I was thinking at gases and came to ballooning.
The main problem with balloons is that they go not higher then a certain altitude.
But what if we could make a balloon with stages.
So lets say we have 3 tanks on board.
1. with helium = 0.2kg/m³
2. with hydrogen = 0.1kg/m³
3. with vacuum = 0.0kg/m³
We have 1 balloon that is filled with helium. As we go higher we steadily replace
the helium with hydrogen and after that we add some vacuum in the top of
the balloon. Would that be doable?

Grtz, Dirk.

[MOD EDIT] This post has been moved from General Discussion to Physical Sciences as it is more suited to this location - Simon Eppig 01/01/2018 @ 22:27 CET