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Say you wanted a ship to have a bubble of atmosphere around it, 800m in diameter with the ship at the center. What sort of gravity (and perhaps magnetic field) would be needed? Would either of these make it impossible for habitation by living plants & animals?

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  • $\begingroup$ First, what ship size are you thinking about. Second, why makes you think gravity or magnetic fields would be harmful to animals on that ship if they do the same effects as on earth. $\endgroup$ – fffred Jan 5 '16 at 18:32
  • $\begingroup$ I'm thinking if the gravity has to be so strong that it prevents movement of animals or growth of plants or the magnetic field so strong that it causes catastrophic cellular damage things would be a no-go. For this exercise assume the ship is an unsealed (open air) sphere 400m in diameter at the center of the 800m sphere of atmosphere. $\endgroup$ – LowellVice Jan 5 '16 at 19:01
  • $\begingroup$ In general there are no physical principles that would allow for such a design. The smallest planet that can hold on to an atmosphere at roughly normal conditions is probably around the size of Mars, as long as it can support a magnetic field. Without that you are looking at smaller bodies with cryogenic atmospheres like Titan that are far from the sun, but even Titan is still somewhat shielded from the solar wind by Saturn's magnetosphere, I believe. In any case, what's wrong with a glass pane? $\endgroup$ – CuriousOne Jan 5 '16 at 19:11
  • $\begingroup$ Does it not make a difference that the atmospheric volume is so much smaller than that of a planet? Could a much smaller mass suffice in holding this relatively tiny bubble of atmosphere? $\endgroup$ – LowellVice Jan 5 '16 at 19:12
  • $\begingroup$ Thanks Jen, I'm seeing a common theme between the two even though I saw that post earlier. It appears that my assumption that a "smaller" atmosphere would make a difference, but I'm getting the impression that it is not the volume of atmosphere that is the key here but the velocity of the particles vs the escape velocity of the object. I'm still trying to understand if a tiny bubble of atmosphere is any different than calculating the possibility of a small exoplanet atmosphere. $\endgroup$ – LowellVice Jan 5 '16 at 20:06
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Rough answer: you need that the sound speed of the atmosphere to be less than the escape speed of your ship at 400 m. Plugging numbers, I get

$$ \frac{(343~\text{m/s})^2\times400~{\rm m}}{2 G}\approx3.5\times10^{17}\,{\rm kg}, $$

or about 3 billion modern supercarriers. That would have to be the mass of the ship, or its equivalent gravity.

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  • $\begingroup$ That's not nearly enough gravity to hold on to an atmosphere, unless it is cryogenic. There are plenty of molecules that are much faster than the speed of sound. For one thing, the rms speed of air molecules at normal conditions is around 500m/s, for another, there are plenty that have five or six standard deviations higher velocities. You are also off by several (3) orders of magnitude in your comparison with supercarriers which have a mass of approx. 1e8kg, it seems. In addition... you would need a really dense material to cram that much mass into a sphere of 400m... $\endgroup$ – CuriousOne Jan 5 '16 at 18:50
  • $\begingroup$ See the comment I added above answering fffred's questions for assumptions of ship size, etc. Let's also assume we have a gravity drive and magnetic field generator that could generate the appropriate forces. Could the fields hold an atmosphere? Would these fields prevent life? $\endgroup$ – LowellVice Jan 5 '16 at 19:04

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