Holographic Sails for Project Starshot? — Homage to Bob Forward

One of the reasons I described Greg Matloff as the ‘renaissance man of interstellar studies’ in my Centauri Dreams book is the continuing stream of ingenious ideas that he develops and delivers through papers and conference presentations. I found the holographic sail concept below fascinating, and would have referenced Bob Forward myself if Greg hadn’t already done it in the text. These two must have been great to hear in conversation! Read on to learn how Greg, a physicist at New York City College of Technology (CUNY) came up with the idea, a process that deftly blended science and art and may provide solutions to some of the more intractable problems posed by Breakthrough Starshot. The author of The Starflight Handbook among many other books (volumes whose pages have often been graced by the artwork of the gifted C Bangs), Greg has been inspiring this writer since 1989.

By Greg Matloff

It was perhaps inevitable that I would be asked to serve on the Advisor’s Board of Yuri Milner’s Breakthrough Starshot, because of my long experience in the analysis of interstellar travel techniques. According to Phil Lubin’s paper on this technology development project, a 50-70 GW laser array mounted atop a southern hemisphere mountain would generate a beam that would be projected against an Earth orbiting ~1 m photon sail for a period of minutes [1]. The sail would be a major component of a ~1 gram wafer-scale spacecraft with a ~0.1-gram payload that would exit the beam after experiencing average accelerations of ~5,000 g. The planned interstellar cruise velocity of the tiny spacecraft is ~0.2c and the voyage time to the Proxima/Alpha Centauri system is approximately two decades.

Image: Gregory Matloff (left) being inducted into the International Academy of Astronautics by JPL’s Ed Stone.

So I attended the first Starshot Advisors Meeting in August 2016 and left with a non-optimistic attitude. Yes, it is possible to design very-high efficiency optical reflectors at the laser wavelength (about 1 micron) to tolerate the enormous thermal load while maximizing acceleration [1]. But these devices tend to be physically thick and massive.

A major problem turned out to be beam-riding sail stability. The sail must be configured to remain in the beam, with its source located on the moving Earth and its terminus directed towards the Centauri system, for a period of minutes. Analysis discussed during the August 2016 meeting and later published revealed that a spherical sail curvature was the best approach to address the beam-riding stability issue [2]. But how would the sail maintain its required spherical curvature during the minutes-duration high-acceleration run?

Finally, rare ~1-micron interstellar dust grains impacting a sail moving through the interstellar medium at ~0.2c pack quite a wallop [3]. So if the spherical sail somehow survived acceleration, it would be a good idea to straighten it post-acceleration to a flat sheet and reorient the spacecraft edge-on to the direction of travel.

Initially, I could think of no way to satisfy all of these requirements. So I encouraged theoretical physicists associated with CUNY to think about ways of increasing graphene reflectivity in response to an expected Request for Proposals (RFP) from the Starshot management team. Because such a development is not impossible, I delivered several papers on the utility of reflective graphene in interstellar solar-photon sailing. In collaboration with other researchers, I also considered toned-down space applications of wafer-scale spacecraft and less intense collimated power beams. Even if the Starshot goal could not be met, I hoped that some major technological advances might come from the decade-duration, $100 million research effort.

But the goals of robotically exploring the planetary systems of our nearest stellar neighbors on voyages requiring a few decades seemed too enticing to simply abandon. So when my partner, artist C Bangs, suggested that I reconsider holographic photon sail coatings, a concept we had collaborated on in 2000-2001, I agreed.

Bob Forward and Holographic Photon Sails

Long before C and I married, we were collaborators. She has generated chapter frontispiece art for most of my books, including The Starflight Handbook. During the summer of 2000, my second year as a NASA Faculty Fellow at Marshall Spaceflight Center in Huntsville Alabama, we attended an International Academy of Astronautics symposium organized by Giancarlo Genta of Politechnico di Torino in Aosta, an Italian alpine city. My participation was concerned with extrasolar and interstellar solar-photon sailing, since NASA had funded my research in the Heliopause Sail Technology Project, under the direction of Les Johnson. C’s role was to curate and hang an art show, “Messages from Earth”, in a medieval Aosta chapel. About 30 international artists contributed work presenting their conceptual message plaques that could be mounted on a solar-photon sail bound for the stars.

