Gas giant atmospheres

(speculativity warning!)

APM will probably enable regular interplanetary spaceflight thus the gas giants will become rather accessible.

Lift

Gas giants do not have no a solid surface.
There are three main possibilities too keep something aloft in the atmosphere of a gas-giant

Flying

Simple airfoils for aerodynamic lift and new APM enabled long time stable means for propulsion can be used.

Balooning

Helium does not work as lifting gas in a hydrogen atmosphere -> hot gas is needed. Practicability depends on the feasibility of light and effective thermal isolation.

Swimming

In bigger depths stiff hollow structures keep afloat at a certain depth. At low pressures (<1000bar) density rises distinctly with pressure. Thus unlike submarines the floating hight is self stabilizing. Ships can swing around equilibrium position.

Note that even when highly pressurized a hydrogen atmosphere does not become all that dense. It shouldn't be hard to estimate feasibility today.

Summary

Keeping things of practical use afloat should pose no problems but super-massive objects (like mountains as often seen in fiction) can not be kept afloat.

Energy

Solar energy is weak so far out in the solar system and only useful for very low power applications.

The easiest accessible source of energy in gas giants is geothermal heat. Simply hang down and or lift up a nanotube-cable (as heat conductor) with ventilated radiators attached at top and bottom. A thermoelectric, thermomechanic or thermochemical converter then produces the required form of energy.

fusion power is the choice for high power and long term energy supply The atmospheres are essentially fuel-pools.

Material resources

Ursanus provides 2,3 % and Neptune 1,5 ± 0,5 % of methane in their atmospheres. Further there are plenty of light non-metal-hydride ices. Given their sizes this is an ridiculous amount of building material. This would allow covering the whole planet in man made flying objects providing a giant area for habitation completely changing the planets appearance.

Human habitation

Beside Jupiter with its very high acceleration of gravity ~10g, highly radiation planet-moon systems and asteroid sucking tendency. Gas-giants actually and surprisingly would provide a rather nice place to live since the gravity of Saturn Uranus and Neptune is near 1g and the atmospheres nicely shield radiation.

At Saturn there is actually a pressure temperature range where a human can go outside without a pressure suit only with oxygen supply pressure temperature range no pressure suit t go outside

Hazards

Asteroid impacts might be more probable and frequent than on earth. The high escape velocities make leaving the planet hard and dangerous.

Atmospheric drag hinders fast transport (well the same is true for earth)

the mysterious depths

The abyss can be used as ultimate garbage can. Everything tossed down is sinking melting and evaporating. (pollution due to incomplete crackup possible?) Still doing this excessively wastes energy and energy shouldn't be wasted even when it seems abundant.

When liquid density pressures are arrived (~1000bar ?°C) density rides with pressure a lot more slowly.