concept

Aerostatic lift

Lift generated by a difference in density rather than by motion — what holds a balloon up, as opposed to the aerodynamic lift that holds a wing up and demands continuous movement through the air. Its magnitude is the enclosed volume multiplied by the density difference between the gas inside and the air outside. The counterintuitive but decisive consequence is that how light the interior gas is barely matters, since every lifting gas is already nearly weightless compared with air: hydrogen beats helium by only a few per cent. What must actually be bought is volume.