An airship, irigible or Zeppelin is a lighter-than-air (buoyant) aircraft that can be steered and propelled through the air using rudders and propellers or other thrust. Unlike other aerodynamic aircraft such as fixed-wing aircraft (airplanes) and helicopters, which produce lift by moving a wing or airfoil through the air, aerostatic aircraft, such as airships and hot air balloons, stay aloft by filling a large cavity, such as a balloon, with a gas less dense than the surrounding air.