Light Beamer | Atmosphere

The atmosphere introduces two effects: absorption (or ‘reduction of transmission from unity’), and loss of beam quality (or ‘blurring of the beam spot’). The transmission of the atmosphere at a wavelength of 1 micron is extremely good, exceeding 90% at high altitude ground-based sites. Going to a high altitude site also significantly reduces atmospheric blurring, which would allow an adaptive optics system to achieve performance near to the diffraction limit.

The effects of atmospheric turbulence on the beam include a broadening of the beam footprint (equivalent to image blurring for telescope observations), random jitter of the beam spot, and intensity fluctuations (or ‘scintillations’). The blurring depends on turbulence and wind profile in the atmosphere. The turbulence amplitude is reduced by a factor of approximately 4 between sea level and an altitude of 5km.

The quality of an image is measured by the Strehl ratio, which reflects the ratio of the peak image intensity from a point source to the diffraction limit of an ideal optical system. The ratio measures phase deviations caused by lens aberrations and atmospheric turbulence. 10m-class telescopes, such as the Large Binocular Telescope (LBT), comprising two 8.4m telescopes, have demonstrated image resolution of 40 milliarcseconds and Strehl ratio of 80% at a wavelength of 1.6 microns.

Breakthrough Starshot aims to achieve the diffraction limit for an optical system of laser beams across 0.2-1km, which is 1-2 orders of magnitude beyond existing demonstrations. There are no fundamental physics limitations to achieving this improvement. A beacon on the nanocraft or near its launch point (for instance on the mothership) could be used to correct for phase variations in real time. The effect of the light beam on the atmosphere could be studied, and corrected for, by adaptive optics, again in real time. Additional beam focusing may also be explored to reduce the beam spot size using pulsed laser filamentation techniques.

Comments (38)

  1. James Hostetter:

    For a ground based array, there's also the problem of filamentation in the atmosphere. A high-power beam through the atmosphere will lead to ionization in the atmosphere and focusing/defocusing of the beam into multiple low power filaments, leading to dramatic reductions in transmitted power. You can end up with GW lasers being reduced to single watts over large distances. You could solve this by putting the array in space (which is really difficult and expensive, and there are problems with power storage and cooling). But there's also an interesting idea out there to use the filaments to create a hot-air waveguide that allows for much more efficient transmission through air. Citation is below:
    Demonstration of Long-Lived High-Power Optical Waveguides in Air

    N. Jhajj, E. W. Rosenthal, R. Birnbaum, J. K. Wahlstrand, and H. M. Milchberg
    Phys. Rev. X 4, 011027 – Published 26 February 2014

  2. Mark Warman:

    @Mr Cooremans

    Re: Solar panel challenges.

    While it may be just slightly out of reach right now, 1MW panels aren't that far away. Needing 100,000 panels poses a larger problem from a manufacture point of view, although still not insurmountable. (With regard to actually putting them in place in orbit, as pointed out before, the SpaceX project may well be able to assist with that issue, depending on how far along they are, and real estate isn't really going to pose a problem except possibly from a maintenance perspective). Having said all that, everything I've been able to find so far statistic-wise had been based on land based arrays and not space based, so what would usually produce 1MW on Earth may actually turn out to be more efficient in orbit. I'm not sure...

    There are some other observations to bear in mind as well; panels in orbit apparently suffer greater degradation over time than those on Earth's surface, heat management is apparently an issue, maintenance may turn out to be an issue (assuming it can't be handled remotely via robot/drone). Mind you, it occurs to me that it may be possible to utilise any heat build up to power turbines of some sort (possible idea below), which may again allow for an increase in generated power (you'd need some method of bleeding that heat build up into a generator, but again that just becomes a challenge to be overcome, with it's own possible applications elsewhere as well).

    Re. nuclear energy production/alternative power sources in orbit:

    Why not both? There's nothing that states we have to exclusively use one type of power source. We could use solar alongside nuclear and any other energy sources that become available along the way. In theory you could design what amounts to a solar oven for heating a liquid that could be used to drive a turbine to generate electricity relatively cheaply (think along the lines of either a magnifying glass focussing the suns rays on a single point of wood to ignite it, or mirrors positioned to collect and direct those rays in a similar way). It'd be big, possibly impractical in the long term, but could be a stepping stone in a valid direction. This would also have the side benefit of redundancy (to an extent anyway); if one form of energy becomes impractical later on, at least there'd be a fallback plan, and rather than a complete loss of power, just a reduction until another source became available.

    As to storage of 100GW for single burst use; I'm not sure how that could be achieved myself, but if solar arrays were to be viable, and could generate a constant 100GW (or thereabouts), would that energy actually need to be stored for long? Would a bank of supercapacitors be a better bet (since they'd discharge faster than batteries would, even though they'd take up far more space)? Would we even need to store energy for that charge, since if we're generating a constant 100GW then the simple act of turning it on should provide the necessary boost? (Which leads me to question how much energy actually needs to be constantly generated; if we're going to be storing energy somehow and releasing it in a short burst, do we actually need 100,000 1MW solar panels? Could we use 50,000 and fire frequent bursts? Could we reduce the required number of panels by 80% and stiore enough energy to release once per day?) Or am I barking up the wrong tree(s) here?

