Braking at Centauri: A Bound Orbit at Proxima?

One of the great problems of lightsail concepts for interstellar flight is the need to decelerate. Here I’m using lightsail as opposed to ‘solar sail’ in the emerging consensus that a solar sail is one that reflects light from our star, and is thus usable within the Solar System out to about 5 AU, where we deal with the diminishment of photon pressure with distance. Or we could use the Sun with a close solar pass to sling a solar sail outbound on an interstellar trajectory, acknowledging that once our trajectory has been altered and cruise velocity obtained, we might as well stow the now useless sail. Perhaps we could use it for shielding in the interstellar medium or some such.

A lightsail in today’s parlance defines a sail that is assumed to work with a beamed power source, as with the laser array envisioned by Breakthrough Starshot. With such an array, whether on Earth or in space, we can forgo the perihelion pass and simply bring our beam to bear on the sail, reaching much higher velocities. Of the various materials suggested for sails in recent times, graphene and aerographite have emerged as prime candidates, both under discussion at the recent Montreal symposium of the Interstellar Research Group. And that problem of deceleration remains.

Is a flyby sufficient when the target is not a nearby planet but a distant star? We accepted flybys of the gas giants as part of the Voyager package because we had never seen these worlds close up, and were rewarded with images and data that were huge steps forward in our understanding of the local planetary environment. But an interstellar flyby is challenging because at the speeds we need to reach to make the crossing in a reasonable amount of time, we would blow through our destination system in a matter of hours, and past any planet of interest in perhaps a matter of minutes.

Robert Forward’s ingenious ‘staged’ lightsail got around the problem by using an Earth-based laser to illuminate one part of the now separated sail ring, beaming that energy back to the trailing part of the sail affixed to the payload and allowing it to decelerate. Similar contortions could divide the sail again to make it possible to establish a return trajectory to Earth once exploration of the distant stellar system was complete. We can also consider using magsail concepts to decelerate, or perhaps the incident light from a bright target star could allow sufficient energy to brake against.

Image: Forward’s lightsail separating at the beginning of its deceleration phase. Laser sailing may turn out to be the best way to the stars, provided we can work out the enormous technical challenges of managing the outbound beam. Or will we master fusion first? Credit: R.L. Forward.

But time is ever a factor, because you want to reach your target quickly, while at the same time, if you approach it too fast, you’re incapable of creating the needed deceleration. Moreover, what is your target? A bright star gives you options for deceleration if you approach at high velocity that are lacking from, say, a red dwarf star like Proxima Centauri, where the closest terrestrial-class world we know is in what appears to be a habitable zone orbit. In Montreal, René Heller (Max Planck Institute for Solar System Research), a familiar name in these pages, laid out the equations for a concept he has been developing for several years, a mission that could use not only the light of Proxima itself but from Centauri A and B to create a deceleration opportunity. You can follow Heller’s presentation at Montreal here.

Remember what we’re dealing with here. We have two stars in the central binary, Centauri A (G-class) and Centauri B (K-class), with the M-class dwarf Proxima Centauri about 13000 AU distant. Centauri A and B are close – their distance as they orbit around a common barycenter varies from 35.6 AU to 11.2 AU. These are distances in Solar System range, meaning that 35.6 AU is roughly the orbit of Neptune, while 11.2 AU is close to Saturn distance. Interesting visual effects in the skies of any planet there.

Image: Orbital plot of Proxima Centauri showing its position with respect to Alpha Centauri over the coming millennia (graduations are in thousands of years). The large number of background stars is due to the fact that Proxima Cen is located very close to the plane of the Milky Way. Proxima’s orbital relation to the central stars becomes profoundly important in the calculations Heller and team make here. Credit: P. Kervella (CNRS/U. of Chile/Observatoire de Paris/LESIA), ESO/Digitized Sky Survey 2, D. De Martin/M. Zamani.

Using a target star for deceleration by braking against incident photons has been studied extensively, especially in recent years by the Breakthrough Starship team, where the question of how its tiny sailcraft could slow from 20 percent of the speed of light to allow longer time at target is obviously significant. Deceleration into a bound orbit at Proxima would be, of course, ideal but it turns out to be impossible given the faint photon pressure Proxima can produce. Investing decades of research and 20 years of travel time is hardly efficient if time in the system is measured in minutes.

In fact, to use photon pressure from Proxima Centauri, whose luminosity is 0.0017 that of the Sun, would require approaching the star so slowly to decelerate into a bound orbit that the journey would take thousands of years. Hence Heller’s notion of using the combined photon pressure and gravitational influences of Centauri A and B to work deceleration through a carefully chosen trajectory. In other words, approach A, begin deceleration, move to B and repeat, then emerge on course outbound to Proxima, where you’re now slow enough to use its own photons to enter the system and stay.

