If we can't build a magnetic bubble for a spacecraft, how about a magnetic tunnel?

If it is impractical to provide an artificial magnetosphere on the ship which would travel to Mars (due to cosmic ray cascades in the material of the ship), what about generating the magnetic fields externally and projecting them into space at a series of waypoints? Or would the distance involved (225 million miles) be too great?
Our planet's magnetic field changes shape constantly due to strong winds from the sun. Image credit:  NASA's Scientific Visualization Studio

Our planet's magnetic field changes shape constantly due to strong winds from the sun. Image credit: NASA's Scientific Visualization Studio

A little while ago we covered some of the main radiation based difficulties of sending people to Mars, and while the solar wind is generally not so troublesome, cosmic rays, which we are shielded from here on Earth, are both more dangerous and much harder to redirect or stop.

Generally we want the outer walls of our spacecraft to be pretty durable, both for airtightness, protection against space junk, and to help protect against the solar wind, which can be stopped by a pretty reasonable amount of shielding. However, as you build up your shield, cosmic rays will start to play a nastier role. While you certainly don’t want a cosmic ray to be able to pass straight through your spacecraft and hit your astronaut unhindered (they’re very energetic particles, the sort that bodies deal very badly with), when a cosmic ray hits a dense object like a wall, it doesn’t just bounce back the way it came from.

Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation, but defeats this purpose with high-energy cosmic rays it simply splits into deadly showers of secondary particles. Image credit: NASA

Standard spacecraft shielding, integrated into hull design, is strong protection from most solar radiation, but defeats this purpose with high-energy cosmic rays it simply splits into deadly showers of secondary particles. Image credit: NASA

It creates a radiation cascade instead; what was one particle is now two, four, sixteen, and beyond, very rapidly, as the particle interacts with the dense material of the spacecraft wall. Sixteen slightly lower energy particles is mathematically worse than one high energy one, and a serious point of concern once we get out of the Earth’s magnetic shielding. So a very reasonable response is to ask if we can bring along our own magnetic shielding, to prevent the high energy cosmic rays from hitting the wall of the spacecraft in the first place. Theoretically, this should reduce the amount of radiation inside the spacecraft cabin, since it would reduce the number of cosmic rays that can make it all the way to the spacecraft shield. The main reason this is impractical right now is simply a logistical one - we don’t have a good way to build a generator for a sufficiently strong magnetic field which is also lightweight enough not to be hard to launch.

Setting up waystations would be an interesting way of approaching the same challenge. If there were a fixed orbital path between the Earth and Mars, and we could build a magnetic tube between the two planets, you could do away with the need to have an onboard magnetic bubble. Because you’re not trying to launch them on the spacecraft, you wouldn’t need to worry about the weight as much, but the magnetic field you’d have to generate would need to be much larger, to guarantee that the spacecraft (within errors) would definitely travel safely through the buffered region. The distances involved here are vast, and so setting up a series of waypoints would almost definitely be unfavorable, at least from an energy consumption perspective. There’s also the question of fueling those waypoints. Are they solar powered? Fission powered? What happens if their solar panels break down or they run out of energy? They’d also have to be able to correct their own orbits in order to be in the right places for the protection of the traversing spacecraft, and at this point we’re looking at a giant electromagnet with rockets, which is a great sounding device to have, but practically speaking, it’s a more powerful version of what we’d like to have on the spacecraft in the first place, and if we can get by with one device instead of several hundred, one is probably better.


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Why does electromagnetism work on quantum scales and gravity doesn't?

Not entirely related to space/astronomy. But why does Coulomb’s law of electrostatic force still work at the quantum level while it’s counterpart Newton’s gravitational force law doesn’t?

Physics questions are also accepted!

Fundamentally, the reason gravity breaks down at the quantum level is because the strength of the gravitational force is much, much weaker than the electromagnetic force. One of the largest outstanding questions in physics is to try and figure out why gravity is so weak, but so far all we’ve managed to do is very precisely measure how much weaker it is. We have a real-world sense of gravity’s weakness, if you stop and think about it - for all gravity holds us to our chairs, if we set a glass of water down on the desk, we expect the glass to sit on top of the desk, not oscillate back and forth until the glass and desk reach a gravitational balance - that’s the power of the electromagnetic force of the atoms in the desk repelling the atoms in the glass.

Once you start going down to quantum scales, you’re dealing almost entirely with fundamental particles of matter - things like protons and electrons, or quarks if you’re going even smaller. If you want to calculate the gravitational force between any two objects, the two pieces of information you’re going to need are the masses of the two things you care about, and how far apart they are from each other. One can then plug in the masses and distances into this handy equation – F=G(m1 x m2)/r^2, where m1 and m2 are the masses of your objects, r^2 is the distance between them squared, and G is the gravitational constant – and out will pop the force the two objects are exerting on each other.

The electromagnetic force works in a very similar way - even the equation looks similar to the equation for the gravitational force. The two things you need to know about for electromagnetism are the charge and the distance - the mass doesn’t play a role here. Our electric force is then written F=k(q1 x q2)/r^2. q1 and q2 are the charges of the objects we’re interested in, r is still the distance, and k is the Coulomb constant. But between the two, we have a very similar looking equation - multiply together the charge or the mass, divide by the distance squared, and scale by some constant.

Here comes the first major difference. The scaling for the electric force is 8.988 x 10^9 N m^2 / C^2. The gravitational constant is 6.674x10^(-11) N m^2 / kg^2. This is twenty orders of magnitude different in scaling. However, one of them is per square kilogram and the other per square Coulomb, and you can’t really compare things with different units (although it can give us a hint as to the direction this is going), so the best way to compare is by doing a calculation.

Let’s assume that two protons are sitting 1 mm away from each other. What’s more important, the gravitational force, or the electromagnetic force?

To work out the strength of the electromagnetic force, we need the charge of a proton (1.6021765 x 10^(-19) Coulombs), a millimeter (0.001 meters), and the Coulomb constant (8.98755 x 10^9 N (m/C)^2). Plugging all that into our equation gives us a final electromagnetic force of Fc=2.307 x 10^(-22) N. Not very large.

However, the same calculation for gravity gives us the following. Plugging in the mass of a proton (1.672621*10^(-27) kg), a millimeter again being 0.001 meters, and the gravitational constant being 6.67384x10^(-11) N (m/kg)^2, we arrive at a gravitational force of Fg=1.867 * 10^(-58) N.

Forces of 10^-22 and 10^-58 Newtons are both extremely small numbers, but we can see that the gravitational force is 36 orders of magnitude weaker than the electric repulsion force. If we were to push our two protons closer together, both forces would get stronger, but since both of these equations use the distance in the same way, the forces would scale up at the same rate, and gravity won’t be able to catch up.

So the problem here for gravity is a combination of the fact that the gravitational constant is twenty orders of magnitude smaller than Coulomb’s constant, and the fact that the mass of any of these particles is much, much smaller than the charge that they bear. The only way to make this better for gravity is to make the mass of the objects you’re looking at much larger; going further down the quantum scale will only make gravity shrink even further into irrelevance.

Something here unclear, or have your own question? Feel free to ask!