Astronautics, or Astronautical Engineering, is the branch of engineering that deal with machines designed to exit or work entirely beyond the Earth's atmosphere. In other words, it is the science and skill of space flight.
The term Astronautics was coin by analogy with Aeronautics. As there is a certain degree of skill overlapping among the two fields, the term Aerospace is often used to explain them both.
As with aeronautics, the restrictions of mass, temperatures, and outside forces require that application in space survive extreme situation: high-grade vacuum, the radiation attack of interplanetary space, the attractive belts of low Earth orbit. Space launch vehicles must endure titanic forces, while satellites can experience huge variations in hotness in very brief periods. Extreme constraints on mass cause astronautical engineers to face the steady need to save mass in the design in order to exploit the actual payload that reach orbit.