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Humans are trying to terraform Mars. The first step is to restart the magnetosphere. To do this, (and any naysayers who think that a magnetic dipole satellite is better can kindly not say so) a swarm of 100-metre parabolic mirrors are built in geosynchronous orbit around the planet. The mirrors are angled to focus sunlight into continuous, intense beams, which vaporise rock and create tunnels through the surface leading to Mars’ core. The beams gradually heat the frozen hearting of the planet, warming it until it melts. This produces a Dynamo that generates a magnetic field, not to mention several volcanoes from the tunnel shafts which will handily warm the planet up.

Assuming the mirrors are 99.6% reflective, (of visible light, at least) and that there are a thousand of these, evenly spaced in their orbits around the planet’s equator, how long would it take for them to tunnel their way to the core, and how long would it then take to melt it?

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    $\begingroup$ I suspect a thousand satellites won't be enough. I mean, mars already receives thousands of square kilometers of light from the sun and has not melted. Unless your solar collectors are absolutely massive very little is going to happen. $\endgroup$
    – sdfgeoff
    Commented Feb 25, 2023 at 20:28
  • $\begingroup$ If anyone is interested, this question seems to be a segue to worldbuilding.stackexchange.com/q/238588/21222 , which had some funny answers :) $\endgroup$ Commented Feb 26, 2023 at 6:14
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    $\begingroup$ @sdfgeoff right...1000 0.1 km reflectors would have about the same collection area as a single 3 km reflector. Mars already receives 5 million times as much sunlight as the entire constellation of reflectors could add. OP, planets are big. $\endgroup$ Commented Feb 26, 2023 at 14:46

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In a previous question you had asked how to tunnel to Mars's core. Willk gave a great answer suggesting a sun gun, so I see why you went with this question here. But the sun gun suggested by Willk would likely be orders of magnitude more powerful than what you mention in the question.

And as per my comments and answer in your previous question, a sun gun will do the digging. But if your goal is anything more sopgisticated than just digging a hole, you will probably be giving yourself a bigger problem than a solution going this way. If you are vaporising a chunk of a planet to get to the core, you either fail like Dutch explains you will, or you keep trying so hard that the ejecta generated by the process will push Mars onto a complete new orbit. Once you get to that point, a magnetosphere will be as much of a priority to your terraformation engineers as landing gear tire pressure is to an airplane that has just crashed in the middle of the Pacific ocean.

If you search WorldBuilding for ways to give Mars a magnetosphere, or at least some other kind of shield against solar wind, you will find many good answers. Here is one suggestion from Terraforming Mars

Creating an artificial magnetosphere around Mars really only needs a superconducting loop placed at the L1 point and energized by a small fission react or even solar panels.

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I am afraid it will never happen. Even assuming that you can vaporise the rock and that the vapor doesn't condense on colder walls of the tunnel clogging it, a hole dug through the crust would collapse under the pressure of the above layers, preventing the beam from reaching deeper, way before you reach the core.

So, you would end up heating up the mantle.

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    $\begingroup$ Heating up the mantle is generous. Heating up the crust a few meters deep is more realistic. $\endgroup$
    – AlexP
    Commented Feb 26, 2023 at 1:50
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This is not in any sense the first step for terraforming Mars, or a necessary step, or even a sensible suggestion of a possible step could be, considering that the core of Mars is already molten.

But aside from that: a 100 m reflector won't make any sort of hole. At the 17000 km distance of geostationary orbit around Mars, it will make an imperceptibly brighter spot about 100 km across. Even sitting on the surface, it might make a small, shallow pit of molten rock before the sun sets and things cool down again. The impact of a thousand of them would be in the rounding error for any terraforming effort.

Even if you did somehow bore a hole to the core and keep that hole from collapsing, the result would just be a fountain of molten iron filling the tunnel until it reaches a column depth equivalent to the rock surrounding the core. You would then have a thousand or so km of molten iron to get through before you could actually deliver any additional heat to the core.

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