Lightsail | Integrity under thrust

To inform the study, a beamer in the 100 GW class was considered. If, for example, 10-5 of the energy is absorbed by a 4mx4m sail, it will be heated by about 60kW per m2, which is roughly 60 times more than sunlight illumination on Earth. This will heat the material but not melt it. Using fully dielectric sails, we can reduce the absorption to less than 10-9 for optimized materials.

Two possible approaches to mitigate the heating challenge have been identified:

1. High reflectivity

Use a material with better than 99.999% reflectivity. Usually, highly reflective surfaces are dielectric mirrors, which are composed of ‘sandwiches’ of material, with each layer reflecting back a modest fraction of the total. Each layer needs to be at least a quarter of a wavelength thick. The weight can be reduced by using a monolayer with high reflectivity at the correct wavelength. Based on recent research, this could be achieved by a ‘hole-pocked’ layer, highly reflective for very specific angles where reflectivity caustics arise. (These caustics occur for wavelengths of light that are actually longer than the sheet thickness.) Adding the holes serves a dual purpose; it reduces the weight of the sail and it could greatly increase reflectivity. This is but one possibility being explored. Modern materials research will explore new materials such as graphene; Breakthrough Starshot aims to take advantage of this rapidly advancing field. The basic Starshot system allows a wide range of options for nanocraft masses and capabilities, all using the same array. This gives it great flexibility in optimizing the science and technology roadmaps.

2. Low absorption

Use a material (such as glass) that has a very low absorption coefficient even when not highly reflective. Such materials are used in fiber optics systems with high power applications. Without the protection of a highly reflective sail, the StarChip electronics would need to be protected from the incoming flux. But this could be accomplished by a combination of geometry (orienting the electronics ‘sideways’ with a low cross-section) and placing a very highly reflective coating only on the sensitive components. These can use the multi-layer dielectric approaches mentioned above, which have already been demonstrated in the lab. Using low absorption sail material, together with a limited use of high-reflectivity shielding for critical electronics, would protect the StarChip without increasing its mass beyond the gram scale. There are a number of high-reflectivity, low absorption materials in existence. For possible fabrication and verification, a demonstrable design of silicon microcubes on a silicon dioxide substrate is under consideration.

As demonstrated by the Japanese IKAROS mission, spinning the sail can reduce wrinkles on its surface. Special attention is needed to avoid impurities and non- uniformities in the sail composition - for example, near mechanical attachments - or accumulation of dust particles on its surface, which could otherwise lead to a localized deposition of energy. There are a wide variety of options allowing optimization of the sail design.

Comments (62)

  1. Dmitry Novoseltsev:

    Please consider supporting a small project to development a family of engines in the nearest future ( for Femto-class spacecrafts for pre-relativistic speeds using related technical solutions.
    Currently, work is underway to organize a profile startup D-Start:
    The proposed engines can be used for experimental development of certain elements of the Breakthrough Starshot technology, and in the "solar petard" engin version - also for experimental study of the movement of high-speed ultralight spacecrafts in the direction of the periphery of the Solar system and beyond.
    Additional information in the group
    All interested parties are kindly asked to contact the author:
    Questions about specific experiments are open for discussion and planning.
    Assistance in attracting investment or grants is welcome.

  2. Breakthrough Initiatives :

    Dear Dr. Novoseltesev,

    Thank you for you bringing this to our attention. Our team and Advisory Committee pays close attention to developments in propulsion and other technologies relevant to Starshot. While we are not currently at a stage to support the particular work you are doing, I will follow your project with great interest. Our next request for proposals will be for the Starshot Interstellar Communications Challenge which focusses on developing novel communications systems for a 1g spacecraft 4.2 LY from Earth. If this topic is of interest to you, please keep feel free to summit a proposal when the RFP is announced – likely in Q3 2020. 

    Very respectfully, 

    Pete Klupar
    Chief Engineer, Breakthrough Initiatives

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