Hacker News Reader

Comment by thatcherc | Hacker News Reader

This is incredibly exciting but a few important details are missing.

The first is how big does the structure need to be? I can buy a 1 Tesla magnet online right now but that's probably not what they're thinking of. Would we need a city-sized coil or something like that?

The second is the time scale. They say that the temperature could rise by 4 Celsius and trigger a greenhouse effect, but is that an immediate effect (10 years or so) or century-scale effect? I'm hoping the scientists put out a paper because I'd love to learn more about the specifics of their proposal.

View on HN

For people asking about the amount of energy we're talking about, the energy density of a magnetic field is |B|^2 /(2mu_0). It appears that the described structure has something on the order of the cross section of Mars, and I'll assume that it's depth is in that ballpark as well. For a fictitious uniform field that would require on the order of 1.6e19 J, which is roughly 1.2 times the total electrical energy output of the US in 2001:

http://www.wolframalpha.com/input/?i=4%2F3+pi+*+(radius+of+m...

http://www.wolframalpha.com/input/?i=1.617%C3%9710%5E19+joul...

Assuming you had no losses, so say ideal superconducting coils, that's how much energy you have to dump into magnetic field. You can do this as slow as you like, so with 1/10 the US electrical energy production it would take 10 years to build up that magnetic field. Hypothetically, if you had thin-film plastic PV cells (like cellophane thin to be reasonable to build and get into space) with 100% efficiency covering the footprint of this system (Mars) you could generate enough power to charge up this field in 471 seconds:

http://www.wolframalpha.com/input/?i=1e19+J+%2F+((solar+powe...)

Of course the big story here is that efficient thin film solar in space could generate obscene amounts of power.

I'm not sure on the depth of the field volume required, so that might make it easier, too.

I wonder if a very large array of solar panels would not also act like a solar sail that would would catch enough solar wind to escape the pull of the L1 Lagrange point.

Not a problem. The L1 point is where the gravitational pull of the Sun is balanced by the gravitational pull of the planet plus the centrifugal force; if you move slightly closer to the sun, its gravity will balance planetary gravity, centrifugal force, and the pressure on the sail.

I think this would happen if L1 was a stable lagrange point, like L4 and L5. That's not the case though, L1, L2 and L3 are unstable. To put it differently, ignoring the centrifugal force, at L1 the gravitational pull of Mars is equal to the gravitational pull of the Sun; when you move the sail towards Mars, the pull towards Mars increases, the one towards the Sun decreases, and the resultant points in the same direction as the pressure on the sail (i.e. towards Mars). Nothing is balanced, in the end the sail falls towards Mars.

No, the fact that it's not stable isn't what's being talked about. It's merely the fact that an additional force from the solar wind would act to shift the equilibrium point of L1 slightly towards the sun.

Makes sense. Thanks for clarifying.

Absolutely this would have an effect: https://en.wikipedia.org/wiki/Solar_sail#Trajectory_correcti...

Presumably, though, some of the power generated could be used to accelerate propellant to compensate.

How does "slowly dumping energy into a magnetic field" work?

Magnetic fields are a form of potential energy. Increasing them requires energy. And decreases release energy, to some mix of electrical current and heat.

For a deeper cut, see https://physics.stackexchange.com/questions/94273/what-force...

Less than 700 million years apparently.

https://twitter.com/humansareawesme/status/83696170574585856...

> is that an immediate effect (10 years or so) or century-scale effect?

We should be so lucky. Planetary scale can be rather large and slow. For instance the oxygenation of the Earth's atmosphere took hundreds of millions of years

https://en.wikipedia.org/wiki/Great_Oxygenation_Event#Time_l...

We don't know how well cyanobacteria fared against its microbial competitors, how efficient the early photosynthetic pathway was, or how large the oxygen sinks were pre-GOE saturation so we have no idea whether planetary oxygenation actually needs to take that long. Without an existing biome that captures oxygen or large bodies of water to dissolve oxides, the process can be many orders of magnitude easier (especially since Mars' atmosphere will take significantly less volume and we might be able to survive just fine with lower air density with a higher concentration of oxygen).

