight but also to find the critical balance between reflectance and emissivity, because we have to build a sail that can survive its acceleration.

What this boils down to is that we are assuming a laser phased array producing a beam that is applied to an extremely thin, lightweight structure, with the intention of reaching a substantial percentage (20 percent) of lightspeed. The laser flux at the spacecraft is, the paper notes, on the order of 10 – 100 GW m-2, with the sail no further from the Earth than the Moon. Lubin’s work at UC-Santa Barbara (he is a co-author here) has demonstrated a directed energy source with emission at 1064 nm.

Thermal issues are critical. The sail has to survive the temperature increases of the acceleration phase, so we need materials that offer high reflectance as well as the ability to blow off that heat, meaning high emissivity in the infrared. The Santa Barbara laboratory work has used ytterbium-doped fiber laser amplifiers in what the paper describes as a ‘master oscillator phased array’ topology. And this gets fascinating from the relativistic point of view. Remember, we are trying to produce a spacecraft that can make the journey to a nearby star in a matter of decades, so relativistic effects have to be considered.

In terms of the sail itself, that means that our high-speed spacecraft will quickly see a longer wavelength in the beamed energy than was originally emitted. This is true despite the fact that the period of acceleration is short, on the order of minutes.

The authors suggest that there are two ways to cope with this. The laser can shorten its emission wavelength as the spacecraft accelerates, meaning that the received wavelength is constant. But the paper focuses on a second option: Make the reflecting surface broadband enough to allow a fairly large range of received wavelengths.

Thus the core of this paper is an analysis of the materials possible for such a sail and their thermal properties, keeping this wavelength change in mind, while at the same time studying – after the operative laser wavelength is determined – how the structure of the lightsail can be engineered to survive the extremities of the acceleration phase.

Image: This is Figure 1 from the paper. Caption: The red arrows denote the incident, transmitted and reflected laser power, while the violet ones indicate the thermal radiation leaving the structure from the front and back surfaces. The surface area is modeled as ?D2; ??=?1 for a squared lightsail of side D and ??=??/4 for a circular lightsail of diameter D. Credit: Santi et al.

The paper considers a range of possible materials for the sail, all of them low in density and widely used, so that their optical parameters are readily available in the literature. Optimization is carried out for stacks of different materials in combination to find structures with maximum optical properties and highest performance. Critical parameters are the reflectance and the areal density of the resulting sail.

Out of all this, titanium dioxide (TiO2) stands out in terms of thermal emission. This work suggests a combination of stacked materials:

The most promising structures to be used with a 1064?nm laser source result to be the TiO2-based ones, in the form of single layer or multilayer stack which include the SiO2 as a second material. In term of propulsion efficiency, the single layer results to be the most performing, while the multilayers offer some advantages in term[s] of thermal control and stiffness. The engineering process is fundamental to obtain proper optical characteristics, thus reducing the absorption of the lightsail in the Doppler-shifted wavelength of the laser in order to allow the use of high-power laser up to 100 GW. The use of a longer wavelength laser source could expand the choice of potential materials having the required optical characteristics.

So much remains to be determined as this work continues. The required mechanical strength of the multilayer structure means we need to learn a lot more about the properties of thin films. Also critical is the stability of the lightsail. We want a sail that survives acceleration not only physically but also in terms of staying aligned with the axis of the beam that is driving it. The slightest imperfection in the material, induced perhaps in manufacturing, could destroy this critical alignment. A variety of approaches to stability have emerged in the literature and are being examined.

The take-away from this paper is that thin-film multilayers are a way to produce a viable sail capable of being accelerated by beamed energy at these levels. We already have experience with thin films in areas like the coatings deposited on telescope mirrors, and because the propulsion efficiency is only slightly affected by the angle at which the beam strikes the sail, various forms of curved designs become feasible.

Can a sail survive the rigors of a journey through the gas and dust of the interstellar medium? At 20 percent of c, the question of how gas accumulates in materials needs work, as we’d like to arrive at destination with a sail that may double as a communications tool. Each of these areas, in turn, fragments into needed laboratory work on many levels, which is why a viable effort to design a beamed mission to a star demands a dedicated facility focusing on sail materials and performance. Breakthrough Starshot seems ideally placed to make such a facility happen.

The paper is Santi et al., “Multilayers for directed energy accelerated lightsails,” Communications Materials 3 (16 (2022). Abstract.