The idea of beaming a propulsive force to a sail in space is now sixty years old, if we take Robert Forward’s first publications on it into account. The gigantic mass ratios necessary to build a rocket that could reach interstellar distances were the driver of Forward’s imagination, as he realized in 1962 that the only way to make an interstellar spacecraft was to separate the energy source and the reaction mass from the vehicle.

Robert Bussard knew that as well, which is why in more or less the same timeframe we got his paper on the interstellar ramjet. It would scoop up hydrogen between the stars and subject it to fusion. But the Bussard ramjet had to light fusion onboard, whereas a sail propelled by a laser beam – a lightsail – operated without a heavy engine. The idea worked on paper but demanded a laser of sufficient size (Forward calculated over 10 kilometers) to make it a concept for the far future. His solution demanded very large lasers in close solar orbits, and thus an existing system-wide space infrastructure.

Forward’s article “Pluto – Gateway to the Stars” ran in the journal Missiles and Rockets in April of 1962 (and would later be confused with an article having a similar name that ran in Galaxy that year, though without the laser sail concept). The beauty of the laser sail was immediately apparent, one of those insights that have other theorists asking themselves why they hadn’t come up with it. Because a beamed sail greatly eases the inverse square law problem. The latter tells us that solar photons aren’t enough because they decrease with the square of our distance from the Sun. Make your laser powerful enough and its narrow beam can push much harder and further.

Image: This is the original image from the Missiles and Rockets article. Caption: Theoretical method for providing power for interstellar travel is use of a very large Laser in orbit close to sun. Laser would convert random solar energy into intense, very narrow light beams that would apply radiation pressure to solar sail carrying space cabin at distances of light years. Rearward beam from Laser would equalize light pressure. Author Forward observes, however, that the Laser would have to be over 10 kilometers in diameter. Therefore other means must be developed.

All the work that began with Forward’s initial sail insights had been theoretical, with authors exploring laser concepts of varying sizes and shapes even as Forward offered fantastic mission designs that could take human crews to places like Epsilon Eridani while obeying the laws of physics. He believed a mission to Alpha Centauri could be launched as early as 1995, triggering interest from JPL’s Bruce Murray, who convened a workshop in 1980 to quantify Forward’s notions and find ways to return a payload to Earth. To my knowledge, the papers from this workshop have never been published, doubtless because the engineering demanded by such a mission was far beyond our reach. Still, it would be interesting to read the thoughts of workshop luminaries Freeman Dyson, Forward, Bussard and others on where we stood in that timeframe.

In 1999 NASA’s Advanced Concepts Office proposed a launch to Alpha Centauri in 2028, a notion that might have been furthered by Jim Benford and Geoffrey Landis’ proposal of using a carbon micro-truss (just invented in that year) that could withstand a microwave beam without melting. Now we begin to see actual laboratory experiments, and in the same year Leik Myrabo subjected carbon micro-truss material to laser beam bombardment to measure an acceleration of 0.15 gravities. See Benford’s A Photon Beam Propulsion Timeline for more on this period of sail laboratory work.

Image: Plan for the development of sails for interstellar flight, 1999. Credit: JPL/Caltech.

So laboratory work explicitly devoted to microwave- and laser-driven sails began 25 years ago and has lately resurfaced through work on sail materials that has developed through the Breakthrough Starshot initiative. Indeed, there are numerous recent papers scattered through the literature that we will be discussing in the future, some containing experimental results from Starshot-funded scientists. It would be helpful for the entire community if this work could be codified and presented in a single report.

But let’s go back to that early lab work. It was in April of 2000 that Benford showed, in experiments at JPL, that sails driven by a microwave beam could survive accelerations up to 13 gravities, while undergoing desorption when the sail reached high temperatures (desorption could have interesting propulsive effects of its own). The effects of spinning the sail were also examined, while Myrabo’s team in that same year experimented with carbon sails coated with molybdenum. By 2002, Benford and his brother Gregory demonstrated in work at UC-Irvine that a conical sail could be stable while riding a microwave beam.

While further work at the University of New Mexico under Chaouki Abdallah and team developed simulations confirming the stability of conical sails under a microwave beam, interest in sails primarily focused on materials in the work of scientists like Gregory Matloff and Geoffrey Landis. Landis’ work on dielectric films for highly reflective sails was particularly significant as materials science kept coming up with interesting candidates — Matloff proposed graphene as a sail material that can sustain high accelerations in 2012, and the examination of metamaterials for the task continues.

When Philip Lubin’s team at UC-Santa Barbara began their work on small wafer-sized spacecraft, it would feed into the concept of the Breakthrough Starshot initiative that was announced in 2016 (See Breakthrough Starshot: Early Testing of ‘Wafer-craft’ Design). Lubin’s work in turn grew out of the Project Starlight and DEEP-IN beamed energy studies his team pursued at UC-Santa Barbara, work that has now been collected in a two-volume set called The Path to Transformational Space Exploration.

A spacecraft on a chip can itself be a micro-sail, as Mason Peck (Cornell University) and team have pointed out in their examination of chips that could use solar photon pressure to move about the Solar System (see Beaming ‘Wafer’ Probes to the Stars). So the idea of miniaturizing a payload and exploiting the potential of laser beaming grafts readily onto the microchip research already underway. It’s interesting that the idea of incorporating the payload into the sail itself goes back to Robert Forward’s Starwisp concept, a kind of ‘smartsail’ whose surface contains the circuits that acquire data. Unfortunately, the Starwisp design had serious flaws, as Geoff Landis would later point out.

We’re still in the early stages in terms of laboratory work focused on sail materials for a lightsail that could carry any kind of payload. Let me quote an interesting new paper on this matter:

Most of the work discussed so far has been theoretical and numerical. Experimental verification of many aspects of lightsails, such as deployment and stability, are difficult to achieve in laboratories subject to Earth’s gravity, and may require extremely powerful lasers and extreme vacuum chambers. Many of the proposed structures are not yet able to be fabricated on the scales required, or rely on material properties that are insufficiently characterized.38 Thus, before full sails can be made, let alone tested, it is imperative that experimental characterizations that can be achieved on Earth be conducted.

This is from a paper by Jadon Lin (University of Sydney) and colleagues called “Photonic lightsails: Fast and Stable Propulsion for Interstellar Travel,” a preprint available here (thanks to Michael Fidler for the reference). We need to talk about the kind of tests needed, and I’ll begin with that next time. We’re headed for the interesting work performed at JPL under Harry Atwater that grows out of a concept some consider our best chance for reaching another star in this century.