Instead of placing many magnetic satellites close to Mars to artificially generate a magnetic field to protect Martian colonists from harming radiation, we could place a magnetic satellite at Lagrangian point L1. The magnetic field does not need to be as strong to deflect the solar wind enough, since the deflection takes place over millions of km before reaching the planet. What would this require to be practical?
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3$\begingroup$ And I just did a quick calculation: sun wind = 400km/s, L1 is about 1000000km from Mars, so you need a magnetic field that induce a perpendicular speed of 1.4km/s. Lets assume that the interaction between the magnet and the ion particle takes place within 10km, you just need 1.7E-6 Tesla to deviate it! We can easilly make 20T magnets powered by solar panel. So the effect will affect a greater cross-section. It seems too easy. Could someone double check my calculation? $\endgroup$– Richard BeaudryMar 6, 2016 at 4:40
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$\begingroup$ Unfortunately a satellite at L1 isn't going to be enough. As Mark Adler indicates in this answer, the solar proton flux is pretty much omnidirectional at Mars. $\endgroup$– HobbesMar 6, 2016 at 19:33
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$\begingroup$ What about protection from other cosmic radiation? Just curious. $\endgroup$– MikeyMar 2, 2017 at 23:13
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2$\begingroup$ Dr Jim Green of Nasa's Planetary science division has proposed positioning a magnetic dipole shield at the Mars L1 LaGrange point to create an artificial magnetosphere to protect against solar wind and radiation. phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html $\endgroup$– johnMJul 6, 2017 at 6:06
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3 Answers
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This answers the question on how to block ions coming directly from the Sun from hitting Mars by a satellite stationed at L1. It does not cover the fact whether such a shield is effective in reducing the radiation level on Mars surface. See e.g. here for more details on radiation.
First, let's have a look at the magnetic field needed. Inside a magnetic field (gray area), the particles follow a circular trajectory (green). The radius depends on magnetic field strength and particle momentum while the total deflection angle also depends on the length of the field.
Using a distance estimate of 1 million kilometer between Mars and L1, and a Mars diameter of 6000 km we see that we need to deviate the solar wind by about 0.3°. Further assuming a thickness of the magnetic field of 10 km we see that the radius of the path of particles in the magnetic field needs to be about 1500 km. Now, we can use the formula for the magnetic rigidity $$B= \frac{m v}{q r}$$ and plug in the known values of mass and charge of alpha particles, we get a magnetic field strength of 5 nT - or about 10000 times weaker than the Earth magnetic field.
Sounds simple enough? Let's have a look at pure geometry. In the drawing all values are given in thousands of kilometers - not to scale for obvious reasons.
Sun (yellow) diameter is about 1.4 million kilometer and we need to fully cover it. A shield (blue) positioned 1 million kilometers from Mars (red) that covers the sun completely has a diameter of: $$R = \frac{R_{sun}d_{L1}}{d_{sun}} = 6100~\rm{km} $$ But, we want to cover the whole surface of Mars (green shield), so we need to add another 3000 km in each direction, yielding a total diameter of 12000 km. That is equivalent to build a ring around the equator of Earth!
Unfortunately it is not enough to build a single coil with this diameter - the magnetic field needs to be perpendicular to the solar wind and the intended direction of deflection. That is, pointing away from the viewer, into the screen.
A single coil of reasonable size is not able to produce such a field that extends over thousands of kilometers along the axis of the coil. A simple way to provide it would be a pair of Helmholtz coils (orange). Again assuming a diameter of 12000 km (which is already beyond the validity of simple formulas as the distance between the two coils needs to be smaller than their diameter) and a strand of copper with a cross-section of 1 square meter, the whole setup would need a reasonable 10 MW of power but weigh about a billion tons, or in other words, the world-wide copper production of 50 years launched to space.
Note that it is not strictly necessary to use a cross-section of 1 square meter. We can always trade mass versus power needed due to increased resistance of a thinner copper "wire": Half the cross-section requires double the power.
