The vast majority of the sun's energy is wasted -- the light and warmth are sent out in a spherical shape, and the planets only pick up those little bits of energy that happen to be going in the right direction.

If we could harness some of the energy that goes in other directions and redirect it toward Mars, we could warm up the planet and unlock the frozen water reserves. Obviously, there's a lot more than that to terraforming the planet, but I think this would kick-start the process really well.

I am envisioning a giant mirror parked at either L4 or L5 lagrange points, catching sunlight and reflecting it toward the planet. From the surface, the mirror would be a second light source; it would basically look like another sun, albeit probably a lot smaller.

My main question is: How big would this mirror need to be in order to have any discernible impact on the Martian climate?

And secondly, what other effects could this project have? Are there any negatives or reasons why it simply wouldn't work?

Finally, a bonus question: Would a similar project work for other locations in the solar system? Europa? Ganymede? Titan?

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    $\begingroup$ To increase the amount of light Mars gets by 50%, you need a mirror to be about 50% the size of Mars. $\endgroup$ Jul 18, 2017 at 15:46
  • $\begingroup$ Would you need 50% more light to make it work? I had imagined that you wouldn't need anything like that much; Mars is cold, but not that cold. $\endgroup$
    – Simba
    Jul 18, 2017 at 15:52
  • $\begingroup$ The amount of energy reaching Mars is enough. The problem is that the atmosphere is too thin to keep it warm. $\endgroup$
    – L.Dutch
    Jul 18, 2017 at 15:55
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    $\begingroup$ @Hankrecords Well, we all know that nuking them from orbit is the only way to be sure. But for just thickening up the mixture a bit, surely a pinch of cornflour would do the trick? :) $\endgroup$
    – Simba
    Jul 19, 2017 at 8:01
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    $\begingroup$ As an alternative, making specific greenhouse gasses in the atmosphere would be easier - something like C2F6, powerful and long lived. $\endgroup$ Jul 19, 2017 at 11:55

3 Answers 3


Your better bet, with a smaller (but still huge) lens (or series of mirrors that form the equivalent of a lens), would be to put the equipment inside the orbit of Venus, keep it synchronous with Mars orbit (this requires fuel; which you might be able to accomplish mostly with solar power; e.g. accelerating atoms to near light speed using electromagnets and emitting them as propellant).

Then your lens can focus light on Mars (planet wide, not to a point!).

Venus is 0.72 AU (Earth is 1.0 AU) and Mars is 1.52 AU from the Sun. Mercury is 0.39; so put your equipment about 0.5 AU from the sun (47 million miles).

The formula for the area of a sphere is $4\times \pi \times radius^2$. So at 1.5 AU vs 0.5 AU, the $4\times \pi$ cancels out, and the ratio of areas is $\frac{0.5^2}{1.5^2}=0.111$, a factor of 9. This means, at this distance, the light that finally reaches Mars occupies 1/9 the area of the disk of Mars. If you wish to double that light; you need a lens (or mirror arrangement) collecting twice as much: But that collection area is roughly the area of a circle, whose area is computed as $2\times \pi \times radius^2$; so you only need to increase the diameter of the mirror by $\sqrt{2}=1.41$ to double the light reaching Mars. (Mirror arrangements can mimic lenses, it is what we do in a reflecting telescope.)

This is still big: Mars diameter is 4212 miles, radius 2106 miles, so the disk has an area $\pi \times 2106^2$ is about 14 million square miles. A Ninth of that would be 1.55 million sq mi; or a disk of radius 702 miles. To double that light, you need a disk of radius 992 miles; or round it up to 1000.

Getting closer to the sun reduces the size requirement, but I am not sure how close you can get to the sun without melting the mirrors and equipment. I'm pretty sure NASA already has materials sturdy enough to serve at 0.5 AU; but if not, invent some fictional materials; it would not be implausible.

added to address comment

The surface area here is quite large; but there is no requirement it be constructed in one piece, or all at once. There are many advantages to building such a construct in pieces. We need 3 million square miles of mirror; but we can engineer this as three million devices of one square mile each. In a disk form this would have a perimeter of 3.545 miles; on Earth I routinely walk that far, for exercise, in about 75 minutes. A square mile is a manageable area, if you are weightless.

Can we manufacture 3 million such devices? Look at how many cars, refrigerators, houses, TVs and cell phones we have manufactured. If the motivation is there, then the answer is yes; especially since most of this is probably a paper-thin shell of reflecting surface, and the real "device" is probably just the size of a gymnasium for a basketball court.


