concept

Ballast

Mass carried for the sole purpose of being thrown away. Because buoyancy is a comparison between displaced air and one's own weight, an aerostat climbs by becoming lighter rather than by pushing harder — so it carries water, sand or waste specifically to dump. The unforgiving part is that ballast is spent once: water released does not return, gas vented does not return. Every altitude change is a withdrawal from a finite account, and an aerostat that has run its account down has largely lost the ability to manoeuvre — which is why buoyant flight is less like steering and more like budgeting.