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What would it take in terms of impact size to interrupt or stop Earth's (or an Earth-like planet's) magnetic field?

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Earth's magnetic field is caused by movements in its core. Its outer core is roughly $2,890,000 \text{ meters}$ below Earth's surface. Therefore, to directly interfere with Earth's magnetic field, this thing is going to have to make a crater $2,890,000 \text{ meters}$ deep.

Issac Newton figured out how to calculate this. The depth $D$ actually doesn't (at least for high-velocity approximations) depend on the velocity of the incoming object, but on the densities of the object ($\rho_o$) and the target ($\rho_t$), and on the length $l$ of the projectile. They are related by $$D=l\frac{\rho_o}{\rho t}$$ Given that the density of an asteroid is about $2 \text{ g/cm}^3$, and the density of Earth is about $5.514 \text{g/cm}^3$, the asteroid would have to have a diameter (assuming a sphere) of $$l=D\frac{\rho_t}{\rho_o}=2,890,000 \frac{2}{5.514}=1,048,240 \text{ meters}$$ By comparison, the object that caused the Chicxulub crater was only about $10,000 \text{ meters}$ in diameter.

Oh, and with a diameter that big, the mass of the object would be $$\frac{4}{3}\pi (1,048,240/2)^3 \times 2,000,000 = \text{something really large}^1$$ Most likely enough to destroy the Earth. So either we're screwed, or we keep our magnetic field. I'd choose the latter.

There are only a few asteroids over $500,000 \text{ meters}$ in diameter, so this thing is probably non-existent in the solar system - unless you count moons and planets. However, they have pretty stable orbits, so I'd say we're okay.


$^1$ I calculated $1.206 \times 10^{24} \text{ meters}$, about $\frac{1}{3}$ the mass of Earth.

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    $\begingroup$ +1. You should add that only 3 or 4 of the known asteroids are big enough (1000+ km diameter) so Earth is safe. $\endgroup$ – Peter M. - stands for Monica Jan 3 '15 at 1:06
  • $\begingroup$ @PeterMasiar Thank you very much; I'll do that. $\endgroup$ – HDE 226868 Jan 3 '15 at 1:10
  • $\begingroup$ Would it be possible for impacts that only reach the mantle to interrupt the convection process, hindering it? $\endgroup$ – Soryu Jan 3 '15 at 2:14
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    $\begingroup$ @reallybigbolide Yes, but they'd have to be pretty deep. Certainly not as deep as this, but much deeper than the Chicxulub crater. $\endgroup$ – HDE 226868 Jan 3 '15 at 2:15
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    $\begingroup$ It's $2000kgm^{-3}$, not $2000000kgm^{-3}$. It's still big enough that there's not going to be any civilisation (possibly not even any life) left to worry about it. $\endgroup$ – frodoskywalker Jan 3 '15 at 20:17
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The only possible way this might happen is if the asteroid had some extremely magnetically active material itself. This magnetic field from the asteroid would then interfere drastically with earth's natural magnetic field.

I'm not aware of any suitable material though (you need it to keep its magnetic properties even through entry, melting, impact, etc.) so you would need to invent something for the purpose.

One option would be to have the material thrown up into the atmosphere by the impact and somehow neutralize the earth's magnetic field for a while.

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