Aerospace engineering, in simplest terms, deals with man-made objects in air and in space. More specifically, it includes two different types of engineering: aeronautical (related to aircrafts) and astronautical (related to spacecrafts). Started originally as aeronautical engineering, it morphed into aerospace engineering when we ventured into space.
The surroundings and its properties change as we go up from the surface of earth and these changes mean that science differs in space (or air) than earth surface. In simpler terms, material required for manufacturing aircraft and spacecraft is different (from each other) and different from what we use to build other transport means on earth. It also means that survival of human beings abroad these aircrafts and spacecrafts will be different. And (apart from other aspects) the propulsion system will also be different. Thus, aerospace engineering makes use of various specializations within science (including material science, fluid mechanics, propulsion, life science, astronomy, etc.) to achieve its goals.
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