  • Greater safety and resilience. The devices can be spread out over a very large area and be widely separated, by hundreds or thousands of miles, making them difficult to attack or sabotage.
  • Configurability. Individual mirrors can be turned to reflect light past Mars, to reduce the amount of extra light reaching Mars.
  • Repairability. A single device, if broken or off-target, is catastrophic. In three million parts, a single broken mirror reduces output by $\frac{1}{3,000,000}$ and is unnoticeable, it can be repaired when you get to it.
  • Gradual construction. I presume these would be manufactured in space, from asteroid material. They could be manufactured over the course of a few decades, and be deployed as finished: For three million devices, that would be just 411 per Earth day; a fast rate but not implausible (and of course you keep the space factory going afterwards, manufacturing replacements or backups or additional capture area).
  • 3-D occupation. The mirrors do not have to be all joined together, as long as they all focus light on the target. Say they are spread over 15 degrees of the radius. For a 0.5 AU (=radius 47 million miles); that would be an arc of 12.3 million miles (say MM). Devices could positioned in a slab 12MM x 12MM and in layers about 6MM deep. This is $8.64\times 10^{20}$ cubic miles; allowing each of the 3,000,000 devices a cube about 66,000 miles on a side all to itself. Remember each is only one square mile of reflective surface; and it is 66,000 miles to the next one. This exact configuration is just for a back of the envelope computation to get the scale right; but clearly we can afford plenty of room to maneuver within the grid for the space-faring workmen, or even the factory itself. It need not be crowded, and no out of control ship, or asteroid, is going to "take out" a large number of our mirrors. And since they are maneuverable themselves, perhaps they can pull in their mirrors (roll them in; or close up like a flower) to avoid any debris they detect would hit their mirror.

The surface area is large, but I think a manageable engineering project for a Martian colony; and it can be engineered for great resilience, and be highly resistance to terrorism or sabotage or accidental destruction. An added function beyond warming Mars: Solar power from close to the sun could be harvested to drive lasers, aimed at Mars orbital receivers, that eventually transfer power to the planet. Because your terraforming equipment and atmosphere generators will also demand giant amounts of energy.

  • $\begingroup$ That's a very nice idea, and it introduces some thought provoking consequences that could make for interesting plot points. I'll have to consider this. The size of disk that would be required is still pretty enormous though. $\endgroup$
    – Simba
    Jul 19, 2017 at 8:21

The L4 and L5 Lagrange points are as far from the planet as the planet is to the sun, so mirrors placed there would be too far away (and therefore would need to be very large).

Giant reflecting mirrors have been proposed as a way to warm up Mars, but they would have to be in (a reasonably close) orbit around the planet. I have no idea what would be best, but supposing you wanted them to direct most of the sunlight toward the poles, you could maybe have a constellation of them in something like Molniya orbits.

The mirrors could help turn the solid carbon dioxide in the poles to gas and increase the greenhouse effect. If there's a lot of frozen subsurface water on Mars, as it seems there is, water vapor would also then contribute to this effect, as well as other gases that are currently trapped in the soil.

This could work for terraforming Mars because Mars is (barely) on the habitable zone and is not too small to retain an atmosphere at relatively high temperatures. You would also have to work to keep that denser atmosphere from being carried away by the solar wind and/or photodissociated, which is in fact what probably happened long ago to Mars.

Heating them up wouldn't work like that on smaller bodies, which upon getting warmer would just lose gases to space. The Jovian satellites in particular are subject to a lot of hard radiation from Jupiter, that would only strip them of their new atmospheres a lot quicker than plain solar wind. And Titan, though reasonably large, has a very dense atmosphere only because it's so cold.

  • $\begingroup$ Thank you. That's really informative. Okay, so we put them in close orbits, my L3/L4 idea seems like it's toast, but that doesn't affect my plot too much. Do you have any idea of the scale of the hardware that would be needed? I'm not too worried about the solar wind issue because my understanding is that it would take hundreds of millions of years to become a problem; am I right about this? Thanks. $\endgroup$
    – Simba
    Jul 19, 2017 at 8:10
  • $\begingroup$ I wouldn't know how to begin, but I'm sure the mirrors would have to be pretty huge. It depends a lot on the distance, the shape of the mirror and the way you'd use them. Parabolic mirrors would focus sunlight efficiently and could be smaller, but then they'd need a rigid structure; sail-like rectangular mirrors could be much larger and lighter but the reflected light would be more scattered. I wouldn't worry that much unless you need in-story details about the mirrors. $\endgroup$
    – pablodf76
    Jul 19, 2017 at 10:26

Terraforming Mars needs much more than just heating it up. You need a stable atmosphere (which may help keeping the warmth in) and to keep that (and to avoid too much radiation on the ground) you also need a serious magnetic field around the planet.

The comment from A. C. A. C. is actually underestimating mirror size by a factor $\sqrt{2}$. In order increase irradiation (e.g.!) 50%, you would need a mirror 70% of the cross section of Mars.

I don't know how much is necessary, but any mirror of that order of magnitude size would represent a wonderful solar sail... this means it will not remain stable in L4/L5. You'll need active steering, so whatever position is OK. Probably a (smaller) mirror lower than L2 and using solar wind to counterbalance planet pull would be a better solution.


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