Gas balloons are inflatable objects that can rise into the air using lighter-than-air gases such as helium or hydrogen. Synonyms for gas balloons include dirigibles, airships, blimps, and zeppelins. Dirigibles and airships refer to large, self-propelled balloons with engines and steering mechanisms, while blimps are smaller, non-rigid balloons that rely on wind currents for navigation. Zeppelins are a type of rigid airship named after German aviation pioneer Ferdinand von Zeppelin. While gas balloons were once a common mode of transportation and used in military operations, they have largely been replaced by airplanes and helicopters. Today, gas balloons are mainly used for sport and recreation purposes, such as hot air ballooning.