It's extremely unlikely that we can pull it off in a human lifespan but I would bet that planetary oxygenation is possible given a few millennia

We have managed to warm the earth considerably in less then a century and that's without trying. I reckon if we gave it our best we could change planetary scale things even faster than that.

That was a byproduct of economically useful work though. If you can think of economically useful work that this project could generate, on an ongoing basis while it's in progress, that's greater than it's ongoing costs that would be great.

> in less then a century

The industrial revolution has been going since before 1800 https://en.wikipedia.org/wiki/Industrial_Revolution

Yes, a magnet has both a peak magnetic field (which is what is usually quoted) and a magnetic dipole moment that is roughly proportional to diameter. The magnetic field at a distance is proportional to the moments, but falls off proportional to the inverse cube of the distance.

So, yes if the magnet dipole were the size of Mars and it needed to be 5mT=500,000nT at 320 Mars diameters, then roughly (ignoring higher order effects) you would need an average 16kT around the surface poles, which isn't possible with current technology.

At 1 Tesla you would want to cover an area 50 times the diameter than Mars? That's pretty fanciful.

OK, either my simple calculations are bunk, or this is a truly crazy idea.

It's not located on/at Mars, it's located at the Lagrange point - which means much like your hand can cover entire mountains at a distance, a smaller magnetic field could shield an entire planet when positioned between mars and the sun.

Based on the drawing that they made the size is at least 1-2 diameters of mars (but there is no field there?). Since the diameter of Mars is ~4000mi that would >10,000km at 5mT.

So with the mentioned 1-2 Tesla starting field, we end up and a cube law reducing the strength 200x at 6 times the distance, and a 1.5km moment. That would require at least 2-3km^2 of 1T magnets. Ignoring the support structure that would have a volume of ~10^10 cm^3 and weigh 100ktons, but only cost $100B on Earth at todays Samarium Cobalt pricing, if the market wasn't highly distorted.

Sounds more reasonable...

sounds surprisingly realistic if you consider that this is a proposal to create an artificial magnetic shield for an entire planet.

Also, isn't there a shockwave effect?

Your hand does not cover entire mountains, only your eyes. The cross section of effective magnetic field has to cover the whole planet.

high altitude nuclear tests have created artificial electromagnetic radiation belts (similar to the Van Allen belt);

https://en.wikipedia.org/wiki/List_of_artificial_radiation_b...

Those also damage electronics far below. In fact, that is the characteristic way of using high altitude nuclear weapons to damage communications over a continental region. But far enough away it would push a little solar wind around, for a short time.

This is where I am confused too the speaker in the video said Earth's magnetic field is 31,000 nanoTesla.

He said the best plan for a shield for Mars is a 500,000 nanoTesla field at L1 which is 320 Mars radii away from Mars.

Maybe I am misunderstanding what he is saying or pronouncing, nanoTesla i.e. billionths of a Tesla?

I imagine there is also a size component involved. The Earth's magnetic field might be 31,000 nanoTesla, but it's spread over an enormous volume. This magnetic shield would have to be much smaller than a planet, so it makes sense that it should have a much strong field.

How do they keep the magnet in place, though? Won't the solar wind pushing against it push it out of there?

I mean, I know that's a stable Lagrange point, but you have to wonder if it could end up moving and or spinning around after enough solar flares hit it and what that would do.

I think the general idea to terraform in a reasonable time is to nuke the poles to release CO2.

Even if one did this, any atmosphere on Mars would be temporary as the solar wind would eventually strip it away. The magnetic field is required for long term viability.

You're not wrong, but that's kind of a non-issue.

It takes on the order of hundreds of millions of years to lose that much atmosphere.

So you are saying we could crash some comets, boil the CO2/ice caps and solve the magnetosphere issue later?

well, this is a serious proposal to solve the magnetosphere-problem! We could build this AND nuke the plant.

Or we could at least use this as an indicator that the problem is solvable over a few centuries and start nuking. Better sooner than later!

Porque no los dos?

Reply on news.ycombinator.com