Recently, the US government announced that sending people to Mars was a long-term objective, and accordingly it is worth looking at some of the hazards. One that often gets airing is the fact that the sun sends out bursts of high-energy particles that are mainly protons, i.e. hydrogen atoms with their electrons stripped off. If these strike living matter, they tend to smash the molecules, as they have energy far greater than the energy of the chemical bond. These are of little hazard to us usually, though, because they are diverted by the earth’s magnetic field. It is this solar wind that is the primary cause of auroras. The solar wind particles knock electrons out of gas molecules, and the light is generated when electrons return. As you might guess, if these particles can knock out enough electrons from molecules to generate that light show, the particle flux would be quite undesirable for DNA, and a high cancer rate would be expected if some form of protection could not be provided.
The obvious method is to divert the particles, and electromagnetism provides a solution. When a charged particle is moving and it strikes a magnetic field, there is a force that causes the path of the charged particle to bend. The actual force is calculated through something called a vector cross product, but in simple terms the bending force increases with the velocity of the particle, the strength of the magnetic field, and the angle between the path and the magnetic field. The force is maximum when the path is at right angles to the magnetic field, and is actually zero when the particle motion is parallel to the field. The question then is, can we do anything about the solar particles with this?
The first option would be to generate a magnetic field in Mars. Unfortunately, that is not an option, because we have no idea how to generate a dynamo within the planet, nor do we know if it is actually possible. The usual explanation for the earth’s magnetic field is that it is generated through the earth’s rotation and the iron core. Obviously, there is more to it than that, but one thing we know is that the density of Mars is about 3.9 whereas Earth is about 5.5. Basalt, the most common mix of metal silicates, has a density ranging from 3 to 3.8, but of course density also increases with compression. This suggests that Mars does not have much of an iron core. As far as I am aware, it is also unclear whether the core of Mars is solid or liquid. Accordingly, it appears clear there is no reasonable hope of magnetizing Mars.
The alternative is to put an appropriate magnetic field on the line between Mars and the sun. To do that, we have to put a properly aligned strong magnetic field between Mars and the sun. The problem is, bodies orbiting the sun generally only have the same angular rotation about the sun as Mars if they are at the same distance from the sun as Mars, or on average if they are orbiting Mars, in which case they cannot be between, and if they are not between all the time, they are essentially useless.
However, for the general case where a medium sized body orbits a much larger one, such as a planet around a star, or the Moon around Earth, there are five points where a much smaller object can orbit in a fixed configuration with respect to the other two. These are known as Lagrange points, named after the French mathematician who found them, and the good news is that L1, the first such point, lies directly between the planet and the star. Thus on Mars, a satellite at L1 would always seem to “eclipse” the sun, although of course it would be too small to be noticed.
Accordingly, a solution to the problem of high-energy solar particles on settlers on Mars would be to put a strong enough magnetic field at the Mars sun L1 position, so as to bend the path of the solar particles away from Mars. What is interesting is that very recently Jim Green, NASA Planetary Science Division Director, made a proposal of putting such a magnetic shield at the Mars-Sun-L1 position. For a summary of Green’s proposal, see http://www.popularmechanics.com/space/moon-mars/a25493/magnetic-shield-mars-atmosphere/ .
The NASA proposal was focused more on reducing the stripping of the atmosphere by the solar wind. If so, according to Green, such a shield could help Mars achieve half the atmospheric pressure of Earth in a matter of years, on the assumption that frozen CO2 would sublimate, thus starting the process of terraforming. I am not so sure of that, because stopping radiation hitting Mars should not lead to particularly rapid sublimation. It is true that stopping such charged particles would help in stopping gas being knocked off the outer atmosphere, but the evidence we have is that such stripping is a relatively minor effect.
The other point about this is that I made this suggestion in my ebook novel Red Gold, published in 2011, which is about the colonization of Mars. My idea there was to put a satellite at L1 with solar panels and superconducting magnets. If the magnet coils can be shielded from sunlight, even the high temperature superconductors we have now should be adequate, in which case no cooling might be required. Of course the novel is science fiction, but it is always good to see NASA validate one of your ideas, so I am rather pleased with myself.