Comment by TTPrograms | Hacker News Reader

Comment by TTPrograms | Hacker News Reader

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For people asking about the amount of energy we're talking about, the energy density of a magnetic field is |B|^2 /(2mu_0). It appears that the described structure has something on the order of the cross section of Mars, and I'll assume that it's depth is in that ballpark as well. For a fictitious uniform field that would require on the order of 1.6e19 J, which is roughly 1.2 times the total electrical energy output of the US in 2001:

http://www.wolframalpha.com/input/?i=4%2F3+pi+*+(radius+of+m...

http://www.wolframalpha.com/input/?i=1.617%C3%9710%5E19+joul...

Assuming you had no losses, so say ideal superconducting coils, that's how much energy you have to dump into magnetic field. You can do this as slow as you like, so with 1/10 the US electrical energy production it would take 10 years to build up that magnetic field. Hypothetically, if you had thin-film plastic PV cells (like cellophane thin to be reasonable to build and get into space) with 100% efficiency covering the footprint of this system (Mars) you could generate enough power to charge up this field in 471 seconds:

http://www.wolframalpha.com/input/?i=1e19+J+%2F+((solar+powe...)

Of course the big story here is that efficient thin film solar in space could generate obscene amounts of power.

I'm not sure on the depth of the field volume required, so that might make it easier, too.

I wonder if a very large array of solar panels would not also act like a solar sail that would would catch enough solar wind to escape the pull of the L1 Lagrange point.

Absolutely this would have an effect: https://en.wikipedia.org/wiki/Solar_sail#Trajectory_correcti...

Presumably, though, some of the power generated could be used to accelerate propellant to compensate.

Not a problem. The L1 point is where the gravitational pull of the Sun is balanced by the gravitational pull of the planet plus the centrifugal force; if you move slightly closer to the sun, its gravity will balance planetary gravity, centrifugal force, and the pressure on the sail.

I think this would happen if L1 was a stable lagrange point, like L4 and L5. That's not the case though, L1, L2 and L3 are unstable. To put it differently, ignoring the centrifugal force, at L1 the gravitational pull of Mars is equal to the gravitational pull of the Sun; when you move the sail towards Mars, the pull towards Mars increases, the one towards the Sun decreases, and the resultant points in the same direction as the pressure on the sail (i.e. towards Mars). Nothing is balanced, in the end the sail falls towards Mars.

No, the fact that it's not stable isn't what's being talked about. It's merely the fact that an additional force from the solar wind would act to shift the equilibrium point of L1 slightly towards the sun.

Makes sense. Thanks for clarifying.

How does "slowly dumping energy into a magnetic field" work?

Magnetic fields are a form of potential energy. Increasing them requires energy. And decreases release energy, to some mix of electrical current and heat.

For a deeper cut, see https://physics.stackexchange.com/questions/94273/what-force...

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