Magnetic Fields and You

Magnets are cool.  Seriously.  They are one of the coolest things ever.  You can stick them to metallic surfaces, you can make them bend electricity, you can use them to MAKE electricity, you can make them hover.  If you have a round one, you can roll it down a whiteboard.  Just ask any five to ten year old what the coolest thing in science is, and magnets will rank right after dinosaurs and lasers (which, let's be honest, shouldn't even be considered in the same league, they reek so thoroughly of coolness).  Another cool thing about magnets: They prevent everyone on Earth from dying horribly of cancer.

Let's look at how this works.  First, it should be noted that the correct answer to the question "How does Earth's magnetic field work?" is "We don't know, but we have a hunch that might be right, assuming our theories about the inner parts of the planet are correct."  That hunch is as follows (according to HowStuffWorks.com here):  The core of the Earth is made of super hot, super pressurized, solid crystallized iron.  Outside the core, Earth's rotation causes some other, slightly less hot, slightly less pressurized, liquid molten iron to spin around the solid iron core, creating a bit electricity and thus a bit of magnetic field.  I'm sure there are much more sciency explanations than this one, but that's the gist.

This magnetic field is really weak (only half a Gauss on Earth's surface, and only 1/8 of that 8000 miles up (which is the diameter of the Earth)), but still strong enough to deflect a large part of the harmful solar wind, forcing it to bend out around the Earth.  Without it, solar wind would annihilate our ozone layer, expose us to the full brunt of solar radiation, and slowly strip away our atmosphere until nothing was left, a process which, on Mars, is currently in its third step.

Which leaves us with a problem.  Mars has no ozone.  Well, actually, that's not too big a problem; we'll be forced to live in domes anyway to trap a useful atmosphere, and the domes will need to be glass so that plants beneath them get light to grow.  Fortunately, normal glass will block most UV rays on its own, and we've already invented various films and treatments (check one of them out here) that can cut out those remaining few rays and leave us safe and sunny.  So, really, the ozone issue is not an issue.  And, since we will be distilling our own atmosphere to fill all of these domes, an atmosphere which the sun cannot strip away, the whole solar wind stealing our air thing isn't too important either.

However, the matter of high energy cosmic rays is still a big problem.  A lot of this problem will occur in space on the way to the planet, but I will deal with that in a later entry on spacecraft design.  For now, I want to limit my inquiries to the colony itself.  According to the Mars Radiation Environment Experiment (MARIE), radiation on the surface of Mars should be roughly equal to that on the International Space Station, at about 100-200 mSv (microSieverts) per year.  To give some context, the maximum recommended lifetime dose of radiation is about 1000 mSv, which would be 10 years on Mars, not counting the radiation the first colonists would experience on the way there.  Cancer risks increase by about 5.5% per Sievert, so at 1000 mSv you have a 5.5% chance, at 2000 mSv an 11% chance and so on.   On Earth, we get about 0.4 mSv of cosmic radiation annually (this does not include Earth-bound radiation sources, like nuclear power plants, and bananas.  Read here for more information.)  So, if we want to live on Mars, chronic radiation is a problem we'll need to solve.

It gets worse.  Not only is chronic radiation a possibility, but without the protection of a magnetic field, the extremely intense radiation from solar flares will hit Mars (and our poor fledgling colony) head on.  Radiation during these events is sometimes over 100 times background levels.  That's a ridiculous 2000 mSv per day, and sometimes these flares last a week of more!  In that week, the lifetime risk of radiation induced cancer for every exposed citizen of Mars would go from 0% to 77%.  A 77% chance of getting cancer.  This radiation dose (.14 Gray in a week) is also high enough to cause chronic radiation syndrome, which is basically all of the nastiness that comes from working in Chernobyl on the wrong day, except drawn out over the course of a year or two.  Not a pretty sight.

