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I was watching a video on Youtube which criticized Elon Musk's idea to use solar power to power the space craft at long distances from the sun such as Saturn. That got me thinking whether it would be feasible to build lasers or some other type of device near the sun which concentrated the light of the sun, which then in turn could be projected at the spacecraft in order to increase its solar panel's efficiency?

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    $\begingroup$ Unfortunately laser beams don't keep their diameter constant over those huge distances. The same is true for concentrated solar light. See the Lunar Laser Ranging experiment. $\endgroup$ – Uwe Jun 13 '18 at 9:12
  • $\begingroup$ @Uwe Yes, so say our laser technology was advanced enough to accurately control the diameter of the laser at distances relative to our solar system, it could be feasible? Or would it matter that the diameter of the laster would be to large if the intensity of the beam would be sufficient? $\endgroup$ – Jonatan Jun 13 '18 at 12:09
  • $\begingroup$ Converting any power source to laser light is terribly inefficient, in the low tens of percent at best. $\endgroup$ – Wirewrap Jun 13 '18 at 13:32
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    $\begingroup$ There is no possibility to keep the diameter of the laser beam small, it is physically and optically impossible. Advance may move technological limits, but not physical. $\endgroup$ – Uwe Jun 13 '18 at 19:00
  • $\begingroup$ @Uwe I disagree. Physics tells us how to keep it small--the generator must be huge. Also, to keep it on target the laser can't go through atmosphere. $\endgroup$ – Loren Pechtel Jun 14 '18 at 2:19
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It's not against the laws of physics, but it's a long way beyond our technology. Basically, how much a laser beam spreads out depends on the wavelength of the light and the diameter of the output lens of the laser (assuming you can make your lens perfect and do a bunch of other tricky things). Suppose you wanted to power a probe at Saturn, which was attached to a 1km diameter mirror. Since you don't want to waste too much power, you need a roughly 1km diameter beam. Let's further assume we're using green light at a wavelength of about 500 nm. Saturn is about 1.4 billion km from the Sun, so you need a laser aperture that is 1.4 billion wavelengths wide, which is about 700m (that means you're actually going to have a 1.7km beam at Saturn, so we'll have to make the mirror a bit bigger).

Making an accurately focused laser that big is basically the same job as making an optical telescope that big, so well beyond what we can do now, but not ridiculous in a century or so.

The further out you want to go, the harder it gets, of course, although shifting to a shorter wavelength would help a bit.

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  • $\begingroup$ A laser beam with 1 km diameter and a power density of 1.5 kW/m^2 (like the solar power density in an Earth orbit) has a total power of 1.2 GW. $\endgroup$ – Uwe Jun 13 '18 at 20:07
  • $\begingroup$ Not to mention aiming it would likely be an issue. $\endgroup$ – Magic Octopus Urn Jun 15 '18 at 11:26

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