The second part of Jim Benford’s examination of Breakthrough Starshot concludes our look at the numerous issues advanced by Phase I of the project. Largely discounted in recent press coverage, the Starshot effort in fact completed a successful Phase I and left behind numerous papers that illuminate the path forward for interstellar flight. This is solid work on everything from laser arrays to metamaterials and the engineering of data return at light-year distances. Read on.

by James Benford

“I have learned to use the word ‘impossible’ with great caution.”
— Wernher Von Braun, after the lunar landing

In this second report, I will describe the major results of Starshot beginning with the mission scenario and then treating each major technical area in terms of how solutions have been resolved and issues retired. In Part 1, I described Phase 1 objectives.

One of the causes of Starshot results not being well-known publicly is that the Breakthrough Foundation has not publicized its events and results during most of its duration. After its completion, substantial reports have appeared, but are not commonly available to the public. There is a final report, but it has yet to be published. There are briefings by Harry Atwater at Breakthrough Discuss and the IRG in Montreal in 2023 [1,2].

The most detailed discussions are in the book Laser Propulsion in Space edited by Claude Phipps, with a system overview by Pete Worden and others, a description of his system model by Kevin Parkin and other aspects of directed energy in space by Philip Lubin, all in the one volume [3]. The Kevin Parkin article is particularly interesting because it contains fully worked-out examples of the possibility of future voyages of humans traveling to the stars in large >100 m sailcraft in future centuries. Note that there are many journal publications produced by Breakthrough Starshot. And there are many papers that have been published since Starshot was put on hold.

The Starshot Mission Scenario has evolved as a substantial improvement over previous beam-driven sail mission concepts. A mothership is launched which houses a fleet of membrane-like sailcraft measuring ~5 meters in diameter and less than a micron thick. The traditional standard laser guide star adaptive optic system can’t be scaled to Starshot-sized apertures to deal with the time–dependent fluctuations due to atmospheric turbulence. The system uses a satellite–based laser which is called the Beacon. It’s in an orbit at the launch time of apogee 200,000 km.

Image: Starshot system geometry. Arrows indicate that the array acquires atmospheric turbulence data from a Beacon and points the beam at the sailcraft. (Courtesy of Breakthrough Foundation.)

The sailcraft are composed of super-reflective metamaterials that stabilize the perturbations that could prevent beam-riding during the propulsion phase. The scientific instruments that are the payload are integrated into the sail. The mission begins as the mothership deploys a sailcraft into space.

Meanwhile on Earth, a phased array of 100 million small lasers turns on, generates ~100 GW of optical power and, using information from a Beacon in high orbit, digitally adjusts the phase of the emitted light to correct for atmospheric turbulence. These small lasers would be manufactured in printed sheets, following the fabrication techniques of the semiconductor industry. This is the means of lowering laser prices.

The single 100 GW beam focuses on the sail and accelerates the sail. Almost no energy is absorbed by the sail’s reflective surface, so imparting force. The sail rides the beam for ten minutes and reaches relativistic speed. It leaves the solar system in less than a week. Soon after acceleration it encounters dust and charged particles, so can be oriented edge-on to avoid such collisions. On arriving at the Alpha Centauri system, it captures images, detects dust and particles and measures fields. The sail transmits data home to an array of optical receivers on Earth, so it begins to arrive four years later. Data return may take decades because of limited data rate. Recall that complete data return from the New Horizons flyby took about a year.

The above figure shows a concept for the sail, about 5 m in diameter. Some studies show that at the velocity under consideration the gas and dust will pass through the thin sail with virtually no damage if it travels face-on. Only the payload would need protection. The sail can also be oriented edge-on in order to avoid such collisions, giving meters of material protection to the center. The payload is around the center, protected from damage due to incoming gas and dust.

Key issues for beam-driven sail systems have been retired by high levels of Starshot research. Most are resolved at the conceptual level. Experiments are needed to verify solutions for these major issues, discussed below.

Can phase be maintained across a large aperture composed of many sources? This is well demonstrated historically for microwaves, principally for radar. For lasers, a new concept has been quantified [4, 5]. Building the hundred million laser emitters into a large array is the driving technical challenge of the project. The principle of the design is to interferometrically link multiple arrays which are phase-locked into modular tiers of larger size. That is, multiple areas which are individually phase controlled would be linked together by interferometry. This approach of linking multiple optical phased arrays is called a hierarchical array. The array design that resulted has laser dimensions and total power levels that are about five orders of magnitude beyond present state of the art capabilities. To control the phase over such a large aperture is the most significant technical challenge to Starshot.

