Holographic sails fixes last technical issues for interstellar laser pushed sails

Greg Matloff and his wife, C Bangs, have researched the use of holographic films for solar and laser pushed space sails. Matloff’s work is described at Centauri Dreams.

Greg Matloff is on the Advisor’s Board of Yuri Milner’s $100 million Breakthrough Starshot project. Matloff has worked for decades interstellar travel techniques.

Breakthrough starshot is considering a roadmap to get to 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. 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.

The art and science of holography has advanced at a rapid pace during the past few decades. Holograms as thin as 25 nanometers have been produced by an Australian-Chinese team. Highly efficient wavelength-selective holographic filters and reflectors have been produced and evaluated. Some modern holograms contain 10,000 holographic layers.

It no longer seems impossible to Matloff 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. Matloff’s 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 [Z. Manchester and A. Loeb, “Stability of a Light Sail Riding on a Laser Beam”, arXiv:submit/1680014 [astro-ph.IM] 29 Sep 2016.], 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.

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