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Normally, hot air balloons are limited to 60,000 ft (related question), the limits being the mass of the physical structure containing the air, and the heat source.

If one constructed a magnetic bubble and filled it with plasma (AKA mini-magnetospheric plasma propulsion) which was kept hot with a really powerful ground based microwave source (~3MW for a 1 ton payload, from my back of the envelope calculations), could it reach space (Kármán line, atmospheric pressure 32 mPa)? Or would something stop it working at a particular altitude, e.g. the plasma would cool down too fast, or become detached from the magnetic field, or the electrical conductivity of the air at some height would neutralise the plasma, or some other reason I don't know yet?

If it is possible, how accurate are my estimates (I don't know how efficiently microwaves would heat a plasma, nor how much power would really be lost by something like this).

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BenRW
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  • It could not reach space. The plasma would be too heavy. Balloons work on buoyancy and well before it hit space it would stop being less dense than the air. – Jim May 15 '14 at 18:03
  • possible duplicate of Do atmospheric physics prevent hot air balloons from ascending over 60,000ft? – DumpsterDoofus May 15 '14 at 18:36
  • The link you provided already answers this question, since it's just a variant of the same physics. – DumpsterDoofus May 15 '14 at 18:37
  • @Jim: Plasma can be made arbitrarily hot, making it arbitrarily less dense than air. – BenRW May 16 '14 at 14:53
  • @BenRW but never less dense than the vacuum of space, which is what it would have to be to lift the container into space – Jim May 16 '14 at 14:55
  • Even if you filled a rigid container with a vacuum, the density of it would prevent it from reaching space – Jim May 16 '14 at 14:56
  • @DumpsterDoofus I've edited the question to be clearer - I'm more concerned about things like the plasma can't be kept hot, or the force between the magnetic field and the plasma isn't anything like strong enough. – BenRW May 16 '14 at 14:59
  • Your updates still don't help. There is no way you can get the density below that of space. The rest of the items in there are all down to whatever device you use to heat the plasma, strength of field etc. so you can't even calculate how high it may be able to get to unless you make a large amount of assumptions. – Rory Alsop May 16 '14 at 15:05
  • @Jim Space is not a perfect vacuum; the air pressure at the Kármán line is 32 mPa, and the the container is a magnetic field. Some of the work being done for mini-magnetospheric plasma propulsion (the same setup, but in interplanetary space) suggests 15-30km radius plasma filled bubbles from a very small source. – BenRW May 16 '14 at 15:09
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    This is to create a magnetosphere for use as a solar sail. It would never get you into space and the 15-30km radius is only doable in space. In the atmosphere, nothing would prevent non-ionized, neutral gas from entering the bubble and eliminating all associated lift – Jim May 16 '14 at 15:15
  • @Jim Thanks! That's the kind of answer I was looking for :) – BenRW May 16 '14 at 15:18

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The most important thing to consider, which I don't see in your calculations at all, is the mass per surface area of the balloon.

You are saying in the comments "the container is a magnetic field", but unless you have a phyiscal container, how can you couple the payload to the bubble?

For the proposal to be reasonable, there needs to be a mechanism to couple the payload to the bubble of plasma, and a consideration of the mass of such a mechanism.

Also, there is no consideration in the proposal of the density of the plasma. What temperature and pressure will the plasma be, and what is the density of the plasma under such conditions?

In summary, the mass of the plasma and mass of the mechanism coupling a payload to the plasma need to be considered before you can even think about saying the balloon could lift a payload.

DavePhD
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  • Thanks for your answer! Based on what I have seen of the mini-magnetospheric plasma propulsion concept, there seems to be a two-way force between a magnetic field and a plasma within that field. I do not know how strong that coupling is. For temperature, I am assuming something in the order of 10,000 K. The overall pressure would, naturally, match the surrounding air — starting at 1.2kPa for 30km, and decreasing to 32 mPa at 100km. The volume would have to be in the order of 1.62 cubic kilometres at 100km for a 1 ton payload. – BenRW May 16 '14 at 15:35
  • but in the magnetic solar sail case that you link to, the source of the magnetic field creating the plasma bubble is part of the spacecraft, whereas you are stating the source is on the ground. You need a way to couple the plasma bubble to the payload, such as having the field source onboard, which would involve much weight. You're not explaining how and why a payload would travel with the bubble. – DavePhD May 16 '14 at 15:45
  • Ah. The heat source could be on the ground, I didn't mean to imply the magnetic field source could be too. I was imagining a device physically the same as an M2P2 spaceship, only switched on in Earth's atmosphere, kept hot by beamed power instead of sunlight, and being propelled by buoyancy rather than solar wind. My written communication seriously needs to be improved. – BenRW May 16 '14 at 15:58
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    ok, so the magnetic field source is on board. How heavy of a field source would you need to confine 1.62 cubic kilometers of plasma? http://www.phys.washington.edu/~sharpe/486/pasko_talk.pdf – DavePhD May 16 '14 at 16:09
  • thanks for the link! Perfect for my needs. If I have correctly understood the equation on page 12, I need 283.6µT on the surface of the bubble. Which means a 10m radius loop carrying 2e9 A. Given the recent superconductor critical current record of 1e6 A/cm^2, I can see that this can't work in our atmosphere. – BenRW May 16 '14 at 16:43