Can a sail material be found which can meet the many constraints on sail acceleration? Most materials effort has been for laser propulsion, where the leading candidate for sail material is silicon nitride. There are no fundamental limits to optimize that material for the key parameters of mass, reflectivity, refractive index, and thermal properties. (For microwaves various types of carbon are preferred, such as microtruss and graphene.)

Can the sail ride on the beam stably? (Feedback is impossible over long ranges.) If not, sails can veer off-course on millisecond timescale. The notion of beam-riding, stable flight of a sail propelled by a beam, places considerable emphasis on the sail shape. Even for a steady beam, the sail can wander off if its shape becomes deformed or if it does not have enough spin to keep its angular momentum aligned with the beam direction in the face of perturbations. Beam pressure will keep a concave shape sail in tension, and it will resist sidewise motion if the beam moves off-center, as a sidewise restoring force restores it to its position. Early stability experiments verified that beam-riding does occur with a conical sail [6].

Experiments and simulations show that conical sails ride a microwave beam stably. The carbon–carbon sail diameter is 5 cm, height 2 cm, and mass 0.056 g.

Beam riding and structural stability is difficult. (a), beam-riding stability, where bold upward arrows depict accelerating beam, light upward arrows the force of radiation pressure, downward arrows the direction of reflected light (b) structural stability methods (c) mechanical issues [7].

Meter-scale shaped sails of submicron, ~100 atomic layer thickness can ride with stability along the axis of the accelerating beam despite the many types of deformations caused by photon pressure and thermal expansion. There is also a requirement for structural stability, the ability to survive acceleration without collapse, and crumpling under acceleration, as depicted in the figure above. And there could be thermal and tensile failure as well as rupture of sail materials. Many studies of this issue have shown multiple solutions.

Stable designs exist for concave shapes and for flat flexible sails with millimeter scale photonic structures to control reflections. (Simple flat sails cannot achieve beam-riding stability because specular reflection produces forces only normal (perpendicular) to the surface.) A considerable advantage of flat sails is that curved sail shapes are more difficult to fabricate at meter scales. However, Starshot has shown that even flat sails can beam-ride by tailoring asymmetric optical properties to produce transverse restoring forces with millimeter-scale photonic structures to control reflections. So a flat sailcraft can be modified to scatter light as if it were curved. For example, the Swartzlander group, in a series of theoretical, computational, and experimental studies, has shown that a flat sail whose reflecting surface is equipped with diffractive gratings is directionally stable [8,9]. Anisotropic scattering of incident light into the grating diffraction orders manifests in optical restoring forces transverse to the membrane, redirecting incident photon momentum to produce beam-riding.

Such metagratings or metasurfaces consist of subwavelength scatterers shaped as disks, blocks, spheres, etc. shape the scattered wavefronts, redirecting incident photon momentum transversely. This provides stabilizing restoring forces and torques. However, adding metagratings makes the sail heavier than the ~0.1 gram per square meter goal. And photonic grating patterns would have to be produced over a large area. The advantage of flat sails will significantly streamline and simplify the fabrication process. The issue is whether such structures can be scaled to manufacture on the size of meters with low mass.

Spin-stabilization will likely be needed to prevent the collapse of sails while acceleration is underway. A beam can carry angular momentum and communicate it to a sail to help control it in flight. Spin can be modified remotely by circularly polarized beams from the ground [10]. It also allows ‘hands-off’ unfurling deployment through control of the sail spin at a distance [10-12]. Spinning them at ~100 Hz rates gyroscopically stabilizes sails against drift, yaw and tilting, allowing numerous shapes to retain their stability. (Circularly polarized electromagnetic fields carry both linear and angular momentum, which acts to produce a torque through an effective moment arm of a wavelength, so longer wavelengths are more efficient in producing spin.)

A final and crucial issue: Can the data be returned from distant space targets at sufficient data rates before the sail moves far beyond the star? For solar system-scale missions this is possible with existing microwave communication technologies. that were realized 50 years ago in the Deep Space Network. For interstellar missions it is possible by using laser communications. Though today’s laser communication systems are far too heavy for Starshot, which instead aims to operate part of its sail as an optical phased array. There are methods of making this likely in future decades [13]. That is because we understand essentially completely the fundamental limits on communication, and our technology today is able to operate very close to those limits.

