Physicists at MIT have created the first perfect mirror. If this is true, could enough mirrors be directed at a space ship like Archimedes "Burning Mirror" would at a boat from automated solar farm in orbit work to power spacecraft: How far can usable light be projected from multiple points in orbit to intersect at a single point in space?

  • $\begingroup$ Are you referring to the advancements made in 1998 that are described here? news.mit.edu/1998/mirror $\endgroup$ – probably_someone May 24 at 4:05
  • $\begingroup$ @probably_someone Awesome I will read it and let you know. $\endgroup$ – Justintimeforfun May 24 at 4:07
  • $\begingroup$ @probably_someone this looks ideal for this task. Now it is a matter of weight and autonomy. $\endgroup$ – Justintimeforfun May 24 at 4:13
  • $\begingroup$ On Stack Exchange, its best to stick to one question at a time. You have three very different ones. You should focus on one, and if that doesn't give you enough information to figure out the others, ask the others separately. They are very different questions $\endgroup$ – Cort Ammon May 24 at 4:40
  • $\begingroup$ @CortAmmon I tried to fix the question into one. I hope this is better? $\endgroup$ – Justintimeforfun May 24 at 4:51

The limiting factor for focusing light in this way is actually not the mirror's material properties but the quality of the grinding of the mirror. To focus light on a point, the mirror must be nearly a parabola (it would need to be a parabola if the sun was infinitely far away... but practically speaking, it's far enough away to treat it as infinitely far).

Parabolas are very difficult to grind. Typically we grind a section of a sphere (which is geometrically easy to construct) and then do clever tricks to grind that out into a parabola. It's hard. When I got to tour the lab which made the Hubble Space Telescope primary mirror, they said that the last round of polishing to get the mirror as parabolic as possible was done with a human finger and some talcum powder. You can imagine how long it must have taken to finish the 2.4m diameter mirror!

Modern telescope mirrors have been playing with active optics, where the mirror has a set of actuators behind it which bend the mirror ever so carefully into the right shape. These can do so adaptively, so if your mirror gets hot on one side and cold on the other, these actuators can mitigate some of the effect of the warping due to uneven heating.

Of course, one thing worth thinking about in this scenario is that no matter how many mirrors you have focused on a point, they cannot heat a target up any hotter than the surface of the sun. This is a thermodynamic issue. If the object were hotter, it would actually radiate so much energy that it would actually heat the sun with the mirrors! For instance, there's no way to heat an object up to the temperature of the core of the sun, where fusion occurs.

So the limit for mirrors is once you have enough solar-multiplication to counteract the radiative heat losses of the object into space. As it turns out, its actually pretty hard to cool down an object in space. Modern satellites have to spend a lot of care dealing with the sun's energy. So you actually don't need a very large mirror in order to heat a point up to rather close to the temperature of the surface of the sun.

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  • $\begingroup$ +1 but if the mirror is 100% reflective that includes infrared light as well to a point? a surface area could be added to the back side as a heat sync. May be you are correct on the parabolic curvature but if each mirror is flat then should not the focal point receive an equivalent size reflection? $\endgroup$ – Justintimeforfun May 24 at 5:08
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    $\begingroup$ @Justintimeforfun If you aren't trying to focus the energy on a point, then flat can be just as effective. The sun is far enough away that the resulting reflected light will be more or less columnated. But it will never reduce itself to a beam that's tighter than the diameter of the mirror in the first place. $\endgroup$ – Cort Ammon May 24 at 5:13
  • $\begingroup$ Of course not, but a solar sail receiving the light could redirect the focused light into a smaller point? $\endgroup$ – Justintimeforfun May 24 at 5:16
  • $\begingroup$ Any lost light or radiation pushes the sail and the reflected is focused into heating fuel like an ion gas? I know I'm reaching but maybe? $\endgroup$ – Justintimeforfun May 24 at 5:19
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    $\begingroup$ @Justintimeforfun It would be interesting to see a solar sail do this for a few reasons. The big one is that its hard to get a good shape of a sail. You're going to have to do a lot of structural things to get the right shape in a very thin piece of material. The other interesting bit is that, as the sail moves away from its target, it's going to have to reshape the mirror to retain focus. Your idea is interesting to me because, while absurdly difficult to accomplish, I can't tell whether there's anything that would get in the way physics wise. It might be possible. $\endgroup$ – Cort Ammon May 24 at 15:02

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