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An unmanned, AI-controlled, terraforming ship arrives at an earth-like planet (i.e earth-sized, rocky, in the goldilocks zone of its star). Unfortunately for the ship's terraforming plans, the planet is tidally-locked, has no magnetosphere and so is at the mercy of the stellar wind and coronal mass ejections. As a consequence, although it is a somewhat-promising terraforming candidate, it has a very thin atmosphere, no surface water and hasn't developed any life at all.

The terraforming ship sets to work, with the first order of business being to protect the planet from being constantly blasted by its star.

The ship's plan is to manoeuvre an almost pure iron asteroid (something like 16 Psyche) to the L1 Lagrange point between the planet and the star, wrap it in a conductive (possibly superconductive) coil and power the coil using solar panels (or a reactor if necessary).

This will place a huge electro-magnet between the star and the planet. The planet will then be in the asteroid's magnetotail and so will be protected to a similar amount as having its own magnetosphere.

Without the stellar wind interfering, the ship will then be able to move on to the next phase of its terraforming plan.

Is this a sound plan? What improvements could the terraforming ship make to it?

[Edit: this question is specifically about placing a planet in the magnetotail of a satellite not how to create a magnetosphere on a planet.]

[Edit 2: I'm specifically interested in if using a ferometallic-asteroid-based electromagnet at the L1 point will work or not. Suggestions of other courses of action are interesting, but IMO don't really answer the question.]

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    $\begingroup$ I don't understand why providing a magnetosphere would be the first order of business. I would be surprised if it was in the first one hundred things to do. The lack of magnetosphere would prevent the planet from retaining an atmosphere over geological time, as in millions and millions of years. It has no impact whatsoever on human, or even on historical timescales. Humans do not care what happens in a million years. There are less than six thousand years between building the pyramids of Egypt and the iPhone 12; in such a tiny timespan, the solar wind will have no material impact. $\endgroup$ – AlexP Nov 29 '20 at 13:16
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    $\begingroup$ Does this answer your question? How could you build an artificial planet-sized magnetic field? $\endgroup$ – AlexP Nov 29 '20 at 13:55
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    $\begingroup$ @AlexP I'd already read that question. I don't think it's the same. It's about creating a planet-sized magnetosphere. This one is about placing a planet in the magnetotail of a satellite. I'm interested in if the magnetotail is a viable alternative. On your suggestion that there's no need to protect a planet from the stellar wind and CMEs – they will have other negative effects on planetary habitability besides stripping the atmosphere on a geological timescale AFAIK. $\endgroup$ – Shimbo Nov 29 '20 at 14:19
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    $\begingroup$ You have read that question, all right, but have you also read the answer? How isn't that answer suitable? (And I don't see what negative effects would those CMEs have which would be prevented by the magnetic field. Magnetic fields are not magical, and a if a CME hits, it hits, magnetic field or not.) $\endgroup$ – AlexP Nov 29 '20 at 14:35
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    $\begingroup$ Most metals are not ferromagnetic... You may want to specify an iron/nickel/cobalt asteroid; a copper, gold, platinum, titanium, chromium etc. asteroid woud clearly be of no particular use. $\endgroup$ – AlexP Nov 29 '20 at 14:53
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This seems like the kind of project that Arthur C. Clarke or Larry Niven would write about!

There are lots of technical challenges in such a project, but if a civilization is able to terraform a distant planet with technology, they might be able to resolve these problems. Two challenges come to mind so I'd like to mention them.

The first one is that L1 is not stable in the long term, but if you are going to capture and asteroid, use solar sails etc. then you have the means to adjust its orbit as needed. You will probably need a constant supply of fuel to keep this going.

Second thing is size and mass. You've chosen the perfect location - L1 is 1.5 million km away from Earth, and comet comas can easily reach two orders of magnitude that length. However the tails are also kinda thin... I do not have the math in me but I don't think an asteroid that is at most 13 km wide would have a magnetotail strong enough to protect a planet. Maybe upscale to something Ceres-sized, or even Moon-sized? This way you might even be able to extract the fuel you need to keep it in L1 from the magnetotail source itself.

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    $\begingroup$ Thanks for those useful insights. That's a good point about the L1 point not being entirely stable and so the asteroid may need its position adjusting over time. About the size of the magnetotail, my understanding is that it's related to the strength of the magnetic field rather than the size of the asteroid. NASA for example have calculated that somehow generating a 2 Tesla field at the Mars-Sun L1 point would create a magnetotail large enough to shield Mars. They don't mention the size of the field generator being important, just its power. $\endgroup$ – Shimbo Nov 29 '20 at 17:22
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Scientists from NASA's planetary division proposed to surround Mars with an artificial magnetic field, with which, in their opinion, the atmosphere on the planet will become denser. The authors of the report propose to deploy an inflatable (gas-deployable) module at the Lagrange point (L1) - a place between Mars and the Sun, where the spacecraft can remain almost indefinitely without using engines. The space module will include deflecting dipole magnets capable of creating a field of 1–2 Tesla (approximately the same magnets are at the Large Hadron Collider).

After that, the field forms a "magnetic tail", which will cover the entire planet. Although the "tail" will be rather weak (small fractions of a tesla), this is enough in theory, since on the Earth's surface the magnetic field is measured by equally small fractions of a tesla.

The authors ignore the cost of long-term maintenance of a space module near Mars, as well as where it will take the necessary energy from. They do not compare this option in terms of cost-effectiveness with other projects of a similar type, for example, the production of SF6 gas on Mars. Even a small concentration of this gas is enough to protect the planet's surface from ultraviolet radiation and create a super-powerful greenhouse effect, which will also melt ice caps (increasing the density of the atmosphere) and return the seas to Mars.

If you consider the first option, for one reason or another, difficult to implement, then you can also simply lay a superconductor solenoid along the equator of Mars and connect it to a powerful current source.

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