The mission objective is to return 100 kB of data. The power requirement on board is driven primarily by the communication needs as well as pointing, tracking and computation. The energy technology is a thin film, radioisotope thermoelectric generator.

Propulsion-oriented scientists usually assume that the mission should be done at maximum speed. But information scientists’ relation to speed is different; they focus on how it affects the data return:

* Slower is better since observations are easier and there is more time in the vicinity of the target star.

* The measure of mission performance is the volume of data returned reliably vs the ‘data latency’ (defined as time from acquisition at Centauri to return to Earth of an entire observational data set).

So from this perspective speed is a secondary parameter except as it influences the data volume and data latency, which will relate to the payload mass, and in particular the communications mass.

Messerschmitt, Lubin and Morrison have studied the minimum data latency that can be achieved for a given data volume, or equivalently the maximum data volume that can be achieved for a given data latency [13, 14]. Generally, they reduce speed for high latency (with the benefit of larger data volume, so larger mass, more instrumentation, and larger data volume).

From this, the key insight that governs the difficult problem of returning data over interstellar distance is that a cost-optimized (meaning cost minimized) system scales as the relation between speed v and mass m: v~1/m^1/4. That means we can have a much heavier communication system onboard. Achieving the data return is more credible. This leads to an optimum mass that maximizes data volume for a given data latency. Future communications research will deal with several probes downlinking concurrently from the same target star. Separating these downlinks (‘multiplexing’, using different formats, polarization, etc.) is very challenging,

That leads to a very significant development conclusion: We would of course develop heavier, lower velocity probes early on as the Beamer is being built out. The Beamer will be built by adding modules of power and aperture over time. It is likely what will happen is that technologies advance, such as sail materials are improved and mass is reduced. As faster solar system deep space missions occur, mass will either drop as the system performance improves or will increase for faster, better data return. That’s the natural development path, leading to faster, better missions.

The on-board pointing system of the sail is also a technical challenge. It must point in the direction of our Solar System, and the beam will be larger than Earth’s orbital diameter, 2 AU. That means a pointing accuracy of a milliarcsecond, about 10 microradians.

Phase I confirmed that short wavelength optical communications can provide the required down-link capability with limited data rate. Low-cost receiver aperture concepts were developed.

System Cost

Before I joined Starshot, I developed an analysis for cost optimization of beam-driven sail systems. In it, the trade-off was between the cost of the sources powering the array versus the cost of the array itself. That was in agreement with the cost of transmitter systems that had been built for interplanetary communications. My conclusion was that the minimum capital cost is achieved when the cost is equally divided between the array antenna and the radiated power [15].

However, Starshot requires more power than can be directly supplied by the normal electrical grid. Therefore, energy storage for the system has to be included, and becomes a substantial cost element [16, 17]. That results in a considerable change in the laser aperture, laser power, and energy storage cost. The result is that the laser cost, which is ~80% of the array cost, becomes the dominant element in the total project cost. The cost trends shown below demonstrate that cost is viable for future fiber amplifiers at ~$0.10/W, and future semiconductor lasers at ~$0.01/W.

The figure below shows that current laser fiber amplifiers and semiconductor laser costs are far too high to afford a Starshot system today. The hope is that economies of scale in the application of lasers to aspects of modern life, for example self-driving cars, will drive down the cost of lasers by economies of scale. In order to reach an affordable level for Starshot, the prices have to fall to order of cents per watt, not many dollars per watt we have today. The points at 2040 and 2050 shows what will have to occur if the cost of Starshot is to be of order 10 billion dollars. That requirement is two to three orders of magnitude cost reduction.

Image: Cost trends for fiber amplifiers and semiconductor lasers.

The Future of Beam-driven Sails

Phase II technical demonstrations, such as laboratory beam-riding sail flights and including orbital sail deployment and sail acceleration, would lead to a firm experimental basis for pilot production of the key sub-systems, leading to the beginning of array construction. That would later lead to precursor missions.

