Aerospace engineering – it's complicated (6,7)
I believe the answer is:
rocket science
(Other definitions for rocket science that I've seen before include "Proverbial activity requiring greater intelligence?" , "Space technology" , "Something very difficult to understand (involving gravity?)" , "Something said to require great intelligence (usually referred to in the negative)" , "Scenic eco-trek (anag) -- complex subject for boffins?" .)