I just read an earlier question about changing the orbit of Mars to be more earthlike. This was shot down because of the massive amounts of energy/time required.

So, if the problem has to do with the greater orbit, there is less energy being absorbed from the sun.

Perhaps we could create a large lens that would concentrate more of the Sun's energy on Mars. At a geostationary orbit around Mars, how big of a lens would be needed to capture the amount of energy necessary to simulate the amount of energy the Earth receives from the Sun?

If this is possible, perhaps the opposite could be used to reduce the energy going to Venus.

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    $\begingroup$ The main problem with this approach is that this object is going to have a mass that is a significant fraction of the planet. Enough to be considered a moon. It will interact gravitationally with the planet once you put it there, speaking of putting it there... how will we do that? It will essentially be like building a new moon, to block/enhance the sun's rays. I don't think we're capable of building a moon, yet, unfortunately. Especially one that is shaped into a lens. Lasers may work if they get more accurate though (look up breakthrough star-shot). $\endgroup$ Jan 16 '20 at 14:58
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    $\begingroup$ For some reason geostationary orbits around Mars are called areostationary orbits. But do you really mean that? Your lens will go through 24 hour day night cycles just like the surface. Maybe you mean in an orbit near Sun-Mars L1 (Lagrange point 1)? In any event, this kind of question about terraforming using extreme technology that's not practical might be better to ask in Worldbuilding SE than here. It's just a thought; there are terraforming questions asked and answered here as well. $\endgroup$
    – uhoh
    Jan 16 '20 at 15:06
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    $\begingroup$ The same question has been asked already on Worldbuilding using mirrors instead of lenses, which is somewhat more realistic (though still extremely unrealistic). @uhoh, Ares is the greek version of the Roman god of war Mars. For the same reason sun-centered orbits are called Heliocentric (Helios = Sun). $\endgroup$ Jan 16 '20 at 15:26
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    $\begingroup$ The largest man-made structure in space, the ISS, is about 100m big. It is in LEO (just above the Earth's atmosphere). Such a lens (rather a mirror, anyways) should be in the size of tens of thousands of kilometers. $\endgroup$
    – peterh
    Jan 16 '20 at 15:32
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    $\begingroup$ @JanDoggen Maybe this one? space.stackexchange.com/questions/3854/… $\endgroup$
    – called2voyage
    Jan 17 '20 at 15:39

It's theoretically possible to use a lens, but in reality it is extremely unlikely. In order stay in one place relative to Mars and the sun it will have to be at the L1 point, which is relatively close to the planet, and as the sun is relatively weak out there the lens would have to be far, far bigger around than the planet itself. This would require technology orders of magnitude more sophisticated than we have now.

Besides, the lens would be massive and take incredible amounts of energy and physical resources to implement, you could use far less resources to build habitats.

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    $\begingroup$ You don't mention that a lens of that size, with a mass which may even exceed Earth would literally redefine what L1 means in terms of the Sun-Earth system. I don't think you could put a planet at L1 and have it be stable if it adds significant gravitational interactions, right? $\endgroup$ Jan 16 '20 at 19:03
  • $\begingroup$ What it comes down to, at best, would be an array of Martio-stationary orbiting mirrors to concentrate sunlight on a relatively small area on the Martian surface. That sort of approach has even been suggested (not very seriously) for Terran energy collection systems. $\endgroup$ Jan 17 '20 at 13:50

This could happen, although a mirror would be more likely for a number of reasons. Lenses typically aren't used for any optical surfaces larger than maybe tens of centimeters, because the glass thickness increases. A lens of such a scale would be absolutely massive. Mirrors, however, are essentially always the same thickness, aside from structural purposes, which don't apply as much in space.

The size of this mirror would be roughly the size of the planet itself. That's quite large, but it would be very thin. Keeping it in place would be somewhat tricky, because the sunlight pressure would tend to push it out of the way. I haven't done all of the math, but it would require a huge amount of resources, and very careful placement to remain stationary.

  • $\begingroup$ I know we're getting into sci-fi here, but how about a gravitational lens instead? $\endgroup$ Jan 17 '20 at 15:45
  • $\begingroup$ Wouldn't work, the size of it would have to be MASSIVE! Like, more than the mass of the Sun. $\endgroup$
    – PearsonArtPhoto
    Jan 17 '20 at 15:51
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    $\begingroup$ @ChrisB.Behrens even the Sun's gravitational lensing "focal length" (it's not a real focus) is way too long to be useful. See space.stackexchange.com/search?q=sun+gravitational+lens and this answer states 550 AU for example. $\endgroup$
    – uhoh
    Jan 18 '20 at 2:01
  • $\begingroup$ So, even if you could create a sci-fi solution, it's simpler to just radiate the energy directly on the target. Not surprising $\endgroup$ Jan 18 '20 at 2:15
  • $\begingroup$ I'm pretty sure it would be placed near but not at the Mars-Sun L1, so it balances gravity and radiation pressure? $\endgroup$
    – ikrase
    Jan 19 '20 at 0:24

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