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 (58)

  1. ENRICO BELMONTE:

    I will show here how conservation of energy and momentum can be achieved as observed in a stationary reference frame (Earth) and in a moving reference frame (lightsail). Doppler effect is neglected in the following example.

    1) Conservation of energy and momentum in stationary reference frame:

    Incident light with frequency 400 THz
    Energy = Ei = h f = 2.6 x 10^-19 J
    Momentum = pi = Ei/c = 8.8 x 10^-28 kg-m/s
    The above are the quantities that must be conserved

    Conservation of momentum:
    Reflected light at 300 THz
    Momentum of reflected light = pr= Er/c = 6.6 x 10^-28 kg-m/s
    Change in momentum = dp = pi – pr = 2.2 x 10^-28 kg-m/s
    dp/2 = momentum of lightsail = momentum of emitted photon = 1.1 x 10^-28 kg-m/s
    p of incident light = p of reflected light + p of lightsail + p of photon
    Conservation of momentum is achieved without heat. The heat appears in the energy equation

    Conservation of energy:
    Energy of reflected light = Er = h f = 2 x 10^-19 J
    Momentum of lightsail = pl
    m = mass of lightsail = 0.001 kg
    Kinetic energy of lightsail = pl^2 /2m = 6 x 10^-54 J
    Momentum of emitted photon = pp
    Energy of photon = pp c = 3.3 x 10^-20 J
    E of incident light = E of reflected light + KE of lightsail + E of photon + heat
    The energy equation is balanced by adding heat, the random motion of atoms

    Discussion:
    Where did the emitted photon come from? When light hit an electron, it jumps to a higher energy level in the atom. The momentum loss of the light is converted to angular momentum gain of the electron. When the electron jumps back to a lower energy level in the atom, a photon is emitted. By conservation of momentum, the lightsail gains momentum equal to and opposite in direction of the photon.


    2) Conservation of energy and momentum in moving reference frame:

    Assume incident light has same energy and momentum as in the stationary reference frame

    Conservation of momentum:
    Reflected light at 300 THz
    Momentum of reflected light = pr= Er/c = 6.6 x 10^-28 kg-m/s
    Change in momentum = momentum of emitted photon = pi – pr = 2.2 x 10^-28 kg-m/s
    p of incident light = p of reflected light + p of photon
    Conservation of momentum is achieved without heat

    Conservation of energy:
    Energy of reflected light = Er = h f = 2 x 10^-19 J
    Momentum of emitted photon = pp
    Energy of photon = pp c = 6.6 x 10^-20 J
    E of incident light = E of reflected light + E of photon
    The energy equation is balanced without heat

    Discussion:
    In the moving reference frame, the lightsail has zero momentum and kinetic energy because the observer is riding on the lightsail. The emitted photon does not accelerate the lightsail because it cancels with the momentum of the incident light which is moving in the opposite direction.


    Conclusion:
    The conservation of energy and momentum in a stationary reference frame is achieved by adding heat in the energy equation. In a moving reference frame, the conservation laws are obeyed without heat. This could be some weird relativistic or quantum effect, or can anybody show a better way to reconcile the results?

  2. michael.million@sky.com:

    'Answer:
    Thank you for your consideration. Silicon is heavy in that we have difficulty building sheets of Silicon much thinner than several hundred adams thick. Which will make the material to heavy also the sail needs to withstand 60,000 g’s of acceleration and silicon is very brittle. With effort I think we can solve all of these problems.

    - Pete Klupar, Breakthrough Starshot'

    Silicon can be made very thin, a few atoms thick, also although silicon is brittle we could have these pyramids of silicon bonded to the back of a much more flexible optical glass material. Now if we leave a very small space between the pyramids so that light has great trouble moving through it we could have great reflectivity with great flexibility. Pyramids offer reduced weight for their thickness, a 1.6 micron thick layer of pyramids has the mass of roughly 1/3 that of the same planar thickness of material.

  3. Breakthrough Initiatives:

    RE:
    Mar 14, 2017 07:59 michael.million@sky.com Posted on: Centauri Dreams

    Answer:
    Thank you for your consideration. If possible could you connect me with folks that can build silicon pyramids of silicon a few atoms thick. I am unaware of laboratory producing these thin sheets of materials. However your idea of building up this metamaterial if it is pyramids or rectangles or cylinders is a good one. Such sheets could provide a material with the required reflectivity and absorptivity required for our work.

    - Pete Klupar, Breakthrough Starshot

  4. michael.million@sky.com:

    Making several atom thick silicon is not that 'difficult', but into pyramids that may be a little early to do at the moment as there is no real commercial drive I can think of. There are no companies that I know of that use atomic thickness pyramids in designs but there is material in the journals about nm constructs. There is also an unknown if we get too small with these pyramids in that their total internal reflective properties may not be effective.

    http://pubs.acs.org/doi/abs/10.1021/nn1000996

    https://www.researchgate.net/profile/Young_Ho_Ko2/publication/275260817/figure/fig1/AS:307946299904000@1450431527596/Figure-1-a-Schematic-image-of-InGaN-DHS-on-GaN-nanopyramids-b-Tilted-view-SEM-image.png

  5. michael.million@sky.com:

    There are however very small carbon and boron cones that perhaps can be made to fit on a glass/silicon sheet or perhaps be used as moulds for optical glass or silicon, but again if they get very small they may lose their beneficial optical properties.

    http://www.jcrystal.com/steffenweber/gallery/NanoTubes/NanoCones.html

  6. Breakthrough Initiatives:

    RE:
    Mar 22, 2017 10:28 michael.million@sky.com Posted on: Centauri Dreams

    Answer:
    There are amazing advances in material sciences these days. I am sure if we can figure out the shape we need the science folks will help us make it.

    - Pete Klupar, Breakthrough Starshot

  7. Breakthrough Initiatives:

    RE:
    Mar 23, 2017 14:06 michael.million@sky.com Posted on: Centauri Dreams

    Answer:
    I think what is important is to not get fixated on one shape but to develop the tools that predict properties of materials so we can design systems to achieve the goals we want.

    - Pete Klupar, Breakthrough Starshot

  8. Adam Crowl:

    A new preprint of relevance:

    https://arxiv.org/abs/1707.08128 Centimeter-Scale Suspended Photonic Crystal Mirrors

    A quote from the preprint:

    "Proposals using large light sails like the Starshot Breakthrough, require mirrors with lateral sizes of 4 × 4 m2
    , thicknesses of 0.05λ, reflectivities of 90 %, ppmlevel optical absorption and a total mass of only 1 g [19].
    Our PhC are designed with a lattice of holes which remove about 30 % of the mass of the membrane. Additionally,
    they are made of LPCVD SiN which has an imaginary refractive index of about 10−6 at 1064 nm and has been shown to withstand high laser powers of around 2.5 · 1011 W/m2 [21] – nearly 2 orders of magnitude more than what is required for the initiative."

    Seems the material is available.

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