Super-habitats

since it would be a matter of getting the gas, id imagine that it
would be like a ram scoop operation. a rocket does a shallow skip
manuver, with the ram scoop open. the scoop would cause a bit of
drag, keeping it in the atmosphere a bit longer that other wise, but
it would still skip out of the atmoshere with a load of gas. it would
use its rockets to stablize its orbit, then using the cold of space
liquify the gas. then it would either make another entry skip or
would dock with a nonatmospheric ship to transfer the gas. Little by
little the atmosphere would be removed. Hundreds of scoops could be
used to gather the atmosphere. They could use ion jets to maintain
sufficent speed to do the skips, regaining the speed during the
orbits. I dont think getting the gas would be that difficult, just
very time consuming.