# Moving asteroids into a planet's orbit to increase rotation to produce a magnetic field

I was doing research on the terraforming of planets with a solid iron core and no magnetic field and saw a few suggested ideas for starting a magnetic field on Mars that got me thinking.

Most of what I read suggested that if you could get a planet to rotate faster, the core temperature would increase and return its core to a molten state, restarting convection and generating a magnetic field. I'm going to use Mars for this example, but I'm looking to apply this to other planets as well.

The common solutions I saw were the following:

• Hit it with asteroids at the right points to give it some spin (could potentially destabilize orbits of Mars and other planets.)

• Slingshot asteroids or planetoids around Mars.

• Move a planetoid with a quarter of Mars's mass into its orbit to use tidal forces to generate the kinetic energy needed.

I'm planning to go with the third approach in this setting, but had an idea.

Would a belt of asteroids with a total mass equal to one quarter of Mars's mass yield similar results on its rotation speed to a singular planetoid?

The thought is, instead of locating and moving an adequate dwarf planet into Mars's orbit and all of the energy required for that, redirect smaller asteroids from our asteroid belt into Mars's orbit over time so there isn't one astronomical upfront cost and money could be saved by reusing equipment on the next few asteroids. I'm imagining a group of automated reusable asteroid tugboats/pushers.

Or would that amount of energy and effort be better spent applied directly to the planet itself?

• There's another recent question about getting a planet to spin faster. I won’t duplicate my remarks on the subject. Jan 30, 2017 at 1:54
• The rough effect can most easily be achieved by running superconducting cables round the planet for a magnetic field and using orbiting foil to reflect light onto the planet. Jan 30, 2017 at 23:24

I would imagine that, the energy needed to perform this could be in wasted efforts. As Levallon stated, these small masses would or could end up bouncing into each other. Possibly sending the masses into the planet or out of orbit causing either the inverse effect or next to no effect. The main issue also would be that, you would have to figure a way to get the timing and trajectory of the masses perfectly so that anything along the same line of rotation is speed locked relative to each other so that they don't end up colliding. You would also have to calculate the trajectories perfectly so that they are EXACTLY on the same line. Any degree off, even minimally to a .0001 would, over time, cause the masses to go off path causing rotation that may not be optimal or even negative to desired effects.

Based on current known technology (not sure how futuristic this would be), it would take a lot of energy to possibly achieve this. The main issue would be finding a body of rock large enough to achieve this so that you only need 1 mass to orbit. Anything that would be large enough is more than likely already orbiting as a moon or you would have to travel the solar system to find an asteroid large enough to make it possible.

Then the logistics of dragging said body back to the planet and launching it so that it orbits fast enough would be also tedious. Even if the planet had a rock large enough already orbiting (but say not fast enough to do the desired results), accelerating it's orbit too fast in relation to the gravity pull could break orbit and now you are stuck with nothing. The other issue would be that, if you achieve maximum allowable acceleration on the rock, without it breaking orbit, there is a chance that this rotation is still not fast enough to achieve desired effects.

There are simply too many things that could go wrong for this to realistically be possible. Of course, you are the world creator and you can have these issues as a possibility to add suspense, but in the end you can choose the ending.

Something similar was asked here about 2 years ago What would be best way to re-melt Mars' mantle and core to revive its magnetosphere and you may want to consider following through this for possible answers.

I would think that you would need a single planetoid to generate the necessary tidal forces to achieve the result you're looking for. One place you might look further would be the tidal effects Saturn's rings on the planet. I suspect that they are negligible due to their uniform distribution ( well, as opposed to a planetoid).

It's an interesting idea though. Maybe you could redirect the smaller asteroids to orbit each other, creating an something of a "dispersed moon," an orbiting group of asteroids with enough combined mass to create tidal forces. Eventually the planet's tidal effect would alter their orbits into ring, so you'd have to make periodic adjustments to maintain it. The orbital resonances necessary to make a stable system might be pretty hairy, and there would always be a danger that one or more would collide or be kicked out of the system.

Just my thoughts.

• If you had the resources to manage a dispersed moon like that, wouldn't it be more practical to just combine them into a single planet? Jan 29, 2017 at 17:38