Working with Michael Hippke (Max Planck Institute for Solar System Research, Göttingen) and Pierre Kervella (CNRS/Universidad de Chile), Heller has refined the maximum speed that can be achieved on the approach into Alpha Centauri A to make all this happen: 16900 kilometers per second. If we launch in 2035, we arrive at Centauri A in 2092, with arrival at Centauri B roughly six days later and, finally, arrival at Proxima Centauri for operations there in a further 46 years. That launch time is not arbitrary. Heller chose 2035 because he needs Centauri A and B to be in precise alignment to allow the gravitational and photon braking effects to work their magic.

So we have backed away from Starshot’s goal of 20 percent of lightspeed to a more sedate 5.6 percent, but with the advantage (if we are patient enough) of putting our payload into the Proxima Centauri system for operations there rather than simply flying through it at high velocity. We also get a glimpse of the systems at both Centauri A and B. I wrote about the original Heller and Hippke paper on this back in 2017 and followed that up with Proxima Mission: Fine-Tuning the Photogravitational Assist. I return to the concept now because Heller’s presentation contrasts nicely with the Helicity fusion work we looked at in the previous post. There, the need for fusion to fly large payloads and decelerate into a target was a key driver for work on an in-space fusion engine.

Interstellar studies works, though, through multiple channels, as it must. Pursuing fusion in a flight-capable package is obviously a worthy goal, but so is exploring the beamed energy option in all its manifestations. I note that Helicity cites a travel time to Proxima Centauri in the range of 117 years, which compares with Heller and company’s now fine-tuned transit into a bound orbit at Proxima of 121 years. The difference, of course, is that Helicity can envision launching a substantially larger payload.

Clearly the pressure is on fusion to deliver, if we can make that happen. But the fact that we have gone from interstellar flight times thought to involve thousands of years to a figure of just over a century in the past few decades of research is heartening. No one said this would be easy, but I think Robert Forward would revel in the thought that we’re driving the numbers down for a variety of intriguing propulsion options.

The paper René Heller drew from in the Montreal presentation is Heller, Hippke & Kervella, “Optimized Trajectories to the Nearest Stars Using Lightweight High-velocity Photon Sails,” Astronomical Journal Vol. 154 No. 3 (29 August 2017), 115. Full text.

Sunshade: A New Trek through ‘Daedalus Country’

Letting the imagination roam has philosophical as well as practical benefits. From the interstellar perspective, consider the Daedalus starship, designed with loving detail by members of the British Interplanetary Society in the 1970s. The mammoth (54,000 ton) vehicle was never conceived as remotely feasible at our stage of technology. But ‘our stage of technology’ is exactly the point the project illustrated. Daedalus demonstrated that there was nothing in physical law to prevent the construction of a starship. The question was, when would we reach the level of building it? For as Robert Forward frequently pointed out, interstellar flight could no longer be considered impossible.

We can’t know the answer to the question, but recall that before Daedalus, there was a lot of ‘informed’ opinion that interstellar flight was a chimera, and that all species were necessarily restricted to their home systems. Daedalus made the point debatable. If a civilization had a thousand year jump on us in terms of tech, could they build this thing? Probably, but they’d also surely come up with far better methods than we in the 1970s could imagine. Daedalus was, then, a possibility maker, a driver for further imaginings.

Fortunately, the Daedalus impulse – and the broader concept of thought experiments that so captivated Einstein – remains with us. I think, for example, of Cliff Singer’s pellet-driven starship, one that would demand a particle accelerator fully 100,000 miles long. Crazy? Sure, but a few decades later we were talking about slinging nanochip satellites in swarms using Jupiter’s magnificent magnetic fields, finding a way to do with nature what was evidently impossible for us to build with our own hands.

Robert Forward used to conceive of enormous laser sails for interstellar exploration, sails whose outbound laser flux would be amplified by an even larger 560,000-ton Fresnel lens built between the orbits of Saturn and Uranus. But I discovered in a new paper from Greg Matloff (New York City College of Technology, CUNY) that it was James Early who introduced another extraordinary idea, that of using a gigantic sail-like structure not for propulsion but as a sunshade. Early’s 1989 paper in the Journal of the British Interplanetary Society specifically addressed the ‘greenhouse effect,’ which even then concerned scientists in terms of its effect on global climate. Could technology tame it?

Once again we’re in Daedalus country, or Forward country, if you will. Imagine a true megastructure, a 2000 kilometer sunshade located at the L1 Lagrange region between the Earth and the Sun, approximately 1.5 million kilometers from Earth. The five Lagrange points allow a spacecraft to remain in a relatively fixed orbital position in relation to two larger masses, in the case of L1 the Earth and the Sun. But L1 is not stable, which means that a structure like the sunshade would require thrusting capability for course correction to maintain its optimum position in relation to the Earth. Bear in mind as well the effect of solar radiation pressure on the shade.

