A hydrogen-fueled airplane made its maiden flight in Germany this past month, emitting nothing but water vapor as it flew across the sky. The electrical system on board is emission-free, and makes a lot less noise than a conventional fuel engine. The plane itself is rather small, with four seats on board. This may seem small, but it represents a major step forward in realizing the dream of sustainable airline travel. The engineers hope to revolutionize regional air travel by providing a zero-emission alternative to airplanes, busses and trains.