Rocket science
Rocket science is an everyday term for aeronautics, or aeronautical engineering, the branch of engineering that deals with vehicles that travel in whole or in part through outer space. It is compared and contrasted with astronautics, the field that covers vehicles travelling within the Earth's atmosphere. There are naturally overlapping areas within the two disciplines, so the modern tendency is to combine the two as aerospace engineering.
The term is also used when speaking of a relatively easy task being made difficult through opposition or incompetence. "This isn't rocket science, folks!"
Rocket scientist describes an aero/astro engineer. It is also used admiringly to refer to any extraordinarily intelligent person, or sarcastically to describe someone who has done something stupid.