Image: Physicist and prolific writer Greg Matloff, author of The Starflight Handbook (Wiley, 1989) and many other books and papers including the indispensable Deep Space Probes (Springer, 2005).

Would a 2000-kilometer shade be sufficient, assuming the intention of reducing the Earth’s effective temperature (255 K) by one K? We learn that solar flux would need to be reduced by 1.5 percent to reduce Earth’s EFF to 254 K. 2000 kilometers does in fact somewhat overshoot the need, reducing solar influx by about 2 percent. That’s a figure that changes over astronomical time, of course, for like any active star, the Sun experiences increased luminosity as it ages, but 2000 km certainly serves for now.

But how to build such a thing? Matloff looks at two versions of the technology, the first being a fully opaque, thick sunshade which would be constructed of lunar or perhaps asteroidal materials. Think in terms of a square sunshade with a thickness of 10-4 meters, and a density of 2,000 kg/m3, producing a mass of 8 X 1011 kg. Building such a thing on Earth is a non-starter, so we can think in terms of assembly in lunar orbit, with the shade materials taken from an asteroid of 460 meters in radius. Corrective thrusting via solar-electric methods with an exhaust velocity of 100 km/s adds up to an eye-opening fuel consumption of 400 kg/s.

But we have other options. Matloff goes on to consider a transparent diffractive film sail (Andreas Hein has recently explored this possibility). Here the sail is imprinted with a diffraction pattern that diverts incoming sunlight from striking the Earth. This is a sail that experiences low solar radiation pressure, its mass reaching 6.4 X 108 kg. But thinner transparent surfaces are feasible as the technology matures, reducing the mass on orbit to 107 kg. Such a futuristic sunshade could be built on Earth and delivered to LEO through 100 flights of today’s super-heavy launch vehicles. Presumably other options will emerge by the time we have the assembly capabilities.

Either of these designs would divert 5.6 X 1015 watts of sunlight from the Earth, energy that if directed to other optical devices would offer numerous possibilities. Matloff considers powering up laser arrays for asteroid mitigation, an in-space defensive system that would work with energy levels much higher than those available through currently envisioned systems like the proposed Breakthrough Starshot Earth-based laser array. A space-based system would also have the advantage of not being confined to a single hemisphere on the surface.

Other possibilities emerge. A laser near the sunshade could tap some of the solar flux and direct it to power stations in geosynchronous Earth orbit, where it would be converted into a microwave frequency to which the Earth’s atmosphere is transparent. You can see the political problem here, which Matloff acknowledges. Any such instrumentation clearly has implications as a weapon, demanding international governance, although through what mechanisms remains to be determined.

But let’s push this concept as hard as we can. How about accelerating a starship? Matloff works the math on a crewed generation ship accelerated to interstellar velocities, with travel time to the nearest star totaling about four centuries. The point is, this is an energy source that makes abundant solar power available while producing the desired reduction in temperatures on Earth, a benefit that could drive development of these technologies not only by us but conceivably by other civilizations as well. If such is a case, we have a new kind of technosignature:

If sufficiently large telescopes are constructed on Earth or in space, astronomers might occasionally survey the vicinity of nearby habitable planets for momentary visual glints. If these sporadic events correspond to the planet-star L1 point, they might constitute an observable technosignature of an existing advanced extraterrestrial civilization.

When considering technosignatures from ET sunshades, it is worth noting that a single monolithic sunshade might be replaced by two or more smaller devices. Also, an advanced extraterrestrial civilization may choose to place its sunshade in a location other than planet-star L1.

There is a Bob Forward quality to this paper that reminds me of Forward’s pleasure in delving into the feasibility of projects from the standpoint of physics while leaving open the issue of how engineers could create structures that at present seem fantastic. That quality might be described as ‘visionary,’ calling up, say, Konstantin Tsiolkovsky in its sheer sweep. Matloff, who knew Forward well, preserves Forward’s exuberance, the pleasure of painting what will be possible for our descendants, who as they one day leave our system will surely continue the exploration of their own ‘Daedalus country.’

The paper is Matloff, “The Lagrange Sunshade: Its Effectiveness in Combating Global Warming and Its Application to Earth Defense from Asteroid Impacts, Beaming Solar Energy for Terrestrial Use, Propelling Interstellar Migration by Laser-Photon Sails and Its Technosignature,” JBIS Vol. 76, No. 4 (April 2023). The Early paper is “Space-based solar shield to offset greenhouse effect,” JBIS Vol. 42, Dec. 1989, p. 567-569 (abstract).

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.