Nature materials -Materials challenges for the Starshot lightsail
A solar sail able to be pushed by lasers to 20% of light speed would need to be about 10 square meters but weigh less than a gram. The sail will have to average out to being 100 atoms thick yet still be able to transmit the force of acceleration. Graphene is one of the strongest materials but it’s also transparent, so it can only act as structural support.
Researchers look into partly reflective materials that have a high refractive index and low absorption. The high refraction allows for the possibility of making light-manipulating structures on the surface of the sail that contribute to its reflectivity. The researchers consider a handful of semiconducting materials that fit the bill, rejecting a number of them because their component atoms weigh too much (like tin). The reflection/refraction of these materials also have to cover a broad range of wavelengths since, once the craft is moving fast enough, the incoming photons will be red-shifted.
In the end, nothing meets the researchers’ full list of desires. So they settled for two out of three and focus on silicon, diamond, and molybdenum disulfide.
The Starshot Breakthrough Initiative established in 2016 sets an audacious goal of sending a spacecraft beyond our Solar System to a neighboring star within the next half-century. Its vision for an ultralight spacecraft that can be accelerated by laser radiation pressure from an Earth-based source to ~20% of the speed of light demands the use of materials with extreme properties. Here we examine stringent criteria for the lightsail design and discuss fundamental materials challenges. We predict that major research advances in photonic design and materials science will enable us to define the pathways needed to realize laser-driven lightsails.