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I've been reading a lot about concepts for protecting astronauts from Galactic Cosmic Radiation on interplanetary voyages. I've read about a lot of different ideas, but all of the ones involving a magnetic field to block the particles seem to use electromagnets, which require lots of power. My question is why can't you use permanent magnets instead of electromagnets? I know there must be some reason for this, but I can't seem to find it.

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    $\begingroup$ Uh is there any way to put the original answer back? I still found it helpful even if part was wrong. $\endgroup$ – Nick Gaug Feb 8 '18 at 4:22
  • $\begingroup$ Undeleting. I'll edit it up too to make sure people don't perceive the wear as something similar to friction. $\endgroup$ – Edwin Buck Feb 8 '18 at 17:00
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    $\begingroup$ NASA says you can do it and you only need one Tesla! physicsworld.com/a/magnetic-shield-could-protect-spacecraft $\endgroup$ – Kevin Jackson Jul 8 '18 at 23:50
  • $\begingroup$ To built a radiation shielding like the Van Allen belts of the Earth, you need weak but huge (distance) magnetic fields. Using permanent magnets you get strong but very small fields. To influence the radiation, you need strong and large fields. An incredible large magnet would be necessary. $\endgroup$ – Uwe Jul 9 '18 at 10:06
  • $\begingroup$ The magnet will provide more shielding than it's magnetic field... Theses are really weak. $\endgroup$ – Antzi Jul 10 '18 at 6:21
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Permanent magnets are pretty heavy, and have a field strength over time distribution such that they lose their magnetism slowly. In a sense, they are not really "permanent" beyond the definition meaning that their field strength degrades very slowly and doesn't require an external power source.

Electromagnets are also heavy, but can be designed to be much lighter than permanent magnets. This includes the addition of a small power source. Their strength can be tuned dynamically, depending on available power. Thus they can survive as long as their power source.

Electromagnets offer lighter weight, tunable output, and a maintenance cycle that avoids remagnetization or bulky replacements. Currently these advantages seem to hold a higher value than the energy they require to operate.

The real problems come into play when one considers the size and strength of a magnetic field required to provide protection in space. I'm not sure if current engineering can make such a field, as the Earth is the current comparison model.

Addressing magnetic wear...

It is a common misconception that a permanent magnet is somehow truly permanent. Elements within a permanent magnet are aligned during the process of magnetizing the magnet. This creates a uniform domain of aligned elements which create the magnetism. Cosmic rays and other distrubances occasionally hit these elements, knocking them out of alignment. As the elements become misaligned, the field weakens.

power graphs of some commercial electromagnets

power ratings of some commercial permanent magnets

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  • $\begingroup$ Permanent magnet may indeed lose their strength but it's not because something "saps their power". You describe them like batteries, which is not the case. 1, 2 $\endgroup$ – DarkDust Feb 6 '18 at 8:29
  • $\begingroup$ Some real progress here, although unfortunately the electromagnet/permanent magnet links don't seem to have a lot of directly comparable graphs or statistics. (And I'm not sure of the equations to turn the megagauss-oersted maximum energy product into "number of cosmic rays absorbed" or "field strength and duration to demagnetize", so I can't run a back-of-the-envelope calculation to see how many years you could expect.) $\endgroup$ – Nathan Tuggy Feb 9 '18 at 3:30
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Magnetic shielding does not shield against gamma rays and neutrons, so some shielding, like drinking and wastewater in a double hull, might be necessary. a few inches of water could cut that radiation almost half and also augment any magnetic shielding as well. The crew will have both drinking and waste water so use what you have for shielding. Fuel may also be a good shield. Solar power for a magnetic shield is plentiful inside the Mars-Solar sphere. For venturing to the asteroid belt and beyond, solar power is not so plentiful. Thin film solar power sails can be light weight and can be unfurled in trans-planetary flight.

As we learned from earlier experiments, the power collection from the sails will have to be done in a carefully engineered way to not stress or collapse the sails. This is especially true in this case since we are creating a strong magnetic field which will create a force on any current carrying wires. To further complicate things a corneal mass ejection will cause a field distortion that will induce currents into these wires possibly blowing circuits and even melting the wires.

Also, aluminum in hulls exacerbate cosmic particle radiation problems with secondary particles, so carbon fiber with polyethylene will perhaps be the hull material of choice. During a solar coronal mass ejection the rocket can turn its tail towards the sun to reduce the effect on the crew. Actually, they will likely do that throughout the flight anyway to shield themselves from solar radiation in general which a directional main radiation source. Additional they could get a little extra photonic push from the solar sails as well.

Of course, this is all speculative. You need the numbers and I hope NASA and SpaceX has them. I'm sure that a good many people will be willing to take a trip to the Moon or Mars even if it means that it will shorten their lives as it appears to have done with the Apollo astronauts. I think SpaceX has the right idea, which is, get there fast to reduce your exposure and the stuff you need to carry like food and accumulated garbage. No, you don't throw the garbage overboard.

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    $\begingroup$ Doesn't seem like much of this answer addresses the question of why electromagnets must be used instead of permanent magnets. $\endgroup$ – Nathan Tuggy Sep 16 '18 at 2:32
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I could think of two other reasons:

  1. Practicality of installation and flexibility: You want your magnets to be strong. But strong permanent magnets are nasty to install (ever tried to pull one of those 1T-surface field mini-supermagnets from your fridge?) and therefore arbitrary field shapes are very hard to achieve.
    If you want a nice custom-made high-field that doesn't interfere with the inside of your spacecraft and protects towards the outside, then electromagnets are a must.

The question remains, why would you use permanent magnets when you can just use some to switch on and off? This way you can react more flexible to situations of increased proton fluxes from the sun vs. quiet phases. Seems way smarter to me.

  1. Shielding properties: Shielding of particles via magnetic fields works via the magnetic mirroring effect. Equating particle kinetic energy and magnetic energy of the dipole used $\mu B$, we can estimate the shielding properties.
    A quick bit of math then shows, that a 1T field with a dipole moment of $0.01-0.1 Am^2$ (I didn't thoroughly search for typical values, only looked up one google result) can repel particles of up to $10^{18}eV$ energy, which according to the cosmic ray energy spectrum would be enough to deflect most of the cosmic ray flux.

But keep in mind that the value of 1T that I've chosen only to simplify calculations here is an absurdly high field strength. A large-scale magnetic field on a ship would have orders of magnitude lower $\mu$ as well as $B$.
And then this would likely be able to shield the solar particle flux, but not the galactic particles (which do orders of magnitude less damage, as their flux is lower).

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