This is to continue expanding the idea of using smaller solar sails, but increasing the energy they receive from the Sun by having a constellation1 of satellites with mirrors + lenses.

For context, I have asked earlier for its possibility here.

I would like to know about the main challenges to make this real, and if there are any existing designs or analysis?

1Not necessarily an Earth orbiting constellation, they could be in various heliocentric orbits as well.

  • $\begingroup$ fantastic, thanks for the edition! $\endgroup$
    – germanio
    Jun 12, 2021 at 15:24

1 Answer 1


If your question is specifically about systems reflecting/focusing sunlight, conservation of etendue places severe limitations on the useful range. In short, just matching the illumination of the sun with perfectly lossless optics will require a system of mirrors or lenses that appears to be as large (in angular area) as the sun from the viewpoint of the sail. For example, to achieve this at the distance of 1 light second from the mirror, with the mirror at Earth's distance from the sun, would require a mirror about 90% the diameter of the moon.

Systems based on lasers can bypass this limitation, beam divergence then only being limited by diffraction, and any limitations on output power or energy conversion efficiency are easily outweighed by the increase in range. Additional benefits are that the right thin film coatings can make a near-perfect reflector of monochromatic laser light (allowing much higher intensities to be used without overheating the sailcraft), and photovoltaics tuned to the laser's wavelength can convert it to electrical power at higher efficiency.

  • $\begingroup$ +1 Another way to explain etendue conservation might be that In order to fry an ant with a magnifying glass, the glass has to look a lot bigger than the diameter of the Sun. I mentioned an f/10 imaging system in my answer to the question the OP linked to, which should give about 100x concentration. Keeping the sail from frying like the ant might be one of the technical challenges if this is happening at 1 AU. Your point about the single wavelength making it easier to make the mirrors extremely high reflectivity (and so low absorption) is spot-on! $\endgroup$
    – uhoh
    Jun 12, 2021 at 15:34
  • $\begingroup$ i.e. one of the challenges is keeping the solar sail from melting. 100x is like 0.1 AU, are there proposed solar sails that are thought to be able to work at that distance? $\endgroup$
    – uhoh
    Jun 12, 2021 at 15:43
  • 1
    $\begingroup$ @uhoh Sail equilibrium temperature would only scalse by the hypercube-root of flux, so even aluminium may still be barely okay at 100x. $\endgroup$ Jun 12, 2021 at 17:53
  • $\begingroup$ nice answer! conservation of etendue is probably the main limitation. Thanks! $\endgroup$
    – germanio
    Jun 14, 2021 at 14:34

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