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If I push a couple of magnetic asteroids into mars gravitational field, could they form an interlocking series of magnetic fields able to protect mars from solar winds?

Size limits for asteroids is any that can become satellites or moons without majorly altering mars regular orbit, or so small that they would cause mars to have Kessler syndrome.

Bonus: If impossible for asteroids could I do it with electromagnets sent from a planet's surface or built into asteroids?

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No

The issue isn't strength, it's size. Earth's magnetic field is remarkably weak. On the surface, Earth's magnetic field is only 0.25-0.65 gauss. Your average household 'fridge magnet is 100 gauss, up to 400X the strength of Earth's magnetosphere.

But Earth's magnetosphere is MASSIVE.

And that's your problem. Mars is a tenth the mass of Earth, but doesn't have a strong enough (per the needs of your question) magnetic field. An asteroid could be very strong, but without all that lovely mass (in the form of a liquid metal core), its field will be very, very small.

Long story short, by the time you have enough magnetic asteroids to produce a sufficient addition to Mars' magnetosphere, they'd be several times (perhaps many times) the mass of the planet. It would all collapse into a new planet.

The same problem applies to manufactured magnets. Strong, but small. Large fields require massive field-generators. You know, planet-sized.

So, no.

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  • $\begingroup$ Oh, thats a shame. $\endgroup$
    – Clay Deitas
    Commented Sep 1, 2018 at 0:19
  • $\begingroup$ Yeah. Celestial mechanics be a harsh mistress, yessireebob. $\endgroup$
    – JBH
    Commented Sep 1, 2018 at 0:22
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    $\begingroup$ According to this abstract, you can place a generator at the Martian L1 point to deflect the solar wind more efficiently. I don't know if it works, though. (Note that the L1 point isn't stable, so your generator will have to station-keep to a certain extent.) $\endgroup$
    – Cadence
    Commented Sep 1, 2018 at 0:26
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    $\begingroup$ @Cadence - and if you're deflecting the solar wind, even if L1 were stable, station-keeping would be necessary. Deflecting particles takes energy. $\endgroup$
    – jdunlop
    Commented Sep 1, 2018 at 0:28
  • $\begingroup$ @Cadence, you'll notice the article didn't go into detail as to how that field would be generated and only vaguely suggests plausibility based on current research to protect spacecraft... a bit smaller than a planet. Nice link, though. Excellent catch. $\endgroup$
    – JBH
    Commented Sep 1, 2018 at 0:33
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Maybe.

You would use 1 magnetic shield, at the L1 Lagrange point. Lagrange points are places where an item can be placed and stably stay as relative to a planet and its star. There are 5. L1 is between the planet and star.

lagrange points https://www.space.com/30302-lagrange-points.html

I was wondering if you could put a magnetic asteroid (or better: a solar powered electromagnet) at L1 and shield Mars that way. NASA has beat me to the idea which is good, as it is pretty wild and I am sure would get thrashed thoroughly here if proposed first here by me. But NASA!

In answer to this challenge, Dr. Jim Green – the Director of NASA's Planetary Science Division – and a panel of researchers presented an ambitious idea. In essence, they suggested that by positioning a magnetic dipole shield at the Mars L1 Lagrange Point, an artificial magnetosphere could be formed that would encompass the entire planet, thus shielding it from solar wind and radiation. https://phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html

So there you go. I suspect that a sprawling solar electromagnetic umbrella might be stronger and bigger than an ferromagnetic asteroid. But maybe a magnetic asteroid would suffice if that is what you had handy.

Magnetic field at Mars L1

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In my answer to this question:

How can I keep an atmosphere on Mars?1

I suggest another possible way to protect the natural or terraformed Martian atmosphere.

It is a rather bold idea, but no more so than terraforming Mars.

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