# How much gravity (and/or magnetic field) would a space ship need to generate to hold an atmosphere around it? [duplicate]

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?

• 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. – fffred Jan 5 '16 at 18:32
• 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. – LowellVice Jan 5 '16 at 19:01
• 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? – CuriousOne Jan 5 '16 at 19:11
• 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? – LowellVice Jan 5 '16 at 19:12
• 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. – LowellVice Jan 5 '16 at 20:06

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