While the Beamer is under construction, many missions become possible that are at speeds lower than interstellar, as well as other applications. The laser driver can beam power to locations in space, such as Earth satellites and space stations. It can deorbit orbital debris. It can drive fast sail missions to the Moon, Mars and the outer planets. At Mars, it could have a second laser array to decelerate the spacecraft, or a retro reflector system, such as proposed by Forward, could reflect a beam from Earth to slow the sailcraft at Mars. And it can beam power to high-performance ion engines.

Development of fast sailcraft that can travel beyond our solar system will enable us to understand the interstellar medium and then, in the fast encounter with other star systems, acquire imaging, spectroscopy, and in situ particle and field measurements.

Beam-driven sails are the only way that probes can be sent to the stars in this century. Completion of Phase II would bring much-increased credibility to the concept by demonstrating beam-riding and operation of a Beamer module in the laboratory. Then the dream of beam-driven interstellar travel could be realized.

Kevin Parkin has even envisioned human beam-driven fast travel to the stars. Accelerating at Earth gravity to relativistic speeds, allowing us to contemplate human travel in future. He points out that human civilizations’ energy production doubled every 40 years since 1800, so that the energies needed for the simplest such missions will be attainable by the end of the century.

Acknowledgements: Figures are by permission of Breakthrough Starshot and Michael Kelzenberg. I also want to thank Kevin Parkin, Dave Messerschmitt and Al Jackson for technical discussions about Starshot.

References

1) Atwater, H. Starshot: from science to spacecraft to missions, Harry Atwater, Interstellar Research Group , Montreal 2023, https://www.youtube.com/watch?v=jV2sNOYzaFA

2) See also same title, Breakthrough Discuss, Harry Atwater, 2023 https://www.youtube.com/watch?v=IrLcllx0LpQ

3. Laser Propulsion in Space: Fundamentals, Technology, and Future Missions, Claude Phipps, ed., Elsevier., Cambridge, MA ,2024.

4. Worden S., Green, W. Schalkwyk, J., Parkin K., and Fugate R., “Progress on the Starshot Laser Propulsion System,” Applied Optics, doi: 10.1364/AO.435858, 2021.

5. Bandutunga C., Sibley P., Ireland M. J., and Ward, R., “Photonic solution to phase sensing and control for light-based interstellar propulsion”, J. Opt. Soc. of Am. B, 38, 1477-1486, 2021.

6. Benford, G., Goronostavea, O., and Benford, J., “Experimental tests of beam-riding sail dynamics” in Beamed Energy Propulsion, AIP Conference Proceedings 664, Pakhomov, A., Ed. 325, 2003.

7. Gao, R., Kelzenberg M. D., and Atwater H. A., “Dynamically Stable Radiation Pressure Propulsion of Flexible Lightsails for Interstellar Exploration”, Nature Comun, 15, 4203. https://doi.org/10.1038/s41467-024-47476-1, 2024,

8. Srivastava P., Chu Y., and Swartzlander G., “Stable diffractive beam rider,” Opt. Lett. 44, 3082-3085, 2019.

9. Chu Y., Tabiryan N. and Swartzlander G., Experimental Verification of a Bigrating Beam Rider. Phys Rev Lett. (123(24), 2024.

10. Benford, G., Goronostavea, O., and Benford, J., “Spin of microwave propelled sails,” in Beamed Energy Propulsion, AIP Conference Proceedings 664, Pakhomov, A., Ed., 313, 2003.

11. Benford, J. and Benford, G., “Elastic, electrostatic and spin deployment of ultralight sails”, JBIS 59 76, 2006.

12. Martin, P. et al., “Detection of a Spinning Object Using Light’s Orbital Angular Momentum” Science 341 537, 2013.

13. Messerschmitt D., Lubin P. and Morrison I., “Challenges in Scientific Data Communication from Low-mass Interstellar Probes”, ApJS 249,36, 2020.

14. Messerschmitt D., Lubin P. and Morrison I., “Interstellar flyby scientific data downlink design,” arXiv preprint arXiv:2306.13550, 2023.

15. Benford, J., “Starship Sails Propelled by Cost-Optimized Directed Energy”, JBIS 66, 85, 2013)

16 Parkin, K., “The Breakthrough Starshot Systems Model”, Acta Astronautica 152, 370–384, 2018.

17. Parkin, K., “Starshot System Model” in Laser Propulsion in Space: Fundamentals, Technology, and Future Missions, Claude Phipps, ed., Elsevier., Cambridge, MA ,2024.