During the reception associated with the art show, C was approached by the late Robert Forward. As many Centauri-Dreams readers will remember, Bob pioneered numerous approaches to interstellar travel during the last few decades of the twentieth century. When Bob reached into his wallet and withdrew a credit card, on-lookers expected that he might be making a purchase. Instead, he asked C how she would affix a message plaque to the sail. She responded that a physical plaque (as was done in Pioneer 10/11), a long-playing phonographic record (as was done in Voyager 1/2) or a computer chip were possible approaches. Bob drew her attention to the white-light hologram on the credit card and expressed the opinion that a low-mass, thin-film holographic plaque could contain a huge amount of information.

After the symposium, C returned to Brooklyn and I rejoined the Marshall sail team. A few weeks later, while C was coincidentally visiting me in Huntsville, we were invited to a lecture by Bob. When Les Johnson introduced him, Bob pointed to C and said: “Fund that woman to do a prototype holographic interstellar message plaque”.

So my small NASA University Challenge Grant through Pace University, where I taught at the time, was reconfigured to support the creation of the hologram. I received no salary from this grant so that the project could be financed. We contacted the Center for Holographic Arts (then located in Long Island City) and the rainbow hologram was completed at that facility with two sculpted figures and four line drawings by C with a transparency of the Apollo 17 image of Earth from deep space that is always visible and supports the holographic images.

We delivered one of the three resulting holograms to NASA Marshall in mid-2001. Another was later purchased by a collector and donated to New York City College of Technology. We use the third for display purposes.

There are seven 2D and 3D monochromatic images on this hologram, representing our solar system, probe trajectory and the human form in a similar manner to the Pioneer plaque. During the summer of 2001, we showed it to many NASA and contractor employees. As part of the research effort associated with the plaque, we participated in simulated space-radiation tests of holographic wrapping paper samples. Holograms are apparently immune to image degradation caused by intense solar flares [4-6].

Image: A view of the Rainbow Hologram created by C Bangs. The hologram contains six images. As the viewer moves from left to right the images transition from one frame to the next. On the extreme left side is a line drawing that places our home solar system on the edge of the Milky Way Galaxy and our planet, third from our sun. The second frame is a line drawing of the female figure holding the payload of the solar sail to demonstrate her size relative to it. The third frame is the sculpted female figure. The fourth frame is the sculpted male figure with his hand raised in what is believed to be a universal greeting. The fifth frame is a line drawing of the male figure. The last frame contains equations that describe the acceleration of the solar sail that the hologram would hypothetically be traveling on. In front of all the images is an photograph of the full Earth visually demonstrating the beauty of our home planet. Credit: C Bangs.

It became apparent to the team that Bob Forward was interested in other applications of holographic solar sail applications than message plaques. As discussed in Ref. 5, it is possible to change the reflectivity of a white-light hologram with a slight rotation. It is therefore conceptually possible to accelerate a solar photon sail from Low Earth Orbit by rotating the sail to reflect sunlight when the Sun is behind the spacecraft and transmit sunlight when the sail faces the Sun.

Current Technology Holography and Project Starshot

During 2017, C and I had several meetings with Dr. Martina Mrongovius, Creative Director of the NYC HoloCenter. The HoloCenter is the outgrowth of the Center for Holographic Arts.

The art and science of holography has advanced at a rapid pace during the past few decades. Holograms as thin as 25 nm have been produced by an Australian-Chinese team [7]. Highly efficient wavelength-selective holographic filters and reflectors have been produced and evaluated [8-10].

Color of contemporary holograms displayed at the HoloCenter seems true to life. If the observer slowly changes position to view an experimental 3D holographic movie, action seems continuous with no breaks. Clearly, a vast amount of information can be stored on a single thin-film hologram. According to the Wikipedia article on holography, thousands of images can be produced and stored per second. Martina reports in a YouTube video that some modern holograms contain 10,000 holographic layers.

It no longer seems impossible to me that the Project Starshot goals can be achieved. One would use a holographic film and expose the image of a filter or mirror that is highly reflective in the laser’s wavelength range. My colleague at Citytech, Lufeng Leng teaches optics. She is quite sure that a hologram of a spherical surface will behave optically like a spherical surface. So the filter or mirror should ideally have a convex spherical shape, from the point of view of the observer (or laser).

If the holographic filter or mirror sail is sufficiently reflective to laser light, the thermal issue should be resolvable. Since the hologram is a flat sheet, it should be tolerant to high accelerations. If the spherical filter/mirror 3D image behaves as discussed in Ref. 2, the sail should self-correct its position and remain in the moving laser beam. If a pair of tiny thrusters are mounted on the anti-laser face of the sail, it should be possible to rotate the flat sail by 90 degrees after acceleration terminates to minimize damage by the interstellar medium.