  3. René Holtackers:

    The Mother ship can be equipped with an adaptive optics system containing a primary mirror (i guess lenses won't permit these high energies) that collects the blurred beams over some square meters and refocuses them on a secondary mirror. The secondary mirror in its place, can shape the exit-beam in cooperation with an instrument that tracks the probe that is being accelerated by the beam.

    Real-time corrective optics are in use for some time in astronomy, so the refocusing by the primary mirror should be feasible.

    Additionally the beam can be shaped to meet the ideal conditions to hit the sail of the space probes in various distances.

    An implication to this refocusing system that came to mind, is (although much heavier than the space probes) that the mother ship's tilt probably must be corrected constantly due to the beam pressure on the mirrors.

    In this way, the mother ship would be a true gun that can prepare, position and shoot the probes sequentially.

    I hope this idea is helpful.


    Given the timescales involved, I would have thought a space-based system would ultimately become more economical. As I recall, solar insolation in earth orbit is about 6 times that obtainable on the surface of the earth even in optimal conditions, and would be available 24x7 with only a simple heliostat (thrusters to occasionally tune the collector orientation). The support structure for space-based collectors could be much simpler than a 1g system.

    I don't have a good idea of how viable space-based mining and manufacturing might be 20 years from now - too much sci-fi and not enough real research, probably. But I thought that some companies were talking seriously about asteroid mining.

  5. Karen Pease:

    No, insolation in orbit is about 1/3rd greater than on the surface, not 4x. It also costs thousands of USD per kilogram to launch things to orbit and increases your assembly and maintenance costs by orders of magnitude, and also introduces a whole host of new technical problems.

    KISS principle at work here - it goes on the surface.

    As for "space mining 20 years from now", how much has technology advanced in this regard in the past 20 years? Or the past 40 for that matter?


    I am not a professional nor do I have professional experience in physics, space travel, or mathematics. I just have taken interest in this project, and I have thought of a few ideas.--

    I think the work you guys are doing with this is truly phenomenal and innovative, but I think that innovation is also a draw back because you are forgetting about some simple engineering techniques that could greatly improve the efficiency of the nanocrafts. For instance the use of reflected light to help generate even more energy hasn't been considered in great detail. Also I think that potentially a propeller or motor of some sort could also help when it comes to controlling the direction of the nanocrafts and steering them. I would also like to point out that the gravitational pull of larger objects could be hazardous to the nanocrafts, but the gravitational force could also be a great asset in the sense that the force could be harnessed (I understand that this would be challenging to say the least, and probably complicate the steering of the nanocrafts) and used as a "slingshot" to move the nanocrafts.

    --Thank you for reading this, and again I am not a professional or even studying this, so I am not

  7. René Holtackers:

    As is written in the challenge text, they want to use a specific wavelength of 1000nm (compatibility with the solar sail of the little star ships).

    I don't know what the solar output for that wavelength is, but it would anyway require an additional bandpass-filter if sunlight is used. And it would be a challenge on its own to find a coating for a filter that can keep its integrity in the heat of a 1 GW lightbeam.

    I guess for the project, a laser produced monochromatic light beam, either generated on earth or in space, is the best option.


    Could we not have high altitude balloons with jet turbines to generate electrically for the lasers, the thrust and thermal output could be used to push the balloons high into the atmosphere lowering absorption issues? We could also have access to most of the sky for stellar targets, it would need careful coordination though.

  9. Jasper Cooremans:

    For now from what I've seen so far in energy production and storage technology I don't think beams in orbit will be a real possibility taking in account the timescale and available budget they have now. Solar energy is not far enough developed yet to be a real possibility for energy production on this scale and solar cells can't actually produce more when you capture more rays, they do until they reach their maximum efficienty. The reason for that is that the materials used to make solarcells have an optimal bandgap where they absorb the fotones at certain frequencies and use them to produce a certain current and voltage. They can't produce more than the theoretical predetermined current and voltage that are determined by the bandgap. As far as energy storage goes with the existing technologies today it is impossible to store this amount of energy in batteries. Superconductors are a better solution but this technology is too recent to implement right now (for as far as I know). Furthermore as far as jet turbines go they need a huge amount of fuel to produce their energy and the energy they can provide electically speaking is still not enough.

    Consulted source: Prof. dr. Johan Lauwaert, "Duurzame energietechnieken", Ghent University, 2014.

  10. Andrew Palfreyman:

    It may well be that, from a project-political standpoint, the use-scope of the beamer array can be significantly broadened by appealing to those more interested in de-orbiting space junk. In other words, by selling the beamer array as dual use we get it deployed that much faster.

Please sign in to be able to add new comments.