Okay, so things are grim.  How do we fix it?  Well, since things will probably take some setting up once we arrive on Mars, the first solution will probably be digging.  Mass blocks radiation.  Lead, as most of us know, is the best single radiation absorber, but 2 meters of dirt or stone will accomplish the same goal as eighteen centimeters of lead, and won't need to be mined carefully or processed before use.  Two meters underground is almost 22 halving thicknesses of dirt, which means that if a solar flare scorches the martian soil with 2000 mSv a day for two weeks, our underground explorers will feel only 0.0005 mSv.  Which is completely harmless.  So, at least at first, humans can escape the radiation on Mars by building and living in caves, which also have the advantage that they do not require the humans to bring too many construction materials with them to Mars (yay saving money!).

But who wants to move to another planet just to become a cave man?  Not me.  I like living on the surface, and seeing the sun once in a while.  Plus, plants need light to grow, and light tends to be scarce in caves.  Of course we can grow our crops underground with full spectrum light bulbs, but it costs energy, energy we could get from the sun, for free, if we could keep the radiation from killing us in our pretty little domes.  How can this happen?  Well, we could make our own magnetic field.  Some people suggest digging into the core of Mars and blasting away with nuclear weapons trying to start a dynamo effect and make a field like Earth's, but I don't buy into the idea; feels too much like Armageddon (an excellent story, but not really related to actual science). So it seems like we should make a whole lot of small magnetic deflectors.  After all, it is a very weak magnetic field that repels the solar wind on Earth, right?

Wrong.  The magnetic field on Earth works because it has a lot of space to work with.  An average deflection of only one degree per thousand miles will mean that everything misses the whole planet if the field takes effect 25,000 miles from the surface (because the diameter of the Earth is less than 25 degrees on a circle with radius 25000 miles, so shifting radiation by 25 degrees in any direction will make it miss Earth).  At that height, Earth would have a field strength of .00098 Gauss, which is very weak, but not negligible.  But, we don't have the ability to make a field that big.  Magnetic fields lose power as a cube of distance, meaning that if you double distance, power drops to 1/8th.

Let's take as an example the strongest electromagnet on Earth, which is 45 Tesla, or 450,000 Gauss, about 900,000 times stronger than the Earth's magnetic field at a range of 0, with a diameter of about two meters.  To put that in perspective, this magnet will literally rip a pacemaker out of your heart from ten feet away.

BOOYAH!

So, how strong is it from 25000 miles away?  25000 miles is 40250 km, or 40,250,000 meters.  This is  very roughly 2m^26 (rounded to nearest power of 2), so the diameter of the magnet doubles 26 times to reach that distance.  That means the strength would be 450000/8^26 Gauss, which leaves you with an infinitesimally small charge (1.5*10^-18 Gauss).  (Note: Only the field strengths at given distances in this section are actually real values.  One degree per thousand miles is a number drawn from thin air.  Actual deflection is not linear, but exponential because as the beam approaches the magnet, the magnetic field gets stronger, and as the angle changes from perpendicular, it also becomes easier to change because there is less momentum fighting against the magnetic field.  The energy of each radioactive particle also greatly affects how much it is swayed, and those energies vary hugely.  So, these numbers are meant only as an example to illustrate the problem, not as actual mathematical solutions to it.)

A possible solution is produce a lower magnetic field, but with a much bigger radius.  Suppose we built an electromagnet with a power rating of 2 Tesla, or 20000 Gauss, but a diameter of 50 miles.  At 25000 miles (roughly 50*2^9), this would yield a force of .00015 Gauss, which is more than 1/6th that of Earth.  So, wider magnets with smaller charges might work.  If you make the magnet with superconductors, then you can run it on much less electricity, and a cooling system is a lot cheaper on a planet which is already quite cold.  It would certainly cost a fortune, and take a very long time to build, probably a decade or more of concentrated effort since the crew would be so comparatively small.  The thing I don't know is how much power it would take to generate a magnet like this; it could require billions of megawatts to just get the thing going.  If the final power input is less than the constant several megawatts required by smaller magnets, then this sort of thing would be a worthwhile project.  If the smaller magnets take less power in the long run, then they are probably the better investment, and in any case it seems likely that they will be employed as a short term solution.  Whatever else is true, it looks like a good number of years on Mars before the magnetic deflector shields are fully up and running.