Image: An artist’s conception of a laser-beamed sail. Credit: Adrian Mann.

The April 2018 Breakthrough Committee Meeting

On April 11, the Starshot advisors met at a Breakthrough facility in the NASA Ames Space Flight Center. While C displayed the prototype holographic message plaque, I presented the case for a holographic sail. We learned that Harry Atwater of the California Institute of Technology and his team are investigating technologies that combine aspects of engineered metamaterials and holography. Most participants agreed that the idea of a holographic sail is promising. Some, including Avi Loeb of Harvard, suggested that experimental validation is required.

A number of experiments should be possible. Some of these could be addressed in response to the Starshot Sail RFP, which is scheduled for release in the near future. Jason Wentworth, a frequent contributor to Centauri Dreams, has informed me that projectiles fired by large naval guns routinely survive very high accelerations. A small thin-film hologram mounted on or in a suitable projectile might demonstrate whether a hologram can survive the requisite ~5,000 g acceleration.

It is not possible today to test a holographic filter’s reflectance and survival in a continuous ~50 GW laser beam. But according to Wikipedia, the inertial-fusion confinement lasers at the National Ignition Facility located at Lawrence Livermore can deliver 500 terawatts for a few picoseconds. Perhaps a test of a holographic sail could be performed at that facility.

If a prototype thin-film holographic spherical filter or mirror is engineered to reflect in the microwave region rather than at the laser wavelength, another test is possible using existing facilities. Beam-riding stability could be demonstrated using the equipment applied by Jim Benford, Greg Benford and colleagues to examine beam-riding stability of a number of sail shapes during 2001 [11].

In any event, the situation is hopeful. Both C and I felt that we were channeling Bob Forward during our presentation. It’s nice to imagine that his shade is smiling and cheering on the efforts of the Project Starshot team.

References

1. P. Lubin, “A Roadmap to Interstellar Flight”, JBIS, 69, 40-72 (2016).

2. Z. Manchester and A. Loeb, “Stability of a Light Sail Riding on a Laser Beam”, arXiv:submit/1680014 [astro-ph.IM] 29 Sep 2016.

3. T. Hoang, A. Lazarian, B. Burkhart, and A. Loeb, “The Interaction of Relativistic Spacecrafts with the Interstellar Medium”, arX1v: 1608.05284v1 [astro-ph.GA] a8 Aug 2016.

4. G. L. Matloff, G. Vulpetti, C Bangs and R. Haggerty, “The Interstellar Probe (ISP). Pre-Perihelion Trajectories and Application of Holography”, NASA/CR-2002-211730, NASA Marshall Space Flight Center, Huntsville, AL (June, 2002).

5. R. Haggerty and T. Stanaland, “Applications of Holographic Films in Solar Sails”, presented at STAIF-2002 Conference, University of New Mexico, Albuquerque NM (January, 2002).

6. G. L. Matloff, Deep Space Probes: To the Outer Solar System and Beyond, 2nd. ed. Springer-Praxis, Chichester, UK (2005).

7. M. Irving, “World’s Thinnest Holograms Could Lead to Thin-Film 3C displays”, New Atlas (May 18, 2017).

8. W. Wang, “Reflection and Transmission Properties of Holographic Mirrors and Holographic Fabry-Perot Filters. 1. Holographic Mirrors with Monochromatic Light”, Applied Optics, 1994, May 1;33:2560-6. doi: 10.1364/AO.33.002560.

9. P. Sharlandjiev and Ts Mateeva, “Normal incidence Holographic Mirrors by the Characteristic Matrix Method”, Journal of Optics, 16, 185-190 (1985).

10. D. W. Diehl, “Holographic Interference Filters”, Ph.D. Thesis, Institute of Optics, Schoolof Engineering and Applied Science, University of Rochester, Rochester, NY (2003).

11. James Benford, Gregory Benford, Olga Gornostaeva, Eusebio Garate, Michael Anderson, Alan Prichard, and Henry Harris, “Experimental Tests of Beam-Riding Sail Dynamics”, Proc. Space Technology and Applications International Forum (STAIF-2002), Space Exploration Technology Conf, AIP Conf. Proc. 608, ISBN 0-7354-0052-0, pg. 457, (2002).