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$\begingroup$ Thanks Asdfex to answer. Why do you need strand of copper with a cross-section of 1 square meter? $\endgroup$ Mar 9, 2016 at 2:59
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$\begingroup$ Thanks Asdfex to answer. Why do you need strand of copper with a cross-section of 1 square meter? And, it does not require to be uniform magnetic field as in a pair Helmholtz coils. All we need to deviate the alpha particles is that at all point of the cross-section, B > 5nT and perpendicular to its speed. Also, in my first message, I set to 10km interaction distance because the particle speed is quite high. Strong magnetic field can propagate farther than that in solar system plasma. The following book mention radius of 120km with 5 E6 Amp *m/kg. $\endgroup$ Mar 9, 2016 at 3:38
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$\begingroup$ So to avoid the satellite to become a magnetic sail, it needs to rotate such that forces cancels out over a complete rotation. Than, let line up 3 such satellites 240km apart, the 2 at each end are rotating and centrifugal force is compensated by magnetic force. Such that they stay at the same constant distance. $\endgroup$ Mar 9, 2016 at 3:47
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$\begingroup$ Depending on the magnetic field generated, we can at least protect a large area on Mars surface. We can synchronize the rotation such that every time the martian settlement is facing the sun(so at noon for their time) their shielding effect is maximum. $\endgroup$ Mar 9, 2016 at 3:54
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The very idea was proposed earlier this month at the Planetary Science Vision 2050 Workshop.
The talk was titled: "A Future Mars Environment for Science and Exploration", J. Green, R. Bamford, et. al.
They combined heliophysics simulation tools with a Mars global climate model to do the simulation. Their abstract suggests placing a 1-2 Tesla dipole at L1 is indeed sufficient to protect and rebuild a Martian atmosphere. However, the scale is not indicated. The full talk specifies 50,000nT measured at one Earth-radius--which is comparable to regenerating Earth's magnetic field and clearly a challenge.
Few details were given on how to generate the artificial magnetosphere field, but in was noted that the field lines must remain parallel with the interplanetary field to function well as a shield.
You can see the full talk here (starts at 1:36:10).
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1$\begingroup$ This is not an answer to the question! The linked material may contain relevant information, but in that case the post must include it. You can edit your post by clicking at the 'edit' button. $\endgroup$ Mar 30, 2017 at 7:02
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$\begingroup$ How big of a 1 Tesla "dipole"? What size? If it's defined in units of Tesla, it should be a coil with some area specified. A 1 Tesla coil with a 1 liter volume will not protect a planet! However, you may want to consider answering the question Is NASA doing research on “mini-magnetospheres” to protect crew from radiation in space? which is more "behavioral" than quantitative! $\endgroup$– uhohMar 30, 2017 at 13:14
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$\begingroup$ Exactly. Their simulations show they need 50,000nT measured at 1 Earth-radius (roughly same as Earth's magnetic field). So this is a huge, Earth-scale field with comparable strength. $\endgroup$ Mar 30, 2017 at 16:30
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$\begingroup$ Thanks Martin, very instructive presentation. I'm glad that someone is simulating it to study the global effect. It's a baby step in the herculus path for terraforming Mars. Thanks all for considering the challenge. The question is now how to make a stable bipole with that strenght in stable orbite at L1. It is a challenge because of magnetic sailing effect. Best regards R.B. $\endgroup$ Mar 31, 2017 at 23:36
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$\begingroup$ @asdfex Do you have thoughts on this? (Sorry for the 2+ year bump ;p) $\endgroup$– T3db0tFeb 18, 2019 at 17:23
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It is not necessary for the artificial magnetosphere to block the radiation. It is sufficient to merely deflect the ions by 0.006 radians in any direction. Maybe 0.01 radians to be safe. The L1 point is a million km from Mars, and Mars has a diameter of 6000 km.
This is a much smaller field than would be required to enclose Mars in a magnetotail.
Perhaps worth considering building one of these for Earth L1 too. A shame to destroy the Northern Lights, but then we don't want to lose our power grid. Maybe spin it up on demand when a storm is detected.